An Introduction to Marine Drilling - Malcolm Maclachlan

INTRODUCTION TO MARINE DRILLING Malcolm Maclachlan

MNI, Master Mariner

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THE ‘AUTHOR

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Malcol~~M~~~achl~~,ur~s ‘i:i,, , , , , bor$mDover, England in 1947 and first worked at sea attheage offourmen when he gained employment illegally on a cross,~:~, ,~,~.~, :~~,~, ; 1 , a, ~ .: , , ,i , ,., ~, ,, channel ferry through a ‘misunderstanding’ with the ship’s master. After training for three, years on the cadet ship HMS ‘Worcester’ he sailed as midshipman with the Blue Funnel Line and gained his second mate’s certificate at theage of20. Heserved as a’navigating officer with several deep sea and coast@ shipping companies, and after qualifying as a Master Mariner held command, of seven, containerships whilst still in his early thirties. He spent a year lecturing in navigation and seamanship at Leith Nautical College before joining the British drilling contractor Houlder Offshore Limited as a control room,operator/mate. He served on the semi-submersible drilling rigs ‘High Seas, Driller’ and ‘Kingsnorth UK’ in the North Sea and aboard the dynamically-positioned diving support vessel ‘Orelia’ in the Persian Gulf war zone. He has been writing and cartooning for many years and became a full-time marine writer and illustrator in 1986, when he was a casualty of the widespread cut-backs in the drilling industry caused by the slumping oil price. He has contrtbuted many articles, short stories and drawings to various marine journals and is currently writing a novel set on a North Sea rig. Married with a young family, he lives in Biggar, Scotland.

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CONTENTS FOREWORD ......................................................................... PREFACE .............................................................................. ACKNOWLEDGEMENTS ....................................................... Chapter 1: The Development of Marine Rotary Drilling

Hand-dug wells .................................................................... Spring pole drilling ................................................................ Cable tool drilling ................... .: ............................................ Rotary drilling ..................................................................... Marine drilling ..................................................................... PRINTED

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11 11 13 18 20

25 25 The parties involved .............................................................. 27 The well owner ................................................................. 27 The operator .................................................................... 28 The drilling contractor ....... ..‘.. ............................................. 30 The drilling contract ........................................................... 31 Supply and service companies ............................................... 35 Government departments .................................................... 36 The costs of drilling offshore ................................................... 42 Well types ........................................................................... 45 Petroleuti geology ................................................................ 46 The formation of hydrocarbons ............................................. 46 Migration of hydrocarbons ................................................... 47 Reservoirs ........................................................................ 47 Anticlines ........................................................................ 47 Fault traps ....................................................................... 49 Stratigraphic traps ............................................................. , 49 Unconformity traps ............................................................ .‘. . ............. 4 9 Exploration methods ............................................. 53 Offshore surveying techniques ................................................. 53 Magnetic surveys ............................................................... 53 Gravimetric surveys ........................................................... 53 Seismic surveys ................................................................. 55 Drilling rig site surveys ...........................................................

Chapter 2: Preparations for an Offshore Drilling Operation

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Chapter 3: Offshore Drilling Platform Types

57 Fixed drilling platforms .......................................................... Fixed platforms with floating drilling tenders G............, .............. 59 59 Self-contained fixed platforms .............................................. 61 Mobile drilling rigs ................................................................ 63 Submersibles .................................................................... 64 Self-elevating (jack-up) platforms .......................................... 71 Semi-submersibles ............................................................. 81 Drill ships ........................................................................ 85 Barge rigs ........................................................................ 89 Chapter 4: The Offshore Rig and its Equipment 89 Basic rig components ............................................................. 89 The drill floor ................................................................... 90 The derrick ...................................................................... 92 The drawworks ................................................................. 94 The blocks, hook & drilling line ............................................ 96 The swivel, kelly & rotary hose ............................................. 99 The rotary table ................................................................ 102 The drilling fluid circulation system ..................................... 112 Drill string motion compensation ........................................... 113 Downhole bumper subs ..................................................... 114 Surface drill string motion compensators ............................... 120 The power plant ................................................................. 123 Drilling equipment .............................................................. 123 API specifications ............................................................ 123 Drilling bits .................................................................... 130 Drill pipe ....................................................................... 134 Drill collars .................................................................... . ....... 136 Stabilizers & reamers ................................................ 139 The drill string & bottom hole assembly ................................ 139 Tubular handling tools ...................................................... 143 Other drilling tools ........................... . ............................... 145 Sub-sea equipment .............................................................. 145 The temporary guide base ................... .‘:.. ........................... 146 The permanent guide base ................................................. 146 The WellheadJcasing hanger system ...................................... 148 Well control & the blow-out preventer stack ............................. 157 The marine riser ................................................................. 162 The riser tensioning system ................................................ 4

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Chapter 5: Drilling Operations

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Running in the hole ............. ................................................ Drilling ahead .......... .......................................................... Making a connection ........................................................... Tripping ............ ............................................................... Running & cementing casing ................................................. Directional drilling ........................ ...................................... Drilling hazards . .:.. ........ ..................................................... Stuck pipe ...................................................................... Fishing .......................................................................... Lost circulation ............................................................... Kicks & blow-outs ........... ................................................ Hydrogen sulphide ........................................................... Weather & ice ................................................................. Drilling operational sequence ................................................ Moving rig onto location & running anchors .......................... Rigging up ..................................................................... ~Running the temporary guide base ...................................... Spudding in & drilling 36” hole ........................................... Running 30” casing & landing the permanent guide base .......... Cementirig the 30” casing .................................................. Drilling 26” hole .............................................................. Running KL cementing 20” casing & running the 18V4” wellhead . Running the 1g3/4” BOP stack & the marine riser .................... Drilling 17%” hole .......................................................... Logging ......................................................................... Running & cementing 133/8” casing ...................................... Making a gyro survey ........................................................ Drilling 12%” hole .......................................................... Logging ......................................................................... Running 8~ cementing 9%” casing ......... .:. . .......................... Displacing the hole to oil base mud ...................................... Drilling 8%” hole to total depth .......................................... Coring ........................................................................... Logging ......................................................................... Running 8z cementing the 7” liner ....................................... Well testing .................................................................... Well stimulation ..............................................................

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166 171 177 ~180 183 189 189 190 193 194 200 200 202 203 203 204 205 205 207 209 210 210 213 215 218 219 219 219 219 219 220 220

Plugging & suspending or abandoning the well ....................... Contingencies & weather ...................................................

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Chapter 6: Marine Operations Basic rig stability _ ............................................................... Displacement & the principle of flotation .............................. The centre of gravity ........................................................ The centre of buoyancy ..................................................... Reserve buoyancy ............................................................ The effect on the CG of adding, removing or shifting weights .... The righting lever ............................................................ The metacentre ............................................................... Ballasting & free surfaces .................................................. Ballasting conditions ........................................................... Rig structure & safety maintenance ........................................ Lifesaving & firefighting equipment ........................................ Work permits .................................................................... Standby boats .................................................................... Rig-moves ......................................................................... Navigation and pilotage .................................................... Towage ......................................................................... Approaching the location .................................................. Running anchors ............................................................. Anchor types .................................................................. Anchor patterns .............................................................. Pre-tensioning ................................................................. The moorings during drilling .............................................. Pulling anchors ............................................................... Dynamic positioning systems ................................................. Rig supplies ....................................................................... Helicopter operations ..........................................................

228 228 228 230 230 230 230 230 234 234 236 237 238 239 240 242 243 244 245 247 250 251 252 253 253 255 259 263

Chapter 7: Rig Personnel & Training ............. Semi-submersible rig personnel .................................. Jack-up rig & drill ship personnel ........................................... Rig personnel training .........................................................

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GLOSSARY OF MARINE DRILLING TERMS

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FOREWORD

Over the years, I have seen many changes in the industry from the period of growth in the 1970’s to our present day recession, I have had .the privilege to witness first hand some outstanding advances in techniques and technology and yet for me at least, drilling remains something of a mystery. Infamous for its jargon, it is as complex as it is fascinating. It’is therefore a particular pleasure to discover, after so many years, a book which is both readable and comprehensive, and which succeeds in revesling to me, as I am sure it will do for all its readers, whatever their level of interest, something of the ‘black art’ of drilling. It also gives me pleasure to think that much of the authors knowledge arises from his connection with Houlders. First as a cadet on the “Worcester” established at Ingress Abbey on the Thames, my Grandfathers home and more recently during his time offshore with the company. Houlder Marine Drilling is the offspring of the shipping company founded by my Grandfather in 1848. Interest in drilling arose in about 1973 as the result of a chance conversation with a Norwegian shipowner who introduced me to Bernard Larsen, another shipowner, who I believe to have originated the conceptual design for the famous H-3 which is illustrated on page 29. Just as the author learned from Houlder, so Houlder in turn had learned from the established drilling industry which in turn had learned from the Moho project. This was to discover by drilling, the composition of the core of the earth. As much as possible was to be drilled through water, and I think that the drillship owes more to the Moho technology than to the offshore shallows of Louisiana. Houlder in turn claims.to have originated the idea of transporting a semi-submersible drilling rig on the deck of another ship. Page 39 shows the High Seas Driller being carried by this method. I conceived the idea and made the preliminary calculations while waiting in the Boardroom of the China National Oil Company in Beiching! I think overall that Houlder has traditionally prospered by innovation and of the innovative approach used for the layout and content of this book is anything to go by, it should be an outstanding success.

Chairman Houlder Offshore Limited

PREFACE This book was’conceived during the ,author’s own quest offshore for the answers to a multitude of questions - questions that are inevitably asked by any curious ‘green hand’ or ‘boll weevil’ in his first wondrous weeks aboard a semi-submersible or jack-up rig. In a complex engineering environment such as an offshore drilling rig only so much can be deduced from a silent observation of the strange and mysterious procedures, and invariably one must repeatedly seek explanations. It is not always easy, however, or diplomatic, to ask a driller to explain the intricacies of his art when he is attempting to make a speedy ‘round trip’ under pressure from a cost-conscious ‘company man’. Nor is a weary roughneck or derrickman likely to want to spend an extra half hour in the messroom after his ‘tour’ describing the arduous work he has just been doing for a whole half day. The drilling jig-saw puzzle can therefore take a considerable time to piece together, and in retrospect I wish now that I had had the benefit of a book on marine drilling to guide me through my first tentative trip offshore. The simple aim of this book, then, is to explain to the new rig hand, to the offshore job applicant, to those on the periphery of the offshore industry who may never have the chance to go out to a rig and see for themselves, and to the interested layman and student on the ‘beach’ the main operations of this fascinating industry. It does not pretend to be a comprehensive or learned study of the oil exploration business, nor does it masquerade as an instructional manual of drilling technology; it is hoped merely to throw a little truthful light on some of the operations of an industry which all too often is portrayed by the mass media as being simply a matter of grim and grimy men heaving and slithering on a wet deck as they struggle to latch massive wrenches onto a steel pipe. That is one interesting facet of marine drilling, but there are many more which are rarely seen by,those not fortunate enough to witness the workings of an. offshore rig. For any technical errors or omissions I can only apologise to~the marine drilling fraternity and ask for their forbearance. Biggar, Scotland March 1987

ACKNOWLEDGEMENTS Thanks are due to Adrian Rose, safety officer, and Ian Edwards, barge operator, both of Houlder Offshore, for their helpful clarifications; to Jim Langley of Brown Brothers for information about motion compensators, and to Gavin Strachan, marketing manager of Atlantic Drilling, for his lucid explanation of drilling contracts. I must also acknowledge the generosity of Phillips Petroleum and BP, who provided many illustrations, and the many companies who sent me research material. Special thanks must also go to my wife Lesley for her encouragement and countless cups of coffee, and to David Gallimore of Dayton’s Publishing who had the courage to back my ide,a form beginning to end.

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CHAPTER 1: THE DEVELOPMENT OF MARINE ROTARY DRILLING Since his earliest days, man has dug holes in the earth’s crust in his search for water, salt and other minerals. Over the centuries his digging techniques have changed and become more efficient, culminating in the method today known as rotary drilling which is almost universally used in oil and gas exploration, both on land and offshore. Rotary drilling with a land rig is a complex business. On an offshore rig it is even more involved, and at the same time is made hazardous by the hostile elements. An appreciation of the innovative way in which oilmen have overcome their difficulties can be gained by looking first at the earlier, simpler drilling methods which led to the development of rotary drilling. HAND DUG WELLS

The age-old traditional method of digging a water well by hand was for one man to pound a hole in the ground with a sharp implement like a big chisel. As the hole got deeper and the cuttings started accumulating at the bottom of the hole, the digger had to load them into some sort of container which an assistant at the top of the hole then pulled out. Digging had to be temporarily halted for this to be done and the well-digging operation was slow and tedious. The walls of the hole had a tendency to cave in as it got deeper, so to prevent this the well had to be lined with some material such as wood or brick as it progressed downwards. These materials were the forerunners of what is now known as casing. SPRING POLE DRILLING

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Hand-digging was slow and dangerous for the digger, especially as he dug through a hydrocarbon-bearing zone and oil or gas started seeping into the hole. A safer and more efficient mechanical method of digging was sought and, according to ancient Chinese manuscripts, one was in use in China as early as the 3rd Century AD. These manuscripts described a method which was really only a logical development from hand-digging. The Chinese ‘drilled’ wells for brine using a percussion system in which a heavy, chisel-shaped bit suspended from a rope was jerked up and down by relays of men bouncing on a spring-board, thus progressively pounding the hole deeper.

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The Development of Marine Rotary Drilling A variation of this method used in Europe and America in the eighteenth century was called ‘spring pole drilling’. A large metal bit was suspended from a flexible wooden pole by a long rope and allowed to drop to the bottom of the well. The bit chewed briefly into the formation, like a man stabbing with a chisel, before the springing pole bounced it back up. As this happened the length of the rope would be extended a little by the ‘driller’ controlling it at the’surface, so that the bit struck with an unvarying force on each successive blow and chewed in a little deeper on each downward stroke. As the bit was reciprocated up and down in this manner the rope twisted slightly and varied the position of each blow on its descent, resulting in a roughly circular hole being dug.

ring pole drilling was the earliest form of mechanised drilling.

The Development of Marine Rotary Drilling Large quantities of cuttings naturally collected at the bottom of the hole, and as this was almost as narrow as the bit, the cuttings had to be removed by a self-shutting container, called a bailer, which was periodically lowered to the bottom. When this method was eventually introduced to western countries iron bits were used, drilling holes of only 2% to 4 inches (7 to 10 centimetres) in ‘diameter. Wells were seldom more than 240 feet (75 metres) deep, but there is evidence of a brine well being drilled to just over a thousand feet (305 metres) in the USA in the early 1840s. To hold back the wall of the hole, this American well was ‘cased’ with lengths of wood shaped into half tubes and wrapped with twine. The Chinese, on the other hand, cased their wells with hollow bamboo sticks, and are said to have drilled to depths of more than a thousand feet by this method. Spring pole drilling was limited by the weight that the wooden pole could repeatedly lift without breaking, and of course this weight included that of the rope, which got progressively longer with the depth of the well. If the rope broke and fell to the bottom of the well with the bit or the bailer attached,, the driller had to improvise a method of retrieving, or ‘fishing’, for the ‘fish’, as the lost equipment was termed. Various gadgets were developed for this purpose and thus the forerunners of today’s efficient fishing tools were devised. In the early part of the nineteenth century, whale oil was the most commonly used fuel for lighting lamps in America and Europe. The US, Britain, Norway and many other countries had large fleets of whalers, and other types of oil were not considered commercially important. In America, ‘rock oil’ was frequently found seeping into brine wells drilled near salt creeks, and at first it was regarded as no more than a nuisance. However, the busy whaling fleets quickly depleted the stocks of whales and it was recognised that sooner or later another source of lamp fuel would be required. The value of the black oil that was often found seeping out of the ground in many places was eventually realised, and this rock oil was henceforth harvested. However, it was to be some years before oil was specifically drilled for. CABLE TOOL’ DRILLING The early nineteenth century saw the rapid mechanisation of many industries in the western world, and about 18.50 a revolutionary new method of drilling called cable tool drilling was introduced. It utilised a steam engine with a crank, giving a long, regular, reciprocating motion to a heavy metal bit on the end of a rope, and it was found to be much more efficient than the old percussion method that utilized a spring pole. The engine was also used

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The Development of Marine Rotary Drilling the hoist the drilling, bailing and fishing tools in and out of the well, and the process of drilling a well thus became much faster and more efficient and was almost completely mechanised, just as it is today. As the cable tool method gained wider favour, a wide range of ingenious devices were developed to overcome problems encountered and make the job of drilling easier and safer. The now familiar drilling derrick, then made of wood rather than steel, but tall enough to house lengths of drilling, bailing or fishing tools, was introduced. Much heavier bits and more robust equipment could now be used with the steam power, and wire instead of fibre rope enabled deeper wells to be drilled. Iron casing, replacing the old wooden sheathing, could now be driven into the hole length by length as the well got deeper, so as to retain the wall and to make it easier to extract any minerals that were eventually found.

An early cable tool rig. These rigs were widely used well into the 20th century and are still found in a few places in America.

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The Development of Marine Rotary Drilling Cable tool rigs were particularly useful for drilling medium-hard rocks, but the softer rocks encountered could not withstand the spudding, or jabbing., action of the bit, and holes often caved in or allowed too much water to seep into them for drilling to continue. A new type of cylindrical steel casing was developed to replace the old iron type, and if caving occurred, lengths of it were screwed together and lowered into the hole to seal the wall of the hole. When the casing reached the bottom, drilling then continued using a slightly smaller bit that could just run through it. As each new problem zone was encountered, so more casing was run inside the last ‘string’, and a correspo,ndingly smaller bit was used to drill out the next section. In those days oilmen were not blessed with the huge variety of tools that now enable them to overcome nearly every kind of downhole problem, so if the cable tool well encountered too many hitches, and so many casing runs had to made that the bit got too small to drill with, the hole would have to be abandoned, often before reaching its target depth. The method was, therefore, slow and inefficient, but to the early pioneers of the oil industry it represented the pinnacle of drilling technology at that time. One of these pioneers, a retired’railroad conductor named ‘Colonel’ Edwin L. Drake, is now famed for supervising the drilling, between June and August 1859, of a cable tool well at Oil Creek near Titusville, western Pennsylvania, USA, that is considered to have founded the oil industry as we now know it. Drake did not actually own the well; a New York lawyer named George H. Bissell owned the land it was drilled on, and his was the idea to drill specifically for oil, since the area had been noted for its surface seepages of oil. In any event, the Drake well, as it is now known, successfully located an oil reservoir at a depth of only 691/z feet (21 metres), and oil flowed at a rate of about 19 barrels a day, or 3,000 litres, which was phenomenal for those days. Bissell became a founder of the Pennsylvania Rock Oil Company, and further successes with cable tool drilled wells made that state the leading oil producing area for the next fifty years. Its lead stimulated the growth of the drilling industry in other areas, notably in Canada, Poland and California. From the oilfields of these areas drillers took their knowledge of the newly successful cable tool drilling techniques all over the world, and the worldwide drilling industry thus began. By the turn of the twentieth century, after fifty years of continuous use and development, it was possible to drill wells by the cable tool method down to about 3000 feet (1000 metres) in favourable conditions. The heydey of the method, however, was the period from 1900 until the great economic depression of the 1930s and many of the best improvements came during this time. Steel derricks replaced the old timber structures, and bits became larger and tougher. The most important development, however, was the

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The Development of Marine Rotary Drilling

introduction of the cementing of casing. This was first done in 1903 when some liquid cement was dumped from a bailer into a well and a string of steel casing was lowered into it. A few days later, when the cement had set, the hard cement inside the casing was drilled out. The cement in the annulus between the casing and the wall of the hole was found to have sealed off a zone of water in the formation, and it had also anchored the casing to the wall of the hole. This success was further improved on when in 1910 a procedure was introduced for pumping a measured amount of liquid cement down a hole. A volume of cement was held between two wooden plugs, spaced one above the other, and was pumped down the inside of the casing, round its bottom end, and up the narrow annulus outside, where it was left to set hard. Although all the cement was expelled from the ‘shoe’ at the bottom of the casing, the plugs were retained inside it, but these were drilled out when drilling resumed with a smaller bit. Again it was found that the cement in the annulus between the casing and the hole had set hard, and henceforth in this manner each section of casing was run into the hole and cemented to surface, thus overcoming many of the formation difficulties previously encountered. Basically, the same procedure is still used today in ‘cement jobs’, and one of its several virtues is that it allows wells to be carefully planned in advance from start to finish in fields where the formation types are known. By 1918 the world’s deepest well, drilled by the cable tool method, was 7,386 feet (2,251 metres) deep,. and the technology of cable tools nearly at its zenith. It was possible to ‘spud’, or start drilling, one of these wells with a 24-inch (61 cm) diameter bit and, if necessary, cement up to sevenseparate strings of casing, from 20-inch (51 cm) diameter down to 5-inch (12.7 cm), as problems in the well were encountered. However, the rotary system, in which the drill bit was rotated under power on the end of a steel tube instead of being reciprocated on the end of a wire, was also being developed, and by then had become the preferred method for drilling holes deeper than 4,000 feet (1,219 metres). It was recognised that the great disadvantage of the cable tool method was that there was no means by which a drilling fluid could be circulated so that cuttings could continuously be brought to the surface, and so that the wall of the hole, together with any oil, water or gasin it, could be held back. . During the 1920s a type of ‘combination rig was developed which could employ both cable tool and rotary methods at different stages of the well when it was thought profitable to do so, but the rotary type of rig was by this time rapidly gaining favour. Up to 1930 there were still far more active cable tool rigs than rotary rigs, and that was the situation obtaining when the pre-

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The Development of Marine Rotary

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The Development of Marine Rotary Drilling

war depression drastically reduced drilling activity all over the world. By that time the deepest well yet drilled by the rotary method was just over 10,000 feet (3,050 metres) deep. When drilling activity resumed after the depression the industry had become leaner, fitter and more competitive, and deeper wells were required to find and exploit new oil reserves to meet the world’s revived thirst for energy. Cable tools, which could rarely drill more than 60 feet (18 metres) a day, had virtually reached the limits of their technology, but rotary rigs, which could, in favourable conditions, drill 2,000 feet (609 metres) in eight hours, seemed to offer unlimited scope for improvement. As a result, after 1930 cable tools never recovered their previous popularity, and they were gradually superseded by the rotary system. The deepest cable tool well ever completed was the Kesselring No. 1 NYS Nat. Gas. Corp., completed in 1953 at 11,145 feet and taking over 2% years to drill. Today the cable tool method is rarely used. ROTARY DRILLING

Rotary drilling evolved from the carpenter’s method of boring a hole in wood with a rotating tool which itself flushed out the cuttings. The derrick, initially introduced for cable tool drilling, was now employed as a simple crane from which a hook, suspended from a block and tackle, was moved up or down, by a wire running over a hoisting drum called a drawworks. The bit was secured to the bottom of a string of steel pipes, and was rotated by power transmitted by an engine on the surface. A rotary drilling machine was patented in 1845 but the system was first used in Texas in the early 1890s in an attempt to solve the soft rock problem that had plagued cable tools there. Up until the end of the 19th century, cable tools were still favoured for most drilling situations. In January, 1901 Anthony Lucas’s well at Spindletop, near Beaumont, Texas, was completed with a rotary rig after several attempts to complete it with cable tools failed due to running quicksand. The quicksand was easily held back with casing when the rotary equipment was brought in and the well finally blew out, producing 84,000 barrels of crude oil a day from a depth of a little over 1000 feet. The ‘gusher’ blew all the drill pipe out of the 60-foot high derrick and shot more than 200 feet into the air above it, proving beyond doubt the value of rotary rigs. On the strength of its Spindletop success rotary drilling soon held sway in the US Gulf coastal areas and was competing strongly with the cable tool method elsewhere. Although the early equipment was radically different to that used today, the concept of rotary drilling is fundamentally unchanged.

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The Development of Marine Rotary Drilling

The Development of Marine Rotary Drilling One of the most significant developments was the introduction of the rolling cutter bit, first made and used by Howard Hughes in 1909. There have been many improvements made since then to Hughes’ original design, but his basic concept remains the same today. Modern metallurgy and components now ensure that bits last many times longer than in those days, and there are now variations on the basic design to suit all types of formation. Bit development has recently been aimed at making the bit match the characteristics of the rock it is drilling, but many bits are designed to cope with a range of various rock types. Most other items of rotary drilling equipment have also undergone great changes since the pioneering days of the method and vast amounts of money have been spent by the oil companies on developing new techniques and ideas to improve the efficiency of drilling. Drilling fluids have been formulated for use in every well condition, and means have been evolved for altering the fluid’s chemical and physical properties as necessary during the circulation process. Fluid circulating systems now have greater capacities and can be more precisely controlled and powerfully pumped. Metallurgical research has discovered ways of combatting corrosion in drilling tubulars and in withstanding the stresses that are imposed from great depths, temperatures and pressures. Downhole equipment has been developed to meet every conceivable need, permitting complex tasks to be performed thousands of feet down a narrow borehole. A tool inside a borehole can only move in three fundamental directions: up, down and round. On the face of it, therefore, drilling holes in the earth’s crust might appear to be a simple job that has been made easy with modern equipment. On the contrary, it remains a highly complex operation demanding grit, determination and much technical expertise and a great deal of money, and nowhere is this more the case than offshore.

MARINE DRILLING Although land drilling for oil has been done for well over a century, it is only since the Second World War that marine drilling has really been in existence as an industry in its own right, and many of the practices now used offshore evolved only in the last twenty years. The first ‘offshore’ wells, however, were a shallow well drilled over the water from a pier at Santa Barbara in southern California in 1897, and a well drilled in 1911 in Caddo Lake, Louisiana, where a steam-powered rotary rig was erected on a wooden, bottom-supported platform.

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The Development of Marine Rotary Drilling

In the 1930s techniques were introduced for drilling in the swamps of Louisiana, USA, in Lake Erie in Canada, and in the large-scale developments then taking place in Lake Maracaibo, in Venezuela. The first floating rig was a simple barge used in 1933 to drill in the bayous of southern Louisiana. All subsequent ‘marine’ drilling, which was at-first confined to swamps and lakes, was carried out using ordinary land drilling equipment mounted on simple, flat-topped, flat-bottomed barges. As the US oil industry rapidly expanded following the end of the Second World War, so it set its sights on drilling in the seas off the US Gulf states, necessitating the development of special craft capable of supporting drilling equipment in a sometimes hostile environment. The first seagoing ‘mobile offshore drilling unit’ of any kind was a submersible platform that drilled in a water depth of only 20 feet in 1948, while the first ‘floater’ was a coverted US Army wartime transport barge that had a rig fitted on an overside cantilever. Many other ‘submersibles’ were built for use in the US Gulf area, these having decks supported by tubular, bottle-shaped columns fitted above pontoons which could be ballasted like those of a modem semi-submersible. These platforms were, in fact, the direct descendants of the modern ‘semi’, although some of them bore little resemblance to today’s heavyduty, harsh environment’ units. The next development after submersibles came in the form of self-elevating barges that had tall legs on which the drilling platform could be jacked up to sit well clear of the water, the feet of the legs resting in or on the sea bed. These craft, of which the first was built in 1954, evolved from floating docks that were used by the US army during the War. The first crude ‘jack-up’ units were used initially in the shallower parts of the Gulf of Mexico and the Arabian Gulf, but the designs grew steadily more sophisticated with the increasing demands of the industry for oil from deeper water, eventually producing units with legs 300 feet high. However, this was still not high enough. After the early makeshift barges and submersibles, converted ships were used to support drilling rigs, but as exploration moved out into deeper water, so fixed platforms were developed that could be towed into position and sunk so that they rested squarely on the drilJing location. At first these, like jack-up units, were only used in relatively calm and shallow waters near the coast, but larger and stronger platforms were later built for the deeper and less tranquil waters further offshore. In 195.5 the first well was drilled by a vessel on which a rotary rig was mounted and by 1957 a further milestone was reached when a well was dril-

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The Development of Marine Rotary Drilling

Mobile rigs of tw jack-up.

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mersible. Bottom: A

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The Development of Marine Rotary Drilling led by a drill ship in 100 feet of water. The equipment used on these occasions was the most advanced then available, but it would look primitive alongside the sophisticated offshore drilling units operating today. The ships and barges then used could only operate in a maximum depth of 600 feet, while submersibles were restricted to 90 feet. By 1960 about 70 barges and converted ships were being used for offshore drilling, much qf this work going on off California. The 1960s saw the introduction of the first semi-submersibles, which could either sit on the bottom like a submersible or float like a drill ship. About 30 of these units were built during the decade, of many different types. The number of jack-up rigs quadrupled at the same time, and the new designs started to utilise the canted legs that are now a common feature of many u n i t s . Purpose-built ocean-going drillships with large storage capacities for fuel, drilling fluids and other supplies were also developed for use in remote areas far from supply bases. They were built with their own propulsion so that they could move themselves between locations and dispense with the need for tugs. In the 1960s drillships were first fitted with one of the most important developments in deep-water drilling technology: dynamic positioning (DP) systems. These systems enabled drillships, and later, semi-submersibles, to maintain position with the aid of computer controlled thrusters that responded to the commands of position monitoring inputs and dispensed with the need for anchors. However, the systems were, and still are, expensive and the vessels using them relatively few. The upward surge in the price of crude oil and the availability of offshore concessions and exploration licences from many countries prompted a frantic spate of rig-building in the 197Os, when an average of 30 drilling units were built each year-more than at any time before or since. This brought the world’s offshore rig fleet up to over 500 units, and with this increase came many improvements in equipment and operating techniques, including the use of surface motion compensators. Having reached a level of about 750 units of which many are surplus to requirements, the 1980s have seen a fall in the number of drilling units built, along with a fall in the price of oil. Offshore technology has, however, kept,, moving ahead. Jack-ups with legs 600 feet long are now able to drill in 450 feet of water, while dynamically-positioned semi-submersibles can now operate in 10,000 feet depths. Offshore drilling is going on in nearly every maritime area of the world, and seems likely to continue and expand as the oil price again rises. Even in the Arctic wastes drilling is carried out from

The Development of.Marine Rotary Drilling

man-made islands - the latest type of submersible - whose huge caissons are floated into position, ballasted and sunk, and fortified against the powerful natural forces within the ice cap. Sea-bed cores have been recovered by a drill ship from a depth of 23,000 feet and experimental well drilling has been conducted in water 13,000 feet, although no commercial oil drilling has yet been carried out in such depths. However, the technology is available to exploit oil reserves in the deep oceans well away from continent&l shelves; it only requires a stronger demand for oil at a commercially viable price to stimulate the drawing board ideas into action.

,

-..,

CHAPTER 2: PREPARATIONS FOR AN OFFSHORE DRILLIN G OPERATION Despite the popular notion of the freedom of the high seas, drilling an offshore well is not simply a matter of an oil company bringing a rig onto a chosen location and starting to make a hole in the sea bed whenever and however it pleases. When an oil company considers that money is worth spending on exploration - and this depends largely on the current market price of crude oil-it must first decide, on the basis of available geological evidence, in what part of the world it will explore, such as in the Pearl River Basin in the South China Sea, or the Davis Strait, off Canada’s Baffin Island, or the North Sea. One region may be more attractive than another, particularly if strong incentives are offered from the host country. After acquiring permission from the host government to carry out the exploratory work, which may take months or even years, large amounts of money and time are spent in carefully examining the sub-sea geology of the chosen area for likely hydrocarbon-bearing locations. When a promising area has been identified by surveys and an available drilling site selected, the oil company must then go through the machinery of applying for the necessary permission from the controlling government authorities to go ahead with the project. This alone can take months or, with’some governments, even years to finalise. Governments have to be sure that the project will be beneficial to their own interests as well as those of the investors, and the safety of the personnel involved and the marine environment has to be regulated by binding agreements with the oil company involved. With an offshore well often costing between 510 million and f20 million to drill, and far more likelihood of it being a ‘dry hole’ than a commercial bonanza, the viability of the well programme has to be very carefully considered before large amounts of investors’ money are ploughed into the project.’ If the chances of success are calculated to be worth the risks involved, finance then has to be raised and partnerships sometimes entered into to spread the enormous costs involved. (Not every offshore well is drilled by a huge multi-national like Shell or Mobil). Then contracts for the supply of a drilling rig and its crew, and for every conceivable item of equipment or service that will be required during the programme have to be sought. Once the rig has been hired and drilling has begun at a cost to the oil company of perhaps g60,OOO a day, time is so costly and precious that not a day can be wasted on waiting for something that should have been ordered months before. With so much at stake, the drilling rig’s operation is naturally controlled extremely rigidly by the oil companies, which largely accounts for the feverish and tense atmosphere often felt onboard.

Preparations for an Offshore Drilling Operation

An operator’s equipment hire costs can be enormous, the drilling rig being just one of many elements in a field development programme.

26

Preparations for an Offshore Drilling Operation THE WELL OWNER

Theoretically, virtually anyone can own an oil or gas well, and private individuals who invest money in insurance companies, pension funds, banks and other institutions may be unaware that they might in fact 1-e a part-owner of one or more wells. However, in most cases the majority owner of an offshore well is almost always an oil company, either one of the big ‘majors’ who have interests in virtually every sector of the oil production, refining and marketing industry, or one of the smaller ‘independent’ oil companies which are purely concerned with production and sale of oil or gas. The independents, which number in their thousands in the US but are relatively few in Europe, are primarily concerned with land drilling, although they have made significant contributions in the development of offshore areas such as the North Sea. About 80% of all the land wells in the US are financed by independents, the remainder being paid for by the majors. These, which include household names such as Shell, Exxon (or Esso), BP, Mobil and Texaco, concern themselves mainly with offshore and overseas operations as far as drilling is concerned, although some majors are highly active in land drilling as well. In many cases offshore, ownership of a well is not by a single company but by a consortium of investors which may include one or more oil companies as well as virtually any type of firm or institution with money to invest in the project. A typical consortium might be composed of: Oil major ‘A’ Oil major ‘B’ Oil independent ‘C’ Power & Light company ‘D’ Textile company ‘E’ Chemical company ‘F’ Oil major ‘G’

with 16.89% of the equity with 14.48% 8.83% 4.50% 3.0% 2.0% 50.0%

Consortia often acquire rights to drill in several areas, and some may stay in existence for a number of years, while others split up after a while with very little drilling activity to their credit. e THE OPERATOR

If the well is owned outright by one company, then that company is known as ‘the operator’, since it would normally operate the well during its production phase when the exploratory drilling has been completed. In the case where a consortium exists to finance the well, one of the participating companies - usually the company with the largest equity ownership - will be

..

Preparations for an Offshore Drilling Operation

designated the operator by mutual agreement. The operator acquires the rights to drill on a location and holds the various licences required, but at the same time he shoulders the huge responsibility for the safety of the hundreds of personnel who will invariably be involved, as well as for the protection of the environment. The operator is, therefore, answerable to the government body which regulates drilling activity in the host country concerned. Most of the operators in countries such as the US and Britain have formed associations through which their own interests can be represented in dealings with governments. UKOOA, the British operators’ association, has about 40 members, most of whom are British subsidiaries of American oil companies.

THE DRILLING CONTRACTOR

In the first half of the twentieth century it was common for oil companies to own their own rigs and to drill their own wells, but nowadays most prefer, for reasons of cost-effectiveness, that specialist drilling contractors do this work for them, both on land and offshore. Drilling contractors have the necessary men and skills, and a vast store of drilling experience, and between them they maintain a large fleet of many different types of rig that any operator, from a giant major to the smallest independent, can call on to tackle any type of drilling job. Even the largest oil company, on the other hand, is unlikely to have a wide range of rig types in its own fleet. Some drilling contractors are active solely in marine drilling, while others own both land rigs and offshore units, which might include fixed platform rigs as well as floaters and jack-ups. Some marine drilling companies own just one elderly barge rig in the US Gulf, while others own large fleets of modern jack-ups, semi-submersibles and dynamically-positioned drillships that are scattered from the Canadian Arctic to the jungle creeks of West Africa. The drilling contractor might own the units he operates, or he might manage some or all of them on behalf of other owners such as finance houses or shipping companies. Some marine drilling contractors have their roots in the shipping industry and diversified their activities into drilling when their traditional cargo fleets dwindled in the mid-seventies. Many others have graduated to the offshore sector from long-established land drilling backgrounds: In most cases the drilling contractor has no equity interest in the well but is contracted to the well operator only to drill the well to a required depth and nothing more. Exceptions do occur, especially where an oil company owns its own rigs, but these are relatively uncommon.

‘I

Preparations for an Offshore Drilling Operation

..

Preparations for an Offshore Drilling Operation The marine drilling contractor might directly employ the entire crew of the offshore drilling unit, comprising the drilling, marine, engineering and catering departments, or he might sub-contract some of the manning out to a crewing agency where this is to his financial advantage. When the price of oil is low, reflected in a slump in drilling activity and the mass ‘stacking’ or laying-up of rigs, contractors need to have the flexibility in their crewing arrangements to accommodate the lower hire rates that working rigs are able to command. The entire catering department, for example, is in many cases nowadays composed of agency personnel who are unlikely tube so well paid as the directly-employed staff of the drilling contractor. THE DRILLING CONTRACT When an operator plans to drill an offshore well, he seeks tenders for the job from a number of selected marine drilling contractors. Three or four months before the well is to be ‘spudded’, or begun, a telex is sent to the contractors outlining the well programme and its requirements in terms of the rig and, drilling equipment. On the basis of the replies from any interested contractors, the operator then sends out the bid documents along with very detailed specifications of the type and capabilities of the rig required and the equipment to be used for the programme. The specification contains numerous detailed stipulations on every matter concerning the rig and its operation, all of which the contractor must be able to meet. The drilling contractor is then able to assess his costs were he to take on the work, and depending on market conditions he makes a bid for the contract, quoting a price that he thinks he can command in the prevailing industry climate. When the market is in his favour, this might far exceed his breakeven level, but more often than not, at the time of writing, it will mean operating at a loss. In early 1987 the break-even point for a typical modern semi-submersible’s operating costs (which exclude bank loan repayments and depreciation) was in the region of $18,000 a day, while many rigs of this type were commanding day-rates of only $12,000, or even less. However, as the price of oil rises, so rig day-rates generally rise, and semi-submersibles have in the past obtained as much as $95,000 a day when the oil price has been high. A rough guide used by some marine drilling contractors is-that they need approximately 10% of the capital cost of the rig to adequately cover all their costs. Thus, when a rig has,been bought for $60 million, about $60,000 a day is needed for profitability. The various contractors in contention for the job make their bids for the contract and the operator evaluates all the bids on their individual economic merit, considering factors such as past performance of a contractor, his ability and integrity, his safety record and the present location of his rig and the time needed to re-locate it.

Preparations for an Offshore Drilling Operation When a rig is being taken over by one operator after the expiry of a drilling contract with another, the new operator’s responsibility normally commences at the time of ‘racking’, or securing onboard, of the last anchor pulled in at the previous well site, immediately before the transit to the new location. It may not be in the best interests of the operator, therefore, to hire a rig that is in every other respect suitable, if it is lying at an uneconomic distance from the planned drilling location. At the same time, a contractor who has the right type of rig lying very near to the new location, and who claims he can complete the well faster than all the other bidders, might not get the contract if his safety record is less than commendable. The document of hire of the rig is called the ‘drilling contract’. This performs the same function as a ship’s charter party, laying down each party’s responsibilities, but it is much more detailed, running to as much as a hundred pages or more. It stipulates, amongst other things, the rates that the contractor will receive for each type of operation during the well programme. There will be a top rate for normal drilling operations, a slightly lower ‘standby rate’ for periods when drilling has to be suspended whilst waiting on equipment, a lower-still ‘repair rate’ for periods of downtime when the contractor’s own machinery has failed, a ‘force majeure rate’ for situations out of the contractor’s control, such as a strike in a supply base, and a ‘moving rate’ for periods in transit between two wells of the same well programme. The hope of both parties, naturally, is that the drilling bit will be on the bottom of the hole for as much time as possible. SUPPLY AND SERVICE COMPANIES In the course of any offshore drilling operation a large number of companies, apart from the drilling contractor, are invariably called upon to perform certain specialised jobs and provide special equipment of one sort or another. The operator may directly contract these supply and service companies, just as he contracts the drilling contractor, or else the drilling contractor sub-contracts them, but in any event, the final cost will ultimately be borne by the operator. Running casing, cementing, mud logging, diving, fishing (or debris retrieval), inspection, and directional drilling are all typical services which are commonly put out to tender, while items such as drilling fluid, cement, fuel and water are amongst the essential supplies which must be received regularly on demand from supply bases. There are numerous firms in the oil industry specialising in the manufacture of individual items of oilfield equipment or the provision of specialist help, and their products and services are described in four large volumes of standard reference catalogues which run to nearly 8000 pages in all and are found in the toolpusher’s office on all rigs. Drill pipe, for example, can be

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31

Preparations for an Offshore Drilling Operation

9.0

COMPENSATION

9.01

ilay Rates:

AND

REWNERATION

~11 rates given ace per 24-hour day or pro rata for part of a day to the nearest half-hour, and shall apply as specified below or elsewhere in this Contract, from the Commencemenr Date until termination of this C""traCf. a) Operating and Moving Rate: us $ This rate shall be payable at all times when no other rate under this Contract applies. b) Standby Rate: 0s s This rate shall be payable if work shall not be capable of being carried out by reason of weather conditions, instructions from COMPANY to cease operations hereunder, the failure or non-operation of COMPANY equipment, lack of any supplies or personnel by reason of delays in COMPANY-provided transportation, or the failure of COMPANY to obtain such lfcences as may be required to permit CONTRACTOR to carry oat operations hereunder in the Operating Area. c) Repair Rate: us $ The first 72 hours of repair time within each calendar month shall be at Operating Rate. Thereafter CONTRACTOR shall be paid the Repair Rate for any unscheduled shut-down of the Unit, excepting periods for routine maintenance or lubrication of the Unit and its equipment, changing of the mud-pump fluid-end parts, repacking swivels, the slipping and cutting of drill line, or pulling of drillstring to If a single repair period shall Last for more than effect repairs. 15 days after the Repair Rare is applied, CONTRACTOR shall after that time receive only 50% of the Repair Rate. d) Force Majeure Rate: US $ This rate shall be payable during any period in which operations are suspended because of Force Majeure, as defined in Clause 12 below. 9.02

InVOiCing a) Within the first 10 days of each month CONTRACTOR shall submit an invoice to COMPANY setting out the sums due to CONTRACTOR under this Contract in respect of the work carried out during the previous Invoices for re-imbursibles shall be submitted as and when month. the relevant information is available to CONTRACTOR. Invoices shall be submitted to the address for COMPANY given io Clause 15 below. b) COMPANY shall pay each invoice within 30 days of the date of the invoice. If COMPANY disputes part of an invoice, the portion not in dispute shall be paid within 30 days of the date of the invoice, and the disputed element shall be paid over as soon as agreement is , reached, wiih interest at the rate of 1.5% per mooch from the due date of payment up to the actual dare of paymenr.

The drilling contract is the legal document of hire ot a rig. ‘I

32

his IS an exrracr.

Preparations for an Offshore Drilling Operation

supplied by more than forty manufacturers, while blow-out preventers are made by over thirty firms. Whenever a problem arises with the equipment supplied by one of these firms, their technicians - ‘service hands’ as they are called - are brought out to the rig to effect speedy repairs. Consequently there may be many personnel on an offshore drilling unit who are only aboard for a few days and who work for a variety of different firms. The operator also has to have craft available to get all this equipment, and the men who will use it, out to the drilling unit. In most areas of the world this means chartering supply boats for the heavier and bulkier cargoes, and helicopters for the men and the lighter, or more urgent, small items. In smooth-water areas, however, crews often travel by fast launch since these are cheaper to hire than helicopters. Two or three supply boats, not necessarily all owned by the same company, might be chartered for the duration of the well programme, while others might be ‘spot chartered’ as required on a single voyage basis. Normally one helicopter company is contracted to provide a regular flight schedule out to the unit, with additional flights being paid for as required when extra personnel or freight have to be transported. In~addition to hiring supply boats, the operator has to provide a safety boat in some parts of the world, to stand by close to the rig whenever it is manned in case of an emergency requiring its evacuation. The standby boat may never actually be used in earnest, but it is an unavoidable expense which must be borne nevertheless as a condition of holding a licence to explore. Re-locating many mobile rigs from a previous location will involve using anchor-handling vessels. These are often dual-purpose ships that may become the rig’s supply boats once their anchor-laying work has finished. Because of their more arduous duties, for which they need much greater power, anchor-handlers normally command higher rates than supply boats, and different rates again will be earned by boats in different power categories. In a particular week in early 1987, for example, an anchor-handler of 12,000+ brake horse power could earn E2,850 a day in the North Sea, while a vessel of 8,000-10,000 bhp could earn E2,650. Large supply boats commanded around %2,50$ while smaller vessels on supply runs could command a maximum of f2,400. Charter rates fluctuate wildly from week to week according to demand and availability of vessels, and rates much higher, as well as much lower, might prevail according to the dictates of the market. As far as the well operator is concerned, he usually wants his boats to be reliable as well as cheap, and he would rather have an expensive boat that can stay ‘on location’ alongside a rig with essential supplies in bad weather than a cheap but unreliable boat that delays the drilling programme. -.-

33

Preparations for an Offshore Drilling Operation

Rigs are constantly hungry for equipment. Top: Casing being loaded. Bo,ttom: Some of the stock of equipment on a semi-submersible’s deck.

Preparations for an Offshore Drilling Operation GOVERNMENT DEPARTMENTS Before the operator can begin any offshore exploration, he must usually obtain permission in the form of a licence from the government in whose waters the proposed well will be. By a United Nations convention, maritime nations have sovereignty over large offshore areas for the purposes of the development of oil and gas resources, and almost every sea in the world is divided by the median lines of all the bordering countries. The North Sea, for example, is shared between Norwdy, Denmark, West Germany, HOIland, Belgium, France and the United Kingdom., each country administering a sea area roughly in proportion to the length of its own bordering coastline. For the purpose of administration of oilfield activity and the award of licences, offshore waters in most maritime areas of the world are divided into numbered blocks. In the British sector of the North Sea these are defined by lines of latitude at ten-minute intervals and lines of longitude at twelveminute intervals, making thirty blocks in each one-degree by one-degree square on a chart. On the British continental shelf all oil and gas exploration and production activity is regulated by the UK government’s Department of Energy, and from time to time a number of blocks will be put up for auction by the DEn in the hope that oil companies will make bids for the exploration and production rights on them. The DEn grants two kinds of licence, one for exploration, which in this context means geophysical surveying and bottomsampling, and the other for production, which includes exploratory drilling. Licences are only awarded after a thorough examination of an applicant operator’s proposed drilling programme and his ability to carry it out safely, as well as his survey work on the area, so that the government will be satisfied that he will operate safely and in the best interests of the nation and its resources.

-b&j t

,. .

117’ ..,+..

2

*i ..A_...

2

Part of a North Sea oilfield map. Several fields are. grouped in one area spanning the median line between the British and the Norwegian sectors. . .. -35

Preparations for an Offshore Drilling Operation Once the operator has been granted his licence to explore and has awarded his drilling, supply and service contracts, drilling can theoretically begin. However, there are usually many other official regulations to be satisfied before he can legally bring his rig on location. While the Department of Energy is primarily concerned with the regulation of offshore exploration and production activity, including the operation of fixed platforms, the safe operation of British floating vessels on the oilfields remains the province of the Department of Transport. The DOT is reponsible for the registry and regulation of all British ships as far as their safe manning and operation is concerned, and as such it has an interest in all British-flag mobile offshore drilling units, wherever they are operating. It is also the examining body for British seafarers’ certificates of competency and it decides the minimum manning scale for individual rigs, so that whatever the unit’s operational mode, whether drilling or in transit, and whether on a long ocean passage or a short shift of location on the same oilfield, the vessel will always be safely manned with a properly qualified crew. This only applies to marine crew; the DOT does not make regulations governing the carriage or certification of any other personnel onboard. The American body with broadly similar official powers to the DOT is the US Coast Guard, which enforces US marine regulations on American-flag rigs working in US and overseas waters. Mobile offshore drilling units are, therefore, treated by government departments like ships in some respects, but like fixed oil installations in others, and the interests of the different government bodies involved sometimes overlap. THE COSTS OF DRILLING OFFSHORE Drilling an offshore well can cost ten times as much as drilling a land well, and an operator’s expenses might well run to $100,000 a day for 100 days or more. In sea areas such as the North Sea or the Canadian Arctic costs are raised due to two main factors. One is the harsh operating environment, which necessitates rigs and equipment which are more robust and therefore more expensive than those needed on land and in less hostile sea areas such as the Arabian Gulf and Lake Maracaibo. The other is the longer time required for drilling the well, due partly to the harsh conditions and partly to the need to use special additional equipment. Minimising the time spent from ‘spudding’ to completion of the well is usually the most important factor to any operator, and oil companies are normally prepared to pay whatever is necessary to obtain the right kind of reliable equipment and services to get the job done expeditiously. The overall costs of any exploratory drilling venture can be grouped under three main headings: initial costs, equipment costs and operating costs. Initial costs cover the preparatory work necessary before any drilling starts, including seismic surveying, the purchase of a licence and the annual licence rental fee.

Preparations for an Offshore Drilling Operation

Top: Seismic survey ships are usually small vessels but contain a great deal of sophisticated equipment. Bottom: A modern semi-submersible rig moving under its own propulsion on sea trials.

Preparations for an Offshore Drilling Operation Seismic surveying is carried out by specialist companies who operate special seismic survey vessels and sophisticated equipment and employ highly trained seismologists and geologists. Although it is quicker and cheaper to carry out a geophysical survey over the sea than on land, it is still very expensive, and oil companies often club together to pay for surveys of large areas before going their separate ways to investigate smaller concessions at their own individual expense. The survey company is usually hired on an area basis, for example to investigate a particular block, and payment is made on the basis of the number of ‘line miles’ of seismic shot. The price the survey company charges reflects not only its own costs but also the importance of its findings to the oil company in accurately determining the position of possible oil- or gas-bearing geological structures, thereby saving the operator expensive but wasted drilling time. As already explained, most offshore operators hire the services of specialist contractors to carry out nearly every function onboard a drilling unit including the actual drilling operation itself, and their own equipment costs are therefore low. A few majors such as Shell and BP, however, still own their own rigs, unlike most oil companies which hire drilling contractors’ units and crews on a time basis. As the quest for richer oil reserves pushes out the frontiers of exploration into more northerly and deeper waters, so offshore rigs are becoming more robust and sophisticated. However, these improvements are reflected in the enormous cost of a new rig. The latest semi-submersibles, which can drill holes 25,000 feet deep in all but the severest weather in water more than 2,000 feet deep, cost more than $110 million to build, even before the drilling tools are put onboard. Their capital cost can be split into three main components: the basic hull, the drilling equipment and the ancillary equipment such as the power plant, auxiliaries and accommodation. The basic hull generally makes up half of the total cost, with the drilling package amounting to and third and the ancillary equipment the remainder. However, these proportions vary with the degree of sophistication of the unit and the unit’s type. With such an enormous investment required, rigs are normally only ordered from building yards on the basis of secure work contracts for a period of months or years ahead. . Whether it is hired from a contractor or owned outright, the largest single cost to the operator is invariably that of the drilling rig. He normally starts paying for its hire as soon as it has completed its previous contract with another operator, and the cost of relocating it is one of the major factors considered in its selection. Towage costs, if these are involved, vary with the length and the difficulty of the tow, but are usually high. In this respect, an

38

,

.-

Preparations for an Offshore Drilling Operation advantage of drill ships over nearly all jack-ups and most semi-submersibles is that they travel under their own power, often at considerable speed, and there are no towage costs involved. For newly-completed rigs built in distant yards, carriage aboard special semi-submersible heavy lift ships, usually operated by specialist Norwegian or Dutch heavy-lift shipping companies, is sometimes chosen in preference to prolonged, difficult and risky ocean towage.

Carriage by special ship is becoming a popular way of transporting rigs over long distances. .

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10

Preparations for an Offshore Drilling Operation

The hire costs of mobile offshore drilling units vary considerably with the type, sophistication and capabilities of the individual unit, with the overriding factor being the current price of crude oil, which determines demand to a large extent. Because of the worldwide slump in exploratory drilling activity caused by the low oil price, day-rates were generally far lower in early 1987 than rates obtaining only a year earlier for similar rigs. Day-rates for semi-submersibles peaked in the period 1980-82 with sums of over $90,000 being the norm for some types, but by the end of 1986, a rig of this type could be hired for less than $10,000 a day, and numerous units of all types were stacked all over the world, keeping hire rates at a ruinous low for many drilling contractors. There were calls for the scrapping of older rigs to relieve the over-tonnaging that had dogged the rig mark~et since the mid-seventies building boom, and this has been put into effect, with the fleet’s size of about 750 units worldwide beginning to show a net loss. The overall cost of hire will naturally depend on the time the drilling contractor takes to drill the well, and this can depend on many factors, not least of which, in the North Sea and other hostile areas, is the weather. Wells in the more placid southern half of the North Sea may only take about 65 days to drill, while 100 or more days might be needed for a well of the same depth further north, where time spent ‘waiting on weather’ may account for a considerable part of the total cost. But the weather is only one of the hazards which may extend the drilling period. Problems in the hole, such as lost and irretrievable equipment necessitating a deviated hole being drilled round the problem,zone are by no means uncommon, and a planned three-month well can easily end up taking five months or more to actually complete. The other operating costs during drilling as far as the operator is concerned are mainly the fees charged by the specialist firms for the services they provide during the drilling operation. These expenses are for supply boats, anchor-handlers, helicopters, materials such as pipe, casing, mud and chemicals, fuel and water, and well and mud logging services. Mud logging and well logging is invariably carried out by specialist companies using their own personnel and equipment. While mud logging can be continuously carried out whilst drilling, well logging sometimes requires the hole to be emptied of equipment like drill pipe, and payment is consequentljr charged on a pounds-per-hole basis. Almost none of the oil companies use their own supply boats, which, in line with the fall in rig hire rates, can, (in early 1987,) be chartered for substantially lower rates than in previous years. The same applies to helicopters, which in the North Sea, the Arctic and many other parts of the world are

40

..

Preparations for an Offshore Drilling Operation

,. ,

Rigs designed for the Arctic are more expensive to build than most.

,

Preparations for an Offshore Drilling Operation indispensable. They are normally hired for a monthly fee, plus an additional hourly rate when the machines are actually in use, on top of which the oil company pays for the fuel used on its flights. Drilling rig crew wages are normally paid by the rig owner or manager, and an operator will only be responsible for the wages of its resident supervisor and a small number of experts which may include a drilling engineer, a geologist and a materials co-ordinator. For the drilling contractor, however, the wage bill is a major cost, since a semi-submersible in drilling mode is normally ctewed by between fifty and sixty personnel, excluding any operator’s staff or ‘service hands’. The driliing contractor also pays for his crew’s food, their insurance and their travel to and from the base heliport, although the crew are transported out to the rig in the helicopters paid for by the oil company. North Sea drilling rig wages at one time reflected the input of expertise contributed by American oilmen in the development of offshore drilling skills and techniques, but in recent times the American element has largely diminished to the point where only a few supervisory staff remain. The wages of North Sea rig crews have fallen behind those of their American counterparts, and have been restrained somewhat by the slumping oil price, but by shore standards they are still high. To keep this in perspective, however, it must be remembered that rig crews often do arduous and unsatisfying work in difficult conditions for twelve hours out of every twenty-four, for fourteen days at a time. On the rigs that they spend half their working lives on their only comforts are good food, clean accommodation, freshly laundered working clothes for every shift and a daily movie show, and many shoreside workers would find an isolated offshore existence in these sort of conditions far from ideal. But virtually every company involved in the drilling business appreciates this, and is usually prepared to reward its offshore employees accordingly. WELL TYPES To most of the personnel on an offshore drilling rig the well being drilled is simply ‘the hole’ whatever its purpose in the operator’s scheme, but every well can be labelled according to its function. Most of the names likely to be met with are listed below. Wildcat Well: An exploratory well drilled in an unproven area, remote from any existing producing well. In some areas only about one wildcat in ten is productive, but even the dry holes yield valuable information from core samples about the geological structures in the area, and on pressures and temperatures to be expected in other wells drilled. The drilling of a wildcat might

Preparations for an Offshore Drilling Operation be in complete isolation from other rigs, ‘wildcatting’ in the same area. Only about yields petroleum, although in the North Sea the US it is one in nine. However, the yield

or there might be other units one wildcat in forty worldwide the figure is one in four and in might still not be ‘commercial’.

Exploration or Exploratory Well: A well drilled in a search for a new reservoir of hydrocarbons. It might be a wildcat well, or a well drilled on an existing field to seek a new productive formation. Discovery Well: An exploration well that produces evidence of oil or gas in commercial quantities. Wells that produce ‘shows’ of uncommercial hydrocarbons are labelled ‘dry holes’ or ‘dusters’. Oil Well: A well that produces hydrocarbons in a liquid state from an underground reservoir. Gas Well: A well that produces hydrocarbons in a gaseous state from an underground reservoir. Sometimes a well that was hoped to be an oil well yields gas instead. If the gas is in commercial quantities it might be sold when equipment is installed for exploiting it. Dry Hole: A well in which no commercially significant evidence of hydrocarbons is found. Dry holes are usually plugged and abandoned. They are sometimes called ‘dusters’. Appraisal Well: A well drilled following the drilling of a discovery well in order to determine the extent of the reservoir or field. Several appraisal wells may be drilled, each close to a discovery well, in order to map out the outline of a new field. In this case they are called ‘step-out wells’ or ‘delineation wells’. Development Well: A well drilled after a discovery well, and usually after several appraisal wells, to commercially exploit an oil or gas field. Most development wells are drilled from fixed platforms built once the field has been appraised. They may be ‘vertical wells’, but are very often ‘deviated wells’. . Step-Out Well: A well drilled close to a discovery well but in an unproven area, so that the boundaries of the producing formation can be determined. Depending on the phase of the operator’s programme, the step-out well might be further classified as a development well, an appraisal well or a delineation well.

43

Preparations for an Offshore Drilling Operation

Preparations for an Offshore Drilling Operation Infill Well: One of a number of wells drilled to fill in between established producing wells on a block or concession. They reduce the spacing between wells in order to increase production from the reservoir. Stripper Well: A well in its later stages of production, when its yield is reducing. For US pricing and taxation purposes a stripper well is defined as a well producing 10 barrels per day or less. The term is more usually heard onshore than offshore, since an offshore well producing as little as this would not be commercially viable. Satellite Well: A well in a field that is being tapped by a production platform’s own wells, but which was drilled independently, and usually vertically, by a mobile rig and later tied in to the platform by a sub-sea production pipeline. Re-Entry Well: A well that is re-entered following earlier plugging for some reason. Wells are usually plugged and abandoned if found dry, or plugged and suspended if ‘commercial’ but not required to be completed at the time of drilling. They might be re-entered and production equipment installed when the price of oil~is more favourable. Vertical Well: A well that is drilled without intentional deviation from the vertical, except perhaps to sidetrack an obstacle. Deviated Well or Directional Well: A well drilled at an angle from the vertical, so that a reservoir is tapped at some distance horizontally from the surface location. Most wells drilled from fixed platforms are deviated wells. PETROLEUM GEOLOGY Many types of crude oil are found in the ground in different parts of the world, but all are natural products of the earth, complex mixtures of chemical compounds called ‘hydrocarbons’ and ‘no&hydrocarbons’. In this natural state crude oil is properly known as ‘petroleum’. Geologists and other scientists have differing theories on the precise origins of petroleum, but there is no doubt that it occurs mainl; in what are termed ‘marine sedimentary rocks’, especially in sandstones, ddlomites and limestones. Like coal, petroleum is a fossil fuel, and since it is an organic substance, organic matter must have been present during the process by which it came to accumulate in the reservoirs it is today found in. This process may never be completely understood, but it is thought to have been a combination of a chemical and bacteria1 action that took place millions of years ago. ..

4s

Preparations for an Offshore Drilling Operation THE FORMATION OF HYDROCARBONS In prehistoric times the earth was covered by dense vegetation and by seas that teemed with minute living organisms, or plankton. As quantities of plants and animals died, layers of dead and decaying organic matter built up on the ground and on the seabed. Meanwhile, sand, gravel and earth was being eroded from the land by the weathering action of wind, water and ice. The resulting silt flowed down the rivers to merge with the decaying plankton, forming sedimentary deposits on the flood plains of the lower river reaches, in the delta estuaries and on the sea bed of the waters beyond the coastline. These deposits continued to build up over the ages, each layer in turn becoming buried by another. Sealed deep inside them, starved of the oxygen that was necessary for the slow decaying process to continue, were the dead marine organisms. The lowest layers of sediment gradually became compressed and heated by the enormous layers above, and eventually they became so compacted that they were tranformed into soft, permeable, sedimentary rocks such as limestone, chalk, sandstone and shale. Under the combined effects of pressure, heat, bacferial action and age the organic deposits in these rocks, their oxidation halted, were eventually tranformed into the substance that we now know as petroleum, and as more and more weight was added above, so the petroleum was forced to seep into adjacent rocks. MIGRATION The newly-formed oil was trapped in the minute pores of the coarse-grained, permeable, sedimentary rock, forming a large ‘reservoir’ in the same way that a sponge can hold a reservoir of water. But since the oil was lighter than water it was able to move, or ‘migrate’, within the rock and displace any water ahead of it. Its natural tendency was to migrate upwards through the pores of any permeable rocks, although it moved perhaps only a matter of a few centimetres each year. Eventually, however, it either reached the surface of the earth and seeped out of the ground, or it arrived underneath a layer of hard, fine-grained, impermeable rock through which it could not pass. In many cases it was possible for the petroleum to continue migrating horizontally below this ‘caprock’, but if the geological structures around the caprock were such that it could not move any further, an accumulation formed underground, rather like air trapped under a cup immersed in water. This trapping, millions of years ago, of large volumes of hydrocarbons thus made possible today’s oil and gas drilling industry which searches for new energy accumulations for humanity. The science of petroleum geology is devoted to discovering where these accumulations are likely to exist.

46

Preparations for an Offshore Drilling Operation RESERVOIRS Petroleum geologists classify hydrocarbon accumulations in three types, termed ‘anticlinal traps’, ‘fault traps’ and ‘stratigraphic traps’. By far the majority of known offshore reservoirs occur in either anticlines or faults, which are known collectively as ‘structural traps’. Examples of structural traps are ‘domes’, ‘faults’, ‘folds’ and ‘unconformities’. In domes and folds the petroleum is trapped at the top of the structure, while in faults and unconformities it is trapped when an impermeable layer lies next to a permeable layer containing the petroleum. ANTICLINES The most common form of trap is the ‘anticline’, which is a formation with an upwardly folded convex structure, looking like a. hummock of sandwiched rocks lying deep underground. In an anticline a coarse-grained, porous, permeable reservoir rock is capped by a fine-grained, relatively impermeable formation such as clay, shale, mari or salt that has folded up to form a dome or inverted bowl, leaving a cavity below the summit in which oil or gas can accumulate. Inside the trap, oil will lie on top of any water present, and any gas will lie over the oil. FAULT TRAPS Fault traps are commonly found where there are sideways displacements of great sections of subsurface rocks. Millions of years ago great stresses caused sections of the earth’s crust to crack, and the two split, or ‘faulted’, faces would be forced to slide across each other so that the formations lying in each face were no longer lined up with each other. Occasionally it happened that a permeable layer moved so that it now faced and was sealed by an impermeable layer which was at a shallower depth. A ‘fault’ trap was thus formed, any petroleum accumulating in it being prevented from escaping by the barrier of the impermeable layer. The existence of a fault trap does not guarantee the existence of oil in the trap, however. Sometimes further movements of the earth’s crust allowed previously trapped oil to leak out, and in other cases oil never reached the trap. This partly explains why so many ‘dry holes’ are drilled after a fault trap has been accurately located by geologists. Large petroleum accumulations in fault traps have been found in the Niger Delta, the US Gulf coast and in SE Asia, and many of the reservoirs in the North Sea are in this type of trap.

Pr4 eparations for an Offshore Drilling Operation

FAULT TRAP

v

ANTICLINE

HYDROCARBON TOP : A fault trap. Bottom: An anticline.

48

RESERVOIRS

Preparations for an Offshore Drilling Operation STRATIGRAPHIC TRAPS Stratigraphic traps are seams of oil trapped inside envelopes of impermeable sealing rock such as impervious clay or shale which prevent further migration of the oil. The shape of the trap is not caused by the structure of the surrounding rock formations but only by the sedimentary process that left a void in the rock while the surrounding sediments became hard. Stratigraphic traps are harder to detect than faults or anticlines, and many of the discoveries in them too date have been accidental. However, as geophyisical survey techniques become more sensitive and accurate, many more will probably be found all over the world. UNCONFORMITY TRAP Unconformities are usually regarded as stratigraphic traps but they are really structural features. In an unconformity trap the reservoir rock is slanted up towards and cut off by a fine-grained, impervious rock that lies across its end, resulting in a wedge-shaped reservoir. Several important North Sea oilfields, as well as the North Slope of Alaska are over unconformities, but they are relatively uncommon compared with other trap types. In some cases the fine-grained rock sealing the wedge-shaped reservoir may actually be the source of the petroleum, migration in this instance having been downwards rather than upwards. EXPLORATION METHODS The objectives of the oil companies in their search for hydrocarbons, whether on land or at sea, are to locate potentially oil- or gas-bearing structures such as the anticlines, faults and stratigraphic traps described above, to test them for hydrocarbons, and then exploit any reservoirs of oil or gas found. Unfortunately, whether or not oil or gas actually exists in a particular area cannot be determined by interpreting geological maps or even by readings of the most sensitive instruments on the surface, but if the right kind of potentially oil-bearing structure can be identified, a chance of finding oil or gas in it may exist. The proof of the matter can only be established then by exploratory drilling. In the early days of land exploration for oil, before the advent of marine dfilling, petroleum geologists were not equipped with sophisticated surveying. devices like those used today, and they had to rely to a great extent on ‘field maps’ to assist in the location of the right types of formation. Field mapping, in which rocks were collected from all parts of the area under investigation, helped to establish the ages and chronological sequence of sedimentary rocks and to determine the position of possible oil-bearing structures.

..

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Preparations for an Offshore Drilling Operation

STRATIGRAPHIC TRAP

UNCONFORMITYTRAP

HYDROCARBON RESERVOIRS Top : A stratigraphic trap. Bottom: An unconformity trap.

. ..

Preparations for an Offshore Drilling Operation

Nowadays, although petroleum geologists have the benefit of highly sophisticated survey equipment, field mapping is still important in the search for oil. At sea, cores are taken by special core-drilling vessels from the sea bed in the area under investigation, and are sent to a laboratory where they are examined, first by palaentologists for fossils, and then by sedimentologists to determine their nature and that of the basin where they accumulated. Geochemists will also test the samples to determine the degree to which the organic matter inside them has been changed by the effects of pressure, heat and age. The findings of these three specialists are together used to reconstruct the history of the sedimentary basin under investigation. To enable particular formations to be dated, which is essential to the search for petroleum, geologists have had to devise a ‘geological time scale’ against which rocks and their fossils could be compared. This time scale covers the periods within the great ‘epochs’ of the earth’s history that petroleum geologists are really interested in - the 600 million years or so from the ‘primary’ or ‘Palaeozoic’ epoch that followed the formation of the first life forms, through the ‘secondary’ or ‘Mesozoic’ epoch that began about 220 million years ago, to the relatively modern ‘tertiary’ and ‘quaternary’ (‘Cenozoic’) epochs. Within each epoch, (sometimes called ‘ages’ or ‘eras’) are ‘systems’ when rocks of different types were laid down. Within the Mesozoic epoch, for example, the Triassic, then the Jurassic, and finally the Cretaceous rock systems were laid down. These are further subdivided into the Upper and Lower Cretaceous, Upper, Middle and Lower Jurassic, and so on, each subdivision often being associated with a particular geographical area where the rock type is found. Oil and gas might be discovered in the rocks of almost any of the epochs and systems, with the exception of the most modern formations of the Quaternary epoch. However, in a particular basin it is more likely to be found in some than others. In the North Sea, for example, numerous fields have been discovered in Jurassic rocks, while only one has been developed in the Carboniferous rocks of the Palaeozoic epoch and none have been found in the Cretacious. , As an exploratory well gets deeper, many different formations of various types of material are encountered, such as claystone, shale, sand and limestone, each of a distinctive colour’and sometimes mixed with other materials. These are the rocks of the different epochs and systems which have been laid down in their chronological order, so that the oldest formations are at the deepest depths and the youngest are at the sea bed. To aid identification by -.-

51

Preparations for an Offshore Drilling Operation

THE GEOLOGICAL TIME SCALE

The geological timescale is a convenient method of dating rocks

Preparations for an Offshore Drilling Operation the geologists on the rig and in the shore laboratories, each layer is given a different name which is often associated with the drilling locality, so that a layer of pink and white coloured limeston~e, grading to marl and encountered by the drill bit in a depth of between 8800 and 9100 feet in the Central North Sea might be recognised as the ‘Ekofisk’ formation, while the chalky white limestpne with grey marl interbeds found just beneath it might be detected as the ‘To? formation. OFFSHORE EXPLORATION TECHNIQUES Whilst on land some idea of the types of underground formations can be gained from the study of the surface geology, it is obviously not easy to obtain rock samples from deep below the sea-bed, and other means of identifying potentially petroleum-bearing structures have had to be developed. Geophysical surveying applies the principles of physics to the study of geology, and this is the means by which nearly all offshore drilling locations are found. Three geophysical survey methods are used to examine formations below the sea bed: magentic, gravimetric and seismic. One or more of these methods is invariably employed to make a detailed survey of any petroleum prospecting area before any drilling commences. MAGNETIC SURVEYS A magnetic survey measures the effect of the magnetic properties of rocks on the earth’s magnetic field, and is carried out by an aircraft carrying an instrument called a ‘magnetometer’. Certain types of rock that contain iron are magnetic, whereas oil-bearing rocks are usually non-magnetic, and variations in the magnetometer readings are used to give an image of the underground layers. This method does not provide very detailed information but it is useful for forming an overall picture of a prospecting area. It was often used to good effect in the early days of North Sea exploration. GRAVIMETRIC SURVEYS Gravity, or gravimetric, surveys measure the effect of variations in the density of different rocks on the earth’s gravitational field. Subsurface rocks exert different degrees of ‘pull’ on the gravitational force at the surface, and ‘gravimeters’ are used to measure these variations. However, this method again provides no detailed information, and is of limited value offshore. , SEISMIC SURVEYS Seismic surveying provides the petroleum geologist with accurate details about depths and extents of layers of sedimentary rock, and is by far the most widely used survey method offshore. The method is based on the principle that different rock types respond with different absorption and reflection characteristics to shock waves which are produced by an energy source

Preparations for an Offshore Drilling Operation released either just below the ground surface or just below sea level. The ‘echoes’ sent back by the rock formations are recorded on the surface by a seismograph (an instrument which measures vibrations) and give a more detailed picture of the subsurface formations than the other geophysical surveying methods. An offshore seismic survey is made by a special seismic survey vessel towing devices capable of generating the necessary shock waves and, behind this, a recording cable possibly two or three miles in length. The energy source is usually an array of many different sized compressed-air guns that look rather like artillery shells, each of which produces a powerful burst of acoustic or sound energy in a different frequency band to its neighbours. These ‘sleeve exploders’ are strung out on a cable called a ‘streamer’ which is several hundred feet long and towed at a depth of several feet below the surface. If the guns are all fired at the same instant they produce an extremely strong sonic pressure wave for a very short duration. The pressure wave travels down through the water and into the underlying sediments and rock structures thousands of feet below the sea bed. Some of the wave energy is reflected directly back to the cable from the rock layers, while some is refracted as it enters or leaves a layer before returning to the surface.

F==--

SEISMIC SURVEY TECHNIQUE

Seismic surveying employs a similar technique to echo-sounding

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Preparations for an Offshore Drilling Operation The recording cable is towed.just below the surface and is fitted with numerous sensitive hydrophones which detect the returning pressure waves. Transducers convert the pressure energy into electrical impulses and these are transmitted along wires inside the towed cable to recording instruments on the ship. The instruments basically measure the time taken for each energy pulse to return, and they relate this to the distance from the energy source to the receiving hydrophone. The voltages received from the numerous channels in the cable are then processed by a computer which converts them to data that can be written on magnetic tape. When the sea survey is complete the magnetic tapes are sent to a processing centre ashore which contains mainframe computers that can efficiently handle large amounts of data, although some vessels are now able to carry out processing onboard, enabling interesting ‘lines’ to be re-shot without delay. After processing the tapes, a seismogram - a cross-sectional view through the earth below the line of the shoot - is produced, and from this interpreters build up contour maps of the structures under the sea bed. These maps are used, like field maps, to pinpoint the positions of potential oil-bearing structures. It is usual nowadays to survey an area and process the data in three dimensions. 3-D surveying is much more expensive than conventional 2-D surveying, but the cost of the survey is normally a relatively small part of the overall exploration budget of the oil company. However, ships are not normally able to carry the powerful mainframe computer equipment necessary to process 3-D data, and 3-D processing usually has to be done ashore. Oil companies often club together to pay for seismic surveys of a large area. that all of them might be interested in, and trading of seismic information goes on to their mutual benefit. But even when the geological structure revealed by seismic or other survey methods looks promising, there is still no certainty that oil is present. Only exploratory drilling can prove whether or not the geological traps located by the surveys actually contain oil. The time now approaches when the operator will have to move his rig in and start , exploratory drilling. THE DRILLING RIG SITE SURVEY Apart from surveys of large areas to detect the presence of structures that might bear hydrocarbons, an offshore operator also commissions a survey of the precise location where he intends a drilling rig to operate. This is also carried out by a seismic survey vessel, and covers an area of perhaps 30 square

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Preparations for an Offshore Drilling Operation kilometres to encompass the scope of sea bed in which the rig’s anchors will be laid and the hole will be ‘spudded’, or begun. The drilling rig site survey reveals information on bathymetry (water depths), sea-bed features such as obstructions, the geology of the sea-bed, anchoring conditions and any problems foreseen for drilling operations, such as the presence of shallow gas pockets. This information helps the operator decide how and where exactly his rig should be positioned to drill down to the target formation.

,

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CHAPTER 3: OFFSHORE DRILLING PLATFORM TYPES

When an operator requires a well drilled, the rig he selects must be capable of doing the job efficiently and safely, but at the same time it must be of a type suitable for the nature of the operation. It would be economic folly, for example, to build a massive fixed platform over the proposed location of an exploration well where there was a ninety percent chance that no hydrocarbons would be found in commercial quantities, so in this case a mobile rig would normally be hired do the exploratory work and a platform might be built on the location later if the well proved commercially productive. In general, therefore, exploration wells are drilled by ‘floaters’, which include semi-submersibles, drill ships and barges, or by self-elevating ‘jack-up’ rigs, while development wells, drilled to exploit a field already discovered, are mostly drilled from fixed platforms. Whether a floater or a jack-up is used for the exploratory drilling depends mainly on the water depth. Jack-up rigs can normally be hired more cheaply than floaters, but if the water is not shallow enough for jack-ups to stand in, they obviously cannot be used. The choice would then lie between a semisubmersible or a drill ship (or barge in some areas), and many factors would be taken into account before the operator made his decision. These would include the water depth at the proposed location, the prevailing weather and sea conditions there, the amount of deck load intended to be carried aboard the rig, and the logistic problems of keeping the unit adequately supplied from a shore base. Even when a particular type of craft has been decided upon, there still remains the question of the capabilities of individual units, since each rig of a particular class may have been modified at stages in its life to fulfil different functions for different operators, and unlike classes of ships, no two rigs of a class are usually exactly alike. Every detail of a rig’s layout and equipment is therefore closely examined before an operator awards a drilling contract. . Generally, offshore drilling platforms can be grouped under three main categories: fixed platforms with floating drilling tenders, self-contained fixed platforms, and mobile units. The mobile units are the ‘rigs’ that do virtually all the exploratory drilling for the oil and gas industry. Fixed platforms are basically production units, but because most have facilities to drill development wells they are described briefly below. . ..

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58

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Offshore Drilling Platform Types. FIXED PLATFORMS WITH FLOATING DRILLING TENDER

In this type, which is seen mainly in the Middle East, the US Gulf, Nigeria and Lake Maracaibo, a small platform is constructed on piles driven into the sea bed over a discovered reservoir, and a drilling derrick and drawworks are mounted on the platform. The drilling tender, which is usually a barge, is moored close alongside the platform and carries the power supply, mud pumps, mud pits, pipe racks, storage space for miscellaneous equipment, and accommodation for the platform crew. A helicopter platform is generally also fitted on the tender. A ‘catwalk’ or ‘bridge’ allows the crew to cross between the two units, while power cables and hoses for fuel, water and drilling fluids are connected from the tender to the platform’s drill floor. After the drilling phase of the development, the platform usually has remote-controlled production and pumping equipment installed, and the tender departs, to be used again at another platform. At one time this was the most common type of offshore drilling platform, but there are relatively few remaining nowadays. They are obviously unsuitable for the more hostile sea areas and would normally only be designed to operate in sheltered waters. A great deal of time would otherwise be wasted in the tender disconnecting and ‘standing off’ to ride out bad weather, especially if it is not self-propelled and requires towing, which most do. They are not, therefore, seen in areas such as the North Sea. SELF-CONTAINED FIXED PLATFORMS

These are massive platforms erected on the sea-bed, and are generally built of either steel or concrete. There are many designs, derived from two basic types: tubular steel structures built on a shore construction site, from where they are floated out to the location and pinned to the sea-bed by piles, and reinforced concrete gravity structures of such enormous mass that, once towed to the location and positioned vertically on the sea bed, they can stand freely. The largest platform of the concrete gravity type reaches nearly 800 feet above the sea bed and weighs more than 600,000 tons in air. Like other gravity platforms it was built, with t,he exception of its steel topside modules, in a drydock, and towed to its location in an upright position and sunk. Most gravity platforms have a ring of cylindrical concrete tanks surrounding their base, which serve as storage for oil awaiting transportation. The tubular steel fixed platforms may be as tall, or even taller than the concrete type, but they are naturally much lighter. They are normally constructed on their sides in a dock and floated to the location either on a barge ...

Offshore Drilling Platform Types

Offshore Drilling Platform Types from which they are tipped to sink vertically, or using steel ‘buoyancy bottles’ attached to their legs. Once on location, the buoyancy bottles are carefully flooded so that the steel ‘jacket’ (as the base structure is called), tips and sinks vertically. Piling is then driven into the sea bed around the legs to pin them. Including the length of the piles, some platforms of this type are more than 1100 feet tall to the peak of the drilling derricks. Drilling from production platforms is mostly ‘directional drilling’ (described in Chapter 5) in which the hole is deviated at a slanting angle after being made vertical for a certain distance. This enables the extremities of some fields to be exploited from one central platform, from which up to sixty directional wells might be drilled. (One platform in the US Gulf has 96 drilling slots.) Horizontal distances from a platform of about two miles can be reached using this technique, which would otherwise necessitate the re-location of a mobile rig many times to drill each well vertically. The largest fixed platforms are able to accommodate two drilling rigs as well as full production facilities and accommodation for several hundred personnel. The power generated onboard these enormous structures is often enough to supply a small city. Electric power for the drilling equipment may be provided by conventional diesel plant, but it is frequently generated by gas turbines utilising gas diverted from the production equipment. The drilling and well control equipment and most of the drilling procedures are similar in most respects to those used on any jack-up rig, and the drill crews are often hired from a drilling contractor who may also operate mobile units. Marine personnel involvement on fixed platforms is usually limited to sea traffic direction and co-ordination and the maintenance of lifesaving equipment, although many installation managers are former shipmasters. As each well is completed on the platform it is then brought ‘on stream’, provided sub-sea pipelines have been installed to transport the oil or gas to shore, or tanker-loading facilities or storage facilities exist at the location. The drilling rig, meanwhile, can be hydraulically jacked over several feet to another ‘drilling slot’ to drill the next well in the development programme. In many instances on platforms the rig or rigs have completed their functions and are removed, or else retained onboard for ‘workover’ (well maintenance) purposes or for use in a later stage of the dev>lopment programme. MOBILE DRILLING RIGS The category of mobile rigs consists of two general types: those which rest on the sea-bed during drilling and those which float. The group of floaters includes semi-submersibles, drill ships and barges, while the bottom-supported rigs are jack-ups and submersibles. .., ‘1

Offshore Drilling Platform Types

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Offshore Drilling Platform Types SUBMERSIBLES

When drilling began in the swamps and marshes bordering the US Gulf states and in the creeks of West Africa, the waterlogged land presented problems for the teams sent ahead of the rigs to prepare the drill sites and their access ways. In many cases, transportation by water was the only answer, and dredgers sometimes had to be brought in to dig shallow access canals.

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Meanwhile the drilling rig, accommodation and storage facilities were mounted on a wide, flat-bottomed, shallow-draught barge which was towed onto the location and ballasted down so that it sat on the bottom, effectively becoming a ‘submersible’. The well was drilled, and the barge was then deballasted to float it out. There was a narrow slit in the barge hull running from the centre of one end to a position just beyond the mid-point of the deck, immediately below the derrick. This not only permitted drilling through the hull but also allowed safe clearance during the tow-out over the wellhead equipment which was left protruding from the swamp bed. Not all these ‘swamp barges’ were designed to be submersible, many being able to drill whilst afloat in shallow water. These are still used in some parts of the world. From 1948 onwards a larger type of drilling platform evolved from the submersible ‘swamp barge’ in which the hulls of the vessel were ballasted and allowed to submerge and rest on the bottom, while the drilling and accommodation deck remained well clear of the water on tall pillars. The hulls were either ship-shaped or were a framework of large-diameter hohow piping, while the deck, which became known as the ‘Texas deck’, was supported by posts or stanchions. In later versions buoyant columns, called ‘milk bottles’ because of their shape, were fitted to provide the necessary stability when submerging or refloating, and today’s semi-submersibles are mostly derivatives of this later design. Many submersibles were built, but few now remain, mostly in the Gulf of Mexico area and Nigeria. They are not usually self-propelled and they are tightly restricted in the depths that they can drill in, which is the main reason why they have been superseded by other types of rig. One unit is able to drill in 100 feet of water, but the largest examples were able to drill with their ‘Texas decks’ 25 feet clear of the sea and their pontoons resting on the botl tom in 17.5 feet of water. Even at such depths there are still considerable wave forces, and ‘scouring’, or the washing away of the sea bed by underwater currents, sometimes occurs around the hulls of submersibles that could eventually upset the plat-

Offshore Drilling Platform Types form if not controlled. Divers are therefore used to reinforce the sea-bed alongside the pontoons with sand bags. Scouring is also a problem for the type of unit that has largely replaced the submersible, namely the ‘jack-up’ rig. SELF-ELEVATING (JACK-UP) PLATFORMS ‘Jack-ups’ comprise about half of all the mobile rigs in the world, and are used for shallow-water drilling. They are self-contained platforms resembling a flat bottomed barge hull with three, four or more vertical legs fitting through openings on the outer hull edges. These legs have ‘teeth’ notched into them and can be raised or lowered by a jacking mechanism on the deck that usually employs a hydraulic or electric rack and pinion arrangement. In its drilling mode the barge hull is raised on its legs well out of the water and serves as the drilling, storage and living platform. It is thus firm and stable and experiences none of the motions due to the sea that affect floaters. After the well has been completed the ‘drilling package’, which includes the drill floor and the derrick, is ‘skidded’ clear of the wellhead, and the barge hull is jacked down the legs until it floats freely. Jacking continues, with the legs now being supported by the floating hull rather than the hull being supported by the legs. The legs are jacked up at least until their bottom ends are sufficiently clear of the sea bed to permit safe clearance during the rig-move to the next location. In its ‘transit condition’ the legs of a jack-up rig can usually be seen towering high above and around the drilling derrick, giving an impression of instability. There have been several accidents during jack-up transits, but the vast majority of rig-moves are carried out in complete safety. On arrival at the next location the legs are jacked down until they touch the sea-bed. The jacking continues after a test period called ‘pre-loading’, the legs penetrating the sea-bed for some distance while the barge hull begins to climb up the legs. When the hull is high enough to be clear of the highest waves expected at the location, the legs are locked and remain in this position until the well has been completed. Most jack-ups have vertical legs, but several hesigns incorporate legs which slant outwards at the bottom to obtain a wider standing ‘spread’ and better stability. The legs are all independently jacked, and their position can be adjusted so that the barge stands horizontal on a sloping sea bed. The legs of some units are fitted with a large, flat steel frame at their lower ends, called a ‘mat’. This affords better stability on some bottom soil types and reduces the danger of capsize due to scouring. ..

64

Offshore Drilling Platform Types

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Cantilever jack-up rigs drill overside.

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Offshore Drilling Platform Types Jack-up rigs are normally stable in their drilling mode, although there have’been some instances of rigs collapsing and sinking when a leg has either sunk into the sea bed or its steel structure has failed. They are, however, designed to withstand strong wind and wave pressures and are usually only at risk from the sea during the critical hours of the jacking phase when the hull is either just leaving or just returning to the fl,oating position, and during bad-weather transits when their low ‘freeboard’ allows water to come aboard the hull easily. Early types were dogged by the excessive penetration by their legs of soft sea beds, but this has largely been eradicated in recent types by the fitting of large feet called ‘spud cans’ or ‘spud tanks’ at the bottom of the legs. HOWever, as the legs of modern rigs get taller the problem of structural bending stresses in them is becoming more difficult to resolve. For this reason some jack-ups’ legs are shortened by dismantling during long transits when there is likely to be much rolling or pitching motion. The deepest water normally operated in by jack-up rigs is about 400 feet, which means having legs almost 600 feet tall to enable the hull to be jacked clear. Most of the 300-plus jack-up rigs worldwide, however, do not operate in water more than 350 deep. The vast majority of these are not fitted with propulsion and have to be towed or carried on special ships between locations. The following particulars are for a typical modern jack-up unit that can drill in water of 300 feet depth. This is a ‘cantilever’ type in which, for drilling, a strong platform that supports the entire drill floor and derrick (the drilling package) is moved outboard on tracks until it is projecting over the after end of the hull. An alternative to this design is for the drilling to be done through a drilling slot in the barge hull. The drilling equipment and its use are described in more detail in Chapter 4 and 5. Length overall Width overall Depth of hull Deck area Leg length Diameter of spud tank Depth of spud tank Distance from centre of fwd leg to centreline of aft legs Maximum distance of rotary centreline from stern edge

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74.75m 86.30m 7.50m 2590 sq.m 133.5otn 14.0m,. 4.0m 65.82m 13.75m

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Offshore Drilling Platform Types

Offshore Drilling Platform Types Projection of leg below hull in normal tow Transit displacement Jacking speed Cantilever beam spacing Maximum water depth Assumed penetration of legs Air gap (between sea and underside of hull) Maximum wave height Surface current velocity Bottom current velocity Maximum drilling depth

5.56m 10,500 tonnes 0.45m/min _ 15.00m 91.4m (300 feet) 1 7 .Otn 14.4m 20m lm/s 0.3mls 7617m (25,000ft)

JACKING SYSTEM: electro-mechanical rack and pinion type, total jacking capacity 9000 tonnes.

POWER SUPPLY: 3 x 12 cylinder supercharged diesel engines driving 3 x 600~ alternators.

.DERRICK: Height 44.8m (147ft); base llm x llm (36ft x 36ft); load capacity 1.39 million lbs. (620 tons)

DRAWWORKS: 2000 hp.

BOP STACKS: 1 x 13-5/8” 5,000 psi stack (annular) 1 x 13-5/8” 10,000 psi stack (single ram) 1 x 13-5/V 10,000 psi stack (double U preventer) 1,x 21-l/4” 2,000 psi (annular) 1 x 21-l/4” 2,000 psi (single ram) 1 x 10,000 psi choke manifold . MUD PUMPS: 2 x 1,600 hp

ACCOMMODATION: 38 x 2-men cabins 2 x l-man cabins 1 x 3-man hospital

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Offshore Drilling Platform Types

Upper dec :k plan view and side profile of a cantilever jack-u

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69

Offshore Drilling Platform Types

Top: The hull section of a jack-up rig. Bottom: A cantilever jack-up rig drilling through a plats form jacket.

Offshore Drilling Platform Types This rig is roughly triangular in hull shape, with the apex of the triangle forming the forward end and the cantilever beams projecting over the after end. The accommodation block stands on the hull just aft of the forward leg, and aft of that are the pipe racks. The drilling package, which is capable of ‘slant drilling’ (see Chapter 5) is further aft on the cantilever beams which extend tom the middle of the deck. It can be skidded completely off the rig and onto a platform jacket so that slant drilling can be carried out from the corners of the jacket, but before transits it is skidded to the centre of the main deck. Beneath the main deck, inside the hull, are large spaces for generating machinery, electrical equipment, mud tanks and mud pumps, and below this level, in an ‘inner bottom’, are numerous tanks for ‘pre-load water’, ‘drill water’, potable (drinking) water, fuel oil, and drilling fluid or ‘mud’. There are numerous designs of jack-up in service but most follow this general pattern. SEMI-SUBMERSIBLES There are several claimants to the distinction of being the designer of the first semi-submersible, but to whoever it belongs the marine drilling industry owes a great deal. Because it can operate in deep or relatively shallow water it is probably the most versatile of all drilling platforms, and for that reason it has become almost a hallmark of the marine drilling industry. ‘Semis’ have drilled thousands of exploratory wells in virtually every sea area of the world and are used in deep and shallow waters from the Canadian Arctic to the South China Sea.

A semi-submersible rig anchored and ballasted down to drilling draft. The standby boat patrols inside her anchor buoy pattern. ..-

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Offshore Drilling Platform Types

,,

A French design for a semi-submersible warship was patented in 1937, and in 1943 a British design was patented for a salvage vessel with twin hulls and five pairs of circular columns, rather on the lines of today’s semi-submersible rigs. But it was the limitation of the jack-up rig to shallower waters that led to the development in the early 1960s of a type of mobile drilling rig that could be used in deep water or, alternatively, resting on the bottom in shallow water. Basically the industry required a type of unit that, whilst having the ability to float and drill in the depths attainable by drill ships and barges, would have better stability qualities than the conventional floating vessels. The column-stabilized semi-submersible, a logical development from the column-stabilized submersible drilling platform, fulfilled the industry’s needs. The foremost problem in drilling in deep water from floaters is heave, the vertical ‘up and down’ motion of a vessel as it rides the waves. One way of minimising heave is to keep the ‘waterplane area’ of the hull to a minimum, and this was achieved in semi-submersibles by retaining the pillar-shaped buoyant deck supports used in submersibles and by having counterbalancing pontoons at the bottom of the columns in which ballast could be kept.

, !,

Many designs of semi have been produced in the last twenty or so years, and most are broadly similar in concept although perhaps widely differing in structure. The more modern designs feature a roughly rectangular working deck with two, three or four vertical circular-sectioned columns fitted beneath the deck at each side, terminating in underwater pontoon hulls containing large tanks for ballast, fuel and fresh water. The columns and pontoons provide the buoyancy to keep the vessel afloat, and some of the tanks in them can be ballasted to submerge the vessel to a sufficient depth to maximise stability and minimise movement in reaction to wave forces, thereby providing an extremely stable platform for drilling from. In addition to providing stability, the columns also support the deck, but they are usually assisted in this function by large struts or ‘braces’ which cross diagonally in fore-and-aft and athwartship directions. Other braces cross between the columns to provide complete structural integrity. , Semis are usually moored on a location by a system of multiple anchors with chain, wire, or chain-and-wire combination cables, but a few are fitted with ‘dynamic positioning’ equipment which dispenses with the need for anchors altogether. The DP units are more expensive both to build and operate, but are sought after for drilling in deep water far beyond the reach of a jack-up rig and a conventionally moored semi.

Side profile of a heavy-duty semi-submersible.

Offshore Drilling Platform Types

Offshore Drilling Platform Types

The fore end view of a semi-submersible shows the braces which strengthen the columns and main deck. ,

74

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I 3 4 ------L-------. Id

Offshore Drilling Platform Types

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Drilling equipment is stored on pipe racks near the drill floor. On this type of rig the derrick is aft of amidships and the pipe racks are forward of it.

Offshore Drilling Platform Types

The main deck and lower hull arrangement of a modern semi-submersible. The starboard pontoon is virtually a mirror image of the port one.

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Offshore Drilling Platform Types

Semi-submersible design has been the subject of much intense engineering effort over the past two decades and the latest vessels are extremely sophisticated compared with older units. Conventionally anchored semis often drill in 2000 or more feet of water, but modern DP units can work in five times this depth without moving off station more than a few yards. Some semi-submersible drilling rigs have been converted to accommodation units, while others have become floating production systems, being used initially to drill production wells where water depths were too great for the safe erection of a fixed platform and then to provide the production facilities usually found on a fixed platform. Semis have also been used to produce from relatively small fields in comparitively shallow waters where fixed platform costs would be uneconomic.

An unusutil type of semi-submersible with a collapsible derrick. Lowering the derrick during transit would considerably improve stability margins.

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Offshore Drilling Platform Types The largest examples of this type displace in the region of 50,000 tons fully loaded, but most semis are about half that size. The following particulars give some idea of the size and capabilities of a typical modern, heavy-duty, column-stabilised semi-submersible drilling rig. The drilling equipment and its use are described in more detail in Chapters 4 and 5. Length overall Width overall Keel to main deck Pontoon length Pontoon width Pontoon depth Corner caissons (4) diameter Centre caissons (2) diameter Main deck, length overall Main deck width Transit draft Survival draft Operating draft Air gap Transit displacement Survival displacment Operating displacement Transit deckload Survival deckload ,Operating deckload

79.3m (260ft) 62.0m (203ft) 35.4m (116ft) 79.3m (260ft) 16.4m ( 5 3 f t ) 7.6m ( 2 5 f t ) 11.6m ( 3 8 f t ) 10.7m ( 3 5 f t ) 72.0m (236ft) 63.4m (208ft) 7.6m (25ft) 15.3m ( 5 0 f t ) 21.3m ( 7 0 f t ) 13.4m ( 4 4 f t ) 15,780 tonnes 22,910 tonnes 25,862 tonnes 4,500 tonnes 2,800 tonnes 3,100 tonnes

POWER SUPPLY: 4 turbocharged diesels each providing 2714 bhp, driving 4 x 600~ alternators. Power is converted to DC by an SCR system, powering drilling equipment motors. Propulsion is provided by 2 x 2000 bhp motors in each pontoon. HEAVE COMPENSATION: Motion compensators have 600,000 lbs capacity with a 20ft stroke. There are 6 riser tensioners, each with 80,000 lbs capacity and a 50ft stroke. Total tensioning capacity is 800,000 lbs. Guideline tensioners with 16,000 lbs capacity and a 40ft stroke are also fit‘ted. DRILLING EQUIPMENT: The derrick is 48.8m (160ft) high, with a base measuring 12.2m x 12.2m (40ft x 40ft). Load capacity is 1.4 million lbs. The crown block is rated at 650 ton capacity and has 7 x 60” sheaves. The travelling block, hook and swivel are each rated for a 650 ton capacity. There is a 3000 hp drawworks driven by 3 x 940hp DC motors.

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Offshore Drilling Platform Types

Many semi-submersible designs have been tried; this one is shaped like a cross and has five pontoons.

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Offshore Drilling Platform Types The rotary table has a 49” opening and is driven from one 940 hp DC motor. It has an 800 ton static load capacity. There are two mud pumps, each with 7” liners and 1600 hp outputs. The unit’s basic tubular outfit includes 25,OOOft of 5” drill pipe, weighing 19.5 lbs per foot. The BOP stack is rated for 15,000 lbs maximum pressure. There are two diesel-powered hydraulic pedestal cranes with a 36.6m (120ft) boom and a lifting capacity of 69 tonnes at 30ft radius and 11 tonnes at 120ft. There is also a crane with a 24.4m (Soft) boom. There are 4 hydraulic double anchor windlasses for 8 x 40,OOOlb weight Danforth anchors and 8 x 5,500ft anchor chains of 3” link diameter, and a permanent chain chaser system is fitted to each chain. Accommodation is provided for 96 persons, in 43 x 2 man cabins and 5 x 1 man cabins which have provision for a second person if necessary. There is also a 3-man hospital. The maximum water depth of 1500 feet for which the rig is designed can be increased by adding extension lengths to the anchor chains, but this reduces the amount of deckload the rig is able to carry. The twin pontoons are roughly ship-shaped and the six caissons (or ‘columns’ or ‘legs’ as they are more often called) have a fairly fat appearance compared with those of many other types. In spite of this, their total waterplane area of only 634 square metres is such that they are somewhat transparent to the waves, giving good stability and motion characteristics. They provide not only stability tanks but also storage lockers for the eight anchor cables and space for large silos containing drilling materials. The power plant in this design occupies a large compartment on the port side of the main deck, while on the starboard side is a mud pump room and a sack store for mud-mixing materials. The mud ‘pits’ or tanks are just forward of the mid-point of the rig, while the accommodation block is ranged along the fore end on two decks. It has an integral ‘pilot house’ (the equivalent of a ship’s wheelhouse) and radio room, and office and recreational space is also provided in it. The drill floor is elevated above the main deck and in this design is slightly aft of the mid-length, near the centre of flotation of the unit where heave is felt least. The pipe racks are situated forward of and below the level of the drill floor, just aft of the upper accommodation deck. Immediately below the drill floor is a moonpool through which drilling equipment is run to the

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Offshore Drilling PIatfom Types

sea bed, and this extends some way aft towards the stern of the rig. Theblowout preventer stack (see Chapter 4 and 5) is housed at the after end of this opening when not in use, and is moved forward on a trackway for runningto the sea bed. This makes the rig alter its draft forward and aft considerably but this, like other stability problems, can be controlled ‘blind’ from the control room situated between the forward end of the engine room and the accommodation. This rig is not dynamically positioned but is typical of the latest generation of heavy-duty, harsh environment, deep water semis now in service with several contractors. DRILL SHIPS

Several marine drilling contractors operate drill ships as well as semis and jack-ups. Because of its conventional ship-shaped hull the drill ship is more prone to movement in a seaway than the semi-submersible, and is therefore subject to longer periods of down-time due to wind and wave action. For this reason drill shins .are more usually (but not always) found working in the smoother waters of the world, while semi-subs can drill in the most hostile environments. This disadvantage is partially offset by the drill ship’s ability to move from one location to the next rapidly and under its own power, with considerable economic advantage. Another point in their favour is that drill ships usually have much greater storage capacity than semi-subs for fuel, water, drilling fluids and other consumables, and they are therefore suitable for drilling in the more remote parts of the world, such as the Arctic, where logistics are a problem. On the other hand, a drill ship often requires a full ship’s complement in addition to drill crew, while this is not always true for a semi-submersible. A feature of many drill ships is a dynamic positioning system which, by means of computer-controlled thruster propellers, can maintain the vessel in a fixed position relative to the sea bed without the use of anchors. These vessels can therefore be used for drilling in very deep water where an anchored unit would experience problems in staying on the location. There are about 80 drill ships in service worldwide, many of which are converted bulk carriers, tankers, general cargo ships and barges. They range in size from about 500 tons to 36,000 tons deadweight, although most are of about 5,000 to 10,000 tons. ..

Offshore Drilling Platform Types

Offshore Drilling Platform Types In most cases the engines are aft, allowing a moonpool to run through the hull at the centre of flotation, roughly at the mid-length, and the drilling derrick is fitted over this on an elevated drill floor. Pipe racks are arranged either forward or aft on the deck and the helicopter pad is sited right aft. The crew accommodation may be in a forward deckhouse or right aft above the engine room. The hull contains mud pumps, mud pits, mud silos, stores, workshops, power plant and tanks for fresh and drill water, fuel and sea water ballast. If the ship is a conventionally moored type there will be large windlasses forward and aft for eight or ten anchor chains, and even DP vessels will still have normal ships’ anchors and cables.

One of the advantages of drillships is that they can usually move between drilling locations rapidly under their own power.

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Offshore Drilling Platform Types Some drill ships, especially of the converted barge type, have a dieselelectric propulsion arrangement in which the diesel generators are housed forward while the electric propulsion motors are aft near the propellers. The following particulars are for a small drill ship of this type, purposebuilt in 1975. 75Oft 20,oooft

Maximum water depth for drilling: Maximum drilling depth:

Intact stability adequate for llO-knot winds and 60ft waves. Limiting conditions for drilling are reached at windspeeds of 50 knots, currents of 2 knots or significant wave heights of 12 feet. Length bp Length oa Beam (moulded) Hull depth Drill floor to main deck Drill well (moonpool) Rotary kelly bushing to sea level Helicopter deck Full load displacement Light ship displacement Draft at full load Variable deck load ate this displacement Tubulars in racks Liquid mud Sack materials Drill water Potable water Fuel oil Helicopter fuel Lube oil Bulk cement Bulk mud

f

362ft 371ft 7oft 22ft 41ft 4ft x 22ft 47ft at 18ft 6in draft 83ft x 83ft 10,060 long tons 5,870 long tons 18ft loin 4,190 long tons 575 long tons 750 long tons 160 long tons 2,940 long tons 250 long tons 1,150 long tons 6 long tons 10 long tons 140 long tons 320 long tons

DERRICK: 36ft x 40ft base, static hook load 825,000 lbs. fitted with a motion compensator. DRAWWORKS: driven by two 800 hp electric motors. MUD PUMPS: Two triplex pumps each driven by two 800 hp electric motors. .. .

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Offshore Drilling Platform Types ROTARY: 49” opening with independent electric drive from 800 hp motor. CRANES: One with 8Oft boom, capacity 22 tons at 20ft radius. One with

lOOft boom, capacity 54 tons at 25ft radius. BOP STACK: One 18-3/4” stack with two double-U rams of 10,000 psi work-

ing pressure. One 5,000 psi annular BOP. POWER SUPPLY: Five diesels each of 1,325 hp. Two tandem units each

drive an 1,850 kW 600V AC generator and the single unit drives a 930 kW 600V AC generator. Total electrical output 4,630 kW. One emergency diesel generator of 335 kW at 480V. DC conversion and distribution is by six 1,200 amp SCR units to drilling, windlass and propulsion motors. AC distribution is by two 1,000 kVA 6001 480V transformers to distribution boards and motor control centres. PROPULSION: Two Schottel thrusters, each driven by two electric motors.

Propulsion output 3,000 shp giving a speed of 7 knots. ANCHOR WINDLASSES: Two double drum chain windlasses aft (for 4

chains), one double drum and two single drum chain windlasses forward, each windlass driven by 800 hp DC motor. ANCHOR CHAIN: Eight lengths each of 3,500ft x 2-3/4” stud link chain. ANCHORS: Eight Vicinay modified LWT type, each of 33,000 lbs weight. BARGE RIGS

These are often very similar to drill ships in all structural respects except for the fact that they are not self-propelled and therefore have to be towed between locations. They are invariably anchored and suffer from the same limitations as drill ships in bad weather. Nevertheless, they are used in many areas of the world, although not in the more hostile environments. ,

Offshore Drilling Platform Types

UJ

II

Offshore Drilling Platform Types

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Offshore Drilling Platform Types

A standby boat maintains a constant vigil near each rig or platform in the North Sea in case of

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CHAPTER 4: THE OFFSHORE RIG AND ITS EQUIPMENT BASIC RIG COMPONENTS

The different types of installations described in Chapter 3 from which marine drilling is conducted might outwardly appear to have little in common except for a derrick rising above each of them, some cranes, a helicopter pad, and some lifeboats. But regardless of the type of drilling platform and wherever it operates, be that in the creeks of West Africa or in deep water amongst the icebergs off North-East Canada, the drilling rig it supports will always be comprised of certain fundamental items of equipment that are common to every one of the thousands of rotary rigs in existence in the world, both on land and offshore. A driller, then, fresh from a land rig in, say, central Africa, would be quite at home with most of the equipment he found on a drill ship working in the Campos Basin off Brazil, while a roughneck working on the drill floor of a fixed platform off Northern Scotland would be equally familiar with most of the drilling equipment on an inland barge rig working in the swamps of southern Alabama. These common basic components, namely the drill floor and derrick, the drawworks and hoist system, the swivel, kelly, rotary hose and rotary table, and the fluid circulation system, facilitate the three main actions that are the main features of the rotary method of drilling. These are: (1) the lowering into and hoisting out of the hole of tubulars and tools; (2) the rotation of a drilling bit in the hole; and (3) the forced circulation of a fluid between the rig and the bottom of the hole. When a power source is added to mechanise these actions, the skeleton of the rotary rig, in theory, is complete, although in practice a great deal of ancillary equipment is necessary to make the rig work efficiently. Much of this basic equipment is found on or around the centre of drilling * operations, the drill floor. THE DRILL FLOOR

The drill floor, or ‘rig floor’ as it is often called, is the work area surrounding the opening in the platform through which tools are run down to the hole being drilled in the sea bed. It is, therefore, the hub of all activity on the drilling unit. ..

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The Offshore Rig and its Equipment

The ‘floor’ is usually about thirty or forty feet square, although its perimeter area is often so cluttered with items of equipment that it looks smaller. It must, in addition to supporting the legs of the drilling derrick, be capable of supporting the weight of the entire drill string when it is ‘set back’ on end inside the derrick, as described in Chapter 5. The opening, called the ‘rotary table opening’ or just the ‘rotary’, is in the centre of the floor. Near it, suspended from wires in the derrick, hang two large wrenches called ‘tongs’. These are used in the connection and disconnection of pipe and are loosely tethered to posts called ‘Sampson posts’. On a semi-submersible rig in drilling condition the drill floor stands in the region of eighty feet above sea level, well above the main deck. This is because the main deck is about fifty feet above the waterline, and the drill floor has to be raised a further twenty-five or thirty feet above that to provide a space beneath where the blow-out preventer stack (a tall assembly of well pressure control valves) can be handled before installation on the sea bed. In one corner of the floor stands the ‘doghouse’, a small, windowed, steel cabin in which the driller in charge of operations on the floor controls the drilling machinery and monitors downhole conditions with an array of instruments. As the driller looks out towards the aperture in the centre of the floor, to his left is a large winch called a ‘drawworks’. The two sets of tongs are connected to the drawworks, as well as being tethered to their Sampson posts. To the right of the doghouse is a tall gap in the wind-proof sheeting that surrounds the floor area. Through this opening, called the ‘V-door’, drilling equipment is hoisted onto the floor from the main deck below, which is connected by a dragway ramp. Facing the doghouse on the other side of the floor is a large manifold of pipes and valves fitted with handwheels which are used in controlling the flow of fluids from the well, and all around and above the floor are the huge girders of the drilling derrick. THE DERRICK

The derrick is the tall, four-legged steel structure that, with the drawworks, provides the ability to lift tools in and out of the well. In a way, therefore, it is a type of large, static crane. It must be capable of supporting the entire weight of the ‘drill string’, a long assembly of pipes and drilling tools, * required for the deepest well for which the rig is designed - possibly more _ than 250 tons - and also of withstanding a certain amount of ‘overpull’ which may have to be exerted on the string should it become stuck in the hole and need pulling free. The weight of the ‘casing’ that will line the hole is another important factor in the derrick design, as a long casing string may weigh even more than a drill string.

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The Offshore Rig and its Equipment

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I

DRILL FLOOR LAYOUT me position ofthe rathole may wry depending cm what power vJ01. are installed.

Top: The drill floor layout typical of most rigs. Bottom: The driller must have a clear view of the floor operations from the dog house.

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01

The Offshore Rig and its Equipment

The derrick is roughly square in section and of such a height that three lengths or ‘joints’ of drill pipe, screwed together to make a ‘stand’ of approximately 90 feet in length, can be pulled out of the hole and ‘racked back’ standing on end in.one side of it. (Handling stands of three joints rather than single joints speeds up drilling operations considerably). This requires a total height of almost twice the length of a stand, since the hoist equipment has to be suspended from the top. The derrick therefore might be 160 feet high, which, added to the height of the drill floor above the sea, would make its peak in the region of 240 feet above sea level in the semi-submersible’s drilling condition, and over 300 feet above the keels of its hulls. A rig might be contracted to work almost anywhere in the world, and in drilling regions such as the Gulf of Mexico, the South China Sea and the North Sea, winds of more than 100 mph are not unknown. The derrick must therefore be capable of withstanding the maximum anticipated windload when the whole drill string - perhaps 15,000 feet of drill pipe and collars - is, racked back inside it. The derricks on many modern floaters and jack-ups are actually rated for a static hook load of over 1 ,OOO,OOO lbs, or about 450 tons, and several-heavy duty units have a derrick load capacity of 1,400,000 lbs, or more than 625 tons. The likely roll, pitch and heave movements of floating rigs are also taken into account when derricks are designed, as these motions obviously put great stresses on the structure. There are several different types of derrick, built for different purposes. One type, found on some offshore rigs but more usually on land rigs, is collapsible, to allow easy removal when necessary. This type is properly called a ‘mast’ and is narrower and appears less sturdy than the usual offshore derrick. In spite of its ability to telescope or j ack-knife, this type of derrick is still capable of withstanding enormous loads. Another type, fitted to a few jackups, can slant to enable a special type of drilling to be done at an angle to the vertical. This is described in Chapter 5. THE DRAWWORKS

The drawworks, which stands at the driller’s left as he faces the drill floor from the doghouse, is a large mechanical or electrically driven winch around the drum of which is wound a thick wire called the ‘drilling line’, or, some* times, the ‘wireline’ (although this can be confused with another type of ,wireline which is important in drilling operations). A large semi-sub’s drawworks of about 3000 hp capacity is likely to be electrically powered, employing three 940 hp DC motors and rated for the weight of drill string or casing required for the deepest hole drilled, in addition to some overpull. The machine weighs close to 40 tons and its hoisting

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The Offshore Rig and its Equipment

A derrickman’s view of the drill floor. The drawworks is to the left, the V-door to the right. ,

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The Offshore Rig and its Equipment

drum is about five feet in diameter. Some older rigs have mechanical drawworks powered by chain drives from engines, but these are being superseded by the electric type. Powerful brake systems are necessary to control the heavy loads the drawworks handles and in addition to the main mechanical band brakes there is also a back-up hydraulic (or ‘hydromatic’) or electric brake, to assist in reducing the momentum of heavy loads. Some newer rigs have electromagnetic drawworks brakes in addition. The drawworks has internal drive shafts with drive chains and clutches and these are geared for changes in speed and direction, all of which are controlled by the driller. The internal workings are enclosed in a steel housing and outside this are two ‘catheads’, or what would be called in merchant ships ‘drum-ends’, fitted on shafts projecting through each side. These are small winch barrels which can be operated independently of the main drive and they are used in the connection and disconnection, or ‘make-up’ and ‘breakout’, of lengths of drill pipe and other tubulars. The cathead nearest the driller is termed the ‘make-up cathead’ or ‘spinning cathead’ and the one on the far side of the housing is called the ‘breakout cathead’. A chain leads from the drawworks mechanism near the make-up cathead to the end of one of the tong handles, while the other tong is connected to the drawworks near the break-out cathead by a wire. Some drawworks have other sets of catheads for handling a fibre rope called a ‘catline’, which is a general-purpose rope used for lifting heavy loads in and around the derrick. In most newer rigs, however, small pneumaticallydriven winches called ‘air tuggers’ do this job. THE BLOCKS, HOOK Bz DRILLING LINE

The drilling line that the drawworks moves is rove through a massive blockand-tackle, or purchase, system suspended from the top of the derrick. The uppermost block is fixed at the peak of the derrick and is known as the ‘crown block’, while the lower one moves up or down and is termed the ‘travelling block’. The tra;elling block of a large rig weighs about 10 tons, and on its lower end is suspended by a handle called a ‘bail’ a very large hook weighing nearly 4 tons. This can swivel, and its body contains a hydraulic dampening device to minimise jarring which could damage the threaded connections of pipe suspended from it. Both the blocks and the hook are rated for a maximum lift of about 650 tons in heavy-duty semis. On some rigs, the hook forms an integral part of the travelling block.

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The Offshore Rig and its Equipment

The sheaves in the blocks are 60 inches in diameter and accept drilling line a little over 1% inches in diameter. A single part of this~ size of wire rope could not take a strain of 650 tons, but when wound over seven sheaves in the crown block and six in the travelling block a ‘mechanical advantage’ is developed which greatly reduces the power required to lift heavy loads. Rigs with less lift capacity might have fewer sheaves in their blocks. The drilling line is a specially-made wire rope of a diameter that varies between 1 and 1-X inches, depending on the rating of the hoist components. Its hauling part is called the ‘fastline’, and this is connected to and wound round the drum of the drawworks. It runs up to one of the sheaves in the crown block and then passes down to the travelling block, back to the crown block, and so on round the purchase system, until the required number of lines are strung for the weight to be handled. The end has to be fixed somewhere, but it is not made fast to either of the blocks as it would be on a ship’s purchase. Instead it is led down to and wound round an anchoring device on the side of the drill floor opposite the drawworks, where it is firmly clamped. This part of the drilling line, between the anchor and the crown block, is known as the ‘deadline’. Just above the deadline anchor there is a ‘tension load sensor’ attached to the drilling line. This measures the strain being taken by the deadline, and the driller can continuously read the weight on a ‘weight indicator’ gauge in the doghouse. Beyond the anchor the surplus line is left wound on the storage reel on which it was delivered from its manufacturers. This is because it will eventually be pulled off the reel and used as parts of the rope wear down in the blocks. The work done by the drilling line is measured daily in terms of the tonmiles which are performed during the various drilling operations. When perhaps 20,000 feet of drill pipe and tools have to be run in and pulled out of the hole at fairly frequent intervals, this can obviously involve an enormous amount of wear and tear. At regular intervals, therefore, when a certain number of ton-miles have been worked, fresh line is slipped off the storage drum and pulled through the deadline anchor. The parts of the drilling line which have suffered most co’ntact with the block sheaves and the drawworks drum can then be shifted so that they no longer chafe, and the surplus length of the fastline on the drawworks end can then be cut off. There are manyvariations in this practice of ‘slipping and cutting’, but on every rotary rig it is regularly done to maximise the life of the drilling line. Over the years, after all, a rig operator will spend more money on drilling line than he probably will on drill pipe. . ..

The Offshore Rig and its Equipment

RIG HOISTSYSTEM

The hoist components of any rotary rig.

THE SWIVEL, KELLY & ROTARY HOUSE

The swivel hangs from the hook on the travelling block by a thick steel looped handle, or ‘bail’, something like the one suspending the hook from the travelling block. In spite of its name the swivel does not actually rotate but it suspends the whole weight of the drill string and, by means of internal bearings, allows its free rotation. As the drill string is lowered or raised in the hole, so the swivel will be lowered or raised by the hoist system above it. Through a curved, rigid ‘gooseneck’ pipe fitted through the top centre of the swivel a leak-proof entry point to the inside of the drill string is provided for the ‘drilling fluid’ that has to be pumped down the hole. This is channeled into the gooseneck by the ‘rotary hose’. The rotary hose is a flexible pipe that carries the drilling fluid under high pressure up from puinps called ‘mud pumps’, which are housed on the main deck level. The hose curves down from the top of a rigid ‘stand pipe’ erected at the side of the derrick, so that it can reach the swivel gooseneck when this is either at its highest or its lowest drilling position. Like all components of the circulating system which are subjected to high working pressures, the rotary hose is given regular pressure tests. If it leaked, the required pressure for efficient drilling could not be maintained.

The Offshore Rig and its Equipment

Suspended from the swivel is a long steel tube called the ‘kelly’. This, is a hexagonal, or sometimes square-sectioned hollow pipe usually either40,, 46 or 54 feet long (40 feet being the most commonly used length). Its purpose is to transmit torque to the drill string from the power-driven rotating device in the drill floor, called the ‘rotary table’, through which the kelly is fitted. This rotation is achieved by means of a bushing, called either the ‘kelly bushing’ or the ‘drive bushing’, through which the kelly fits closely. The kelly bushing fits in turn by means of either four pins or else a square base into the ‘master bushing’ (sometimes called the ‘rotary bushing’) which sits in the wide opening of the rotary table. When rotary drive is applied by,a power source, the master bushing turns, driving the kelly bushing and the kelly round with it. If the drill string with a drilling bit on its end is attached to the kelly, the bit will also rotate. The kelly is free to slide up or down through the kelly bushing allowing the drill string to be simultaneously rotated and lowered or raised. In this-way the bit it able to drill a hole. -

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Left: Hexagonal and square kellys. Right: The swivel suspends the kelly and simultaneously allows it to turn.

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97

The Offshore Rig and its Equipment

Top: Pin and ware drive roller kelly bushings. Bottom: The corresponding types of master bushings.

At both the top and the bottom of the kelly, between it and the swivel and drill string respectively, a valve is installed to serve as a type of manual blowout preventer. These ‘kelly cocks’ allow local control of drilling fluid to protect the swivel, standpipe and mud pumps against well pressure, while at the same time preventing fluid loss when disconnecting the kelly. A similar but portable valve called the ‘drill pipe safety valve’ is kept handy on the drill floor for quick insertion into the string below the kelly should any sign of backflow from the well oFcur. The kelly can then be reconnected to it and well control operations can be effected. The lower end of the kelly is connected to the upper end of the drill string by a tubular tool called a ‘kelly saver sub’. This is a short threaded pipe fitted onto the end of the kelly so that the kelly’s threads are saved from wear. The saver sub can easily be replaced, but the kelly can not.

98

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The Offshore Rig and its Equipment THE ROTARY TABLE

The rotary table is fitted below the level of the drill floor deck and only the opening in its top is normally visible at the centre of the floor. If the table were uncovered it would look like a large rectangular steel box with a drive shaft emerging from one end. Inside the box, which weighs about 11 tons, is a well-oiled geared drive mechanism with bearings on which a cast steel turntable revolves. Into this turntable fits the master bushing which, in turn, accommodates the kelly bushing. On older rigs the rotary table might be powered from the drawworks by a chain and sprocket drive, but on most modern offshore units it is electrically driven by an independent tiotor. The motor’s power provides the torque that is transmitted, via the master bushing, kelly bushing and kelly, to the drill string and the bit.

Top: Th .e only part of the rotary table normally seen is its top, which is level with the Bottc 3131:: The moving parts inside it must be kept well lubricated.

drill floor.

99

The Offshore Rig and its Equipment Rotary tables are rated according to the size of their opening, their load capacity and the maximum allowable rotation speed. A typical deepwater, heavy duty rig’s table might have a 49%” opening in which the master bushing is turned at a maximum of about 325 rpm provided by a 940 hp DC motor, and is able to support a static load of 800 tons. The table opening is made so wide to allow large-diameter pipe and sub-sea equipment to be ‘run’ from the drill floor. Otherwise the entire rotary would need to be removed during certain operations, and this in fact happens on some rigs.

Kelly (Square or Hexagonal)

Pin Drive

Square Drive

The two usual arrangements of kelly and bushings.

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The Offshore Rig and its Equipment

When pipe is being run in or pulled out of the hole the ‘bowl’ of the master bushing in the rotary table also accommodates devices called ‘slips’. These are tapered steel wedges which are inserted by hand and jam the drill pipe firmly in the rotary while lifting appliances called ‘elevators’ are disconnected or connected. The individual wedges are connected by hinges so that they can wrap around the pipe to provide friction over as much area as possible. In depth they are a little over a foot long, but various types and sizes are used for different grades of pipe. The area around the rotary table sometimes gets extremely mucky and slippery from wet drilling mud, so it usually has a non-skid surface, or else fibre mats are used for a foothold. LIP SEGMENT

HANDLE’ &,, LEFT

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THANDLE PIN W/COl-ER PIN & WASHER

SLIP SEGMENT’ LEFT

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HANDLE

‘SLIP SEGMENT

/ HINGE PIN W/COlTER PIN

Rotary Slip Parts RETAINING RING

Rotary slips get pleanty of wear and tear and have to be carefully maintained.

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101

The Offshore Rig and its Equipment THE DRILLING FLUID CIRCULATION SYSTEM Unlike its predecessors, the rotary method of drilling depends on the circulation of a special fluid down to the bottom of the hole and back up to the drilling unit for a number of important purposes. The main functions of this drilling fluid, or ‘drilling mud’ as it is commonly known, are: (1) to exert a pressure on the formation being drilled through. This pressure must be slightly higher than the formation pressure so that any gas, oil or water under pressure in the formation can be prevented from making an unplanned entry into the hole. Formation pressures increase with the depth of the hole, so drilling mud is carefully ‘weighted’ with special ingredients so that the ‘hydrostatic head’ of the mud column just overbalances the expected ‘bottom hole pressure’; (2) to prevent the wall of the hole from caving in, or ‘sloughing’, and creating a blockage. This is achieved by the pressure of the column of drilling mud acting against the wall of the hole and coating it with a layer of strained solids called ‘filter cake’ or ‘mud cake’. This happens as some of the liquid in the mud escapes by filtering off into porous formations in the wall, and it helps to preserve the well of the hole intact. Ideally the filter cake should not be so thick as to obstruct the passage of the drilling tools through the hole, but at the same time it should not be so thin as to allow the exit, or filtration, of too much liquid from the drilling fluid into the formation; (3) to remove drill cuttings from the bottom of the hole and from the cutting surfaces of the bit and carry them up to the surface for examination and disposal. The heavier and thicker, or more viscous, the drilling fluid is, and the faster it circulates. the more able it is to carry cuttings. There is an optimum viscosity, however, which should not be exceeded; (4) to help maintain the maximum drilling rate comptable with safety. To do this the drilling fluid must be of such a ‘weight’, or density, that it will only just prevent the entry into the hole of gas, oil or water from the formations being drilled through, and no more. The heavier the fluid is made, the safer the well might be, but drilling progress is hampered by over-heavy mud; (5) to lubricate and cool the bit. Just as pressure increases with depth, so does the downhole temperature, and a bit drilling constantly under a heavy load generates very high temperatures which can adversely affect its performance. Much of this heat can be absorbed from the bit by the drilling fluid, which also ‘oils’ the cutting teeth as they chip away at the rock;

102

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The Offshore Rig and its Equipment

(6) to keep cuttings in suspension when circulation is stopped for any reason. It is essential that a smooth flow of cuttings is returned to the surface, and they cannot be allowed to all fall back and clog the bottom of the hole if the mud pumps stop. To hold them in suspension the drilling fluid must be formulated so that it stiffens or ‘gels’ when at rest, and then become fluid again when circulation is resumed. Certain special ingredients ensure that this happens; (7) to have properties of electrical conductivity or resistivity that will assist in obtaining ‘electric logs’. This is important to enable downhole conditions to be monitored continuously with instruments on the surface. All these functions of the drilling fluid are enabled by highly complex mixtures of minerals and chemicals that are added to a base ingredient which is usually water, but sometimes diesel oil. A different formulation of ‘mud’ is made up for each stage of a well, so that the different formation types, depths, pressures, temperatures and many other factors are all planned for. Usually a specialist ‘mud company’ is engaged to devise the whole mud programme for the well, and one of their own highly trained ‘mud engineers’ stays onboard to organise it. The operator who has hired the rig has the final choice of circulating system used during the well programme, but this is determined mainly by the characteristics expected to be found downhole. However, certain basic components are common to all systems. The heart of any drilling fluid circulation system are the mud pumps (sometimes called ‘slush pumps’), of which there will usually be two, and sometimes three aboard a large rig. These are usually reciprocating, gear driven, dual piston, positive displacement pumps that force the drilling fluid at high ‘pressure up to the standpipe in the corner of the derrick. The fluid then goes through the flexible rotary hose into the swivel, and down through the hollow kelly and drill string to the bottom of the hole where it is expelled at high velocity through nozzles in the drill bit. There it picks up drilled cuttings and is forced to return to the rig through the annulus between the drill pipe and the surrounding casing, finally flowing up through a large pipe connecting the rig and the sea bed to be retrieved on the rig. l

The breakdown of the mud pumps during drilling could have serious consequences, so they must be reliable. If one pump were to stop for any reason, the other might be able to cope alone in some conditions, but there is always a danger that the formation pressure might exceed that of the drilling fluid column. The pumps must therefore be extremely robust and capable not

The Offshore Rig and its Equipment

DRILLING FLUID CIRCULATION SYSTEM

The drilling fluid circulation system. The same mud is circulated repeatedly, so it needs constant purification.

only of prolonged service under heavy loads, but also of handling abrasive, sand-laden fluids without ill effect. Large mud pumps each of 1600 input horsepower are commonly used on the more sophisticated rigs that can drill the deeper wells of 25,000 feet and more. Their pistons work in seven-inch cylinder liners at 120 strokes per minute and they can pump nearly 800 gallons of drilling fluid a minute at pressures in the region of 2500 - 3000 psi. Each pump is charged, or ‘primed’, by smaller centrifugal pumps and is driven by two electric motors mounted on the pump. Maintaining the mud pumps in good order is a responsible job, and usually a drill crew member called the ‘derrickman’is allotted this duty. He is often helped by the ‘assistant driller’.

The Offshore Rig and its Equipment

Mud pumps are the heart of the circulation system.

The efficient progress of the drilling programme depends to a great extent on the use of the right drilling fluid for the formation being drilled through, and on its correct treatment on its return to the rig before it is pumped back down the hole. One of the most important stages in this treatment is the removal of the cuttings brought up to surface. If this is not done the cuttings will be returned to the bottom of the hole when the fluid is recirculated and they will be reground, slowing down the progress of the bit. In addition the cuttings must not be allowed to accumulate in the fluid, since they would adversely affect its carefully designed properties and harm machinery and drilling tools: The cuttings travel with the returning mud through a pipe on the rig called the ‘flow line’ or ‘mud return line’, and they are poured onto a mechanically vibrated screen called a ‘shale shaker’. The mesh of the screen allows the smaller particles to fall through along with the liquid component of the mud, but it traps the larger rock fragments. The liquid eventually gets recycled; while the rock cuttings gather at the lower end of the sloping shaker, where they are removed for dumping overside or, in some drilling areas, removal by barge. When normal drilling is in progress and the shale shaker is in operation, one of the roughnecks is normally assigned the duties of ‘shaker hand’, keeping the machine working efficiently without getting clogged up,

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The Offshore Rig and its Equipment

A double deck shale shaker can cope with more solid material than a single deck machine.

which sometimes happens when the rig is lying at an angle which restricts the flow of fluid, or when a sticky type of clay is drilled. The temperature at the bottom of the hole is high, so the cuttings are hot, making the mud warm and the shaker room steamy, as well as extremely noisy. From the shaker the strained fluid is fed through a sequence of other machines which remove the finer particles. The ‘desander’ traps grains of sand and other abrasive cuttings that can damage the mud pumps and the insides of drilling tubulars and drill bits, while the ‘desilter’ removes siltsized particles that not only cause wear in machinery but also increase the weight of the mud to the detriment of drilling rates. Other machines called a ‘mud cleaner’ and a ‘centrifuge’ are then used to separate the very fine drilled solids from the expensive solid constituents of the mud, which are then returned to the clean fluid before it is passed to a receiving tank for settling. One more item of equipment completes the separation p!ocess before the fluid is finally ready for pumping back down the hole. Apart from corrosive oxygen, the returning mud often carries small bubbles of entrained ‘formation gas’ with it. If this ‘gas-cut mud’ is recycled down the hole, it could eventually lead to a dangerous situation called a ‘blow-out’. It is therefore removed in a ‘de-gasser’, which often incorporates a vacuum pump or else a pump that throws the liquid mud into a tank at such high speed that any gas gets separated from it and vented off.

106

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Top: A desander. Bottom: A desilter. Both are essential components of an efficient drilling fluid circulation system.

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The Offshore Rig and its Equipment

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108

The Offshore Rig and its

Equipment

When the drilling mud has passed through all these machines it is collected in a receiving tank called a ‘mud pit’. There it can be treated as required before being transferred back to a suction pit to await recirculation down the hole. There are usually four or five mud pits grouped near the mud pumps in a large, cavernous space on the deck below the drill floor called the ‘mud room’. Suspended in each pit from an overhead beam is a long shaft with an electrically driven propeller on its end. These ‘agitators’ are used to prevent the mud from settling out, so that the weight of the fluid will be kept uniform throughout the tank. In most cases offshore the drilling fluid is a liquid, but other fluids may be used in special situations where the use of tnud is undesirable. Air, gas or foam (which is a combination of liquid and air) might be used, and these can all give extremely fast drilling rates. However, they require a lot of special compressing equipment, and in most cases offshore, ‘drilling fluid’ simply means semi-liquid, greyish-brown ‘mud’. Despite its name, drilling mud is a very complex liquid. Most drilling muds are basically colloidal suspensions of clays in a liquid base, and they look like dense grey soups, but they can have a wide range of chemical and physical properties which are carefully adjusted by the mud engineer, or ‘mud man’ as he is often called onboard, to meet the different conditions in the hole. He has a small laboratory near the mud room and he monitors the returning mud for changes in its properties such as gel strength, fluid loss and viscosity, that might retard the bit’s penetration rate or otherwise adversely affect drilling performance. With the derrickman’s help he then treats the mud in the pits by mixing various additives to alter its chemical and physical characteristics. In the majority of cases, the mud is water-based. Fresh water, salt water or sea water might be used, according to the mud programme formulation. Sea water can, of coure, be readily pumped aboard an offshore rig, and limited quantities of fresh water can be distilled from it by special machinery, but the large volumes of fresh water, or ‘drill water’ as it is called in this case, that are required for mixing mud, have to be brought out by supply boat. Certain salts or other solids may be added to the drill water for different ’ purposes when the mud is being made up, in many cases to increase the -weight, or density, of the mud. The mud weight has to be continually monitored and altered to control the various formation pressures encountered in the different layers drilled through, and it can range from the density of fresh water (8.34 pounds per gallon) to more than 20 lbs/gal when very heavy ‘additives’ are used. However, in general, the lightest mud possible in the prevailing well conditions is used.

The Offshore Rig and its Equipment

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Clockwise Rotation

1 .75 x lmpellkr E$a~eter Irecomyendedl

1

An agitator prevents mud from settling out in the pits.

110

The Offshore Rig and its Equipment

The usual material used for weighing up mud is barite (barium sulphate), a fine greyish-white mineral powder which has a specific gravity of 4.2 - 4.3. This means that, for a given volume, it is a little over four times as heavy as fresh water. Large quantities of barite (sometimes spelt baryte or barytes) are stored in dry powder form on offshore rigs in special tanks called ‘P’ tanks. When some barite is required for mixing, the derrickman opens certain valves and the rig’s compressed air system, often providing a pressure of about 120 psi, drives the powder into a mixing hopper in the mud room. From there it can be added to the liquid base in a measured quantity to the directions of the mud engineer. Under certain conditions an oil base mud is preferable to a water base mud, perhaps due to the sensitivity of a formation to water, or where the special properties of oil base muds are necessary to overcome a particular problem, such as high bottom hole temperatures or a tendency of the drill pipe to stick to the wall of the hole. In this case diesel oil is usually used, again transported out to the rig and pumped aboard by supply boat. Handling pipe spewing water base mud (during an operation called a ‘wet trip’) makes drill crews dirty and wet, but diesel oil base mud is doubly unpleasant as it can cause skin complaints, so they are normally compensated for having to use it. In some areas, including the North Sea,, low toxicity oil base muds are now used.

The denrickman mixes additives with drilling mud. The mud engineer tells him what treatmc :nt the mud needs to restore its proper characteristics.

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111

The Offshore Rig and its Equipment Other types of mud might also be used from time to time, such as ‘lignosulfonate muds’, ‘lime muds’ and ‘gyp muds’, each having its own peculiar properties and uses for overcoming formation problems. However, these are not so commonly used offshore as water and oil base muds. With a drill floor and derrick, a hoist system and a rotary in which a drilling string is turning with drilling mud flowing inside it, the basic rotary rig is complete. This rig might work effectively on land, but to compensate for the motion of the sea, which makes a ‘floater’s’ drill string rise and fall, a further fundamental item of equipment has to be used.

DRILL STRING MOTION COMPENSATION To do its job, the drilling bit must be permanently on the bottom of the hole with a constant weight applied to it while it rotates. But because a semi-submersible or a drill ship or floating drilling barge heaves up and down with the surface of the sea, some means has to be provided to prevent the bit from intermittently lifting off the bottom and landing heavily down on it as each passing wave moves the vessel vertically. This might not be necessary in the US Gulf swamps where there is little water motion, but it is a frequent problem in areas such as the North Sea or the South China Sea where high swell waves are frequent. Even if the vertical movement were only slight and the driller attempted to keep the bit steady on the bottom by raising and lowering the hoist, the result would be that the weight of the bit was constantly varying, with a consequent slowing of the drilling operation. This was a problem that the pioneer offshore drillers recognised from the earliest days of deep water drilling, but systems were surprisingly not developed to effectively combat it until the 1960s. The systems now used are known as ‘motion compensators’.

,

There are two main methods of motion compensation in use on drilling rigs: ‘downhole bumper subs’, and ‘surface motion compensators’, although within each system there are many variations in design from the different manufacturers. Early systems of vertical motion compensation involved the use of a tor~ que converter in the hoist mechanism, so that as the vessel moved up and down more line would run on or off the drawworks drum as required. Bumper subs, or ‘slip joints’ as they are sometimes called, were also used in the drill string, just above the heavy ‘drill collars’ that were placed between the drill pipe and the bit.

112

The Offshore Rig and its Equipment

As floating drilling vessels evolved, the combined use of torque converters and bumper subs for vertical motion compensation was replaced by total dependence on bumper subs. Then, in 1970, one manufacturer began te&ng a surface motion compensator in the hoisting assembly just below the travelling block. This was a hydraulic/pneumatic cylinder arrangement that ‘stroked’, or reciprocated, as the vessel moved. This unit was the forerunner of today’s surface motion compensators that are now used by most deepwater rigs. However, downhole bumper subs are still used in many cases. DOWNHOLE BUMPER SUBS

A bumper sub acts in a similar way to a car’s shock absorber. It is basically a cylinder filled with a compressible fluid, usually having a five-foot stroke, and one or more of them can be inserted in the ‘bottom hole assembly’ just above the drill collars to compensate for the vessel’s vertical movement. This position is the point of ‘zero weight’ in the drill string. Above it the drill pipe is supported in tension from the rig, and below it the drill collars and other components of the bottom hole assembly are in compression and are applying weight to the bit. The driller adjusts the hoist to give just the right amounts of ‘weight on bit’ and tension in the string. If a heave of less than five feet is being experienced, one bumper sub in the string might be enough. But if the heave becomes greater than five feet and only one bumper sub is in the string. the drill string has to be pulled out of the hole and more bumper subs added until the amount of heave can be conIf a heave of less than five feet is being experienced, one bumper sub in the string might be enough. But if the heave becomes greater than five feet and only one bumper sub is in the string. the drill string has to be pulled out of the hole and more bumper subs added until the amount of heave can be contained by the combined stroke lengths of the series of bumper subs.This, of course, will cause a serious loss of drilling time, unless the bit needs to be pulled for some reason at the same time. Another difficulty is that as the bit cuts deeper into the formation, the driller has to be careful to lower the hoist far enough to allow the bumper subs enough play to do their job. But if the ‘rate of penetration’ of the bit is fast, the bumper sub can reach the limit of its stroke, or ‘drill off’, which can result in the bit jumping off bottom if the vessel heaves upwards on a wave, with a subsequent severe jarring on the bottom as it descends again. This can cause not only leaking seals on the bumper sub but also broken teeth in the bit, and with a diamond-studded bit or core head, serious and expensive damage can occur. If the drilling rate is slower than expected on the other hand, the bumper sub can close up, resulting in damaging compression of the drill pipe above it and too much weight being put on the bit. .-

113

The Offshore Rig and its Equipment

These disadvantages are overcome through the use of surface motion compensation. SURFACE DRILL STRING MOTION COMPENSATORS The surface drill string motion compensator fitted to the modern rig does the same job as downhole bumper subs, but more efficiently. It consists of a pneumatic or a pneumatic/hydraulic cylinder and piston arrangement fitted at some point in the hoist system inside the derrick. There are ttvo systems of surface compensation of this kind in general use on offshore drilling rigs: ‘travelling block compensators’ and ‘crown block compensators’, but both work on the same basic principle. Compressed fluid is fed from a resevoir to cylinders in which pistons operate. The fluid acts on the pistons, and the piston rods lift a proportion of the weight of the &ill string which is dependent on the fluid pressure. This can be adjusted to support only as much of the drill string’s total weight as is necessary to.leave the

The Offshore Rig and its Equipment

desired weight on the bit. As the rig moves up and down, so the compensator pistons ‘stroke’, raising or lowering the drill string to maintain a nearly constant weight on the bit. The stroke of each cylinder is usually in the region of 20-25 feet, and this dictates the total amount of heave which the compensator can absorb while the rig is still drilling, but in practice drilling is often suspended long before the maximum stroke has been reached. The difference between travelling block and crown block compensators lies mainly in the positioning of the compensator cylinders. In the travelling block system the cylinders are attached to the travelling block which moves up and down within the derrick on vertical rails. This entails having flexible pipe to carry the fluid from the pressurised reservoir bottles on the deck below to the moving cylinders. In the crown block system the cylinders are fixed at the top of the derrick above the crown block, and the crown block is moved up or down on the lower ends of the compensator piston rods with each heave of the vessel. This system has the advantage of all the pipework being rigid and thus not so liable to failure as the flexible lines on the travelling block system.

A 1,500,ooO lb capacity travelling block motion compensator.

The Offshore Rig and its Equipment Another type of crown block compensator uses a single cylinder fixed between beams lying across a platform (called the ‘water table’) near the top of the derrick. The piston rod extends upwards from the cylinder and has on its upper end a large sheave through which chains are rove. These hang down on either side of the cylinder and support the crown block which moves up and down as the piston strokes with vessel movement. This type of assembly has a maximum load of 800,000 lb compared with the same maker’s travelling block compensator, the maximum load of which is 600,000 lb. Both are available with strokes of either 20 or 25 feet.

A travelling block compensator slides up and down on rails in the derrick as the rig heaves, keeping the drill string steady.

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The Offshore Rig and its Equipment

Ic

117

The Offshore Rig and its Equipment

A crown block compensator is fixed at the water table underneath the A-frame.

118

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COMPENSATOR SJAtjCW BANK

BANK 7

BANK 2

CROWN MOUNTED COMPENSATOR

ACCUMULATOR

AIR COMPRESSOR AND DRYER

ACCUMYLATOR

H

POWER UNlJ

The crown block compensator system.

The Offshore Rig and its Equipment Each of the manufacturers incorporates variations in the basic design of the compensator systems. The fluid in the reservoir bottles in some types is compressed air which is passed to an accumulator in which pneumatic pressure is converted to hydraulic pressure, oil then being forced into the compensator cylinders to act on the pistons. However, another design employs pneumatic pressure throughout the entire system. There are also types of travelling block compensator which employ only one cylinder (called simplex compensators), but as this is a more complicated system, double, or duplex, cylinders are more common. There is no doubt that surface motion compensators play a major role in marine drilling, many operations of which would be much more difficult without them. They allow the driller to vary the weight on the bit by simply adjusting the air pressure, which is important when formations of frequently varying characteristics are being encountered, or when ‘coring’ (removing rock core samples from the hole) or ‘turbo-drilling’ with a special fluid-driven bit. Surface compensators also increase the service life and penetration of bits, and save tripping time to effect repairs to bumper subs and change damaged bits. Initial ‘spudding-in’ of the hole is easier, and landing the ‘blow-out preventer stack’ on the ‘wellhead’ (see Chapter 5) with part of its weight taken by the drill string motion compensator is made less potentially hazardous than by allowing all of the stack’s weight to be taken by the riser tensioning system. ‘Fishing’ for lost or stuck pipe is also safer and easier, since the fishing tool can be more precisely controlled in its descent to contact with the ‘fish’, and ‘milling’, or grinding of debris or pipe is made easier in bad weather.

THE POWER PLANT On most offshore drilling rigs the mud pumps, the rotary table and drawworks are all powered by direct-current (DC) electric motors. A source of DC power is therefore required, and this is provided on the latest semi-submersibles by a set of four large 1Zcylinder diesel engines which each develop in teh region of 3,000 brake horse power. On this type of unit the engines are not housed in the bottom of the hulls, where a ship’s engines would be, but in a machine room on main deck level. ’ Each engine drives a shaft-mounted 600-volt alternator which generates the AC power which is used for most services around the rig. Some of this alternating current, however, is fed to silicon-controlled rectifiers (SCRs) which convert the AC to DC. This is then fed to the motors on the drawworks, rotary table and mud pumps as required. -.-

120

ml ” BUS BARS I I

DRAWWORKS

I

44oYSCR SYSTEM

p.&jR MUD PUMP 2 1:1> DC MOTORS

TRANSFORMER ‘

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The power generation system on a semi-submersible. Most modem rigs use silicon-controlled rectifiers for AC-to-DC conversion.

The Offshore Rig and its Equipment

If the drilling rig is a self-propelled type, DC currrent is more than likely also used to power the large electric motors that drive its propellers and, possibly, its anchor windlasses. This current also comes from the SCR bays. On a semi-submersible there are usually two motors to each of the two propellors, and these are driven through reduction gearboxes, situated with the motors in the pontoon hulls. In addition to the main engines there is also an emergency diesel generator housed in a separate space. This cuts in automatically should there be a complete failure of the main generating plant, and provides temporary services while repairs are effected. It is not powerful enough, however, for normal drilling to continue and only supplies essential emergency services. .-

122

The Offshore Rig and its Equipment As on a large ship, the job of maintaining the power plant belongs to a team of mechanics, motormen and electricians led by a chief mechanic. These engineers also maintain~mush~of the drilling equipment whose servicing is beyond the ability of the drill crews. Drill crews are expected, however, to do much of the maintenance work on the machinery they use. DRILLING EQUIPMENT Every MODU (mobile offshore drilling unit) carries its own basic outfit of drilling equipment which generally comprises drill pipe, heavy-wall drill pipe, drill collars, tongs, slips and elevators for these tubulars, and casing handling and running tools. A deep water semi-submersible might be equipped with 25,000 feet of five-inch diameter drill pipe in addition to fifty or sixty drill collars, each 31 feet long but of various standard diameters (perhaps 91/z”, 8” and 6V2”). In addition the rig will have its own sets of tongs, slips and elevators for handling this pipe as well as slips and elevators for handling and running casing. Items such as drilling bits and casing which are specified by the operator for the well programme are normally supplied by him, and are brought out to the rig when required. All this equipment is made to what are called ‘API specifications’. API SPECIFICATIONS The vast majority of all tubular drilling equipment, and much other oilfield equipment besides, is nowadays manufactured to the standards of the American Petroleum Institute - the API. In the early days of rotary drilling it was never certain that components would mate uniformly, and much confusion and lost time resulted. The industry set up the Institute to standardise as inuch equipment as possible, and the API is now universally recognised as the leading industry authority in this field. DRILLING BITS The spearhead of the rotary drilling system is the drilling bit. During the drilling of a well many bits of several different types are used, depending on factors such as well depth, formation types encountered and their hardness, and the occurrence of drilling problems. But since a bit-change can pnly be made if the entire drill string is pulled out of the hole and then run back in - a long operation called a ‘round trip’ - then it is most important that a: durable bit of the correct type for the formations to be drilled are selected before running in the hole. When drilling a wildcat well the formations may not be known until cuttings return at the shale shaker, but in a development well they should be known from previous wells drilled. Good bit selection is a compromise of many factors, but in general a bit is required that will give a ..

123

The Offshore Rig and its Equipment

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The Offshore Rig and its Equipment

good rate of penetration (or ‘ROP’) and drill for a long time without wearing down or getting narrower in gauge (which can cause sticking problems for the next bit used). Obviously then, the choice of bit has a significant effect on the time taken to drill the well, and therefore on its total cost. Many types of bit are manufactured, but the ‘rolling cutter rock bit’ is regarded as the basic type and the most popular. It has three main components: the bit body, the rotating cutters, and the bearings that the cutters turn on. The cutting elements are rows of teeth on the surfaces of steel cones which rotate, the apex of each cone pointing in towards the centre of the lower end of the bit. As the cones turn, the teeth on each cone fit neatly between the teeth on the adjacent cones, so that the three cones all turn smoothly withoht jamming. The teeth are either milled from~the steel of the cone itself or they are made individually from very hard tungsten carbide and inserted into small recesses made in the cones. Tungsten carbide is a costly mineral almost as hard as diamond, so ‘tungsten carbide insert bits’ are much more expensive than steel-toothed bits. The three cones are mounted on bearings which extend from the bit body, and at the top of this are screw threads for attachment of the bit to the ‘bottom hole assembly’ of the drill string. In some cases the apexes of the cones do not point exactly to the centre of rotation of the bit body but are all slightly offset, which produces a gouging, tearing action which is useful for. drilling soft formations. For harder formations the degree oft offset is reduced or eliminated so that the bit is more truly rolling, and this produces a crushing action which is more effective on this type of rock. The centre of the bit body has a hollow channel, and between the cones, in the underside, are one or more nozzles called ‘jet nozzles’, through which the drilling fluid is expelled at high velocity. The nozzles’ gauge can be altered to give different velocities and hydraulic horsepowers, but the jet of fluid must in any event be capable of keeping the bottom of the hole free of cuttings and of creating enough turbulence around the bit to keep the rolling cutters clean. The design of the rolling cutter rock bit has hardly changed since the original conception and introduction of the first bit if this type - a two cone model - by Howard Hughes in 1909, but modern technology has improved both its cutting surfaces and its bearings. Nowadays these bits are made to API specifications in diameters from 3 ,/4” to 26”, and within that size range no less than twenty-five standard sizes, as well as some non-standard sizes, are available. Each standard size is made in a range of types for drilling different types of formation ranging from the softer sandy shales, clays and salts

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The Offshore Rig and its Equipment through lime and dolomite to hard quartzitic sands, abrasive quartzite and granite. The size and thickness of the various bit compenents varies with the hardness of the formation intended to be drilled. For the softer formations a lighter bit weight is required so smaller bearings and thinner shells are used, allowing more space for long, narrow teeth. For harder formations on the other hand, that require a heavy ‘weight on bit’ to be applied by the driller, more robust bodies with heavier bearings and stubbier teeth are necessary. STEEL TOOTH BIT with Sealed Journal Bearings

CARBIDE TOOTH SIT with Sealed Journal Bearings Compact Land

Outer End ofToat

__

Inner End ofToat

Bear F l a n k Front Flank

ROCK BIT ELEMENTS

Special Metal = Inlays

I m‘7

O-ring Seal

Marking on Top of Shank Rit _.__ Sire -.. Bit Assembly Nu,

Lubricant Reservoir Cap

’ Shank Bit Type

-..-..I

Rock bit components. Bits have to be tremendously robust

The Offshore Rig and its Equipment

Top: steel tooth and tungsten carbide insert rock bits. Bottom: Bit cones are designed to rotate on different axes.

SOFT FORMATION CONE DESIGN

OFFSET

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127

The Offshore Rig and its Equipment Tungsten carbide insert bits were originally designed purely to cope with very hard formations that made the life of a steel toothed bit uneconomically short. Nowadays, however, improved metallurgy and tooth design means that tungsten carbide bits can be used on any type of formation, and small variations in the shape of the tungsten carbide inserts can accommodate a wide range of hardnesses. Where extra-long bit life is required and where a scraping action would be more efficient than a cutting action, diamond bits are used which have numerous industrial diamonds set in a rigid body. A diamond bit has three main parts: a shank, a matrix crown, and the diamonds, which are set into the crown. There are no moving parts since the complete matrix crown with its inset diamonds does the same job as the cones of a rolling cutter bit. Because of this the shape of the matrix crown is very important to the bit’s performance. Generally, a rounder shape is used for harder formations while a more tapered, conical shape is used for soft and medium-hard rocks. The matrix is made from tungsten carbide to resist the heavy abrasion it suffers, as well as erosion from drilling fluid passing through at high velocity. This is forced out of holes made in the central area of the cutting face and passes between the diamonds and the formation. Like other gems, the weight of industrial diamonds is measured in carats, and the stones used in diamond bits vary from one carat to l/20 carat in weight. The size, type and number of diamonds used in a bit varies with the formation type the bit is intended for, the largest stones being used for soft formations while smaller stones are intended for the harder rocks. While the bit is drilling through hard rock many stones will probably be lost from the matrix, but those that remain are recovered and inserted in another bit by the manufacturers. Needless to say, diamond bits are extremely expensive, but their costs have to be balanced against their efficiency in terms of feet drilled per hour. Of the several bits that are likely to be used in the well, some may be able to drill hundreds of feet, while others last for just a few feet which may take many hours to cut through. Eventually components such as the teeth or the bearings will deteriorate and need replacement, but the bit manufacturers generally try to ensure that all components have similar lifespans. For his part, the driller endeavours to put the correct weight on the bit and adjust the speed of rotation correctly, as both these factors also affect bit life. In addition he must be careful not to damage the bit when running into or pulling out of the hole and to this end a plate called a ‘bit breaker’ is used in the removal or connection of the bit with the bottom hole assembly. This fits into the bowl of the rotary table and grips the bit in specially made recesses while

128

The Offshore Rig and its Equipment Natural Diamond Bit

Polycrystalline Diamond Compact Bit

Primary Fluid Courses

Carbide Studs

Crowfoot Ports

/ Fluid Courses with-I%* ycr yrialiinr Converging/Diverging Diamond Cutting NOZZkS ElWVXlt3

’ Diamond Pads with Diamonds

Standard Gage I

Carbide Inserts

Shank

-Breaker Slot

API Connection

Diamond bit components. The diamonds can de replaced in the matrix.

the assembly is revolved. Should it be discovered on pulling out that the bit has some part broken off, a fishing operation will normally have to follow, incurring much lost time and expense. Just as a shipowner’s aim is to have his .-

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The Offshore Rig and its Equipment

Bit breakers are used to umcrew bits without damaging them.

vessel carrying cargo between ports for the maximum possible time, so the well operator wants the drilling bit on the bottom for the maximum possible time, and time spent fishing for ‘junk’ is naturally abhorred. Fishing is explained in Chapter 5.

DRILL PIPE Drill pipe is used to transmit rotary torque from the rotary table down to the drill bit, and to provide a passage for drilling fluid to the bottom of the hole. It is therefore one of the most important items of rotary drilling equipment, and because of the high standards to which it is manufactured for its relatively short but stressful life, it is expensive and forms a large part of the total capital investment of’the drilling contractor. Drill pipe of various API standard diameters is commonly used, and pipe is also graded according to weight and quality of steel. Standard diameters in use offshore are 23/s”, 2’/8”,3W’, 4”, 4%” and 5”, with 5” being the most widely Gsed on floaters, Rigs are rated according to the maximum depth to which they can drill, based on the length of drill pipe they carry, and when comparing rigs it is necessary for a client to know what size the rating was based on, since narrower diameter pipe weighs less per foot and would therefore impose less stress on the derrick and hoist equipment than the same length of wider pipe. A typical outfit for a modern semi-submersible might be 25,000 feet of 5” Grade S135 drill pipe weighing 19.51 lbs/ft.

1;o

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The Offshore Rig and its Equipment

Pin and box drill pipe tool joints are standard in the industry

high pressure, and if this is not spotted it can eventually cause a connection to fail. Great care is taken not to damage threads when making or breaking connections, and frequent inspections are made for signs of erosion-caused by leaking tool joints and other stresses that weaken the pipe or tool joints. Before use, threads are normally cleaned and inspected, and a special zincbased thread compound is applied to the pipe. Thread protectors made of rubber or rubber-like material are always screwed on to the tool joints when moving and racking pipe. _

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131

The Offshore Rig and its Equipment There are three ranges of length under API standards: 18 to 22 feet, 27 to 30 feet and 38 to 45 feet, but offshore the most common lengths in use are 31 and 4.5 feet. Three joints of 31-foot pipe, or two joints of 45foot pipe are usually ‘made-up’ to form ‘stands’ that cut down the handling time when running in or pulling out of the hole. In addition to these standard lengths there are also short ‘pup joints’ which are used to make up a required length of drill string. These come in lengths such as 5, 10, 1.5 and 20 feet. The ‘wall thickness’ of all these joints is about %a”. Each joint of drill pipe is basically a tube with a short, wide section at each end called a ‘tool joint’. One tool joint has an internal or ‘female’ thread, called a ‘box’, while the other has an internal or ‘male’ thread, called a ‘pin’. When two joints are connected, the pin is screwed into the box, tongs being used on the tool joints to apply the necessary torque. The tube section is always manufactured as a unit, and the tool joints are welded on afterwards. They are able to withstand high torsional and tensile stresses, and have ‘shoulders’ under which elevators can be attached for lifting. Any joint of drill pipe must be expected to occasionally support the entire weight of the drill string when this is hanging from the hook with the bit offbottom. At this time the string is held in tension with the greatest tension at its top, since the weight of the rest of the string is below this part. During drilling the bit rests on the bottom and weight is applied to it by the drill collars which make up part of the ‘bottom hole assembly’. Some of the tensile load is therefore removed from the top of the string at this time, but when the string is pulled out of the hole the tension again comes on the top joints. Because of this variation in tensile loading, drill pipe must be capable of withstanding great vertical forces in the hole. At the same time, however, it must be pliable. If a ‘drill stem’ (as the string of drill pipe is correctly known) could be seen rotating in a hole, it would appear to bend in many places. This is desirable because the drill pipe must be able to follow the meanderings of the bit as it heads towards the target location; too rigid a drill stem would get stuck in the hole. However, the pipe should be kept permanently in tension by the drill collars at the bottom of the string, and should not be allowed to compress and buckle. The powerful twisting force, or torque, imparted to the drill pipe by the rotary table and the kelly is another stress which every joint of drill pipe must be capable of enduring. In a crooked hole the pipe may not be able to rotate freely and there may be a tendency for it to break or twist off, necessitating a long and costly fishing operation, so it is vital that only pipe in top condition is used down the hole. The thread of the tool joint is often the first part to fail, due to ‘washing out’, or metal erosion from leaking drilling fluid under

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The Offshore Rig and its Equipment

Heavy wall drill pipe is distinguishable from ordinary drill pipe by its thickened centre portion

Heavy drill pipe, called ‘heavy-weight’ or ‘heavy-wate drill pipe’ (HWDP), is also carried by all offshore rigs as an intermediate weight grade of tubular that can be used in the transitional area between the pliable drill stem and the heavier and stiffer drill collars in the bottom hole assembly. Fifteen or twenty joints of HWDP might be used in vertical holes, and up to thirty in directional wells. The wall thickness is usually about one inch, and while the weight of a particular grade of ordinary 5” drill pipe might be 19.5 ‘lbs/ft, that of heavy-weight pipe of the same diameter could be 48 lbs/ft. A further type of drill pipe is made from aluminium, but is not commonly used offshore and not accepted as API standard. It is more expensive than steel drill pipe but has applications where its light weight is an advantage, such as where helicopters have to be used to transport the pipe and where extra pipe has to be carried to extend the depth capacity of the rig.

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133

The Offshore Rig and its Equipment DRILL COLLARS With’ a wall thickness of about two inches, drill collars are the heaviest drill string components, and a number of them are always used in the bottom hole assembly to add extra weight and stiffness above the drill bit. This helps to keep the drill pipe in tension, and therefore straight, and reduces the chance of the pipe buckling, although the collars themselves bend when put in compression. Enough collars are used to give the required amount of load on the bit plus the additional weight required to keep the drill pipe in tension, and the whole assembly of connected collars might extend for several hundred feet above the bit, forming the stiffest part of the drill string.

Flush-walled and spiral drill collars are both commonly used.

Drill collars are usually of uniform diameter throughout their length and are either smooth-sided or spiral-grooved, the latter type reducing the tendency for the assembly to become stuck in the hole by having less surface area in contact with its wall. There is an internal channel for the passage of drilling fluid, and box and pin threaded connections at the ends l&e those of drill pipe. The ends are without bulbous tool joints however, since the wall of the pipe is thick enough to absorb most of the knocks it receives. Connections of collars are made and broken with tongs, each two connected collars presenting a smooth surface across their connection, and cross-over subs, which have different gauge threads at each end, are used to connect collars of different sizes.

134

The Offshore Rig and its Equ@men&

Top: Drill collars stacked on the pipe racks. Bottom: Spiral collars are easily discernible on the left from the top of the derrick.

‘.q

135

The Offshore Rig and its Equipment

Drill collars are manufactured to API specifications in a variety of weights for a range of widths varying from 2 7/8” to 12” in outside diameter. The weight of an 8” collar might typically be 160 lbs/ft, the joint being 31 feet in length. Collars are subject to the same sort of stresses as drill pipe,,and the same care and attention is given to their maintenance and handling, including lubrication and the use of thread protectors. STABILIZERS & REAMERS

Depending on the company man’s requirements, various special tools may be used with the collars in the bottom hole assembly. Stabilizers and reamers are hollow-bore tubular tools that have fin-like blades mounted on sleeves on their bodies that contact the wall of the hole some distance above the bit. Some stabilizers have spiral blades while others have vertical straight blades, and the blades may be either non-rotating or rotating with the drill string. They are positioned in the assembly at strategic points and they both have functions in straight-hole and directional drilling.

.

Stabilizers may

136

rigid or rotating spiral sleeves

The Offshore Rig and its Equipment

ROTARY TABLE \

* DRILL STEM

DRILL STRING ’

COLLARS TENSIONING DRILL STEM NEUTRAL POINT (ZERO WEIGHT)

.A-

T COLLARS COMPRESSING BIT

BOTTOM HOLE ASSEMBLY

Drill string components. Heavy collars are always placed just above the bit.

137

The Offshore Rig and its Equipment

An elevator latched onto a stand of pipe in the rotary. In the left background the kelly stands in the rathole. On the right is the choke and kill manifold.

1

3

8

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The Off’shore Rig and its Equipment The purpose of a stabilizer is to guide the drill bit and force it to rotate in the centre of the hole, thus keeping it as near vertical as possible and reducing unintentional angle changes. One or more are often used, usually below the drill collars, to stiffen the assembly when required, such as when deviation from the vertical has to be kept to a minimum. At other times stabilizers may be inserted at some point in the drill string to provide a fulcrum on which the drill bit can pivot to provide a build-up of angle in a required direction and to a required degree. A reamer is somewhat similar to a stabilizer but has rolling cutters instead of blades. It is designed to enlarge a hole already drilled bye a bit positioned below it, especially in hard rock and where the bit is experiencing hole tightness, and one or more may be used in the string. While it is performing its reaming function the reamer also acts as a stabilizer, although the wall contact area is not as great as that of proper stabilizers. Stabilizers have a limited amount of reaming ability, but they are not so suitable for hard formations, THE DRILL STRING, BOTTOM HOLE ASSEMBLY AND DRILL STEM The connected lengths of drill pipe that are run into the hole are collectively called the ‘drill string’, although often this term is applied to include the drill collars as well. The string of drill collars, stabilizers, reamers, heavy-wall drill pipe, other subsidiary items, and the drill bit, is together termed the ‘bottom hole assembly’ while the entire assembly, from the bit to the swivel, is correctly termed the ‘drill stem’. The order in which the ‘BHA’ is made up is normally laid down by the company man to suit the formation it is designed to drill through. TUBULAR HANDLING TOOLS On any rig’s drill floor will be found a variety of tools that make the work of handling drill pipe, collars, casing and other tubulars less arduous. ‘Elevators’ are collar-like gripping devices used for lifting tubulars by their ends. They hang from long steel bars called ‘links’ that are suspended by loop-shaped ‘bails’ attached to the hook/swivel unit. There are different types and sizes of elevator for each kind of tubular, most of them having some sort of locking gate arrangement in their body into which the tubular can be inserted.

l

‘Tongs’ are basically large wrenches for applying torque to pipe. Again there are numerous patterns of tong, some manually operated and others hydraulically or pneumatically operated. Two basic manual examples will be found hanging by wires from the derrick near the rotary; these are the makeup and break-out tongs used in making and breaking connections of drill

The Offshore Rig and its Equipment

A manual tong. Two of these hang from the derrick by wires so that they can quickly be swung round and latched onto tubulars when required to make or break a connection.

pipe in association with the make-up and break-out catheads on the drawworks. Larger, power-driven tongs are usually used for torquing-up heavier tubulars, and sometimes machines such as an %on roughneck’ are used to dispense with most of the manual involvement. . At the other end of the spectrum, a spinning chain is thrown around the tubular to be made up or broken out on many rigs, to start the turning process before the tongs are applied. However, these are dangerous and are outlawed in some oilfields. ..

140

The Offshore Rig and its Equipment

.

Top: A hydraulic torque wrench can grip upper and lower parts of a connection and apply preset torque to make or break it. Below: The machine can move towards the rotary table when required.

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141

The Offshore Rig and its Equipment

,

Top: A hydralulic spinning wrench can reduce drill crew fatigue and improve efficient :y. Bottom: An ‘iron roughneck’ saves manual work.

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The Offshore Rig and its Equipment

A ‘kelly spinner’ is a power-driven device installed above the kelly cock and below the swivel to enable the kelly to be connected and disconnected quickly without the use of chains or tongs. A power-driven pipe spinner, sometimes called a ‘spinnerhawk’ (although this is actually a trade name) is also sometimes used to spin up drill pipe without the use of chains. Slips are wedge-shaped, steel, hinged segments that are inserted by hand between the rotary master bushing and a tubular in order to temporarily suspend a string in the rotary when the hoist is disconnected. Their use is described in Chapter 5. A ‘spider’ is a large ring-shaped block with gripping surfaces on its inside circumference. Some spiders are used in place of slips when running largediameter pipe like casing and riser, while other spiders serve as a housing for inserted slips when running tubing or drill pipe. The larger types are often hydraulically or pneumatically operated, while others are manual. Riser spiders have large internal clamps that grip the marine riser before it is lowered through the rotary (see Chapter 5). The drill floors of modern offshore rigs are equipped with large and powerful telescoping arms that can guide tubulars through the V-door and into the mousehole, stabilize pipe when stabbing and making up or breaking connections, move pipe between the rotary and setback during tripping, guide the kelly in and out of the rathole and guide and stabilize casing and marine riser. Many other ingenious handling devices that are designed to make the drill crew’s life easier are made by oilfield equipment manufacturers, and there is a trend towards the complete automation of the roughnecks’ and derrickman’s job. The basic equipment briefly described above is tried and tested, however, and the vast majority of rigs will be using it for many years to come. OTHER DRILLING EQUIPMENT

A vast range of other implements, too many and varied to be described in a book of this scope, is used on a drill floor and elsewhere on the rig at different times during the well programme. Oilfield equipment manufacturers make numerous designs of tpols for drilling, cementing, downhole surveying, logging, fishing, coring and many other functions, and some trade names have won so much popularity with drilling personnel that they are even used to describe the products of other makers. Thus a rig’s BOP hydraulic fluid may not be made by Koomey, Inc., even though it is commonly known as ‘Koomey fluid’, the ‘drift indicator’ commonly known to the drill ..

143

The Offshore Rig and its Equipment

crew as a ‘Totco’ might in fact be made by a firm other than Totco and the ‘iron roughneck’ could be another maker’s version of Varco’s well known hydraulic spinning and torque wrench. MOTOR

RING GEAR \

I

UPPER BEARING, TORQUE ARRESTOR REDUCTION ’ /

LOWER BEARING MOTOR SHAF CLUTCH /

PINION GEAR’

-SUB SHAFT

,

Top: A motorised kelly spinner. Bottom: A hand-operated spider for casing, tubing and drill pipe. Spiders are mechanical slips.

The Offshore Rig a& its

Equipment

SUB-SEA EQUIPMENT & THE MARINE RISER

Spudding, or starting to drill an offshore well, is slower and more complex from a floater than from a jack-up rig or fixed platform because various items of guiding and well-control equipment first have to be positioned on the sea-bed. These large pieces of hardware are lowered through the moonpool of a semi-submersible or drillship, and their connection at the sea-bed is remotely controlled from the drill floor with the help of underwater TV cameras. The procedure of running this equipment is described in Chapter 5, while the individual items are described below. THE TEMPORARY GUIDE BASE

After the drilling unit’s anchors have been deployed on the new location the first item lowered is the temporary guide base (the TGB). This serves as a foundation for all the other sub-sea equipment that will follow, and as an anchorage for the guideline cables on which that equipment will be run down to the sea-bed. The TGB is a circular, octagonal or square, flat, steel frame of about 100 square feet in area which has compartments in which ballast materials can be placed. The unit alone usually weighs about four tons, but it is heavily weighted with bags of cement, barite or other heavy materials before being lowered to the sea bed on the end of a string of drill pipe. A special tool for releasing the TGB when it is in position is fitted to the drill pipe string, and this connects with a slot in the steel guidebase frame. On the underside of the frame four spikes project to dig into the sea bed and firmly anchor the unit.

A temporary guide base with its running tool. The prongs dig into the sea bed and the compartments can be loaded with weighting materials.

The Offshore Rig and, its Equipment Four wires are attached to the edges of the TGB, and when it has been landed these are tensioned up and used for guiding other items of equipment down to their locations above the TGB. There are also two smaller lines for running TV cameras down for monitoring operations from the doghouse on the drill floor. The guide line tensioners are large cylinders, installed in the cellar deck on the rig, that contain pistons which.are positioned by air pressure. The amount of tension required on the guide lines is simply set by adjusting the air pressure. In the centre of the TGB frame is a wide circular aperture with a funnel shape projecting above it into which the bottom of another frame - the permanent guide base - will fit. The drill bit will commence drilling and all subsequent .downhole operations will be conducted through this aperture.

THE PERMANENT GUIDE BASE The permanent guide base (PGB) is another heavy steel frame, about 3 tons in weight and square in shape, that has a wide central aperture and a tall post on each corner through which the four guidelines run. The PGB serves as a landing seat for the wellhead and as a guide for drilling tools and the blowout preventer stack which is eventually located above the wellhead. The posts are used to locate the stack, which has arrangments to accept them within its own frame. The PGB is run down the guide lines to connect with the TGB, and there is a funnel-shaped projection around the aperture on its underside that inserts into the TGB’s funnel-shaped top aperture and ensures an accurate fit.

THE WELLHEAD/CASLNG HANGER SYSTEM The wellhead is a large, cyl’indrical device housing several internal fittings called ‘casing hangers’ that are designed to suspend the required number and sizes of casing and tubing strings that will be used in the well. The wellhead/ casing hanger locates through the holes in the two guide bases and fits into the top of the conductor casing after this has been run. It projects above the PGB, and is designed to connect with the BOP stack which is later run above it.

146

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The Offshore Rig and its Equipment

.w

147

The Offshore Rig and its Equipment WELL CONTROL & THE BLOW-OUT PREVENTER STACK If the drilling fluid, or ‘mud’, is maintained at the correct weight to overcome every different formation pressure encountered, the well should theoretically be safe. If the formation pressure were allowed to exceed that of the mud column, however, well fluids would enter the hole and start forcing the mud out of it. This situation is known as a ‘kick’, ,and if it is not quickly brought under control it can lead to an extremely serious situation called a ‘blow-out’. When this happens, the entire column of mud is expelled with great force from the hole, along with the drill string which has a tendency in that situation to behave rather like a lot of spaghetti instead of strong steel pipe. This is followed out of the hole by either gas, oil or sea water, and often a mixture of these, in a great gush which envelops the rig. If the gas is allowed to ignite, which can easily happen when there is live electrical and hot mechanical equipment working nearby, the entire rig and its equipment, which perhaps cost more than $100 million, can be completely destroyed, with an attendant risk of serious loss of life. Blow-outs have been responsible for some major catastrophes in the drilling business, and although they are much more rare nowadays than in the past, they always attract unwelcome publicity that can give the wrong impression of the industry as being one surrounded by constant danger and unsafe practices. For all these reasons every possible step is taken to prevent kicks and blowouts from occurring. The drilling mud forms the first line of defence against kicks and blowouts. The second, and last, line of defence is the blow-out preventer stack. This is a collection of large, high-pressure valves which is fitted on the top of the wellhead in a vertical tier and which can be controlled remotely from any of several positions on the drilling unit. Although outwardly the BOP stack on a deep-water floater appears fairly unremarkable, it is an enormously expensive precision tool that can withstand pressures of up to 15,000 psi. Because of the intricacy of its numerous working parts a dedicated ‘sub-sea engineer’ is employed by the drilling contractor to maintain it and its control system in top condition. Through the middle of the BOP stack is a hole wide enough for large drilling tools to pass up and down during the course of normal operations. The width of the opening is determined to some extent by the stage at which the stack is intended to be first used in the well programme. An 18%” stack is quite a popular size, but this can obviously not be used until wide-diameter bits have drilled 36” and 26” hole (see Chapter 5). When a kick or blow-out threatens the rig and the BOP controls are operated, large and powerful devices are closed together to seal off the hole and

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The Offshore Rig and its Equipment

.

.

The Offshore Rig and its Equipment MARINE CONNECTOR

FLEX LOOPS

EMERGENCY SOP RECOVERY SYSTEM

LOWER MARINE RISER PACKAGE STRUCTURAL FRAME W l T H FUNNEL DOWN RETRACTABLE POST EXTENSIONS

ORIENTATION KEY ACCOMMODATES UP TO +I.50 MISALIGNMENT

SPIDER BEAM LANDING ARMS

EXTRA-HEAVY STRUCTURAL POSTS WITH INTEGRAL GUIDE RIBS

FLANGED POSTS FOR DISASSEMBLY OF STACK BETWEEN RAM SETS REMOVABLE LOWER STRUCTURAL POSTS

A BOP stack complete with lower marine riser package,

150

I

The Offshore Rig and its Equipment

C/osing action begins.

Seal around drill pipe.

,

Shut-off sea/-no drill pipe. The working action of a spherical BOP as pipe is ‘stripped’, or pulled out, t h[rough it

152

The Offshore Rig and its Equipment

prevent the passage of well fluids up to the rig. Arrairgements have to be made for sealing the hole either when drill pipe is in it, or when it is empty, and different types of preventer are incorporated in the stack for use in every different situation. The topmost preventer in the stack looks like a large steel pot from the outside and is called the ‘annular preventer’ or, sometimes, the ‘bag preventer’ or ‘spherical preventer’. This can seal off the annulus between the preventer housing and any type of tubular that happens to be inside it. It can also seal off the hole completely if there is nothing inside running through the preventer at the time. Inside the cylindrical steel body of the annular preventer is a large ring of steel-reinforced rubber or similar material, called the packing unit. Below this are pistons which are moved upwards by the forced entry of hydraulic fluid into small ports. As the pistons push the packing unit upwards, the rubber-like material is forced to expand in towards the centre of the hole, sealing tightly around the tubular running through it, or, if no tubular is present, completely blocking the space. On high-pressure sub-sea stacks there are often two annular preventers, one above the other, so that there is complete back-up, or ‘redundancy’ of these vital elements.

,

;

iower Housing

A spherical, or annular, BOP.

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151

The Offshore Rig and its Equipment

Below the annular preventers are three, and very often four sets of ‘ramtype preventers’. These ,also work under hydraulic pressure, which forces large steel rams to slide together horizontally to provide a seal. One of the sets of rams, called the ‘blind rams’ has flat faces that butt tightly together when there is no tubular in the hole. Another set is called the ‘pipe rams’, and these have a vertical semi-circular channel in their ends of the same diameter as that of the drill pipe being used on the rig at the time. When there is pipe in the hole the pipe rams are able to seal tightly round it, thus preventing well fluids from passing up the annulus. The stack has to be brought to the surface and the pipe rams changed whenever a different size of pipe is going to be used, but normally the same size is used for the duration of the well programme. Roundhead ram shait R a m s;aff seal

I

I

&&n assembly

Cylinder head

Wear

A ram-type BOP. Several sets of rams are incorporated in any stack

ram shafl seal

-,

The Offshore Rig and its Equipment Another type, usually the top set beneath the annulars, has sharp blades that can, if necessary, shear right through any drill pipe that is in the hole. Normally the annular preventers alone would be used to attempt to control the well pressure, and if they were not enough, the pipe rams would be used, with the shear rams being operated only in the last resort. The controls for all the preventers are in the doghouse, with miniature replica panels usually at two other places on the rig, such as in the toolpusher’s office and at the forward lifeboat station. While the preventers stop the surge of well fluids up to the rig, bubbles of gas which are responsible for kicks (and ultimately blow-outs) can be circulated out of the drilling fluid system through narrow pipe connected to the side of a cylindrical device called a ‘drilling spool’ at the base of the tier of

Semicirl Xhr top

SUPPOrt pIat

suppon plate

Shear rams

closed

These shear rams are designed to cut pipe with 3000 psi hydraulic pressure applied.

EMERGENCY BOP RECOVERY SYSTEM FLEX LOOPS

BOP REENTRY FUNNEL ASSEMBLY

/ MARINE RISER CONNECTOR

CHOKE AND KILL LINES WITH RETRACTABLE STABS

_ BOP STACK

UNIFLEX JOINT LCHOKE 8 KILL VALVES GUIDE FRAME

HIGH-ANGLE RELEASE H-4 CONNECTOR

GUIDE/ FRAME

H-4 WELLHEAD C O N N E C TOR

1 Rig&: The BOP stack. Left: The lower marine riser package. These are separated for maintenance on the rig,

E w E a F m 9” E. jj cc

The Offshore Rig and its Equipment preventers. This pipe is called the ‘choke line’ and it is connected to a large manifold of valves on the drill floor called the ‘choke manifold’. On the other side of the spool from the choke line is a ‘fill line’ and this enables heavy drilling fluid to be pumped into the well to quell, or ‘kill’ a kick. Both lines are often duplicated for full redundancy.

The choke and kill manifold is installed at the side of the drill floor where it can be operated automatically or manually.

The complete tier, or ‘stack’, of preventers is encased in a large frame twenty or more feet high, and the entire unit may weigh nearly 200 tons in air. It can cost several million dollars to renew, and it therefore has to be handled very carefully when being lowered into position from the deck of the ,. rig where it is assembled and tested. As previously mentioned, the size of the hole through the BOP stack can not be varied, so it dictates the stage at which the stack can first be used in the well programme. To allow for maximum safety coverage, manufacturers often offer two types of stack systems - single and double - which can be used

The Offshore Rig and its Equipment

at different stages. The first sizes of casing used to line holes drilled in deep offshore wells are 30-inch, followed by 20-inch and 133/s-inch. Single stack systems, which are the most frequently used on large floating units, generally have an 18% inch aperture and are rated for maximum well pressures of 15,000 psi. If this size of stack was set in position at the beginning of drilling (‘spudding in’), large tools and casing could not pass through it, and every time some device had to be run into the hole the stack would have to be ‘pulled’. An 18% inch high pressure stack can therefore only be used after the 20-inch casing has been set. A dual stack system, on the other hand, employs two BOPs. One has a 21%” wide aperture but a low pressure rating of only about 2090 psi, and this can be used following the running of 30-inch conductor casing until the 13%” wide string of casing known as the ‘surface string’ is set. It is then replaced with a 13%” size stack of high pressure rating for use until completion of the well. The BOP stack is part of the rig’s standard equipment and is not supplied by the operator like other sub-sea items are, so the two stack system may be the only option on some units. However, on most of the larger and newer rigs only a single stack system is used. The upper part of a large, 18% inch high-pressure BOP stack, which is often called the ‘lower riser package’, can be detached from the lower section in an emergency once the well has been shut in. It provides a connection for the control ‘pods’ which contain the numerous actuators that work the hydraulic BOP valves. The pods, which on the outside look like small boxes, contain a mass of delicate control equipment which is connected to the surface control panels by a thick rubber hose containing scores of electrical wires. This is wound off a large reel in the cellar deck by the moonpool. Usually there are two pods, one being a reserve unit. Also on the lower riser package are mounted accumulator bottles which contain supplies of hydraulic fluid for immediate use in an emergency, and a large device called the ‘riser flex joint’ or ‘ball joint’. This provides a flexible connection between the top of the BOP stack and the lower end of the marine riser which is attached to it. If the hydraulic control system fails, an emergency acoustic control system employing transponders mounted on the stack can be operated remotely from the surface. , THE MARINE RISER

Some form of conduit has to be set up between a drilling vessel and the well so that tools can be easily run down to the hole and so that drilling mud can circulate back from the hole to the rig. The ‘marine riser’ performs this job and is one of the most important elements of any floating drilling operation.

._)

1.57

2 Types BOP stacks: single & double (dual)

single: high pressure ( 15000 18 3/4"

double (dual): has in real tow BOPs 1st one (surface string low pressure (abt. 200 large size 21 1/4" (use

2nd one: 13%" size of the high p

The Offshore Rig and its Equipment

Top and bottom: Joints of marine riser take up a lot of space, but must be carefully stowed on floaters. These are on drill ships.

158

,

The Offshore Rig and its Equipment Because of its great length, which makes it very heavy when it is full of mud, and because of the strength of the wave, tide and current forces that act on it, the riser must be strong and well-designed, especially at its couplings. It must also be capable of withstanding great external pressures should the drilling fluid completely escape from it for any reason. It is therefore a very expensive part of the rig equipment which has to be safeguarded by very carefuluse. The marine riser is composed of a series of long, wide-bore pipes whose ends are connected so that they form a tightly sealed and smooth passage from the point of suspension beneath the rotary table to the BOP stack on the sea-bed. The joints must be of sufficient diameter to allow free passage of the largest downhole items that will be run, such as casing and casing hangers. On a BOP stack with an 1g3/4” aperture, a 20” OD riser would probably be connected, this having approximately the same internal diameter as the BOP aperture. The standard riser joint is 40 feet long, but pup joints of shorter lengths are also used to make up the exact distance between the top of the stack and the underside of the rig. Each joint is ‘stabbed’ into the one below it and secured by special connectors which tie the two flanged ends together as it is run to the sea bed through the rotary. A special riser spider is used to support the riser when this is being done. Because the sea-bed connection at the BOP is rigid when the riser is set up, whereas the top connection, at the rig, is moving in relation to this, some means of allowing free vertical movement has to be incorporated into the system. This is achieved by the insertion of a ‘telescopic slip-joint’ near the top of the riser, just below its connection with the underside of the drill floor. As the vessel heaves up and down in the seaway the upper part of the slip joint moves inside the lower part, which is static in relation to it. Viewed from the deck of the rig at the moonpool it may thus appear that the lower part of the riser is moving, whereas in fact it is the other way round. At the bottom of the riser, just above the connection with the BOP stack, is the ‘riser flex joint’, often called the ‘ball joint’, which allows a certain amount of horizontal play in this part of the riser in response to wave action. On some very deep water rigs the riser is fitted with a second ball-joint in the surface wave area beneath the slip joint’. Running down the outside of the entire length of the riser are two narrow, rigid pipes called ‘choke and kill lines’. These connect with the BOP stack and are used in well pressure control operations.

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lC9

The Offshore Rig and its Equipment

TELESCOPIC JOINT

MARlNE JOINT

RISER

MARINE RISER CONNECTOR

Left: Marine riser system components. Right top: Flanged riser couplings. Bottom: Choke and kill lines run along the outside of each joint of riser.

Although in most cases the BOP is run together with the marine riser, there are certain circumstances when this is not practicable. If shallow gas exists in formations just under the sea-bed, there is a risk of this gas blowing out unless sufficient equipment can be installed to contain its pressure. However, the rig’s BOP stack can not be used at the initial stages of drilling as its aperture is too small for very large bits to enter. At the same time the short length of 30-inch conductor pipe that is the first casing string run might not

i60

”

-‘.’

The Offshore Rig and its Equipment be capable of withstanding the pressure of the shallow gas, even if a largeaperture BOP stack could be installed above it. To avoid the risk of the gas blowing out through the drill floor, where ignition would be possible, a pipe called a ‘diverter’ is installed at the top of the riser, and the riser is connected not to the BOP stack but directly to the conductor casing by means of a ‘pin connector’. The diverter leads from the riser to both outboard sides of the rig, and has valves so that any gas coming out of it can be fed to the downwind side. Pin connectors and diverters are often used in the North Sea and other drilling areas where shallow gas is frequently encountered.

A hydraulic latch, or pin connector, is used to connect the riser system to the conductor housing . when drilling without BOPs through shallow gas sands.

Various measuring devices, including hydro-acoustic beacons, are used to monitor the angle the riser makes with the vertical, so that the position of the rig can be adjusted to minimise stresses on it. In most deep-water situations the riser will not make a perfectly straight, vertical line from the rig to the

The Offshore Rig and its Equipment sea-bed but will curve somewhat with the forces acting on it. The beacons are attached to the riser, and sometimes to the BOP stack, and the angle and azimuith readings obtained from their pulses is recorded on a visual display in the control room. THE RISER TENSIONING SYSTEM In water depths of up to about 200 feet the whole marine riser can be freestanding, with no means of support other than its connection with the vessel. In deeper water, however, there is a risk of the riser collapsing due to the weights and environmental forces acting on it, and some means of supporting it therefore has to be employed.

The lower part of the riser is supported by tensioner wires in the moonpool. These wires are often called ‘Rucker wires’.

162

.,.,

The Offshore Rig and its Equipment

The usual means is by a system of wires held in tension in a similar way to the drill string motion compensator system described earlier in this chapter. The riser support wires are connected to the upper end of the lower, rigid, part of the riser, just below the slip joint, and compressed air is fed from a bank of reservoir bottles to tall cylinders in which pistons operate. On the ends of the pistons are large sheaves forming part of a purchase arrangement through which the tensioning wires are rove; when the pistons move the wires in the purchase move. To dampen their action the pistons are balanced with high pressure air to absorb energy on one side, and compressed hydraulic fluid on the low pressure side. They have a stroke which is less than the maximum allowable heave by an amount determined by the mechanical advantage of the purchase system. Thus a 20-foot heave could be translated to a 5-foot movement of the pistons. The cylinders are usually arranged in pairs diagonally opposite each other, so that an even tension is maintained on all wires.

STAINLESS STEEL ROD VALVE

GLAND BEARING AND SEAL

I- HYDRAULIC CUSHION

‘TIE DOWN II

1 1 TO RISER

A/R BANK

The riser tensioner system.

The Offshore Rig and its Equipment

The amount of tension required on the lines and the number of lines connected will depend on factors including the total length of riser deployed and the density of the drilling fluid inside it. As drilling progresses the hole gets deeper and downhole pressures get higher. Heavier mud therefore has to be used, and this increases the strain on the riser. This is compensated by attaching more tensioner wires and by gradually increasing the air pressure in the tensioner cylinders. Each of ten ‘rucker wires’, as the tensioner wires are sometimes called might, for example, have a tension of 22,000 lbs on them.

/

/ I

From the side of the moonpool it often appears that the lower part of the riser is heaving. In fact the upper part and the rig is moving while the lower riser is steady.

164

-w

CHAPTER5 DRILLINGOPERATIONS Because of the enormous costs involved, marine drilling is a twenty-fourhours-a-day, seven-days-a-week operation that may continue for anything from a few weeks to several months before ‘total depth’ (TD) is reached and the well can be ‘completed’ or ‘plugged and abandoned’. Even Christmas day is a normal working day on most floaters which, it should be remembered, could be costing an operator anything from $10,000 to $95,000 a day just to hire, quite apart from the other well costs he incurs. The actual drilling is done by drill crews who work ‘back to back’. The usual pattern is that under the supervision of the senior toolpusher (and at night the junior toolpusher or ‘tourpusher’), two crews, each comprising a driller and about five men, each work a daily twelve hour shift on the drill floor and associated areas. The assistant driller, derrickman and three roughnecks forming the drill crew are helped as and when required by three general purpose ‘roustabouts’ and their foreman, who is normally the crane operator. Specialists like directional drillers, casing crews and technicians from any of the drilling equipment manufacturers are flown out to the rig whenever they are required, while a rig mechanic and an electrician remain on call to effect emergency repairs to drill floor and mud room equipment that the drill crew itself cannot cope with. Apart from these secondary jobs the mechanic, electrician and motorman also have to maintain the power supply that is essential to keep the rotary rig functioning, and many other people in various background workplaces around the rig similarly make an essential contribution to the drilling operation which is focussed on the drill floor. While the driller physically controls the drilling from his control panel in the doghouse, the company man has to see that the drilling programme is carried out efficiently, with the minimum of delay ,and hazard, to the requirements of the operator who is paying for the well. He therefore has an enormous responsibility, with authority and remuneration to match, and a new hand aboard a rig soon becomes aware that what the company man says, goes. Through the toolpusher he sees to it that everyone onboard is attuned to the fact that in Ihe drilling business time equates directly to money, and the less time wasted the better, both for the operator’s pocket and the contractor’s reputation. Many rig operations therefore tend to be carried out at an extremely fast pace which does not always appear compatible with complete safety. Because of the nature of the procedures and the heavy and powerful equipment used there are inevitably some accidents, but most rig crews are generally highly trained through long and repeated practice of their own

. ..*

,lhrl

Drilling Operations specialised tasks and through their training courses. To the casual onlooker, then, some of the physical manoeuvres on the drill floor, such as making connections of drill pipe, might seem almost automatic and mechanical. RUNNING IN THE HOLE When the entire drill string is to be run into the hole it will normally already be standing inside the derrick structure in several orderly rows in the ‘setback’ area to one side of the V-door. When it was last pulled out of the hole the string was disconnected not in single joints, but in ‘stands’ of three joints (or sometimes two) with the male threaded ‘pin’ ends lowest, and this is how it was ‘racked back’ in the fingers.

166

Drilling Operations A three-joint stand of any 31- or 30-foot tubular is called a ‘treble’ (or ‘thribble’ an an American rig), and a stand of two 45-foot joints is called a ‘double’, each stand making 90 feet or thereabouts in length. This length reaches to the ‘fingerboard’ high above the drill floor, where the top ends of all the stands have been pulled into slots between the fingers by the derrickman, who works leaning out from the adjacent monkeyboard with a harness round his midriff. The kelly, with the swivel, the kelly spinner, the kelly cock and the kelly bushing attached to it, has meanwhile been stored in the ‘rat hole’, which is a deep tubular storage recess that projects down beneath the drill floor. The rotary hose is still attached to the swivel, so that to reconnect the assembly for drilling, the swivel simply has to be picked up by its bail and fitted into the hook, while the lower end of the kelly (which is protected with the kelly saver sub) has to be screwed into the topmost joint of drill pipe. But first the drill string has to be ‘run in the hole’. The bottom hole assembly (the BHA), perhaps a thousand or more feet of heavy tools, is first made up as it is passed item by item through the rotary table opening. The bit, stabilizers, collars, jars, subs, heavy-wall drill pipe and any other components included in the carefully designed BHA that will make the next section of hole are in turn pasted with thread compound and screwed togther, or ‘made up’, and lowered through the rotary by elevators which hang by links from the hook/block assembly. As each item is lowered through the rotary by the hoist, the drawworks brake is applied and slips are inserted, or a spider is used to provide a holding grip while the next joint is connected. The weight of the entire string could be more than 200 tons, and can sometimes cause the steel of the slips to fuse to the rotary bowl, so the roughnecks apply ‘dope’, or zinc-based thread compound, to the backs of the slips before they insert them. TO the BHA the stands of drill pipe that comprise the drill stem are then connected stand by stand, and these are lowered through the rotary opening. Again, the appropriate type of elevators are fitted by the derrickman round the shoulders of the upper tool joint of the stand to be lowered, and the stand is released from the fingerboard. The hoist lifts the stand a little while the roughnecks on the floor ninety feet below guide its lower end over to the rotary and stab it into the tool joint box of the tubular held there in the slips. The connection is then spun up, usually by a pneumatic pipe spinner, but on some rigs by a spinning chain which is thrown around the stabbed pipe and pulled off again by the make-up or spinning cathead. Finally the correct torque is applied to the connection by tongs, which might be of the manual type or else pneumatically operated.

.._)

167

Drilling Operations

Slips are inserted manually around a tubular to grip it in the rotary during the making or break@of a connection.

The break-out tong is secured round the lower of the two tool joints. The end of its arm is attached by a wire to the break-out cathead shaft on the drawworks, but this wire remains slack during the make-up operation, since the break-out tong on’ this occasion is only meant to ‘back up’, or hold the joint firm while the connection is tightened. The make-up tong meanwhile is applied to th& top joint and the end of its arm is connected with the makeup cathead shaft-on the drawworks by a chain which is wound tight to torqueup the connection. The arm of the break-out tong tends to turn clockwise when the chain is pulled, so a wire called a ‘snub line’ which is secured to a nearby ‘Sampson post’ on that side is permanently fixed to the break-out tong to prevent it from turning round. There is also a snub line leading from the make-up tong arm to a Sampson post on its own side, but this remains

Drilling Operations slack during making-up. Also connected to the end of the make-up tong arm is a torque gauge with a transmission line leading to the doghouse. The driller is thus able to read what torque is being applied to the connection as he controls the make-up cathead.

USE OF MAKE-UP TONG

During make-up, the joint is torqued-up by the make-up tong chain. The break-out tong is prevented from turning round by a snubline.

Having tightened the connection to the correct torque, the weight of the complete string is taken by the hoist, the slips are removed and the stand is lowered ninety feet into the rotary until the tool joint of the uppermost joint of pipe is two or three feet outside the rotary. The drawworks brake is again applied, the slips are inserted between the rotary bowl and the pipe to grip it, and the elevators are released to pick up the next stand. This process is repeated for each of the stands, which may number more than two hundred in a deep well. The complete cycle of operations for each connection takes only a few minutes, yet rig operators and oil companies are constantly seeking ways of reducing the connection time to save costs.

Drilling Operations

When the last stand of drill pipe is in the slips, the kelly, with the swivel, kelly spinner, kelly cock, kelly bushing and saver sub attached, is lifted out of the rat hole and screwed into the drill pipe by the pneumatically operated kelly spinner, finally getting torqued-up by t’ongs. The driller starts the mud pumps to begin circulating drilling fluid through the string to the bottom of the hole, and he lowers the kelly until the kelly bushing engages with the rotary master bushing. A roughneck stationed at the shale shaker announces ‘returns at the shaker’ and the driller starts the rotary turning and eases the drawworks brake off to lower the bit to the bottom of the hole. When it touches bottom, ‘drilling ahead’ commences.

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170

Drilling Operations DRILLING AHEAD With the bit now making hole, the driller makes fine adjustments to the mud pump pressure and,rotary revolutions, and uses the weight indicator and the hoist controls to put the correct weight on the bit. As the bit chews into the rock the hoist has to be lowered inch by inch to maintain the proper weight, and various other gauge readings on the doghouse control panel have simultaneously to be monitored, including return mud flow, mud pump pressure, pump strokes per minute, rotary torque and mud pit volume and others.

From the doghouse control panel the driller monitors numerous data feedbacks.

,

Depending on the hardness of the rock and the rate of penetration (the ‘ROP’), drilling ahead like this may continue for anything from fifteen minutes to several hours before most of the kelly has been lowered through the rotary. If the kelly is the usual forty-foot length, about thirty feet of it can be ‘drilled down’ before a new joint of pipe will have to be inserted in the string. That operation is called ‘making a connection’.

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171

Drilling Operations

MAKING A CONNECTION When most of the kelly’s length has descended through the rotary table the kelly is pulled back up until the first tool joint of drill pipe shows two or three feet above the table. The roughnecks insert the slips into the annulus between the drill pipe and the rotary bushing, and the weight of the drill string is eased off the hoist. The connection between the kelly saver sub and the drill pipe is then loosened with tongs and unscrewed totally by the powered kelly spinner. A single joint of drill pipe will previously have been hoisted from the pipe racks on the main deck up the dragway ramp to the drill floor, where it now stands ready in the ‘mouse hole’ near the rotary table. This is another tubular recess, like the rat hole but a little shorter. The threaded pin end of the kelly saver sub is inserted into the top end of the mousehole joint and the driller

Tubulars are positioned by crane on the V-door dragway ready for hauling up to the drill floor by air tuggers.

172

..

Drilling Operations

activates the kelly spinner to spin up the connection. The roughnecks then attach the two sets of manual tongs to the kelly and the drill pipe and tighten the connection to the correct torque. Then the driller lifts up the kelly, pulling the attached new joint of pipe out of the mousehole, and the roughnecks guide its lower end over to the rotary where they stab and make up the connection in the usual way with the tongs. The driller then raises the hoist a little to allow the roughnecks to withdraw the slips, and the new joint is lowered through the rotary until the kelly bushing again engages with the master bushing. The mud pumps are restarted, the bit is then lowered to the bottom, and drilling resumes, the whole operation having taken only a few short minutes.

When a connection has to be made, the kelly, complete with kelly bushing, is detached and lifted out of the rotary. Here the roughnecks are’about to break the connection.

171

Making a mousehole connection. Left: The kelly is lifted from the rotary. Centre: The slips are inserted. Right: The connection is loosened with the tongs.

,

Making a mousehole connection. Left: The kelly saver sub is stabbed into the new joint of drill pipe. Centre: The connection is torqued-up by the make-up tong. Right: The new joint of pipe is lifted from the mousehole.

Drilling Operations

TRIPPING

The period of ‘drilling ahead’, or ‘making hole’ as it is called on American rigs, may last uninterrupted, apart from making connections, for several days or just a few hours, depending on the hardness of the formation. Sooner or later, however, the bit will need replacing. The entire drill string then has to be pulled out of the hole and unscrewed stand by stand, the old bit changed for a new one, and the string made up again and run back in. ‘Making a round trip’ is a long and tedious operation that might take a drill crew their entire twelve-hour tour if the hole is deep and they have come ‘on tour’ just at the wrong time. The kelly is first pulled up and the slips are inserted around the drill pipe in the rotary bowl. The connection between the drill pipe and the kelly is then loosened and broken, and the kelly is lifted into the rat hole, where it is left complete with the kelly spinner, kelly cock, kelly bushing and the swivel, to which the rotary hose is still attached. To break the connection the roles of the two sets of manual tongs are reversed from the make-up situation. The break-out tong with its wire, which pulls in an anti-clockwise direction, is used to loosen the connection, so it is now,,applied to the top tool joint, while the make-up tong holds the lower tool joint firm with its snub line preventing it from swinging round anti-clockwise. Its chain hangs slack meanwhile. SAMPSON POST

.

The break-out tong loosens pipe during tripping-out while a snub line prevents the make-up tong from turning.

Drilling Operations The connection is broken and spun loose and the pipe elevators are lowered and attached to the shoulders of the tool joint projecting from the rotary table. The stand of pipe is hoisted out of the hole until the tool joint of the next stand is in the correct position for unscrewing, and the slips are again inserted.

A ‘wet trip’. This would be particularly unpleasant for the roughnecks when oil base mud was being used were it not for the help of the racking arm.

The derrickman, meanwhile, has climbed to the monkeyboard and has put on his harness. He leans out and catches hold of the top’of the stand of pipe as it comes near the monkeyboard, and guides it into oneof the slots in the fingerboard, making sure that the pipe will be racked in the correct order. The roughnecks on the floor below, meanwhile, guide the lower end of the stand towards the set-back area where the driller sets it down. The derrickman unlatches the pipe from the elevator and the procedure is repeated for the next stand of pipe.

178

7’

._

Drilling Operations The driller has to be careful not to pull pipe up too quickly and cause ‘swabbing’, or a suction effect on the formation that could introduce well fluids into the hole, and he also has to ensure that the volume of pipe that has been removed from the hole is replaced with the same volume of.properly weighted mud. This is pumped through the fill line and the trip tank (or ‘possum belly’) level gauge on the drill floor will tell the driller whether there is any net increase in the tank level, which could be the first indication of a kick.

The driller lowers the kelly into the rathole.

When all the drill string except for the last drill collar and the bit has been racked back, the bit’is lifted clear and the bit breaker designed to fit it is placed in the rotary. The bit is inserted into the bit breaker and the break-out tongs are applied to loosen its connection with the collar. The bit is unscrewed by hand and the drill collar is lifted off. The bit can then be removed from the breaker and examined for wear. the new bit is lubricated and made up to the collar and the whole procedure reversed until the drill string is once more assembled and drilling ahead can be resumed. ..

170

Drilling Operations RUNNING & CEMENTING CASING The large diameter hole which is drilled to start a well has to be lined as soon as possible with steel casing. This prevents the wall of the hole from caving in, or ‘sloughing’, and stops sea water entering, or drilling mud escaping from the hole into the surrounding formations. The short, topmost string of casing also provides a firm base and an anchorage for the BOP stack and for the longer strings of narrower casing which will be run later to line the lower sections of the hole. Casing is designated by its outside diameter, namely 3t)“, 20”, 133/a”, 103/4” and g5/a”, and each joint, which on average measures 42 feet in length, has to be extremely strong to withstand high well formation pressures as well as compressive and tensile stresses.

Casing has to be carefully landed, arranged and marked on the pipe racks.

Drilling Operations

In shallow development wells the first, or ‘outer conductor’ string of 30” casing may be run to a depth of only about 150 feet, but deeper wells, especially exploration wells, may be cased to a depth of 1000 feet or more before drilling starts in the next section of the well. The conductor, and all other casing strings, is firmly anchored by filling the annulus between the casing and the wall of the hole with cement. The cement is not poured in from above, but pumped from below. A thin cement ‘slurry’ is pumped down into the casing pipe and a rubber plug is set on top of it. Drilling fluid is then pumped down on top of the plug, which then acts as a piston inside a cylinder, forcing the cement down the inside of the open-ended casing and up the annulus between the casing and the hole wall. When the rubber plug reaches the shoe at the bottom of the casing string, pumping is stopped and the well is left standing for several hours for the cement to set hard. This period is known as ‘waiting on cement’ (WOC) and gives the drill crew an opportunity to do repair and maintenance jobs around the rig.

SEA BED $-

6

OUTER

CONDUCTOR

INNERCONDUCTOR

&

A TYPICAL NORTH SEA CASING DESIGN

A typical casing design for a North Sea well drilled from a floater. Jack-ups employ a ‘foundation pile’ which,is driven in to connect the sea bed with the rig.

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181

Drilling Operations

JOB IN PROGRESS

JOB COMPLETED

A simplified cementing system. Most cement jobs employ ingenious tools for pumping the cement by stages to ensure complete penetration round the annulus.

Drilling is then resumed with a smaller bit: a 26” bit to fit through the 30” outer conductor casing, a 17?‘? bit to fit through the 20” inner conductor, a 12%” bit to fit thiough 133/a” surface string, and an 8V2” bit to fit through the 9%~” intermediate casing. If test cores or other indications look encouraging, a further string of 7” production casing or a ‘liner’ is cemented at or near the bottom of the hole, and various tests are conducted on the formations in the hope that well fluids can be persuaded to flow up to the rig. Narrow gauge ‘tubing’ may then be run down from the surface inside the casing and liner to channel the fluids up to the test equipment.

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Drilling Operations DIRECTIONAL

DRILLING

Most offshore wells drilled by mobile rigs are drilled vertically, or almost vertically, but often it is necessary to drill directionally, or at an angle to the vertical. This type of drilling is done routinely from fixed platforms. When a platform is installed to exploit an offshore discovery, some of the development wells drilled from it will be vertical, or nearly so. But to reach the horizontal extremities of a surrounding oilfield is more difficult. A floater or jack-up rig might be used to drill an isolated satellite well that could be tied in with sea-bed production pipe to the central platform, but to drill the required twenty, thirty or even sixty development wells from mobile rigs and then tie them in by pipelines would be inordinately expensive. Numerous directional holes are therefore usually drilled from fixed production/drilling platforms to tap the fields around them, and the techniques then employed are basically the same as those used when a floater has to drill directionally for some reason. This might be, for example, to penetrate a reservoir at a certain angle to take advantage of natural formations, or to by-pass an obstacle blocking the path of the vertical well, or to drill a relief well into a well that has suffered a blow-out. Directional drilling required special skills, techniques and equipment, and very often it is done on a floater under the guidance of a highly trained and skilled directional driller who will be specially hired by the operator.

Directional drilling can be employed for a number of purposes. In most cases from a floater it is to sidetrack an obstacle.

Drilling Operations

Normally a directional well is drilled vertically for a short initial distance which is cased before deviation is begun from the ‘kick-off point’. Any of a variety of drilling tools could be used from this point, depending on the characteristics of the hole required. Most frequently offshore a ‘downhole drilling motor’ is used in conjunction with a ‘bent sub’, but in some cases a long, tapered steel wedge called a ‘whipstock’ is set on the bottom to start the hole in the desired direction. The downhole drilling motor converts the hydraulic energy of the mud stream in the drill pipe into mechanical energy to turn the bit. Two types of motor are used, one a turbine, the other a positive-displacement motor, both of which force the drill bit to turn without any rotation of the drill string being needed. However, the rotary is normally kept turning slowly whilst the ‘mud motor’ or ‘turbo-drill’ are being used in order to prevent any sticking of the drill pipe to the wall of the hole. Interestingly, the Russians are reckoned to be the most common users of mud motors.

Some of a directional driller’s tools. Left: A mud motor. Right: A drilling turbine in three sections. .._

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A whipstock can veer a bit away from the vertical and make it drill a window through casing.

The tapered shape of a whipstock is designed to force the bit away from the,vertical as the bit drills downwards. This long tool is attached to and carried down to the bottom of the hole by the drill string, then unlatches automatically, allowing the bit to travel on past it and make the ‘pilot hole’ for the deviation. Once this has been done the whipstock can be recovered, and the motorised bit makes the new directional hole unaided. Special direction-seeking instruments are incorporated in the bottom hole assembly to orientate the whipstock in the desired direction and later, following its withdrawal, to check the course of the hole from time to time. .-

1

8

5

Drilling Operations The correct make up of the bottom hole assembly is most important, and numerous variations can be employed to either make the bit maintain direction or build up angle as required. Where there is room in the hole, stabilizers are used to provide a pivot at the desired point, while ‘packed hole assemblies’ that effectively fill the space created by the bit ensure that the BHA stays rigid and straight. Frequent surveys are made with the magnetic or gyroscopic compass-like instruments, and the bit can be steered with amazing precision to its target or round an obstacle. If magnetic survey instruments are used, the drill collars that they are run inside can not be made of magnetic material, so special non-magnetic collars, sometimes called ‘monels’ are used. These are shiny, like stainless steel.

. .

t

Top: A monel, or non-magnetic drill collar. Bottom: This gyro instrument package has an outside diameter of only 2% inches.

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Drilling Operations There is a wide range of survey tools on the oilfield equipment market, many of them employing a technique in which a miniature compass card looking rather like a small fairground rifle target is fitted to and punched by a narrow projectile which is dropped down the hole. This is attached to and retrieved by a thin wire called a ‘sand line’ which runs down through the drill string, and from the card the angle of inclination and the direction of the bit’s path can be read. One such instrument commonly used to check the drift of vertical holes offshore is called a ‘Totco’ after the name of its manufacturer.

Left: A gyro survey instrument. Right: A measurement-while-drilling (MWD) tool sends pulses back to the rig through drilling mud. .. 10-l

Drilling Operations Once the bit is on its deviated path, the directional tools may only be needed occasionally to correct large deviations from the desired course of the well. Alternatively, the build-up of angle continues towards the desired maximum, which may be 30 or 40 degrees or more, the rate of build-up of angle being about 2 or 2% degrees per 100 feet. Once the desired angle is obtained it is usually maintained steady until the well is completed, as this allows a ‘stiff’ assembly to be used which results in faster drilling. When directional holes are drilled to relieve wells blowing out, the target area for the bit is usually at the base of the blowing well just above the penetrated reservoir, and drilling begins from a location a thousand or more feet away horizontally on the surface. When the blowing well is finally penetrated, large quantities of heavilyweighted drilling fluid are pumped down the relief well to kill the flow in the main well. Piloting a wandering drilling bit towards a narrow hole thousands of feet down and to one side is a technical operation that calls for the highest degree of skill and experience, and some of the names of these drillers are almost legendary in the industry. Another type of direction drilling, called ‘slant drilling’, is done by a small number of specially-equipped jack-up rigs. Here the derrick is slanted to an angle of about 30 degrees, so that the hole is deviated from the surface and not after some depth of vertical hole has been drilled. This allows greater angles to be ultimately obtained and enables deviated wells to be drilled in relatively shallow depths. Slant drilling might be useful, therefore, in exploiting a wide reservoir lying under shallow water with the minimum number of platforms being built. So that the slanted well is begun from the position of what will eventually be a production platform, the jack-up rig is positioned close alongside the new platform’s basic structure, or ‘jacket’, and ‘skids’ its whole drilling package onto the jacket, through a corner of which drilling then takes place. On completion of the first wells, the drilling package can be skidded around to another corner on the jacket to drill other slanted wells, and can also be straightened up to drill vertical wells before finally being skidded back over to the jack-up rig. Once back on the jack-up the drilling package can drill overside in the normal manner from its cantilever beams. The cost of drilling a directional well from a mobile unit or a platform is normally greater than that of drilling a vertical well of the same hole depth. This is because of the slower drilling rate and the time required to make surveys of the course of the well, and to correct the course where necessary.

188‘

L This MWD log shows the decrease in weight on the bit as a result of sharp dog legs in its path. >

About 30% additional time is required on average for a directional hole. The timescale required for drilling a typical directional well from a semi-submersible is outlined in the final section of this chapter. DRILLING HAZARDS

Few, if any, offshore wells proceed from spudding to completion without hitches of one sort or another to disrupt the programme, and operators invariably make allowances in their timescales for ‘contingencies’. The most common hindrances to offshore drilling operations are ‘stuck pipe’, ‘fishing’ operations and having to ‘wait on weather’. More serious but fortunately less frequent are conditions known as ‘lost circulation’, ‘kicks’ and ‘blowouts’, and the occurrence of a dangerous gas called hydrogen sulphide. STUCK PIPE

If the hydrostatic pressure in the column of drilling fiuid, or mud, is greater than that of surrounding formations, there may be a-tendency for the drill string to be drawn into the side of the hole and stick. This happens especially wheer_re there is a large build-up of filter cake, or mud cake, on a porous formation wall, and when rotation of the drill string has been stopped for some reason. This might be during a mechanical breakdown or the taking of a directional survey, or even during the few minutes it takes to make a connec. .. r’on

Drilling Operations Con. Other causes of sticking may be jamming of the drill collars in grooves, in a deviated hole, and the bit grinding trying to work underneath a mound of

sloughing, or caving-in of the walls, called ‘key-seats’, worn in the bends on a broken part such as a cone, or uncleared cuttings.

In an attempt to avoid stuck pipe the drill string is kept moving for as much of the time as possible, either in rotation or vertically. Stabilizers are used in the drill string to keep the tubular part of the string away from the wall of the hole, and drill collars with spiral grooves are often employed in place of smooth-sided collars. The surface area in contact with the wall of the hole is less on spiral-grooved collars than it is on the smooth-sided type, and the grooves allow better circulation of the drilling fluid to lubricate the sides of the hole. The fluid itself is made as light as possible while at the same time slightly overbalancing the formation pressure, and various additives can be used to make the mud cake less viscous. Sometimes stuck pipe can be freed by quickly ‘spotting’ a slug of pumped oil down the inside of the drill pipe and up the annulus to the sticking point, and allowing this to soak into the mud cake while pulling and ‘jarring’ on the string from above. The driller can exert a certain amount of ‘overpull’ on the string with the hoist, and he can activate the ‘jars’ if there are any incorporated in the drill string. Jars, or ‘bumper jars’, work by releasing stored energy when they are either compressed or tensioned, and very often they will be effective and the string will come free. If these methods are unsuccessful, the point of sticking can be determined by a small instrument called a ‘free point indicator’ that is run down the inside of the drill pipe. This can tell the driller whether the extra pull and torque he is applying to the drill string is affecting the part of the string opposite the instrument, and starting at the bit, the device is moved up the inside of the string until the first free point above the sticking area is identified. The instrument is then withdrawn and in its place a small explosive charge is lowered to a position inside the first tool joint estimated to be above the free point. When the charge is fired electronically from the surface the threads of the joint should loosen enough for the free section of the string to be ‘backed off’, or unscrewed, by rotation of the drill string at the rotary table. This part can then be pulled out of the hole. Attempts can then be made to recover the stuck part of the string still inside the hole using special ‘fishing’ techniques. FISHING Fishing might be attempted to recover a length of stuck pipe, a broken tubular, a section of casing, an item lost down the hole through the rotary aperture, broken bit cones, old packers or anything that will impede the progress

190

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Drilling Operations of the bit. Undrillable items are termed ‘junk’, while an object that a operation is aimed at connecting to is called a ‘fish’.

fishing

A wide range of ingenious tools have been developed through long experience of generations of drillers to loosen and retrieve fishes. Jars, or ‘bumper jars’ can be used to exert a very heavy or very light jolt to a stuck item, and a ‘fishing string’ can be specially made up to position these just above the fish. Cutters can slip over drill pipe or tubing and remove whole sections. Small fragments of metal debris, such as broken bit cones, or metal tools that have been dropped through the rotary might be removed with a powerful magnet.

An overshot can latch onto the top of broken pipe.

Drilling Operations

Mills of many different shapes can grind down drillable items such as pieces of cement, packers, and other items that cannot otherwise be fished out. Small milled fragments can then be washed up into the interior barrel of a ‘junk basket’ which has a grinding shoe on its lower end. ‘Rotary shoes’ of many types can ‘wash over’, or slip over, a fish and grind an annulus outside it, freeing it from the wall of the hole or the casing in which it is jammed. Larger fishes, such as stuck pipe, are gripped by an ‘overshot’, which grips pipe on the outside, or by a ‘tap’ or ‘spear’ which screws its tapered end into the open end of a pipe and grips it from inside. Some small items of junk may simply be milled and left embedded in the wall of hole, if they are not likely to impede drilling.

Round Nose Mlfl WNh Regular Pin Connection

Taper M/II WIkh R.gulw PM Connrction a n d With Fl~Mng Neck

J u n k M/l/ Round NQII Mill With Reguler Wlfh Regular P/n Conn.ellon a n d Pin C o n n e c t i o n a n d With Fishing N e c k With Flrhln# Neck

A range of mills for grinding down hole debris.

Fishing is expensive ‘downtime’ as far as an operator is concerned, and drillers take every precaution to avoid having to do it. Trying to engage a small, irregularly shaped piece of broken metal that is out of sight thousands of feet down a narrow hole is extremely difficult and often unsuccessful, but if the fish cannot be gripped it might mean having to deviate the well round it, or, at the very worst, having to abandon the well completely. Nowadays less time is spent fishing than formerly, especially in deep wells, because it is often more economical to simply back off and ‘sidetrack’ the fish. Sidetracking is a technique used to by-pass an obstacle that might involve milling through a section of casing, sealing the casing below the kick-off point with cement, and drilling ‘directionally’ round the fish. An explanation of directional drilling is given in the previous section in this chapter.

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Drilling Operations

BASKET BODY

CATCHERS

A junk basket collects fragments of debris such as broken bit teeth.

LOST CIRCULATION

When more mud is circulated down the drill pipe than returns through the flowline, a situation called ‘lost circulation’ exists. Apart from being costly in terms of lost mud and time, it can also be dangerous because it makes well control difficult and can quickly use stocks of mud which are necessary for full control. The drilling fluid, or mud, must be maintained at a pressure which will contain the pressure of any fluids present in the formation - what is termed the ‘formfttion pressure’. When a very porous formation, or fissured rock, or rock such as limestone that contains cavities, is encountered and there is a pressure drop, the build-up of sealing filter cake on the wall of the hole may not prevent the escape of the drilling fluid into the formation. This may happen particularly when the mud has been weighted up to exert an over-pressure on another, deeper part of the hole that has a higher formation pressure.

193

Drilling Operations

As the mud escapes into the formation circulation reduces or ceases, evidenced to the driller by the level in the trip tank falling, and it becomes difficult to continue logging the well by normal mud-logging techniques. Drilling may then become impossible, but even so, more mud must be added to the hole to maintain enough hydrostatic pressure to over-balance the greatest formation pressure in the hole. If the high-pressure zone takes charge, there could be a ‘kick’ which, if not controlled, could eventually lead to a full scale ‘blow-out’. Heavy materials called ‘lost circulation materials’ (LCM) that plug the pores or fissures of the formation are added to the drilling fluid to control fluid losses when they occur. Mica, cellophane flakes, ground walnut shells, olive stones, oyster shells, coconut fibres and many others materials can be used as LCM, and quantities of these are kept handy in the sack room or mud room for mixing with mud in an emergency. The extra heavy mud containing them is pumped on its normal circulation path down the hole and deposits the LCM in the pores and fissures, blocking them against the exit of the mud. If the LCM does not stem the flow into the weak formation, cementing the fissures may be required, or a string of casing may have to be run to seal off the formation before drilling can be resumed and the mud weighted up again to deal with the lower, high-pressure formation.

KICKS & BLOW-OUTS

A kick is a sudden surge of drilling fluid up the annulus, caused by the formation pressure encountered by the bit exceeding the hydrostatic pressure exerted by the column of drilling fluid. A kick might be a gas kick, a salt water kick or an oil kick, depending on the particular well fluid entering the hole, but all types of kick are dangerous and have to be guarded against. If mud of too low a density, or ‘weight’ is used, or if the drill crew fail to keep the hole full of mud when tripping out, a kick can occur. Theoretically, as drill pipe is pulled, the mud level in the trip tank should fall by an amount equal to the volume of withdrawn pipe, and this volume has to be replaced to maintain the hydrostatic head. If the pumping rate has not increased and the trip tank level does not fall but rises, it signifies to the driller that a kick is probably occurring. Similarly, if the returning flow rate through the flow-,, line increases even though the pumping rate has not increased, a kick may be happening. Pulling the drill pipe too fast when the bottom hole assembly is so tight in the hole that it acts like a piston and ‘swabs’ or sucks at the formation is a frequent cause of wells flowing unintentionally. The driller has to guard against all these possibilities and he has a range of warning devices

194

Drilling Operations

available to him including a simple board and pointer gauge on the drill floor that shows the level in the trip tank. One sign of an impending kick is a ‘drilling break’ in which the bit suddenly starts cutting much faster than before, indicating that it has entered a softer formation that might contain well fluids. These can exist at almost any Ievel, but the deeper they are, the greater the pressure at which they will be released from the formation. Gas bubbling from the formation and rising in the returning drilling fluid can also warn of a kick, although lightly ‘gas-cut’ mud is commonly dealt with simply by removing the small gas bubbles in the degasser installed near the shale shaker. Gas kicks are the most difficult to contain, since gas that is compressed into a small volume at the bottom of the hole expands rapidly and greatly as it rises to the rig. As it does so it pushes drilling fluid from the hole, causing a rise in the pit level. With less weight of drilling fluid in the hole, there is less hydrostatic head to contain any further gas bubbles, and the passage of these is made even easier. The situation can get steadily and quickly worse if the first kick is not effectively dealt with, the ultimate consequence being a blow-out. Well fluids entering the hole are not always dangerous, and the driller’s, toolpusher’s and company man’s combined experience will dictate whether or not the quantities involved can be tolerated. Raising the mud weight to combat a kick, or taking other preventive measures, may only slow drilling down unnecessarily, so the situation is usually very closely monitored before any radical decisions are made. Drilling with a lower mud weight actually improves penetration rates, since the lower mud pressure allows the rock to break away under the impact of the bit teeth more easily, so the decision to weight up the mud cannot be taken lightly. However, if the correct decisions are not taken, the consequences could be calamitous. A blowout is an uncontrolled escape from the well of formation fluids either gas, salt water or oil, or a combination of these. Because of the everpresent risk of gas being ignited a’s it envelops a rig, blow-outs are potentially the most dangerous and disastrous hazard facing a drilling rig, either offshore or on land. Numerous rigs have been destroyed over the years and many lives lost as a consequence of wells blowing wild, although fortunately this happens much more rarely now than in the past, when a ‘gusher’ such as the one at Spindletop was often considered a sure sign of success for the drilling venture.

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Blowout fires like this are rare, but could easily happen if safety awareness was not uppermost in the minds of offshore personnel.

The danger is increased offshore by the difficulties imposed by the sea in preventing blowouts and in escaping from a blazing drilling rig or platform when well control is lost. The cost of regaining control is often very high and a great deal of oil and gas may be wasted before the well is successfully capped, with serious sea pollution being likely. A rig costing a hundred million dollars to build, quite apart from the equipment installed on it, can quickly be turned into a twisted and charred heap of scrap by a blow-out, and a floater or jack-up may even sink, or a platform collapse as a result of one. Not surprisingly, therefore, a great deal of effort goes into well safety measures, and preventive equipment costing many millions of dollars is installed in the knowledge and hope that it may never be used in earnest. A blow-out can develop relatively slowly as a result of a kick not being properly dealt with, or, if the formation pressure is much greater than that expected, it can happen so quickly that there is no time for preventive measures .

i96‘”

Drilling Operations

HYDRAULIC CONNECTOR

ANNULAR PREVENTER

I I

SHEAR RAMS PIPE RAMS

I

I

I

I

I

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I II

I

PIPE RAMS PIPE RAMS t

I HYDRAULIC ~ CONNECTOR

[

BOP stacks can be arranged in different configuration. Most offshore stacks are of this type.

Drilling Operations There are several reasons why a kick might not be ‘killed’, allowing a blow-out to happen. If the BOP stack is not installed at the time, or if one stack is being changed for another, or if the controls are operated incorrectly, it might not be possible to prevent a blow-out. Even if the BOPs have all been closed, the well might still flow up the outside of the casing of this is not properly cemented, or if there are cracks in the formations around the cement. High fluid pressures at the bbttom of a well can actually break rock open and make a path to the sea-bed outside the well. This type of blow-out is especially difficult to deal with, and can reduce the stability of a drill ship or semi-submersible if gas is bubbling into and reducing the density of the sea water underneath it. Theoretically this alone could be enough to sink the drilling unit.

The first indication of a kick is usually an increase in the mud level in the pits, but an increase in flow may be apparent before this. The normal practice is for the driller to stop the mud pumps temporarily, thereby stopping circulation. If mud returns continue with the pumps stopped, then the well can be assumed to be flowing. The driller then pulls the kelly up and closes the BOPs around the drill pipe, while the toolpusher and company man are called to the drill floor. Over a period of minutes they watch how the pressures and volumes of mud change, and they calculate the weight of mud needed to stop the flow. This ‘kill weight’, which is made by adding heavy substances such as barite to the mud, should be enough to just over-balance the formation pressure, and with the correct weight of mud and the correct procedures, the kick can usually be killed.

Kick-killing procedures usually involve balancing the bottom hole pressure with the kill mud so that no more formation fluids can enter, while keeping one or more BOPs closed to protect the rig, and circulating the pressurised fluid out of the system. There are several methods of doing this, the one used depending on the particular circumstances of the kick. The weighted mud might first be pumped down the hole to displace the overpressured well fluid, or the kick might first be circulated out and the heavy mud then pumped in, or both might b$ done at the same time. To maintain a backpressure in the hole the driller adjusts the choke on the choke manifold to reduce the orifice that the returning mud flows through. As the weight of the mud is increased and the difference between the formation pressure and the hydrostatic head pressure decreases, the choke is widened until the two pressures are evenly balanced and the choke is completely open. The well is then ‘killed’ and the BOPs can be opened and drilling can be resumed with caution.

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i ,,:,STUD BOLTED HEAD,

HYDRAULICALLY ACTUATED LATCHING DOGS

CONTRACTOR VALVE SLEEVE

CLOSING CHAMBER ,

The diverter allows a floater to safely vent off shallow gas from either side without shutting in the well.

Drilling Operations HYDROGEN SULPHIDE Hydrogen sulphide, or H$3 or ‘sour gas’ as it is often known, is a colourless, toxic, highly flammable, heavier-than-air gas that is one of the most dangerous of the various well fluids that may be encountered in drilling. A mixture one part of H2S to 1000 of air is fatal to man, but in spite of its pungent, offensive ‘rotten eggs’ smell at lower and less dangerous levels of concentration, it deadens the sense of smell and is not detectable in more dangerous volumes, which can lead to a false sense of security. Continuous exposure to concentrations of only 400 parts per million can be fatal over a period of minutes, and a very brief exposure to it at full toxicity can cause rapid death, so strict precautions are therefore taken against the chance of rig crews being exposed to it in wells where it is likely. Aside from its other unpleasant qualities, H2S is also very corrosive, and the mud used in the hole has to be specially treated to resist its damaging effects. All the drilling equipment used, including the blow-out preventers, mud pumps and tubulars, has to be of a quality sufficient to withstand the corrosion so that no single component will fail in service due to corrosion. Very detailed instructions are given to crew members about their actions in H*S situations, and when the well is predicted to yield sour gas, orders are generally given for the removal of all facial hair. This enables tight-fitting gas masks to be worn in an emergency, and all hands are issued with one of these sets. HIS gas detectors are fitted in the shale shaker and drill floor areas, the alarms being monitored on the drill floor and in the control room. As soon as the presence of H2S is suspected, tests are made on the atmosphere with a portable gas detector, with the tester himself wearing a,self-contained breathing apparatus set. Far from being a waste product that has to be got rid of, H2S is in fact a valuable constituent of crude oils and is essential for the production in the refinery of sulphur. It is, therefore, an unpleasant fact of drilling life that has to be put up with, just as bad weather sometimes is. WEATHER & ICE After a sustained period of strong winds, large swell waves are created which cause a floating rig to heave up and down. Moderate heaves are accommodated by the telescoping action of the riser slip joint, but when the rig heaves to the limit of the stroke of the slip joint assembly, drilling has to stop and a period of ‘waiting on weather’ (WOW) commences. There is little that can be done about this and the two drill crews usually take advantage of the break from routine to do maintenance on their equipment. Downtime due to weather is commonplace in areas like the North Sea and Arctic and during

200

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Drilling Operations

the local hurricane seasons in many other parts of the world, but it rarely affects units working in enclosed seas where large swells do not have room to build up. In the most severe weather conditions, when there is a danger of overstressing of the marine riser, the rig has to detach itself from the wellhead. This ‘hanging off operation involves pulling up the drill string inside the last string of casing and hanging it off on ,a special tool that is lowered down inside. A special tool with a non-return valve prevents any flow up the inside of the drill pipe, and sometimes packers are set to seal the annulus. The upper part of the drill string can then be pulled, having first displaced the drilling mud from the marine riser with sea water. Then a blow-out preventer can be closed and the lower marine riser package can be detached from the rest of the BOP stack. On stacks with guide lines this means having to cut the lines or disconnect them by some other means. Many drill ships and the few dynamically positioned semis that commonly drill deep water wells use stacks without guide lines, where the marrying of the upper and lower packages is achieved with the aid of hydro-acoustic transponder locating signals. In this case, unlatching is more simple. The riser does not have to be pulled back to the rig, but is left~suspended well clear of the sea bed while the unit rides out the storm.

specially

Drilling Operations Unlatching also has to be planned for by floaters working in Arctic regions where icebergs frequently threaten collisions with drilling rigs. These units use supply boats and helicopters to scout for approaching bergs or growlers, and sometimes even a massive berg can be steered clear by the supply boats. When collision cannot be avoided, the rig has to unlatch, leaving the BOP stack standing on the sea bed. To protect the stack from the bottom of deepdrafted bergs, special holes are often dug in the sea bed before drilling operations commence. This increases the costs of Arctic drilling ventures, which are the most expensive of any offshore, but it is vital to preserve the BOP stack and the well that it caps. DRILLING OPERATIONAL SEQUENCE From one rig to another there will always be differences in the drillingprocedures used and the sequence in which equipment is run, depending on factors such as the type of rig, the operator’s standard practice, the water depth, the presence of shallow gas pockets, etc. On fixed platforms and jack-up rigs, for example, the wellhead and BOP stack are not installed on the seabed as with semi-submersibles and drillships, but on the cellar deck of the rig. This obviously affects the order of running of these and other items of equipment. Many other procedures, however, are common to all rig types. Because of these variations, the main operations in drilling a typical offshore well (if there is such a thing) can perhaps best be illustrated with the aid of a hypothetical exploration well programme for a semi-submersible operating in the central part of the North Sea. Total depth for this well is to be 14,200 feet, and it is to be tested for production if any hydrocarbons show in core samples. The main features of the well programme and the timescale involved are as follows: Operation

Days

Move rig, run anchors & rig up on location Drill 36” hole to 160’ below sea bed Run & cement 30” casing Drill 26” hole to 1500’ true vertical depth Run & cement 20” casing Run B-3/4" BOP &riser Drill 17%” hole to 7003’ TVD (7219’ measured depth) Run electric logs Run &cement 13-3/s” casing Run gyro survey

2.0 1.0 1.0 1.5 1.0 1.0 10.0 1.0 1.5 1.0

Cumulative days 2.0 3.0 4.0 5.5 6.5 7.5 17.5 18.5 20.0 21.0

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Drilling Operations

Drill 12-r/+” hole to 12150’ TVD (12636’ MD) Run electric logs Run & cement 9%” casing Displace hole to oil base mud Drill & core 8%~” hole to Total Depth (14203 TVD = 14686 MD) Run electric logs Run & cement 7” liner Test well (3 zones) Plug & suspend well Allowance for contingencies&waiting on weather

19.0 2.0 2.0 0.5

40.0 42.0 44.0 44.5

20.0 3.0 2.0 28.0 3.0 20.5

64.5 67.5 69.5 97.5 100.5 121.0

The well is planned to be deviated at 3500’ TVD, in the 17%” hole section. . The operational sequence is described m more detail below. MOVING RI,G ONTO LOCATION & RUNNING ANCHORS

Rig-moves and anchoring operations are described in Chapter 6. When anchors have been set and the rig is ready to begin drilling operations, very precise final ‘surface location’ co-ordinates are measured with satellite navigation equipment from 40 passes of a navigational satellite. This information is vital in determining how the drilling bit must travel to reach the ‘target location’ more than 14,000 feet down, where it is hoped to penetrate a reservoir. The path is not a straight vertical line, and every deviation must be carefully planned and charted, just as in surfaces navigation, and an accurate starting position must therefore be known. A tolerance of 20 metres from the desired location is allowed by the operator, and the rig must be proved to be within this radius. RIGGING UP

This basically involves preparing the rig for drilling. Offshore rigs are kept in more of a state of preparedness for drilling than are land rigs, which usually have to be ‘rigged down’, or dismantled, moved on trucks or caterpillar tracked vehicles between locations and then re-assembled. On a semi-submersible most of the drilling equipment is ready for working at any time at short notice. Drill pipe, however, will have to be brought up to the drill floor from the pipe racks where it is stowed during the rig-move, and fresh mud will have to be mixed. Two hundred tons of drill pipe stood in the derrick setback, as well as approximately the same weight of mud, could seriously diminish the amount of stability the rig has during the critical phases of deballasting and ballasting, so the entire drill string is normally laid down for the transit and mud pits emptied. Many other small jobs have ot be done, but the rig should be ready for work almost as soon as the anchoring operations are complete.

Drilling Operations

RUNNING THE TEMPORARY GUIDE BASE (TGB) The temporary guide base, the heavy steel frame that will serve as the foun* dation for other sub-sea equipment is first weighted up with bags of cement ~ or barite and lowered to the sea bed on the end of a string of drill pipe. The end of the drill string is fitted with a special running tool that releases the guide base when it is in position. When this has been done the four guide lines and two TV camera lines are tensioned up from the rig and the underwater TV camera is deployed to monitor further operations from the doghouse.

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The hole opener is steered down to the temporary guide base by a utility guide frame.

SPUDDING IN & DRILLING 36” HOLE TO 160’ BSB

After the TGB has been positioned, a 36” diameter bit called a ‘hole opener’ is lowered to the sea bed inside a ‘utility guide frame’ that runs down the four guide lines. This large bit enters the central aperture in the TGB and the guide frame automatically detaches itself. A relatively short section of hole is now drilled with the hole opener; in this particular well drilling stops at 160’ below the sea bed (BSB). Sea water is initially used as the circulation fluid through the drill string, and this returns with the drilling bit’s cuttings to the sea bed where the cuttings are deposited. Every thirty feet an ‘inclination survey’ is made to ensure that the hole is vertical. After the hole has been drilled it is normally filled with a mixture of water and bentonite to make a thick gel substance to prevent ‘sloughing’, or caving-in, and filling. The hole opener is then pulled out of the hole and back up to the rig. . RUNNING 30” CASING & LANDING THE PERMANENT GUIDE BASE U--W

30” conductor casing, sometimes called the ‘outer conductor’, is next run into the 36” hole to prevent sloughing. With the permanent guide base installed on its top it also serves as the landing base for the blow-out preventer (BOP) which will be run later.

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Drilling Operations

The complete drilling rig. 1: Motion compensator. 2: Riser and guideline tensioners. 3: Diverter. 4: Riser tensioner. 5: Riser connectors. 6: Lower marine riser package. 7: B O P stack, 8: Wellhead and guide bases.

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A team of ‘casing hands’ contracted by the operator is flown out to the fig for this and all subsequent casing jobs, and they have their own special equipment, such as power tongs for connecting the joints of casing. The casing is brought out by supply boat some days before the anticipated running operation and the joints are laid on the pipe racks for inspection and numbering. As each joint is required for running it has to be hauled up the ramp from the pipe racks and through the V-door onto the drill floor, where the thread protectors are removed, the threads lubricated and special casing elevators are attached to lift the joints into the rotary opening. The first joint run is the round-ended ‘float shoe’ that will be at the bottom of the string. Again the utility guide frame is fitted, this time to hold the shoe as it travels down to teh aperture in the TGB. Each joint of casing is ‘stabbed’ into the next lower one just before this enters the hole, one of the casing hands guiding the pipe from the ‘stabbing board’ erected about 20 feet above the drill floor. ‘Centralisers’, which keep the casing string in the middle of the hole and allow a more even distribution of cement, are fitted round the joints as they are run, and ‘scratchers’ might also be fitted. These are pronged devices that stick out from the joints and contact the wall of the hole to remove mud cake and improve the cement bond. Centralisers and scratchers are sometimes collectively referred to as ‘jewellery’. Before the last joint of 30” casing is run, the permanent guide base (PGB) is attached to its top, leaving about five feet of casing protruding above it. This will provide an anchorage for the next string of casing, which will be 20” wide. The PGB, with the casing suspended from its aperture, is lowered on a special running tool to the sea bed. The guide lines running through the four posts on the PGB guide it into position and it slots into the TGB’s aperture with a funnel-shapped bottom projection that guarantees an accurate fit. CEMENTING THE 30” CASING

Now the casing has to be anchored to the wall of the hole. This is the first of several ‘cement jobs’ that will be performed during the course of the well programme. However, as the two conductor strings (30” and 20”) are short compared with the ‘surface’, ‘intermediate’ and ‘oil’ strings, their cementing is less complex, although a good bond must still be achieved: A drill string is run into the 30” casing, with a special ‘stinger string’ through,,which cement will be pumped extending it to the ‘shoe’ at the bottom of the casing. Cement is pumped through this at high pressure until returns are seen by the TV camera coming out on the sea bed. The cement is then left a few hours to set (called ‘waiting on cement’), after which the permanent base for the main drilling operations should have been established on the sea bed. Sometimes

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Dmig OjMmatiOM cement will not have completely surrounded the casing all the way to the sea bed, and a ‘top cement job’ has to be performed from the outside. The need for this is verified by making up a string of five-inch drill pipe with a few joints of 3%” drill pipe on its end to make a nozzle, and ‘washing down’ the top of the cement outside the 30” casing between the casing and the PGB frame with sea water. This operation is monitored by a TV camera carried by the drilling rig’s remote operated vehicle, which is unmanned and controlled from the surface by an ROV ‘pilot’. In this case the cement has reached almost to the sea-bed so a ‘top job’ is not required.

An ROV enters the water. These hesemachines machinesare aremade madeininaawide widevariety varietvof of shapes shauesand andsizes, sizes.few few of them if any looking anything like conventional submarines.

DRILLING 26” HOLE TO 1500’ TVD

A hole now has to be drilled for the ‘inner conductor’ casing, which is 20” in diameter. This will actually require a hole 26” wide, and a 26” bit of a type appropriate for the nature of the formation is selected. The bit is again guided down to the top of the PGB by the utility guide frame, and once again the circulating fluid is discharged onto the sea bed, since no riser has yet been run to enable it to return to the rig. For, about a day and a half the 26” bit drills to a true vertical depth of 1500 feet, which might not equate with the measured depth (MD) if the hole deviates from the vertical at all. Later sections of the well will be made to deviate, but this particular section is not. To make sure it stays vertical, inclination surveys are made every 200 feet.

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Drilling Operations The prevent ‘sloughing’ (caving-in) the hole is filled with a gel-water fluid which is pumped down through the drill string. Following this, the bit is pulled back to the drilling unit. Five and a half days of the well programme have now elapsed. The BOP stack used by this rig has an 183/4” aperture, so it cannot yet be used for controlling any downhole pressures. In some drilling areas surface gas pockets are a problem, and in this situation the marine riser could now be run with a device called a ‘pin connector’ which attaches with hydraulically-locked pins to the outside of the conductor housing. At the top of the riser a T-shaped pipe called a ‘diverter’ channels any gas that flows out to the lee side of the rig. Shallow gas is not a problem in this area, however, and the pin connector does not need to be run. RUNNING & CEMENTING 20” CASING, & RUNNING THE 183/k” WELLHEAD The shoe at the bottom of the 20” inner conductor casing is guided into the aperture in the PGB by automatically-detaching arms on the utility guide frame, as with the 30” outer conductor. 1500’ of 20” casing are run, to the top end of which is attached the wellhead, a long, cylindrical device with internal fittings called casing hangars that suspend the various sizes of casing and tubing strings that will be run during the remainder of the well programme. The upper end of the wellhead, which has an internal hole diameter of 183?&” in this case, is designed to closely latch onto the 18%‘4” BOP stack when this is run, making a gas-tight connection. The wellhead is run with the last joint of the 20” inner conductor casing, after which the casing is cemented into the hole through the stringer string, with cement returns again being discharged to the sea bed. RUNNING THE 183/i" BOP STACK & THE MARINE RISER When the inner conductor casing string and wellhead have been set and cemented, the BOP stack, in this case a 15,000 psi stack with an 183/4” aperture, is run attached to the lower end of the 21”-bore marine riser. This will act as a conduit for tools and for drilling fluid and cuttings returning from the well. Each joint of riser, including standard 40-foot joints and shorter pupjoints, is taken in turn from the pipe deck and hoisted up to,. the drill floor where it is fed into the 49%” opening where the rotary table master bushing is normally located. The heavy BOP stack, located on its test stump just forward of the moonpool in the cellar deck, has been re-assembled following maintenance by the sub-sea engineer and tested to its working pressure. It is now lifted off the test stump and hoisted over the moonpool by an overhead

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A quick welding job is done in this rig’s cellar deck before the stack can be deployed. Lost time costs a lot of money, so delays are minimised.

beam trolIey arrangement. Sitting in the moonpool on the ‘spider beams’ the stack isconnected to the ball joint and the bottom joint of riser, and is then lowered by the hoist as each additional joint of riser is connected on the drill floor. The stack weighs in the region of 190 tons in air and is worth several million dollars, so its descent is very carefully controlled, with the toolpusher usually in attendance. (Stacks have been known to be dropped to the sea bed rather than lowered).

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Drilling Operations As the stack nears the wellhead it becomes liable to damage by landing heavily or bumping, since the rig will be heaving up and down to some degree if there is the slightest sea running. As the final joints of riser are added on the drill floor, therefore, the riser tensioner wires are connected just below the slip joint, and part of the load is transferred to them. A proportion of the total load is thus taken by the rig’s surface motion compensator system while the remainder is held by the riser tensioning wires, enabling the stack to be landed with the minimum of jarring.

A BOP stackwith lower marine riser package connected runs down the gui de linesand enters the water. Ttme choke and kill lines are clearly visible.

The BOP stack is latched onto a special connector on the wellhead, which both supports it and provides a gas-tight seal. After it has been landed, all subsequent strings of tubulars and casing will run through the N3/4” apertures in both the stack and the wellhead. Once the stack is in position on the wellhead it is hydraulically pressure-tested to ensure a good seal.

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At the top of the riser, above its telescopic slip joint and underneath the drill floor, a diverter is fitted. This is a large housing in which an emergency sealing device can shut off the vertical 21”.wide access to the drill floor and divert well fluids to either a narrow flowline outlet or a vent line outlet. The vertical access can be sealed around the kelly, drill pipe or casing that happens to be running through it when it is closed, so it is really a form on annular blow-out preventer. Drilling fluid returning form the well normally passes out through the flowline to the shale shaker, and this is the same line through which mud is delivered from the fill-up line when the drill string is being tripped out of the hole. If any gas flows up the riser, the vertical access is sealed at the top and the flowline is closed, thus diverting the gas to the vent line. From there it can be directed~ by an automatic deflector to pass overboard on either the port or starboard side, depending which is downwind. All the controls for the diverter are in the doghouse. DRILLING 17%” HOLE TO, 7003’ TVD (7219’ MD)

The cement-filled shoe of the 20” casing is next drilled out with a 17%” bit, after which a 17%” hole is drilled to a vertical depth, in this well’s case, of 7003’. However, not the entire distance is drilled vertically. At 3500 feet TVD there is a ‘kick-off point’ where the drill bit will be deviated by directional drilling techniques. By the time the bit arrives at a level 7003’ directly below the sea bed it will actually have travelled 7219’ along the hole. This is termed the ‘measured depth’. (Directional drilling is explained more fully in a preceding section of this chapter.) From the kick-off point drilling is done by means of a bit rotated by a downhole drilling motor or ‘mud motor’, and a device called an ‘MWD tool’ is incorporated in the drill string. This ‘measurement while drilling’ tool enables directional surveys to be made and their results transmitted to the surface by pulses that travel up through the drilling fluid. A digital readout in the doghouse tells the driller the angle and direction that the bottom hole assembly is taking and he can adjust it accordingly by varying the weight on the bit or the mud pump pressure. This hole section shoufd take about 10 days to drill, or longer if downhole difficulties arise. Gumbo,.a type of sticky clay that tends to clog equipment both downhole and in the shaleshakers, is a problem here, but the mud is specially conditioned and its flow rate increased to combat it. About 1100 gallons a minute of mud weighing around 9 ppg (pounds per gallon) are pumped by the two mud pumps. Due to the sticky clay, ‘swabbing’ is also a problem when the drill string is pulled out of the hole. If the string is pulled . . 112

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Drilling Operations

Hole deviation equipment for ‘kicking off’: whipstock, anchor, packer and mills.

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DriWng Operations too fast it tends to suck the wall of the hole inwards, which could lead to formation seawater or other fluids in the formation bursting into the hole. A ‘kick’ of this sort has to be avoided, so the driller is careful to withdraw the string slowly, filling it with mud as he does SO through the fill-up line. He monitors the amount of swabbing and fill-up by watching the level of mud in the ‘possum belly’ or ‘trip tank’. Normally as the drill pipe is withdrawn the mud level should fall, since the pipe left will displace less volume, and the driller keeps the level constant by adding mud as required. LOGGING When 7003’ TVD has been reached the well is logged with electric and sonic wireline logging devices to determine conditions in the hole before running casing. Before logs can be run, however, mud must be circulated through the well to remove any cuttings, and to allow the mud to be ‘conditioned’. While this is being done casing running equipment is brought onto the drill floor, checked and lubricated. Well logging is done by expert technicians employed by a specialist company which maintains a permanent wireline logging unit onboard. This is installed at the far end of the rig in line with the V-door, so that the long wireline that runs off its storage reel can pass directly up to the drill floor where it enters the rotary. There are many different techniques of well logging, such as electric, sonic and radioactivity logging, each employing special instruments which are attached to the wireline. This is lowered to the bottom of the hole and then brought slowly back up, the devices ‘reading’ different types of data as they pass each formation and recording the data on electrocardiogram graphs which can be interpreted by the geologist and drilling engineer.

RUNNING & CEMENTING 133/s” CASING About 7,000 feet of 13%” ‘surface string’ casing is now run and cemented to seal the wall of the hole so far drilled. The contracted cementing company maintains a powerful cement mixing and pumping plant onboard, called the ‘cement unit’, and this firm is not usually changed by a new operator. The plant cons&s basically of a large mixing hopper and water pipes leading to a powerful pump which delivers the mixed cement slurry to a cementing head on the drill floor where it enters the casing. The amount of slurry required is very carefully calculated from a knowledge of the width and depth of the hole and the size of the casing, and various additives are mixed with it to a precise formulation that is monitored by sophisticated equipment.

Drilling Operations

TUBING PROFILE ChLIPER

CASING PROFllf

CALIPER

CASING MINIMUW

Remaining WI ThiCknes

Caliper logs measure. internal diameters and profiles to detect damaged areas.

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l.O.CALIPER

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@a-Log Tubing Profile Caliper

Da-Log Casing Profile

Caliper

lx-Log Minimum 1. D. Caliper

There is a vast range of downhole logging devices on the market. These are caliper tools.

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The company man usually supervises the cement job, while the cementer operates the pumping machinery and the bargemaster controls the valves through which the dry cement powder is delivered to the jet mixing hopper from the rig’s ‘P’~ tanks. The whole operation, which is extremely noisy and dusty, has to be done rapidly and without hitches if the cement is not to start setting before pumping is complete. It is vital to do a good ‘job’ the first time, as a lot of time can be expended if a remedial ‘squeeze job’ has to be done later to fill up unfilled spaces. There are many variations in the downhole equipment used for cementing and the procedures in their operation, some of which are quite complex. Basically, however, they all have the same objective: to allow cement to be pumped from the bottom of the casing string into.and up the annulus between the casing and the wall of the hole. Since it is difficult to know exactly what is happening to the cement inside the casing at any given moment during the cement job, movable plugs are usually used to contain the cement as it travels down to the bottom of the casing. In one of the most simple methods a lower plug travels ahead of the pumped cement, pushing any drilling fluid ahead of it out of the casing like a piston. Pumping the cement at high pressure from the surface continues and this plug eventually seats on the casing shoe, unable to travel any further. However, this does not prevent the cement from escaping, as continued pumping ruptures a seal in the shoe. The cement then starts flowing out through a non-return valve and passing up the annulus between the casing and the wall of the hole. In most systems a certain amount of cement is left to set inside the bottom of the casing above the shoe, this being drilled out at the start of drilling of the next hole section. However, too much cement must not be left to set inside the casing, and some way of expelling the bulk of it has to be used. At a predetermined moment, therefore, the cement pump is stopped and a top plug is released above the last of the cement. Drilling fluid is then pumped down at high pressure by the mud pumps, pushing the top plug and the cement below it down. The cement continues flowing out through the valve in the shoe until the top plug lands on the bottom plug, which is termed ‘bumping the plug’. The back-pressure on the mud pump then rises, indicating that all the cements has been expelled from inside the casing’. After a ‘waiting on cement’ (WOC) setting period of several hours the casing is pressure-tested. MAKING A GYRO SURVEY

A ‘gyro survey’ is now made by a wireline-run instrument to check the inclination and direction of the deviated hole at a specific depth. In an uncased hole a magnetic survey could be made with ‘single shot’ or ‘multi shot’ tools,

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Drilling Operations

the information being recorded on film discs which are read at the surface. Magnetic survey tools can not be used here, however, as the steel casing would cause errors in readings. As with magnetic survey tools, both single shot and multi-shot instruments are. used in gyro surveying. Tool designs vary but often employ a cylinder only about two inches wide, in which a gyroscope, a small compass card, a battery pack, a camera and film discs are included. DRILLING 12%” HOLE

The cement shoe of the 13%” casing is now drilled out with a 12%” bit and directional drilling continues with the mud motor for approximately 19 days to a depth of 12150’. This is the longest period of drilling planned. LOGGING

When 12,150 feet has been reached mud is circulated down the hole to clean out any cuttings, and the mud is conditioned to restore its quality. Electric logs are then run, as in the 17%” hole. RUNNING & CEMENTING 9%” CASING

This is done as with the 133/a” casing, with a pressure test following the cementing. This casing string is sometimes termed the ‘intermediate string’. DISPLACING THE HOLE TO OIL BASE MUD

The water base mud is now replaced by oil base mud which is either mixed in the mud pits from diesel oil kept in one of the rig’s pontoon tanks or else is pumped aboard ready-mixed from a su@ply boat. It has to be used because the shale formations that will next be drilled through have a tendency to slough, or cave in, with water base mud. Oil base mud might also be used if the formations were sensitive to water, or in formations where there was a tendency for drill pipe to stick due to differential pressures. DRILLING 8V’z” HOLE TO TOTAL DEPTH

8%” bits are now used to drill to total depth (TD) of 14,200’. Measurementwhile-drilling (MWD) devices are used every 500’ during this final phase of drilling to determine the accuracy of the bit’s travel as it nears its target. This involves only a brief pause in drilling, when ;ensors in the instrument cause a transmitter to send pulses up through the mud to the rig, giving a digital readout of angle and azimuth in the doghouse: When TD is reached the ‘pay zone’ or zones should have been penetrated, but there is not a ‘gusher’ like there would have been in the old cable tool wells. The well is still controlled by the weight of the drilling mud, and any formation fluids present in the pay zones will not be able to flow until special casing called ‘liner’ has been set through the zones and has been ‘perforated’ to allow access for the fluids to

Drilling Operations

the rig. But this liner will not necessarily be run unless the results of a ‘coring’ programme yield encouraging results. CORING

A ‘core barrel’ with a ‘coring bit’ at its lower end is now fitted to the bottom of the drill string and approximately 760 feet of core samples are removed from the formation at a rate of 6 to 8 feet an hour. These will go ashore for laboratory analysis and will largely determine whether or not the test programme will be run. Unlike a rock bit, the coring bit has a cylindrical matrix with a wide central aperture through which the core passes to the barrel interior. The cutting surface of the matrix is studded with a large number of industrial diamonds which are of a type and arrangement to suit the formations in the pay.zones. The core barrel is made up in 30 foot sections to a total length of 90 feet and lowered into the rotary opening on the end of a drill string which is rotated in the normal way. The geologist, assisted by the mud loggers, carefully removes the thin, cylindrical core sample from the core barrel and after making his own tests packs it in long tubular boxes for transporting by helicopter ashore, where it will go to a core laboratory for analysis. The core looks and feels like nothing more than a cylinder of ordinary grey or brown rock, but it may contain vital clues as to what hydrocarbons lie in the formations beyond the wall of the hole, and in what quantities they exist. In spite of souvenir-hunters’ requests, nobody is usually allowed to keep any of the core, as it is obviously of possible value to a rival operator. Another method of coring is sometimes used in exploration wells, called ‘sidewall coring’. This involves the firing of small core barrels horizontally from a cylindrical tool called a ‘core gun’ at selected depths, so that the barrels embed in the adjacent formation. The barrels are attached to the gun by steel cables and when the gun is withdrawn they pull samples of the formation out with them. This particular period of coring runs its full course for a further four days, as initial lab analysis of the first 60-foot core sent to town indicates that the well is worth testing. Otherwise the well would have been plugged and suspended, which would take abbut another six days. LOGGING

Following coring the well is again cleaned out by circulating and there is a three-day period of well logging of various kinds to determine hole and formation conditions.

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Coring equipment: Left: Conventional and wireline core barrels. Right: Diamond coring bits for medium-hard and hard dense formations.

Drilling Operations RUNNING & CEMENTING THE 7” LINER

Core analysis having yielded ‘shows’ of hydrocarbons in the pay zone formations, the operator has decided to run a 7-inch liner in which test equipment can be installed to allow well fluids to flow to surface under contra. This liner looks much like any other type of casing, but it does not run all the way from the well bottom to the sea bed like ordinary casing. Instead it is suspended from the bottom of the deepest string of casing run (the 9%” in this case) by means of a liner ‘hanger’. The liner is cemented at the same time as it is run, and a packer or plug is set at its top to isolate the test zones inside it from the cased hole above. If a string of narrow tubing is now run down through this packer with special valves that allow the controlled entry of well fluids, it will be possible to channel pressurized well fluids to surface under control. But to allow the fluids access to the inside of the liner, it has to be perforated. This is one of the operations in the period of ‘well testing’. WELL TESTING

Perforating the liner in three zones, each separated by a packer, and subsequent production testing may take approximately two weeks, perhaps more. For this specialised operation a contracted well test crew comes out to the rig with a great deal of test equipment that they set up on the deck. Flare booms are swung out from the sides of the rig and burners are installed on their ends, while pressure vessels and tanks to contain the well fluids are connected to pipework on deck and pressure tested. The target pay zones are sealed off by the liner and by the packers in between each zone so that fluids from one zone will npt leak into another. Perforating of these zones is done by electronically firing a cylindrical ‘gun’ that is lowered by wireline into the desired zone, where it fires either steel bullets or explosive charges of high-pressure gas. These shoot horizontally through the liner and the cement behind it and penetrate the wall of the hole, allowing access for the formation fluids to the string of test tubing that has been run in. During the firing, the rig and all ships in the close vicinity have to go into a state of ‘radio silence’ when all radio-frequency equipment that might affect the firing mechanism is electrically isolated. During this period no radio communications are possible with the outside world. In another type of well test, a ‘drill stem test tool’ (DST tool) is run on the end of a string of drill pipe to TD. When it,lands on the bottom a packer installed in the string at a level above the test zones automatically seals against the wall of the hole and valves open to allow well fluids access to the drill string. At the same time, a clock mechanism starts a graph in the tool which records the pressures experienced against a timescale, allowing analysts to later determine what pressure variations occurred at different

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,

Typicar slew? valve PC T test system.

Typica, full bore PCT test system.

Pressure-c operated well-testing equipment is run down the hole on a string of drill pipe. Internal valves can 1be opened and closed as required from the surface.

Drilling Operations

A typical offshore well-testing layout on a floater. This takes up a lot of space, and special ships are sometimes used to test wells drilled from small platforms.

Drilling Operations

stages during the test. The packer meanwhile seals off the test zones from the drilling fluid which is pressurizing the annulus higher up. After perforating, oil or gas will hopefully enter the test string of narrow tubing that is run down through the packers and will travel up to the test equipment on deck and out of the end of the flare boom where it will be ignited. There is a chance of some oil pollution, especially if unburnt oil drips from the end of the burner, so the rig standby vessel is made ready for pollution control duties, in which towed boards together with a special dispersant chemical will break up any oil on the water surface. In case the drips of oil go unnoticed in darkness, flaring-off is not allowed to start until daylight in some regions. The heat from the flare can be intense, so a water spray ‘deluge’ system that protects the sides of the rig is activated by the ballast pumps during flaring-off. It is a hard but accepted fact of life in the drilling industry that only about one wildcat well in ten flows ‘commercially’, so the chances are that the hole will be dry and will have to be plugged and abandoned. However, there are two methods by which flow can be encouraged, if hydrocarbons are known to be in the formations but seem reluctant to emerge. These are called ‘well stimulation’ techniques. WELL STIMULATION

Stimulation of offshore wells can be done either from the rig, requiring a lot of special equipment and materials to be brought out, or by ships designed specially for the job. The two methods normally used for well stimulation are ‘fracturing’ and ‘acidizing’. Fracturing involves splitting the formation rock by pumping a liquid down the hole at a very high, carefully calculated pressure that might be as much as 15,000 psi. Along with the liquid, sand is pumped down, and this works into the fissures in the rock, opening it up. When the liquid pressure is relieved, the solids, called ‘proppants’, hold or ‘prop’ open the cracks, allowing any well fluids inside to flow. Sometimes mixtures of water and alcohol, or blends. of other liquids and solids may be used, or foam or nitrogen gas may be injected, depending on the nature of the formation. . Acidizing the formation eats away any soluble material clogging the pores of the producing formation rock, allowing the fluids to flow more freely. It is done by pumping a mixture of water and hydrochloric or other acid down the well, and is sometimes used in conjunction with fracturing. Both these methods are sometimes used to stimulate a new well to flow, and they are often employed during ‘workovers’, when the job of the drilling rig is not to

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A successful well test? Gas bums from the flare boom of a semi-submersible rig. This does not mean a bonanza, however.

drill a new well but to ‘overhaul’ an existing well and restore or improve its productivity. In this particular well, which is a wildcat exploration well, neither method of stimulation is employed, however.

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Drilling Operations PLUGGING & SUSPENDING OR ABANDONING THE WELL

If well-testing proves the reservoir productive to a profitable degree, the swell will normally be plugged and suspended to await development of the field at a later date. This might be years from the drilling of the first expioratory wells, depending on the operator’s or his consortium partners’ plans. Alternatively, if production equipment is to be installed straight away, production tubing will have to be run from the sea bed through the packer separating each producing zone, so that each of the three zones can produce independently of the others. This is called a ‘multiple zone completion’. Later a production wellhead will be installed, either on the sea-bed from where it will be tied in to a nearby platform, or on the platform itself if this is erected over the well. If the well does not flow, or if it flows at an unprofitable rate, it will probably be abandoned, which involves sealing it with a cement plug and removing all wellhead equipment, leaving the sea-bed in a clean condition. Since it is very expensive and can be used again, as much casing as possible will be removed from the well, which involves using a tool called a ‘casing cutter’ to mill each string free below the wellhead. The alternative to cutting the casing is blasting it free with explosives, and these are often used offshore. When the casing has been freed the wellhead can be salavaged along with the two guide bases. Under some countries’ regulations the sea bed must be left absolutely clear of debris, so the rig’s divers, or an ROV or a small submersible are deployed to inspect it, sometimes making a video film of the area. CONTINGENCIES & WEATHER As described in the ‘Drilling Hazards’ section earlier in this chapter, very

few wells are drilled exactly to schedule offshore. Machinery may break down, equipment may get stuck in the hole, and large seas and swells that produce big heaves can mean periods of ‘waiting on weather’. The operator of this particular well has allowed more than 20 days for ‘contingencies’, and this figure might be increased in the winter. One period of operations when delays might occur is during the anchoring of the rig at the location. This and other marine and associated operations are described in the next chapter. .

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CHAPTER 6: MARINE OPERATIONS Regardless of its unconventional shape, the mobile drilling unit that provides the platform for the drilling rig and its equipment is, fundamentally, a seagoing vessel. As such, the rig is subject to the same elemental forces and the same scientific laws governing its flotation and stability as every other vessel on the high seas. It therefore requires a small corps of professional seamen to monitor and control its stability and to provide the marine support services that are essential to the efficient working of the drilling department. While drilling operations on a floater are conducted by the drillers and drill crews under the leadership of the senior toolpusher, marine operations during the drilling programme are in the charge of the bargemaster or barge captain. He is on call 24 hours a day, and to assist him he has a deck foreman, or ‘bosun’, and one or more seamen. On some rigs there may also be a ‘barge engineer’ who deputises for the bargemaster at night. The control room operators, who are often ‘signed on’ as the certified mates of the vessel, are also part of the marine crew, and their primaryfunction is to monitor and control the stability of the drilling unit. RIG STABILITY Rig crewmen other than the bargemaster and the control room operators are not normally required to have a knowledge of the principles which govern the behaviour of their rig in the water. However, an appreciation of the rudiments of stability can be useful to a toolpusher or a driller when a heavy item such as a BOP stack is being handled, or when a semi-submersible is being ballasted or de-ballasted through its critical phase of draft, which can cause impressions of instability. Ship or rig stability is basically applied mechanics, and if the theory of torque (or forces and levers) as applied to tongs or the rotary table can be understood, understanding stability should not be a problem. DISPLACEMENT AND THE PRINCIPLE OF FLOTATION One of the most fundamental principles of stability is that a floating vessel of any kind displaces its own mass, or weight, of the liquid in which it floats. Its displa‘cement, or true weight, is therefore equal to that of the volume of water occupied by its underwater structure. For example, if the unit displaces 20,000 cubic metres of fresh water, which has a specific gravity of 1.0 (i.e. a density of 1 metric tonne per cubic metre), then its own weight will be 20,000 tonnes. If the draft at which the unit floats is increased, then the displacement will also increase.

‘228”

Marine Operations

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Semi-submersibles are designed for steadiness even in the roughest seas.

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229

Marine Operations THE CENTRE OF GRAVITY The weight, or displacement, of the rig is due to the force of gravity acting on its mass, and this force is said to act vertically downwards through a point termed the ‘centre of gravity’ (the CG). The position of the CG will depend on the distribution of weights about the rig. If most of the weight is low in the structure, as when operating with ballast tanks mostly full, the centre of gravity will be relatively low, but if there is a lot of weight high up, as when there is a large deck load and the drill string is set back in the derrick, the CG will be relatively high. If the CG is allowed to get too high the unit will become unstable, and if it is too low it will also result in an unsatisfactory condition called ‘stiffness’. THE CENTRE OF BUOYANCY For a vessel to float in water it must receive an upthrust force equal to its displacement. This upthrust is called ‘buoyancy’, and it is reckoned to act vertically upwards through a point called the ‘centre of buoyancy’ (the CB), which is at the geometrical centre of the underwater portion. When the rig is at rest and lying upright in the water, the lines drawn upwards through the CB and downwards through the CG are coincident. If the vessel is inclined (i.e. heeled or trimmed) away from the vertical, its underwater shape will obviously change somewhat. The buoyancy force will still act vertically upwards through the CB, but the position of the CB will move to a new point at the geometrical centre of the new underwater shape. RESERVE BUOYANCY The volume of all the watertight spaces above the waterline that are permanently sealed and occupied by air are collectively known as the rig’s ‘reserve buoyancy’. Every vessel must have some reserve buoyancy, as when large waves are supporting its ends but not its middle, the hull sinks down to displace the same volume as it would in calm water. If there were no watertight spaces above the normal waterline the vessel could be overwhelmed by the sea. The air held in the watertight compartments inside the columns of a semi-submersible forms reserve buoyancy, just as the air in spaces in the hull, the forecastle and the afterpart of a drill ship does. If an empty compartment is punctured and flooded below the waterline, perhaps by a supply boat, the volume of buoyancy that it lost to the sea is then made up by some of the reserve buoyancy, and the drilling unit’s draft will alter accordingly. THE EFFECT ON THE CG OF ADDING, REMOVING OR MOVING WEIGHTS The position of the centre of gravity is changed, either intentionally or otherwise, by altering the distribution of loads on the rig, and any addition or removal of loads also causes the CG to move in a definite direction. From its

Marine Operations

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WEIGHT lATERTIGHT FLAT

+ VOLUME OF DISPLACEMENT

BUOYANCY

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A semi-submersible’s centres of gravity and buoyancy normally lie somewhere beneath the main deck.

initial position the CC always moves directly towards a loaded weight, and directly away from the position formerly occupied by a removed (or ‘backloaded’) weight. If a weight is shifted from one position to another on the rig, such as from the drill floor to the pipe racks, then the unit’s CG, which may be many feet below the main deck, will move in a line parallel to that line of movement. The amount of the CG’s movement within the drilling rig will depend on the weight of the object loaded, backloaded or moved, and on the distance at which this action took place from the CG. In the same way, then, that a heavy force applied to a long tong handle results in more torque than a light force applied to a short tong handle, a large weight moved a great distance from the rig’s CG has more effect on stability than a light weight moved close to the CG. Thus if the BOP stack was run from the moonpool and landed on the sea bed, the drilling unit’s CG would move away from its original position, i.e. upwards, a considerable distance, whereas ten tons of drill water transferred from one tank to another near the CG would not cause much change in trim or heel. A heavy weight lifted by a crane and moved a short distance across the deck could have the same effect on the movement of the CG as a lighter weight moved from one end of the deck to the other. .*

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Loading and shifting weights on the rig affects the positions of both the centre of gravity and the centre of buoyancy.

Wherever the new position of the CG finally is after these weight distribution changes have been made, the weight of the rig will still act vertically down through that position. Virtually everything that can be moved on a floater affects the resultant position of its centre of gravity, and the evenness of distribution of the weights around the decks, on the drill floor and in the pontoons or hull, perhaps amounting to as much as 15,000 tons, is important to the maintenance of a sufficient and safe ‘margin of stability’. Even the tension in each anchor chain must be taken into account, since this acts just like a weight at each point of pull on the deck or columns. If the tensions are not correctly balanced, they can create an angle of heel, as can a strong wind and icing-up of the rig structure in cold weather. THE RIGHTING LEVER If the position of the unit’s CG is altered by a change in the distribution of loads, then the rig will heel or trim until the underwater shape has altered enough for the lines drawn up through the CB and down through the CG are once again coincident. However, if the unit is heeled not by adding, remov-

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ing or shifting weights but by some external force such as the wind or waves, then the position of the CG will not change. As the unit heels to this external force its underwater shape alters, and the CB moves towards the new geometrical centre of that shape. The vertical lines of force through the CG and CB will not now be coincident, and a ‘lever’ effect is introduced which will tend to right the unit until the two lines are again coincident. The amount of leverage is measured by the horizontal distance separating the vertical lines of buoyancy and gravity, and this is called the ‘righting lever’ or ‘righting arm’. In stability diagrams the righting lever is normally represented as a line joining points G and Z. Just as a long tong handle can exert more leverage than a short one, the longer the righting lever, GZ, the more the inclination of the rig to right itself. The righting force being exerted on the righting lever is the rig’s own weight, i.e. its displacement. When this value, in tons, is multiplied by the length of the righting lever, the result is the value of the ‘righting moment’. This is really a sort of rotary torque being applied to the whole rig. The values of righting levers for different angles of heel can be plotted on graphs for any kind of vessel, and control room operators on drilling rigs have these diagrams available in their units’ stability manuals.

7 RIGHTING MOMENT

THE CG MUST ALWAYS BE BELOW M FOR POSITIVE STABILITY

When the rig is inclined by an external force a righting lever is set up which tends to restore it to its original position.

I

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233

Marine Operations THE METACENTRE When a drilling rig is heeled over, if the vertical line representing the force of buoyancy is drawn up through the CB to meet the unit’s vertical centreline, the point where the two lines meet is termed the ‘transverse metacentre’ (M). For any particular draft at which the unit floats the position of the metacentre is fixed, whereas the height of the CG, it must be remembered, can be altered. The distance between the centre of gravity (G) and the metacentre (M) at any given time is termed the metacentre height (often called ‘the GM’) and this is very important to stability. If the unit is to be stable, then the CC must always be below the metacentre, so that the metacentric height has a ‘positive’ value. If the CG is ever allowed to coincide with or rise above the metacentre, so that there is either a zero or negative metacentric height, a dangerous unstable condition will exist that could lead to the unit’s capsize. Generally, a weight added at a high level or removed from a low point will reduce the GM, while a weight added at a low point or removed from a high level will increase it. When a weight is suspended from a high point, such as the crown of the derrick or the end of a crane jib, the drilling unit’s CG moves towards the point of suspension, not towards the actual object suspended. This is one of the factors to be considered in stability calculations when large loads are suspended from the crown block. BALLASTING & FREE SURFACES The CR0 checks the distribution of weights onboard and makes daily calculations to ascertain the unit’s metacentric height. From the control room, ballast is pumped in or out or transferred between tanks to adjust the position of the CG as items are loaded, backloaded or moved. Some ballast tanks, usually the ones at the far ends of the pontoons of a semi-submersible, are used for trimming the unit, i.e. tilting it either forward or aft, and for heeling it to either port or starboard, since these are furthest from the CG and therefore have the most ‘moment’ effect. These tanks will probably therefore be left ‘slack’, or only partially full, most of the time. In general, however, most ballast tanks are kept either ‘pressed up’ or as empty as possible. This is done to restrict the movement of free liquids onboard, since any liquid ‘free surface’, especially if in a wide or long tank, has the effect of moving the CG of a vessel upwards and reducing its metacentric height. Even the drilling fluid in the mud pits has a detrimental effect on stability for this reason, and mud is therefore usually dumped before a rigmove. Where there have to be free surfaces, they are best confined to narrow or short tanks, and this fact is important during ballasting or de-ballasting operations. There is much more loss of stability during the filling of a

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Minimising free surfaces is important when rigs have to operate in rough seas,

large tank than there is during the adding of the same volume of water to be divided between several smaller tanks. This can be shown simply by cornparing on the one hand the difficulties in carrying a breakfast tray with half an inch of water lying on it, and on the other hand in carrying the same amount of water in the small compartments of a refrigerator ice tray. In the routine operations of a floating drilling unit there are numerous situations that call for ballast to be pumped in or out, or transferred between tanks. As the mud pits are filled and emptied, supplies loaded or backloaded, the drill string run or pulled out of the hole, or casing run, adjustments to the trim or heel of teh rig will have to be made. In general, however, the drilling draft must be kept constant, so the unit’s load displacement must not exceed the tonnage applicable to this draft. This requires the careful checking of all the weights onboard at any given time. In preparation for a semi-submersible’s rig-move from one drilling location to another, almost all the ballast is pumped out from the pontoon and column tanks so that the rig lifts up to float at a level about the top of the pontoons, which reduces water resistance and weight and increases speed. During the de-ballasting operation the amount of stability, measured in terms of

Marine Operations the metacentric height (the GM), passes through a critical phase when it is severely reduced, allowing the unit to become very ‘tender’ and susceptible to somewhat unpredictable ‘flopping’ movements. This occurs just as the waterline reaches the bottom of the columns, when the rig’s ‘waterplane’ shape is changing from being a number of small circles (the column sections) to two or more wide and long, relatively ship-shaped hulls. During this phase, which is repeated in reverse during ballasting-down at the new location, it is important to have as little free surface as possible, and this calls for careful planning and execution of the tank ballasting sequence. BALLASTING CONDITIONS There are four ballasting conditions for which a semi-submersible drilling rig is normally prepared: ‘drilling’, ‘transit’, ‘survival’ (or ‘storm’) and ‘damage’. Going from drilling draft, which might be 65 or 70 feet on a large rig, to a transit draft of between 20 and 25 feet involves pumping out several thousand tons of ballast, which takes several hours. Because of the changed underwater shape at transit draft the metacentre is very high in relation to the CG, and there is consequently a large and safe margin of stability when this draft is finally reached. Survival draft is a compromise between having sufficient draft for the extra margin of stability needed in very bad weather, but not so little air gap beneath the main deck of the unit that there is a danger of seas coming aboard. Often a draft of 50 feet is chosen to be used when winds of about hurricane force are expected. Because of the design of their sub-structures, semi-submersibles at transit draft are unable to take severe punishment from very rough seas, and if necessary they are ballasted down to survival draft during a rig-move storm. A damage condition exists when a watertight compartment is accidentally flooded, causing the unit to incline and to sink lower in the water. If so much inclination (either list or trim, or both) is caused that the top of one of the columns becomes submerged, the situation becomes much more serious because the chain lockers, where the anchor chains are stowed, can start flooding through their top entrances below the windlasses, causing the unit to eventually capsize. Normally, however, semi-submersibles are designed so that the flooding of any single watertight compartment should not cause a capsize, If a semi is damaged and lists or trims badly, ballast can be removed from the low side, or, of this is impossible, it can be pumped into spaces on the high side. Vents, watertight doors and other openings are closed to slow down the flooding process where possible, and these have to be maintained

236

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At transit draft then pontoons of a semi-submersible are awash. It is safer and easier to tow the rig like this than when it is ballasted down to a deeper draft.

in efficient working condition, which is another function of the marine crew. RIG STRUCTUliJl AND SAFETY MAINTENANCE

A mobile offshore drilling unit is similar to a ship in many aspects of its construction as well as its stability, and it therefore has to be maintained much like any ship, even being periodically dry-docked. The transport depart: ments of most flag states certify the construction of drilling rigs and lay down rules for the maintenance and inspection of their structure and safety equipment, and this work is very often undertaken on their behalf by a ‘classifica-

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Marine Operations tion society’ such as Lloyd’s Register, Det norske Veritas or the American Bureau of Shipping. Drilling rigs, like ships, are ‘classed’ by these classification societies when they are built and their ongoing maintenance has to comply with the classification society’s rigorous standards for a unit to remain ‘in class’. This is partly to satisfy the rig’s insurers, amongst others, that the unit is structurally sound and seaworthy. Routine maintenance work is usually the job of the marine and engineering departments under the bargemaster and chief mechanic, and often a programme of ‘planned maintenance’ and inspection is followed throughout the rig’s life. Hull compartments have to be checked for watertightness and frequent tank soundings are taken by CROs to ascertain whether any spaces are leaking. Structural members such as cross braces can develop cracks due to the great strains imposed by heavy seas and by the deck load distribution, and visual inspections are made by the marine crew in areas known to be prone to damage. With proper weight distribution essential, the CROs aso make ‘torsion checks’ when they calculate the rig stability, to ensure, that each quarter of the rig’s structure is bearing no more than its due share of the total load. Aside from this, the unit has to be kept from rusting, and large quantities of paint are used to preserve the steelwork from the hostile elements. Periodically, surveyors from the classification society and from the flag state’s shipping department will closely examine the rig and issue certificates attesting to its safety of construction and equipment. One aspect that the surveyors are always interested in is the maintenance of the lifesaving appliances and firefighting equipment. LIFESAVING & FIREFIGHTING EQUIPMENT The upkeep of safety~equipment is the responsibility of both the marine and engineering departments. It involves amongst other things periodically running lifeboat engines, testing the fire pump, water sprinkler and deluge systems, halon and CO1 gas extinguishing systems and other firefighting equipment such as fire extinguishers and foam monitors. .

-.,

The lifesaving and firefighting equipment on a drilling rig is much like that on any large ship. Lifeboats, inflatable liferafts, lifebelts, lifejackets, smoke floats, distress rockets and flares form the main lifesaving appliances, with enough capacity in the boats for at least twice the total complement. The lifeboats are totally enclosed and self-propelled, and they are self-righting in case of capsize. They are normally made of fibre glass and powerful water spray nozzles fitted outside their cabins enable them to remain surrounded by burning oil for several minutes without harm to their occupants. Most

238

Marine Operations

Helicopter landings and take-offs can be hazardous. Foam monitors are installed around the helideck perimeter and a crewman stands by in a firesuit.

designs are conventionally boat-shaped, but circular capsules are used on some rigs. The firefighting equipment usually consists of a high-pressure fire main with hydrants and hoses through which sea water is pumped by a powerful fire pump which can be backed up if necessary by the ballast pumps. There are gas, flame and smoke detectors in all appropriate places and accommodation spaces are fitted with water sprinklers, while machinery spaces are covered by halon and carbon-dioxide systems. Every working or living space in a drilling rig has a suitable type of fire, extinguisher located nearby. WORK PERMITS

Many of the routine operations carried out aboard an offshore drilling rig or platform pose risks to the health and safety of the personnel concerned, and drilling contractors and platform operators have devised systems of ‘work permits’ to ensure that the correct procedures are followed in dangerous situations.

.”

239

Marine Operations Although different flag states and different companies may have their own systems, three types of permit are in general use on semis and jack-up drilling units in the North Sea. A ‘hot work/entry permit’ allows jobs such as welding, oxy-acetylene burning, grinding, flaring-off and the inspection of enclosed spaces to be done only if conditions are safe. Obviously a drill crew would not be allowed to ‘slip and cut’ the drilling line if gas was present in pockets near the drawworks or elsewhere on the drill floor, so the atmosphere in the work area has to be tested with an analysing instrument before a permit can be obtained. Similarly, an electrician who wants to work on high voltage equipment has to apply for an ‘electrical work permit’, and a job which involves a crewman being suspended overside requires an ‘overside work permit’. A typical job of this sort might be repairing a TV camera on its guide lines in the moonpool. The permit holder would have to wear a life-vest, be connected to the rig by a safety harness and line, and would have to have a watchman in attendance. One other condition of having a permit to work overside would be that the rig standby vessel was stationed close to the drilling unit with its emergency crew closed up for immediate action. STANDBY BOATS Not every maritime nation with offshore drilling activity requires the use of standby boats, but in the British and other sectors of the North Sea they are mandatory whenever an installation is manned. The standby boat, which

Many of the older standby boats are converted side trawlers. Their good sea-keeping characteristics make them ideal for this job, especially in rough weather.

240

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Marine Operations

must be able to accommodate the entire complement of the installation, has to stay within five miles of the location, and whenever required has to put its rescue crew on alert and come in to ‘close standby’. Most of the ships used for these duties in the British sector are converted middle-water trawlers that switched to standby work when they were no longer able to fish profitably on their traditional grounds, but the newer vessels are purpose-built. They all carry fast inflatable boats, marine and aeronautical VHF radios and anti-pollution equipment, and their crew are trained in rescue techniques which they practice frequently in ‘man overboard drills’ using a dummy launched from the rig or platform. The only situation when a standby boat might not be legally required to be in attendance is when the rig is not engaged in either exploration or production. Transits between locations fall into this category, and these are explained in the next section. RIG-MOVES A transit, or ‘rig-move’ as it is commonly known, is the passage of a mobile

offshore drilling unit from one location to another, and at this time the marine department comes into its own. The transit may be made under the rig’s own motive power, under tow from one or more tugs, or even with the drilling rig being carried onboard another vessel. Drill ships invariably shift location under their own propulasion, while most jack-up rigs are not self-propelled and have to be towed. Semi-submersibles are usually fitted with thrusters which may be powerful enough for normal propulsion, or which might only be used for making small adjustments of position at the location. Regardless of the power of their thrusters, however, most semis shift location under tow, using their thrusters to assist the towing vessel. In this way, speeds approaching those of conventiona ships are possible on some types in fair weather. An increasing number of drilling units, especially jack-ups, are now moved over long distances on the strengthened decks of special semi-submersible ships adapted to lift very heavy weights. The carrying ship, which is often a converted tanker which has had extra reserve buoyancy tanks fitted, first submerges its main deck, allowing the rig to float &er it and be positioned, and the ship is then de-ballasted and lifts the r&clear of the water. Some semi-submersibles weighing, more than 20,000 tons have been carried in this manner, usually from their building yards in the Far East to delivery locations in the West, and on some voyages two or three jack-up rigs have been carried as one cargo. A small number of Norwegian and Dutch shipping companies in particular have made this a specialization.

Marine Operations

Before the rig-move of a jack-up, the barge hull is jacked down its legs until it is afloat. Then the legs are jacked up and the tow can begin.

A semi-submersible’s transit normally requires the platform to be de-ballasted so that only the pontoons are in the water. This reduces the vessel’s weight and the amount of resistance from the sea, and increases the transil speed. It also reduces slamming of the waves on the horizontal and diagonal braces that support the deck and columns, which could cause serious damage. A drill ship proceeds from one location to another much like any other conventional ship on a voyage, some vessels making speeds of 15 or more knots. A jack-up rig’s hull, meanwhile, has to be floated for a transit, as described in Chapter 3, and the legs must be raised well clear of the sea bed.

242

.d

Marine Operations Jack-ups can normally only make transits in good weather conditions due to their vulnerability to storm damage, and in some cases their legs have to be cut and sections stowed on deck to avoid over-stressing when rolling or pitching. Alternatively, the legs may be left intact but specially strengthened for the voyage. Any type of rig’s transit through a coastal region normally has to be approved by the local authorities, and transits are not usually begun during bad weather conditions when towage may be difficult and there is a risk of the tow parting and the rig running aground or getting out of control, Care is always taken to secure all movable equipment before any transit, and navigation and stability are closely monitored throughout the operation. The weather is very carefully watched, and often a special weather forecasting service for the oil industry is employed to give prognoses for the route ahead. No standby vessel is normally in attendance during the transit, although helicopters are still usually able to fly on and off the rig for normal crewchange or emergency purposes. NAVIGATION & PILOTAGE During the transit, whether it is a local location shift on the same field or a long ocean passage, a drilling rig is navigated like any conventional ship, the the bargemaster in command and the control room operators (who are often signed on as the mates), or other marine personnel being enlisted for watchkeeping duties in the pilot house. Although the bargemaster or the contractor’s marine superintendent may supervise the tow and subsequent anchoring operation, professional towmasters are often hired from specialist marine consultancy firms to oversee the towage and mooring operations. This applies particularly in the case of a jack-up rig which often has no qualified bargemaster in its crew. However, on a self-propelled semi-submersi-’ ble or a drill ship, the bargemaster is always legally in command. A semi-submersible’s navigation equipment, like that of most other coastal’vessels, normally includes gyro and magnetic compasses, radar, echosounder, radio direction-finder and Decca Navigator. Satellite navigators, Loran-C or Omega may also be used for long-distance hauls where the Decca Navigator system is of limited use. This equipment is housed in the pilot house which, although perhaps not as spacious or sophisticated as a large merchant ship’s bridge, performs exactly the same functions. As the tow progresses, the rig and the towing vessel each regularly fix their positions and compare results before making any necessary adjustment in course or speed. The rig, with its propulsion assisting the tug, is able to steer either by a conventional wheel and rudder arrangement, or by independent

i43

Marine Operations adjustment of the power in each thruster, made by throttling or by varying the pitch of thruster blades. TOWAGE The tow is normally made by one or two multi-purpose anchor-handling/tug/ supply boats (AHTSVs) of adequate power measured in terms of ‘bollard pull’ as well as in engine horse power. These vessels are hired by the well operator and may later be used in a supply role when the rig is drilling. Although the tugs are in control of the rig’s movement, they work under the directions of the drilling rig’s bargemaster, or the towmaster if there is one.

A TYPICAL RIG TOWAGE ARRANGEMENT

A typical one-tug towing arrangement for a semi-submersible. When two tugs are used, one pulls a chain/wire towline from each pontoon.

3.44

Marine Operations

Semi-submersibles are usually connected to the tug by a chain-and-wire bridle arrangement, either attached at the forward ends of the pontoons or at the main deck level on the forward columns. This is a permanent fixture on the rig and can be recovered in the event of a breakage with a wire connected to an air tugger winch. The length of the towline is extended as speed is gathered, and shortened towards the end of the voyage, with the bargemaster and tugmaster remaining in constant communication about this and other aspects of the tow.

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In heavy weather during the transit, when the rig’s lower structure is taking a pounding from the sea, it might become necessary to submerge the unit to survival draft. This can usually be done in a matter of an hour or so, but it slows down progress drastically and would normally only be done if an alteration of course is not a better option. APPROACHING THE LOCATION

The well location, and the precise intended position of each of a semi-submersible’s anchors, is selected by the well operator on the basis of the findings of geophysical surveys made well before the rig-move. Sometimes, especially at deep-water locations, the survey ship or a supply boat will lay a pattern of buoys to mark the location and anchor positions. However, in areas of frequent bad weather or strong tides there is always a possibility that these will not float over their intended positions, so quite often the location position is determined by highly accurate electronic navigational equipment.

I j I

The operator’s marine representative generally liaises with the drilling contractor’s marine staff on the matter of the mooring plan, discussing difficulties that may arise with the operation such as the close proximity of other rigs’ moorings or of an oil or gas pipeline or a telecommunications cable, etc. The operator’s marine representative, who may in some cases be a hired consultant, accompanies the rig during the rig-move and the mooring operation to ensure that the client’s requirements are met. Very little positioning toler%nce is normally permitted by an operator in any direction from the planned location, and sophisticated electronic position monitoring equipment, including a satellite navigator receiver, is usually hired together with an operating technician for the duration of the operations. As little as twenty metres.radius from the surface location might be allowed, plotted on location co-rdinates measured to within one tenth of a second, both of latitude and longitude. (It is int~eresting to note that a tenth of a second of latitude equates approximately to only ten feet). As well as a geographical fix, location co-ordinates are also determined in

Marine Operations metres and hundredths of a metre by a system known as the UTM system. It is doubtful, of course, whether such precision of positioning can be maintained throughout the well programme, except perhaps by a jack-up rig, since a floater is constantly moving in response to the forces of wind and tide, but these parameters demonstrate the stringent requirements that operators expect to be met by a marine drilling contractor. The final heading of the rig is also very important, as this must take account of the prevailing weather conditions at the location for the period during which the rig is expected to be anchored so that drilling, helicopter and boat operations and any flaring off of gas can all be done safely. The towmaster needs to know exactly what line to approach on, and precisely where to place the rig and each-of its acnhors. Factors such as wind, current and tidal flow all play a critical part in the morring operation, and the towmaster must allow for chnages in them during the several hours’ duration of the operation. The engine power and manoeuvrability of the tug and anchor handling vessels also has to be considered, of course, but this will have been thoroughly investigated before hiring them.

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Marine Operations

:

The final approach to the location, which may be about five miles from the point where the tow is shortened up, is normally made under tow with assistance from the rig’s own thrusters, which, in the case of a twin-pontoon design of semi, are like a ship’s propeller at the after end of each pontoon. The aftermost anchor on the weather side is normally the first to be deployed, and while the majority of the anchors will be carried out from the rig and then dropped by the AHVs, this anchor is dropped by therig itself as it passes over the anchor’s sea-bed position. The rig therefore approaches the anchorage from windward along teh bearing of intended deployment of this anchor, making due allowance for the amount of drift caused by the wind, while the towing vessel attempts to prevent the rig from drifting downwind too much. A semi-submersible’s anchors are normally numbered clockwise, with the forward anchor on the starboard side being Number 1. In the case where the wind is from the port side of the location, therefore, the approach heading will be along the line of the port aft anchor, which is named Number 5. When the wind is from the starboard side, Number 4 will be dropped first.

RUNNING ANCHORS

About a mile from the first anchor drop position, a pennant buoy attached to the first anchor is dropped in the water. The anchor is ‘walked out’ slowly under windlass control to a position about 100 feet above the sea-bed, the rig meanwhile slowly approaching the drop position. As soon as this is reached, as indicated either by bearings of the buoy pattern or by the electronic navigation equipment, the after weather anchor is dropped by the rig (in the case where the wind is from the port side, Number 5, or where the wind is from the starboard side, Number 4). A typical length of chain deployed on locations in the central North Sea would be 3800-4000 feet, so the rig will run on for this distance, or approximately eight cables (0.8 nautical miles), before arriving on the location. Another anchor is normally passed by a rig crane to an attendant anchorhandling vessel just before the first anchor is dropped, but the rig is slowed down using its own proptilsion rather than by either of these anchors. The dropped anchor’s chain is paid out at a controlled speed, the towing vessel meanwhile pulling the rig steadily towards the drilling location and controlling the heading. On arrival at the intended location the towing vessel and the dropped after weather side anchor together keep the rig more or less on the location while fine adjustments are made to the rig position and heading, and final preparations are made for running the other anchors. ..

247

Marine Operations

Marine Operations ,.(, ,. ,

To save time, it is normal practice for each AHV to load the required amount of equipment for the number of anchors she will be required to run before running operations actually commence. A sufficient length of wire pennants, together with shackles, to reach from a surface buoy to the sea bed can then be spooled onto each boat’s wind drum, ready for connection to the anchors when these are eventually passed to the vessel from the rig. Usually a ‘chain chaser system’ is employed by rigs to help anchor handling vessels run and recover their anchors. A large steel ring called a ‘chasing collar’ encircles the rig’s anchor chain close to the anchor and in its stowed position is suspended from the deck of the rig by a heavy ‘chaser pennant’. This pennant is passed to the AHV, which can then pull the rig’s anchor away from its bolster stowage position towards the vessel’s stern roller and carry the anchor out to the dropping position, while the rig pays out the anchor chain. When the required amount of chain is out from the rig, the boat can lower the anchor to the sea bed, with a marker buoy being attached to the chaser for later recovery. When the anchor is to be recovered, the AHV picks up the buoy and the chaser pennant and drags the chasing collar along the chain until the collar slips round the anchor shank. The anchor can then be lifted by the AHV while the rig hauls in the chain. When the anchor pennant is first lowered to the AHV by the rig crane it is secured on a large device on the deck called a ‘pelican hook’, and the crane’s hook is released from the pennant. The AHV’s crew then secure the connected lengths of pennants that are wound on the winch drum to the anchor pennant, and the tension on the anchor pennant is transferred from the pelican hook to the winch. When the AHV is ready the rig lowers its anchor from its bolster, transferring its weight to the pennants on the boat. About twice the water depth is paid but on the anchor chain, the AHV heaving the anchor close up to her stern roller and the&acking back about ten feet. She then lines up on the heading given by the rig towmaster and steams quickly out on this heading while the chain is paid steadily out over the rig’s windlass to keep it as straight as possible. When the correct length of chain is out according to the anchoring plan, the windlass brake is applied and the AHV connects the buoy to the pennant and lowers the anchor to the sea bed, meanwhile steaming ahead to maintain tension on the chain. With the anchor on the bottom the buoy is released and the AHV comes in to the rig with the cQllar sliding round the chain, and the pennant is racked on the rig. T O ensure accurate positioning of the anchors, special laser guidance equipment is sometimes set up on the pilot house top and on the decks of the AHVs, so that very precise courses can be maintained by the vessels when steaming out to the drop point. -. _

249

Marine Operations With the rig held roughly in position on the. location by the tug and the weather after anchor, the weather bow anchor and the second stern anchor are normally run, preferably simultaneously by two AHVs. Thereafter procedures vary according to individual operators’ practices, but a typical sequence, with the wind on the starboard side, might be as follows:1. Drop first anchor (No. 4). 2.i On arrival at location, run weather bow anchor (No. 1) and second stern anchor (No. 5). 3. Run second bow anchor (No. 8). 4. Release towing vessel from tow bridle. 5. Run breast anchors (Nos. 3,6,2, and 7). 6. Add back-up anchors (‘piggy backs’) to main anchors where required. 7. Pre-tension (see below). ANCHOR TYPES As with all marine equipment, there are several different designs of anchor in common use offshore, each having its own benefits and drawbacks. Different anchors are suited to different sea-bed conditions, but drilling rigs, like ships, are equipped with one set of anchors and chains when they are built and these are not normally changed, although the chains might be lengthened for a deep water drilling contract.

.

The usual types of anchors found on semi-submersibles are called ‘stockless’, that is, they do not have the long upper horizontal bar that characterizes the ‘Admiralty pattern’ or ‘common’ anchor. There is instead a long, straight ‘shank’, at the lower end of which is a ‘head’ with two ‘flukes’, one on either side of the shank. These are hinged so that they can swivel about 30 to 50 degrees and dig into the sea bed when a strain is exerted on the end of the shank. Some types have a stabilizing bar fitted through the head so that the anchor is prevented from turning on its side, which would stop the flukes from embedding. On the top of the shank is the anchor shackle to which the chain cables is attached, and at the lower end, on the head, is another shackle for connecting the pennants that secure the surface buoy or to attach a secondary, tandem anchor. The weight of an anchor for a large ‘semi’ would be in the region of 30,000 lbs or 13 tons, and its ‘holding power’ about eight times that weight. The length and weight of the chain paid out is an important factor in the holding ability of the system, as is the amount of tension put on the chain once it is laid.

250

Marine Operations ANCHOR PATTERNS

There are several geometrical arrangements of anchor chains which can be used for mooring a drilling rig, all of which are designed to maintain the unit within a tolerable lateral distance of the location. The pattern used depends on the type of rig, its structural shape and the number of anchors and chains or wires fitted, and on location considerations such as the strength of currents and tides and the existence of pipelines and other moorings. One of the most common arrangements on a rectangular-decked ‘semi’ is for eight anchors (two from each corner of the deck) to be laid with 45 degrees bet-

Some popular anchor patterns. Many factors influence the operator’s choice of mooring design.

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I

Marine Operations ween each, the forward starboard (or Number 1) anchor being laid 221/2 degrees to starboard of the rig’s heading. Another popular ‘spread’ employs angles of 27r/2 degrees and 6214~ degrees between the heading and Numbers 1 and 2 anchors respectively, this pattern being mirrored on both sides and at each end. PRE-TENSIONING When all the anchors have been deployed in their correct positions, and the rig is within the limits of tolerance set for the location, the rig is ballasted down and the anchors bedded in firmly by tensioning up each chain to a ‘proving load’. This is held for about ten minutes and is witnessed by the operator’s marine representative. Should the proving tension be insufficient to prevent the anchors dragging, back-up anchors called ‘piggy-backs’ may have to be laid in tandem with th,e main anchors; usually a number of backup anchors are kept available on the AHVs during the mooring operation. If the chains all hold with the proof load applied to the satisfaction of the marine representative, they are then slackened off to a working pre-tension suitable for the depth of water. A high working tension will keep movement around the well location to a minimum, but will result in stiffness of the mooring system, with an attendant risk of an anchor breaking out of its bed in bad weather. On the other hand, too little tension will give elasticity which may allow unacceptable movement of the marine riser when this is connected. Normally, 5% of the water depth is considered the maximum offset from the vertical for the marine riser and the ball joint, in order not to overstress them.

Left: A semi-submersible’s corner-mounted mooring windlass for two chains. Right: The two anchor chains are guided through fairleaders just above the bolsters at the base of the columns. Pulling from this point they are less likely to pull the rig over.

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Marine Operations THE MOORINGS DURING DRILLING

The anchor chain tensions are constantly changing as wind, tides and currents exert forces on the rig. They therefore need continually monitoring in case any of the chains near its maximum pre-tension or breaks out. Once that happens, a chain reaction sometimes occurs as intolerable strains come on the other weather-side anchors, resulting in several chain failures and the rig bearing down over the lee side chains. Load cells are generally fitted at each windlass, and remote read-outs of the chain tension are usually available in the control room and the pilot house, as well as in the local windlass control cab. Chain tensions are normally read every four hours, or hourly or constantly during bad weather. On occasions, such as when the BOP stack is being ‘pulled’, the bargemaster has to move the rig slightly off location to avoid damage to the well head in the event of the stack being dropped. This is done by slacking down the chains on one side and heaving in those on the other, until the required distance of offset has been moved. When the BOP stack is again being connected, the rig is moved back onto location in the same way. PULLING ANCHORS’

At the completion of the well programme, the anchors are pulled and the transit is made to the next location. Any piggy-backs deployed are normally recovered first, although in good conditions they may be recovered at the same time as the main anchors, The chain tension on all anchors is first slackened off a little and the rig is de-ballasted to its transit draft, which takes several hours. The anchor handling vessel approaches the buoy of the first anchor and lassoes it, connects it to her winch wire and hauls it in over her stern roller. If no buoy has been used, the chaser pennant is passed to the AHV, which then ‘chases out’ to the anchor with the chasing collar. The pennant wires are then connected to the winch wire which the boat hauls on, and the anchor is broken out of the ground. The AHV lifts the anchor close up to her stern roller while the rig starts heaving the chain in, the AHV manoeuvring in to the rig with it. When there is still about 200 feet of chain out from the rig the AHV slacks off her winch drum, allowing the eye of the anchor pennant to engage in the boat’s pelican hook, and the rig continues to haul the anchor in until it is bolstered. The boat then passes the chaser pennant back to the rig where it is secured on deck. Once all the piggy-backs are home, the breast anchors (Numbers 2,3,6 & 7) are usually pulled before the towing vessel, which may be used initially to ..

253

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@ Crane passes chaser pennant to boat. Boat chases out to anchor.

@ Boat heaves anchor to stern roller. Rig heaves chain in. Boat carries anchor to rig.

@ Anchor bolstered. Boat passes chaser pennant back to rig and moves to next anchor to be pulled.

Pulling anchors. This is usually easier and quicker than laying them.

254

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recover some of these anchors, is connected to the towing bridle. The bow anchors and the weather stern anchor are then recovered, the towing vessel meanwhile preventing the rig from falling back over the wellhead, and finally the lee stem anchor is recovered. The tow can then begin, DYNAMIC POSITIONING SYSTEMS

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To avoid having to use an anchor spread, and to enable them to drill in deep water where anchors could not be used, dynamic positioning ,systems are fitted to some deep water drilling units, including some semi-submersibles as well as many drill ships. ‘DP’ is also widely used by diving support vessels, as well as a small number of supply boats and coring vessels. It was first introduced in the early 1960s as a result of the demand for drilling in deeper water where anchoring was difficult or even impossible. The purpose of a DP system is to automatically hold a vessel in a desired position without the use of physical restraints such as ropes, cables or anchors, ‘automatically’ meaning in this case without manual inputs to control corrective manoeuvres. This is achieved by feeding computers on the vessel with information about its heading (obtained from gyro-compasses) and about the position relative to one or more fixed points, either on the seabed, on a fixed platform or on the land. The computers rapidly calculate the amounts of thrust required to correct deviation from the intended position and generate control signals to individual thrusters sited on the vessel’s hull. This enables forward, backward and swivelling movements of the vessel to be made, virtually eliminating any ahead/astern surge or sideways sway, and ders, or beacons, are deployed by the DP vessel on the sea bed to transmit Some early DP systems used only a bare minimum of thrusters and a single analogue computer, the reference point on the sea-bed being a clump weight suspended from the vessel’s side by a tautly tensioned wire. By measuring the angle of the wire from the vertical a reasonably accurate position could be maintained. However, if the equipment failed and the vessel moved very far off location, serious damage to or complete failure of the marine riser and drill string could result, with serious implications for the well programme’s time and costs. Full redundancy, or the duplicating of all essential system components such as measuring devices, computers and thrusters was therefore introduced, and this is the norm today. Modern DP e’quipment is highly sophisticated, and very expensive. Taut wire devices are still used by most, if not all DP vessels, but for operations in deep water, hydro-acoustic position reference systems are used in addition. These operate without physical connection with the sea bed and are not therefore so liable to physically break like taut wires. Small transpon-

Marine Operations

ders, or beacons, are dpeloyed by the DP vessel on the sea bed to transmit a signal which is detected in hydrophones mounted on the vessel’s underwater structure and then passed to the computers for resolving into two positional co-ordinates. A third system used in conjunction with taut wires.and acoustic transponders in certain circumstances employs pre-positioned radio transmitters mounted on some fixed structure to transmit signals to receiving antennae on the drilling vessel, the range and bearing of the fixed structure being measured and fed into the computers. This is similar to the rig positioning equipment sometimes used on arrival at a drilling location. This system could obviously not be used where there is no fixed reference point within its range, but is normally available as a back-up to the other two systems.

A dynamically-positioned rig is equipped with computer-controlled thrusters which are constantly working to maintain station over the well.

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Marine Operations

TRA”Sa WNDER The DP system aboard a semi-submersible rig. Its high installation and operation cost makes it one of the factors which an operator has to weigh against using a conventionally-moored rig.

..

257

Marine Operations

Most DP vessels nowadays also have sensors that measure wind direction and speed which the computers take into account before determining the amount of corrective thrust required. The most obvious advantage of DP in a drilling operation is that the vessel is not restricted in its operating depth except by the limitations of the drilling equipment; DP is as effective in 20,000 feet of water as it is in 200 feet. The system can be put into operation in a matter of an hour or less after arrival at a drilling location, compared with the eight hours or more that is often necessary for running a spread of anchors, and when the well has been completed the vessel can move off location in even less time than it took to set up in DP. This is particularly important on rigs that are operating in iceberg areas, where hanging off and unlatching may be necessary at short notice to move clear of an approaching berg. A short shift of position can be made in DP, but it takes only a matter of minutes to come out of DP for a conventional shift, and setting up again in DP normally takes only half an hour or so. Another advantage for a well operator is that anchor handling vessels do not have to be hired. With boats working a conventional spread of eight anchors the total time spent anchor-handling during a well programme may run to 2-3 days, or, where there are difficult sea-bed conditions and bad weather, even weeks. The final advantage, which is more for the benefit of a drill ship than a semi-submersible, is that DP permits easy and frequent changes of heading. These are essential in drill ships for the reduction of movement in a seaway, their hulls being more responsive to heavy seas and swells than those of semisubmersibles. These considerable savings, mainly in time, obviously affect the cost of the well programme. However, they have to be weighed against the increased cost of hiring a DP rig. This is the one major disadvantage of a DP system, the initial cost of the system far exceeding that of a conventional eight-point anchor arrangement. On top of the capital cost of the equipment, with its full duplication, there is also the cost of running sufficient machinery to have power to operate the thrusters, which has to be generated in addition to the power required for drilling equipment and other services. It has been said that a DP system represents about 20-25% of the total cost of a DP rig, or about 4-5 times the cost of a spread anchor system.’ DP rigs therefore command considerably higher day-rates than other rigs. They also need skilled DP operators to monitor and operate the sophisticated control equipment and take speedy remedial action should a fault develop. DP therefore cur-

258

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Marine Operations rently has only limited demand in marine drilling for special operations such as drilling in very deep water or in ice regions. RIG SUPPLIES From the time the unit is anchored on its drilling location until it has completed the well programme and once more pulls anchors, a steady stream of supply boats will keep it stocked with the numerous goods it needs to drill the well and feed its crew. Loading and discharging (‘backloading’) the boats is the job of the roustabouts under the leadership of the crane operator and the direction of hte bargemaster. It is done at night as well as day, and usually in as bad a state of weather as the master of the supply boat considers is pru-

Older and less powerful supply boats often tie up to the rig they are working, running sternlines up to the rig’s deck and maintaining tension on them with their engines, or alternatively dropping anchors, then backing in and running sternlines out. The more modern boats with powerful engines and side thrusters are able to stay on location without the use of mooring lines, and a select few have DP equipment. Even though some of them are quite large ships by some standards, from the deck of a semi-submersible a supply boat looks small and vulnerable to the sea. Their crews who handle the numerous containers, skips, mud hoses, slings of casing and drill pipe and a wide variety of other cargo on the boats’ afterdecks often have to work waist-deep in water which pours over the stern and bulwarks and rushes dangerously up and down the deck. Accidents occur, in which case the injured crewman is brought aboard the rig for treatment by the medic. Below its upper deck a supply boat normally has a number of tanks for carrying the bulk powders and liquids required for drilling operations: water, fuel oil, oil base mud, base oil, barite, bentonite and cement. These fluids are pumped up to the rig through flexible ‘bulk hoses’ which are lowered down to the boat, while deck cargo is lifted off by the rig’s cranes. Every item has to be manifested for customs purposes, and the rig storeman tallies each loaded and backloaded cargo. Supply boats can often accommodate up to twelve passengers, and personnel transfers are normally made between the boat and the rig by a ‘personnel basket’ which is a circular platform with netting fixed between its perimeter and the lifting bridle. Baggage is placed inside the netting, while the passengers, wearing lifevests, stand round the edge of the basket gripping the netting as the crane hoists them overside.

,

Marine Operations

260

Marine Operations

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4

Top and bottom: Supply boats operate in all weathers and have to be handled with skill.

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Marine Operations

Deep-drilling rigs with seventy or more crew call for frequent stores deliveries. Food and small items of equipment comes out in containers which can be left onboard until emptied. ,

Supply boats attending drilling units in Arctic regions sometimes have an unusual extra duty: steering icebergs out of the way of rigs. The boats encircle a threatening berg with a buoyant fibre rope and deviate the berg’s course with sustained use of engine power.

262

Marine Operations HELICOPTER OPERATIONS Although fast crewboats are used for personnel transport in some smoothwater regions of the world, the vast majority of rig crews travel to work by helicopter. There are estimated to be about 2,500 of these aircraft employed by the oil industry around the world, half of which are in the USA. A wide range of aircraft types is used for offshore flights, from the 44-seat, twin rotor, 47,000 lbs Boeing-Vertol Chinook to the 4-seat, 3,350 lbs Bell Jet Ranger. In the North Sea and many other drilling areas the most popular type is the Sikorsky S-61-N, which can carry 24 passengers and weighs over 20,000 Ibs fully loaded. The loading of passengers and freight from a rig in the North Sea is the responsibility of the, Helicopter Landing Officer (the HLO) who might be the bargemaster, the deck foreman or some other person appointed by the Offshore Installation Manager (the OIM). The HLO’s duties include ensuring that the helideck is clear for landing and is equipped with all the necessary lights, netting and fire appliances, and that there is a protective-suited crewman standing by with a foam monitor while the aircraft is on deck. Passengers have to be prevented from walking near the tail of the helicopter, where the small, vertical tail rotor is a danger, and their baggage has to be removed from or loaded into the aircraft’s hold space.

Thick

netting helps helicopters to stay on the helideck. This aircraft is a Super Puma.

Marine Operations

White the aircraft is empty it may require refuelling, and this is done using carefully inspected, clean Jet A-l aviation spirit from ‘helifuel’ tanks carried by the rig. Full tanks are brought out by supply boat when required and the empty ones taken back into ‘town’ for replenishing. Communications with helicopters as they approach or leave a rig are maintained by the radio operator, who passes details to the pilot of wind speed and direction, air temperature, barometric pressure, the amount of roll, pitch and heave, the unit’s heading, the sea state and cloud coverage. For a 75man semi-submersible working in the North Sea there might be three scheduled flights a week for regular crew-changes, with extra flights as required by the well operator, who sometimes might require some urgently needed item of drilling equipment transported rapidly. At speeds of about 120 mph, offshore helicopters are about ten times as fast a supply boats. Helicopter passengers travel wearing rubberised ‘survival suits’ which are designed to retain body warmth in the event of ditching. The procedures for evacuating a ditched helicopter have to be learnt by all offshore workers at special courses. These and other matters relating to rig personnel are described in the next Chapter.

264

CHAPTER 7: RIG PERSONNEL & TRAINING The full complement of a mobile offshore drilling unit normally comprises the drilling contractor’s own rig crew, a small number of personnel employed by the well operator and several ‘service hands’ who carry out specialised contracted work onboard such as diving, cementing and mud logging. Apart from this permanent manpower, other temporary personnel often travel out to a rig for a few days to do a special job or to make an inspection of one sort or another, so the ‘personnel onboard’ figure tallied by the medic/clerk is constantly changing. The crew list below gives some idea of the numbers of personnel that might typically be found aboard a semi-submersible rig drilling in the North Sea. OIL COMPANY PERSONNEL:

DRILLING CONTRACTOR’S CREW:

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Operator’s Representative 2 Geologists Drilling Engineer Materials Co-ordinator Senior Toolpusher/OIM Junior Toolpusher Bargemaster Chief Mechanic 2 Radio Operators Medic/Clerk Chief Steward Chief Cook Night Cook 5 Stewards 2 Control Room Operators Storeman Sub-seaEngineer 2 Drillers 2 Assistant Drillers (A/D) 2 Derrickmen 6 Floormen (Roughnecks) 6 Roustabouts 2 Crane Operators Deck Foreman (Bosun) Maintenance Roustabout (AB) 2 Electricians

OPERAmJNS MANAGER l,CALLDRILUNGOPER.4mmS

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I PERSONNEL& TRAlNlNGMGR. - - - - - -

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SHORE STAFF

I I SUB-SEASWT. I/C SUB-SEAEOUPT. - - - - -

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I RIGS”PERlNTENDENT IIC DRILUNG - - - - - - -

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MARINESUPT. CATERING IIC MARlNE MANAGER - - --------_

ENGlNEER SUPT. I/C MACHINERY ----________ CHIEF MECHANIC I/C RIG MACHY.

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I COOKS STEWARDS

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DEPARTMENTAL BREAK-DOWN ON A SEMI-SUBMERSIBLE DRILLING RIG

The departmental structure in a marine drilling company. It differs considerably from a ship- owner’s manning structure.

OFFSHORE STAFF

Rig Personnel & Training

Instrument Technician 2 Mechanics 2 Motormen Welder OTHER CONTRACTOR’S PERSONNEL: Mud Engineer Cementer 4 Mud Loggers Diving Supervisor 3 ROV Pilot-Technicians 64 TOTAL COMPLEMENT: The total figure of 64 is often increased to around 80, or more on larger rigs, depending on the operations. During a rig-move, the total ‘POB’ may be as low as 40, and if the rig is ‘stacked’, only a handful of maintenance personnel might remain onboard. The jobs of the personnel in the normal complement listed above are described below: OPERATOR’S REPRESENTATIVE: The ‘company man’, as he is normally

called aboard the rig, is the operating company’s on-site superviser who oversees the entire well operation on their behalf. He may be directly employed by the operator, but is often a self-employed consultant, with many years of oilfield experience at all levels, who is hired through an agency. He is on call 24 hours a day, and has an office with accommodation attached aboard the rig. Every morning the company man is in contact with the operator to pass a verbal report of operational progress and to receive instructions. He dictates when supply boats are to be brought alongside, what tools are to be used down the hole, and much else besides, working in close liaison with the senior toolpusher. GEOLOGISTS: Oil companies normally directly employ their own

geologists, one or more of whom might be onboard for the duration of the drilling and well testing period. The geologist examines cuttings returned from the well and sends samples to the shore laboratory for analysis. He , works in a small office or ‘shack’. DRILLING ENGINEER: The company man is usually an experienced tool-

pusher who has supervised many drilling operations. However, he sometimes needs the technical assistance of an engineer trained in mechanics and hydraulics, and a drilling engineer may therefore be carried for some drilling operations.

Rig Personnel & Training

The company man makes the onboard decisions on behalf of the operator.

MATERIALS CO-ORDINATOR: The logistics of supplying the many needs of an offshore rig are complex, and some, but not all, oil companies employ a materials co-ordinator to keep a tally of their equipment and organise supplies, liaising with the materials base ashore. The ‘matsman’ works to the company man’s instructions and in liaison with the rig storeman. He is a source of much useful information for the new rig hand who is confronted by a lot of strange equipment. SENIOR TOOLPUSHER: The senior ‘pusher’ is the supervisor in charge of the drilling department, and is responsible to the company’s drilling superintendent for the day-to-day drilling operations on the rig. He may also be the appointed Offshore Installation Manager (see below) under British government regulations. He is on call 24 hours a day although he normally works a regular day-shift from 0600 to 1800, and does offshore tours of duty of either 7 or 14 days (or up to 28 days if the vessel is on an overseas location). He is responsible for the safety and welfare of the drilling department personnel, and for the safety of drilling and related practices on the installation. If he is also the OIM his responsibilities are wider.

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268

Rig Personnel & Training JUNIOR TOOLPUSHER: Night operations, from 1800 to 0600, are nor-

mally under the supervision of the junior, or night toolpusher, who, like the senior toolpusher, is normally an experienced driller. He may alternatively be known as the ‘tourpusher’ (‘tour’ being pronounced ‘tower’). BARGEMASTER: The bargemaster is the legal master of the drilling vessel

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and is in charge of the marine department onboard. On some rigs he may also be the legally-appointed OIM (see below) but if the senior toolpusher is charged with this responsibility the bargemaster is responsible to him for all marine matters affecting the barge. He is responsible for ensuring that there are the required number of marine crew aboard to satisfy manning regulations, for stability and ballast control, for assisting the toolpusher in safety matters, for loading and backloading supply vessels, and for the operation of bulk cargo storage and delivery systems. On a British self-propelled mobile rig the bargemaster must be qualified as a Deck Officer Class 1 (Master Mariner), which necessitates long training and experience in merchant ships. On an American rig he must have either a Master, Any Gross Tons Upon Oceans Licence (usually called an Unlimited Master’s Licence) or a Master of Column Stabilized Drilling Rig Licence (Industrial Master’s Licence). To obtain the ,former certificate the American bargemaster or barge captain must be experienced aboard large merchant ships, but for the latter qualification he need only have served in drilling rigs, and not necessarily in any previous marine role, although he must have undergone a training course. CHIEF MECHANIC: The engineering department is under the supervision of a qualified 1st Class Engineer, who ensures that all rig machinery functions and is maintained properly. He is usually an experienced merchant ship engineer and has team of mechanics, electricians and motormen to assist him. As well as power-generating machinery the engineering department has much other plant to look after, including some drilling equipment. On a jack-up rig the chief mechanic may also be the jacking engineer.

RADIO OPERATORS: A rig’s radio room is much like that Gf a large mer-

chant ship, with satellite communications, MF and VHF radio telephones, telex, facsimile and helicopter communication equipment usually being installed. Operators are normally former merchant ship radio officers and are kept busy sending reports of the well’s progress to ‘town’ and receiving numerous instruction messages daily. When a helicopter is sceduled, they pass rig weather information and liaise with the aircraft during its flight. -.., 360

Rig Personnel & Training

The radio room on a rig is much like that of any large modern ship

MEDIC/CLERK: In the busy heavy engineering environment of a drilling rig there are inevitably accidents to personnel and a qualified nurse is therefore employed to treat minor injuries. More serious cases are sent ashore or to a large production platform by ‘Medevac’ helicopter. While he is not treating patients the medic also does routine rig office paperwork, such as compiling crew and passenger lists, and organises accommodation arrangements.

270

Rig Personnel & Training CHIEF STEWARD: Catering for sixty or more personnel calls for careful

stores planning, especially when supplies may be delayed due to bad weather. The chief steward, or ‘camp boss’ as he is known on some rigs, may double as the chief cook in some companies, although managing the catering department is normally a full-time task. COOKS: There are usually two cooks, one on a day shift, the other on !

i,

nights, when bread is baked. They work in a galley which is mechanised as far as possible, like that aboard any large ship. There is usually a galley steward to assist the duty cook. STEWARDS: Rig catering staff keep accommodation and messrooms habit-

able and assist in the galley and elsewhere to the chief steward’s requirements. The whole shift’s working clothes,‘which get extremely dirty on a rig, are washed at the end of each tour, with a steward operating the laundry service. The catering staff may be directly employed by the drilling contractor, or may work through an offshore crewing agency. CONTROL ROOM OPERATORS: The ballast control room of a semi-sub-

mersible IS normally manned by qualified mariners who monitor the rig’s stability and operate pumps to trim the rig or adjust its draught. The CROs also monitor marine traffic, engine alarms, fire and gas alarms, watertight doors and bilge alarms, and keep track of weights loaded and backloaded and quantities of bulk materials used daily. Bargemasters are usually promoted from the rank of CR0 . STOREMAN: A rig must carry numerous spare items of equipment for dril-

ling, engineering, electrical and marine uses, and the documentation of stores and equipment loaded or backloaded by supply boat is done by the storeman. He works in the rig store and around the decks where he tallies equipment. , SUB-SEA ENGINEER: The BOP stack and its remote controls are main-

tained and tested by the sub-sea engineer, who is a specialist in hydrauliic engineering and control systems and often has a marine engineering background. He is also responsible for running the other items of sub-sea equipment. Jack-up rigs do not need a sub-sea engineer, as they do not use the same sea-bed devices as floaters. -.-

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The sub-sea engineer can be asked to work long hours when the BOP stack has to be brought to the surface for repair.

DRILLERS: The driller is the supervisor of the drill crew and is responsible for conducting drilling operations from the drill floor. He operates the hoist, rotary and mud circulation equipment from a small control &bin known as the dog house, and operates the BOP controls in an emergency. No formal qualifications are usually required to become a driller other than the industry’s own certification in subjects like well pressure control, but several years’ experience is necessary at all levels in the drill crew. Sometimes a specialist ‘directional driller’ may be brought out to advise the company man during directional drilling operations.

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Rig Personnel & Training ASSISTANT DRILLERS: The A/D assists and understudies the driller,

relieving him on the drawworks brake at times, and supervising jobs around the rig that the driller cannot attend to. He is usually responsible along with the derrickman for the maintenance of the mud pumps and other circulation system equipment.

For twelve hours a day each of the two drillers is at the drawworks brake in the doghouse. ,

.

DERRICKMEN: The top ends of stands of drill pipe are manipulated into or

out of the set-backs by the derrickman, who leans out from a small platform called the monkeyboard that projects high over the drill floor. When not ‘tripping’ the derrickman is responsible for maintaining the mud pits and mixing mud to the specifications of the mud engineer. His work is physically demanding, like that of the floormen. ..v

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Rig Personnel & Training FLOORMEN: The ‘roughnecks’, as the floormen are usually called, are the

hands who do the hard physical work of running in and pulling out tools from the rotary on the drill floor. Their job, and the derrickman’s, is usually the most physically demanding and dirty of any on the rig, but to become a toolpusher, rig floor experience is necessary. ROUSTABOUTS: The roustabouts are general purpose hands who may be

called upon to do almost any type of manual work aboard the rig, although they mainly work in the drilling and marine departments. A roustabout might be called upon to relieve a roughneck on the drill floor for a meal break, handle pipe being loaded from a boat, wash down the decks, paint, or drive the sack room fork lift truck. Most roustabouts aspire to jobs on the drill floor.

One of the roustabouts ‘jobs’ is marking conductor casing ready for running in the right sequence.

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Rig Personnel & Training CRANE OPERATORS: The crane operator not only drives the cranes but is

also usually foreman of the roustabout squad. He must be familiar with every item that might be required to be lifted up from the main deck to the drill floor, and hes has to be expert a judging when to land expensive equip ment on the heaving deck of a supply boat 70 feet or more below his crane cab, perhaps at night and in driving snow. DECK FOREMAN & MAINTENANCE ROUSTABOUT: The deck fore-

man, or ‘bosun’, is the experienced seaman in charge of the marine deck crew, which on a semi-submersible in its drilling mode might only consist of one other seaman to comply with government regulations. On long ocean passages the deck crew would be increased to meet a minimum regulatory manning scale. ELECTRICIANS: The rig electrician is normally kept busy repairing the

numerous items of electrical equipment onboard, from the main SCR power distribution system to the galley toaster. He has a workshop on the main deck but can often be found in the control room or engine room. INSTRUMENT TECHNICIAN: The numerous control and measurement

devices aboard a rig, including items like the weight indicator, fire and,gas alarms, ballast tank gauges and sub-sea TV cameras are maintained by the instrument technician. On a DP rig there may be a specialist DP technician in addition. MECHANICS: The rig mechanics are the equivalent of a merchant ship’s

engineers, and most come from merchant navy backgrounds. They are responsible for maintaining the main engines (usually diesels) in smooth working order, as well as drilling machinery, air compressors, heating equipment and many other auxiliaries. MOTORMEN: The motorman is the shift mechanic’s assistant, responsible

for minor repairs and maintenance and for making routine checks on machinery. Amongst other jobs he tests bilge alarms, checks lubricating oil consumption and records engine data. WELDER: There are always modifications and repairs being made to the

steelwork of a rig, whether in the form of a new ladder, the installation of a new piping system or the replacement of worn out deck grating. The welder is in constant demand by all departments and keeps welding equipment stationed at handy positions around the rig as well as in his welding shop. .”

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Rig Personnel & Training MUD ENGINEER: The ‘mud man’, as the mud engineer is usually called,, is

responsible for carrying out the well operator’s mud programme, which has been carefully formulated to keep the well safe from kicks and blowouts at all times and to maximise drilling efficiency. He supervises the mixing of mud, analyses it on its return from the hole and treats it with additives to change its physical or chemical properties as appropriate for the particular formation being drilled through. The mud engineer is normally employed by the same specialist firm contracted to the operator that supplies the mud. CEMENTER: The cementer’s job is to operate the powerful cement pump-

ing machinery during ‘cement jobs’, and to maintain the equipment in between jobs. Cementing is a service carried out by a specialist firm contracted to the operator, and their plant remains permanently onboard the rig. MUD LOGGERS: The mud loggers monitor the mud circulating back from

the bottom of the hole with special equipment installed in a portable logging shack. If hydrocarbons show in the mud, they can determine the depth at which they exist by means of recording instruments. DIVING TEAM: If underwater work is required by the well programme it

may be done using a hired remote-controlled or remote-operated vehicle (an RCV or ROV), a small manned submersible, or a squad of saturation divers who live for limited periods in saturation chambers, pressurized by a mixture of oxygen and helium. Diving is normally contracted out to specialist firms who install their plant on the main deck over a small moonpool. WIRELINE OPERATORS: The wireline equipment on which logging

devices are run down the hole is owned by a specialist contractor but the wireline winch unit is permanently installed on the rig. The wireline company’s technicians operate the equipment during logging runs. OTHER VISITING PERSONNEL: Frequently when the rig is on location,

shore-based personnel will have to come out to the rig for short periods. Items of equipment may need repair, overhaul or inspection, some part of the rig may be use for a mandatory periodic survey, or well casing or some special tool may have tb be run in the hole. The temporary personnel may therefore include casing hands, government inspectors and surveyors, and service technicians, and accommodation must be made available for them. Certain shore-based employees of the rig operator may also appear onboard from time to time. The ‘rig superintendent’ oversees the day-to-day management of the rig from his office in ‘town’, and periodically he may ..,

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Divers descend to their working depth in a presswised bell. The netting in the background is beneath the rig’s moonpool.

want to inspect the rig to ensure that efficient onboard management is being carried out. The ‘marine superintendent’ is the rig operator’s marine department head and the company’s nautical advisor, and he may attend a rig-move from one location’ to another, when he might assist the bargemaster with anchor retrieval, the towing operation, and subsequent anchor running. The ‘base safety officer’ ensures that company and government safety regulations are being compiled with onboard the units in the contractor’s fleet, and periodically makes an offshore inspection. He keeps accident statistics and often issues safety bulletins.

Rig Personnel & Tr&@ THE OFFSHORE INSTALLATION MANAGER (OIM): l.,h&r Section 4; of

the Mineral Workings (Offshore Installations) Act 1974, a British rig must have an Offshore Installation Manager. The OIM is legally responsible to the government’s energy minister for the safe operation of the installation, and for the safety, health and welfare of all personnel on or working in connection with it during offshore operations. He is also charged with ensuring that all operations and the use of equipment are undertaken by, or under the supervision of, a suitably qualified person. It is left to the installation’s operator to decide which person should be appointed to this important position, and large fixed platforms will normally have a person onboard whose duties are solely those of the OIM. On a mobile rig, however, either the senior toolpusher or the senior member of the marine department (the bargemaster) will normally be OIM. In the case where the senior toolpusher~ is appointed as OIM, there will usually be provisions for the transfer of OIM’s powers and responsibilities to the bargemaster in two sets of circumstances. One is when drilling operations have been completed and anchors can be pulled - in other words when the rig is temporarily to become a ‘ship’; the other is when an emergency situation arises that endangers the seaworthiness of the rig. In this case the bargemaster formally advises the OIM that he considers drilling should be stopped, and transfer of responsibility is made, with an entry in the official log books, for the duration of the emergency. Rigs drilling on many overseas locations do not require installation managers unless stipulated by local regulations. In many drilling areas, local labour is hired to perform drill crew jobs up to the level of derrickman, and the entire catering and roustabout squad may be recruited in the host country* HELICOPTER LANDING OFFICER @LO): An HLO is required on all

British offshore drilling rigs under government regulations to take responsibility for the control of all helicopter operations affecting the rig. These include refuelling, keeping the landing area clear, controlling the movement of personnel on the helideck, manning fire-fighting appliances and, usually, acting as despatcher for freight. The HLO may be the bargemaster, the deck foreman or some other responsible person appointed by the OIM. JACK-UP RIG AND DRILL SHIP PERSONNEL

The crew of a jack-up is not usually very different to that of a semi-submersible. Instead of a bargemaster, who is required on self-propelled rigs, there

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may be a ‘barge operator’ to attend to marine affairs, though he need not necessarily be a qualified mariner. The jacking machinery is normally operated by the chief mechanic. Most of the personnel on a semi-submersible, with the exception of the sub-sea engineer, will be found on a jack-up rig. Drill ships have a larger marine crew than semi-submersibles, and if the vessel is dynamically positioned she will have a team of DP operators who double as navigational mates. The ship’s master is in command, although the OIM may still be the senior toolpusher. RIG PERSONNEL TRAINING

The oil companies operating in the North Sea and in many other parts of the world usually insist that all personnel travelling out to their installations or the rigs in their hire have a minimum standard of certified safety training. Completion of an Offshore Survival Course and an Offshore Basic Fire Course are the usual basic requirements in the British sector of the North Sea, although certain personnel will have to be qualified in their own special fields. A driller, for example, needs a certificate to prove that he has attended a course in well pressure control, and marine crew have to possess certificates of competency to hold certain positions such as master, mate and AB. Training courses for offshore personnel are offered by many institutions and Britain such as nautical and technical colleges, but there are also organisations that cater specifically for the needs of the marine drilling industry. The Offshore Petroleum Industry Training Board has many years of experience in fire-fighting, drilling and oil production training, and operates a fire training centre at Montrose in eastern Scotland on behalf of 13 major offshore operating companies involved in the North Sea. Instruction is given by experienced professional firemen, and trainees on the basic fire course are put through four rigorous days of exhausting exercises which bring them into close contact with real fires of major proportions. The centre runs some thirty courses in all, ranging from the basic training course to more advanced tuition in, for example, fires involving hazardous substances and helic6pter fire-fighting, all the courses being developed in consultation with the offshore industry. Virtually every British and many foreign oilmen working in the North Sea have completed one or more fire courses at the Montrose centre. Also at Montrose is the OPITB’s Drilling and Production Technology Training Centre, where experienced oilmen teach raw recruits to the indus-

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Offshore personnel training at a special centre in Norway.

Rig Personnel & Training

try the basic rig floor skills, as well as running advanced courses in subjects like well pressure control for drillers and toolpushers. At Aberdeen the DPTI’C maintains an advanced drill floor simulator where trainees can gain experience in handling drilling problems such as deviated holes, stuck pipe and oil, gas or water kicks. At Montrose the Centre has a complete drilling rig erected over a 3000 foot training well, and a second 2000 foot well is used for wireline training operations. The Offshore Survival Centre run by Aberdeen’s Robert Gordon’s Institute of Technology commands equal status in the eyes of offshore workers with the Montrose facilities, and for most North Sea hands the memory of their survival and helicopter underwater escape course is long and vivid. The centre runs 1.5 different safety courses for the offshore oil industry, as well as for the Merchant Navy, the fishing fleet and aviation, and it has an enviable world-wide reputation. On the 4day offshore survival course, which encompasses helicopter escape training, students learn first how to launch, handle and survive in the types of enclosed lifeboats that all offshore installations are now equipped with. There are lectures on various aspects of survival including physiology, survival suits, resuscitation and first aid, and the trainees are then introduced to the difficulties of wearing a cumbersome survival suit in the water during a ‘wet drill’ in a special environmental tank. This is in preparation for the fourth and final day, which provides the severest test of students’ stomachs and nerves. Every student, including non-swimmers, has to undergo the helicopter escape training, and those who fail to follow the precise commands of the instructors the first time the mock-up helicopter fuselage is immersed in its tank have to repeat the exercise until they perform it correctly. The simulator is first immersed upright, but on the second run is capsized so that students have to swim out through their allotted exits. The final session of the course is an abandonment exercise in which students jump into the environmental tank which is transformed into a dark, rain-lashed and wind-swept cold ‘North Sea’ with the aid of ‘special effects’ including a powerful wind generator. A liferaft has to be righted, then boarded, and the trainees experience a little of the discomforts of these craft in realistic conditions. Although marine crew such as bargemasters and control room operators normally hold merchant navy master’s and mate’s certificates of competency

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before they are appointed to a drilling rig, they may have to undergo special training in the operation of dynamic positioning equipment or in semi-submersible stability, which is somewhat different in theory to monohull ship stability. This type of training is given at certain nautical colleges, although the DP training may be done at the system manufacturer’s plant in some cases. Another training course often attended by rig personnel is a first aid course run by the various ambulance associations. In addition there are numerous courses for individual crew members who need special training appropriate for their job, such as the Offshore Installation Manager’s course and the Helicopter Landing Officer’s course. The most serious risk facing almost any vessel at sea is that of fire, and this is particularly true aboard a drilling rig where routinely conducted operations can, if not controlled properly, pose a threat of fire from themselves. For this reason, onboard fire-fighting training is given great importance. Musters and drills are normally carried out weekly, sometimes during the night as well as in daytime. Similarly, everyone aboard a rig is expected to know how to operate the rig’s survival craft, and lifeboat drills are usually held together with the fire drills. There may be some instruction on first aid or resuscitation techniques given during either of these drills by the rig medic, and crews sometimes practice handling stretcher cases at the same time. Drill crews are frequently exercised in operating the BOP controls so that the closure of the preventers can always be effected in the fastest possible time should this become necessary.

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GLOSSARY OF MARINE DRILLING TERMS Few other industries have developed a terminology as colourful as that of the drilling industry. Marine drilling terms have largely evolved from the language of American oilmen with inputs from the nautical and engineering professions, and the ‘green hand’ or ‘boll weevil’ invariably finds many of the words used offshore strange to say the least. This glossary is by no means exhaustive, but contains many of the terms most frequently heard offshore. -AA.B.: Abbreviation for Able Seaman. A member of the marine crew aboard

drill ships and many semi-submersibles. He might be called a ‘maintenance roustabout’ in some companies. ABANDON A WELL, TO: To abandon operations on a well because it is

proved incapable of producing profitability, or because of obstacles that have arisen during its drilling. Before abandoning, as much casing as possible is normally retrieved and cement plugs are set to prevent leakage of fluids from one formation to another inside the well. Finally the well is plugged near the surface with cement, the wellhead removed and the sea bed left clear of debris and obstructions. ACCOMMODATION BLOCK: The quarters deckhouse on a rig. It also

provides office space as well as storage, catering and recreational areas, which often include a cinema and gymnasium. ACCUMULATORS: Large steel bottles used to store hydraulic fluid under

high pressure from compressed nitrogen or air, for hydraulically operating the blowout preventer in an emergency. Other types hold compressed air. ACIDIZING: A well stimulation process in which a mixture of water and

hydrochloric or other acid is applied to the wall of the hole to dissolve material obstructing the flow of hydrocarbons into it from the producing formation. ACOUSTIC TRANSPONDER: An underwater electronic device which

emits sound waves so that its position can be determined from receiving equipment on the surface. Transponders are attached to the marine riser and BOP stack so that their positions in relation to the drilling unit can be monitored from the surface. They are also used in dynamic positioning systems for the DP vessel to reference on and in guidelineless drilling for guiding subsea tools to their sea-bed positions. Sub-sea BOP stacks often have emergency acoustic controls in addition to hydraulic controls.

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Glossary of Marine Drilling terms ACTIVE PIT: The mud pit in which mud that is mixed, treated, conditioned

and ready for use is stored. Mud returning from the shale shaker is passed into this pit after being cleaned. A-FRAME: The peak of a drilling derrick’s structure, above the crown

block. It is shaped like the letter ‘A’. AGITATOR: A paddle suspended in a mud pit by a shaft from an overhead

electric motor. It is rotated to stir the mud and prevent it from settling out. AIR DRILLING: A drilling method that employs compressed air as the dril-

ling fluid instead of a liquid. AIR GAP: The height above the sea surface of the underside of a drilling rig

or production platform. AIR TUGGER: A small, pneumatically-driven winch used for general pur-

pose hoisting. There are tuggers on the drill floor, in the cellar deck by the moonpool, near the towing bridles and in several other places around a rig. ANALYSIS, CORE: See core analysis. ANALYSIS, MUD: See ,mud analysis. ANCHOR: A heavy hooked appliance used with chain cable, or sometimes

wire or a combination of chain and wire, to secure semi-submersibles, drilling barges and some drillships to the sea-bed. A ‘spread’ or pattern of eight or ten anchors is usually laid symmetrically from the rig with the assistance of anchor-handling vessels. Jack-ups are equipped with anchors like most other seagoing vessels, but are not moored on location with them. Also a device for securing the deadline to the drill floor. See ‘deadline anchor’. ANCHOR BUOYS: Floating buoys attached to a rig’s anchors by pennants,

used to mark the positions where the anchors were laid for identification and , to facilitate rapid anchor retrieval. ANCHOR CHAIN: Heavy linked chain connecting a vessel to its anchors. A

typical semi-submersible might have eight anchors, each fitted with 4.500 feet of 3 inch chain cable, the inboard ends of which are secured in chain lockers inthe rig’s vertical columns. The diameter refers not to the width across the links but to the width of the steel bar forming the links.

Glossary of Marine Drilling terms ANCHOR CHAIN TENSION: The amount of strain an anchor chain regis-

ters, measured in tonnes, tons or kips (1 kip = 100,000 lbs). Chains are liable to over-stress in bad weather, strong tidal streams or fast currents, and are monitored regularly by rig watchkeeping personnel. ANCHOR DRESSING: The operation of attaching back-up appliances, such

as a piggy-back, to an anchor to increase its holding efficiency. ANCHOR-HANDLING VESSEL: A ship specially designed for laying and

retrieving rig moorings, equipped with large winches and special equipment for connecting and disconnecting heavy anchors and chains. The engines of the most modem AHVs produce more than 14,000 bhp. Anchor-handlers often double as supply vessel and/or tugs. ANCHOR PATTERN: The geometrical spread of anchors laid from a mobile

drilling unit. Any of several different patterns may be selected to suit conditions at the location or the type of unit. ANCHOR PILE: A long pile set into the sea bed for the purpose of tethering

a floating drilling unit. Piles are sometimes,used in situations where conventional anchors can not be used. ANCHOR-SETTING: The laying of anchors from a mobile drilling unit.

This is a complex procedure which may take many hours or even days. ANNULAR PREVENTER: A device in a BOP stack that can seal around

irregular-shaped objects, such as drill pipe, that pass through it. It is activated by hydraulic compression of a reinforced rubber or rubber-like packing element, ANNULUS or ANNULAR SPACE: The space surrounding any tubular sus-

pended in the hole. During drilling the circulation lfuid flows up the annulus between the drillpipe and the wall of the hole, or, when the well is cased, between the drillpipe and the casing. During a casing cement job a cement slurry is pumped from the bottom of the casing up the annulus between the casing and the wall of the wellbore. ANTICLINE: A type of hydrocarbon reservoir in which impkrmeable, fine

grained formation rocks have folded upwards to form a convex shape over porous, permeable reservoir rock. This is the most common type of oil or gas trap. A.P.I.: The American Petroleum Institute, the organisation responsible for

setting the majority of the standards used worldwide in the drilling industry.

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GLOSSY of Marine Drttttng terms APPRAISAL WELL: A well drilled following the drilling of a discovery

well, to determine the extent of the oil or gas field. ASSISTANT DRILLER: The member of the drill crew ranking below driller

but above derrickman, whose duties are to understudy the driller and relieve him at the drilling controls in the doghouse when required, and to supervise jobs around the rig on behalf of the driller. Abbreviated A/D. AUTOMATIC CATHEAD: The drawworks mechanism to which the make-

up chain and break-out line are attached.

-BBACK-LOAD: Cargo sent in to a supply base from a rig. It may include rub-

bish skips, empty containers, surplus casing, damaged equipment, etc. BACK-LOADING: The operation of loading a supply boat from a rig for its

return journey to the supply base. This is done by crane and, if bulk cargo is involved, with bulk hoses. BACK-OFF, TO: To unscrew one threaded joint of pipe from another. In

fishing operations explosive charges are sometimes set off against stuck tubulars to loosen the joint threads so that the upper joint can be backed off. BACK-UP: To hold one joint of pipe firm while another is being screwed

into or out of it. BACK-UP POST: The Sampson post to which the snub line of the back-up

tong is attached. BACK-UP TONG: A mechanical wrench suspended from the derrick by a

wire and used to hold the box, or lower, end of a joint of tubular firm while the pin, or upper, end is unscrewed from it or screwed into it. When making a connection the break-out tong backs-up the pipe, and when breaking a connection the make-up tong holds it firm. BAG PREVENTER: An alternative name for the annular blow-out pre-

venter, q.v. BAIL: A thick steel loop-shaped handle fitted to the top of the swivel for

attaching it to the hook. Bails are also found on the end of elevator links.

Glossary of Marine Drilling terms BALLAST: Seawater loaded into special tanks to add weight to a vessel for

stability purposes. Semi-submersibles are ballasted down to the draught that gives optimum stability compatible with safe wave clearance during drilling, and are de-ballasted during transit from one location to another to reduce weight and the water resistance of the hull. A large ‘semi’ might have capacity for more than 12,000 tons of sea water ballast. BALLASTING: The operation of adding ballast to a drilling unit to increase

its displacement and lower it in the water. The opposite of ballasting is deballasting, q.v. BALLAST PUMPS: Powerful pumps used for pumping ballast into, out of or

between ballast tanks for stability purposes. They are usually remotely operated from the control room of a semi-submersible. Their capacity may be as much as 600 tons per hour. BALL-JOINT: A flexible, pressure-tight device installed at the lower end of

a marine riser, just above its connection with the BOP stack, to allow a limited amount of horizontal movement of the drilling unit. Sometimes called the ‘marine riser flex joint’, or simply the ‘flex joint’. In some riser systems a second ball joint is used at the top of the riser. BARGE: A term sometimes used to describe a mobile offshore drilling unit

of any type. More properly applied to a flat bottomed non-self-propelled vessel. BARGE CAPTAIN: The American name sometimes used for a bargemas-

ter. BARGE ENGINEER: The crewman in charge, or alternatively second-in-

command, of marine operations on some drilling units. In units where a bargemaster is required, the barge engineer may be the bargemaster’s deputy. The name is something of a misnomer as the barge engineer is usually a deck department seaman. BARGE OPERATOR: The crewman in charge of marine operations on a

jack-up rig. He is sometimes the only trained seaman in the crew of that typ’e of rig. BARGEMASTER: The mariner in charge of the marine department on a

semi-submersible drilling rig and the legal master of the vessel. British-flag rigs must have a qualified master mariner as master. On some foreign rigs the bargemaster, or ‘barge captain’ or ‘barge engineer’ as he may be known,

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is sometimes a member of some other discipline (e.g. the drill crew), who has gained a restricted licence to hold this position. BARGE RIG: A drilling rig on a non-self-propelled barge. Commonly used

in shallow water regions such as the US Gulf, West Africa and Lake Maracaibo. BARITE, BARYTE or BARYTES: Barium sulphate, a mineral used to

increase the weight of drilling mud. Its specific gravity is approximately 4.2, i.e. barite is 4.2 times as heavy as fresh water. Quantities of barite are transported to rigs by supply boats in bulk powder form and stored in special ‘P’ tanks until required for mixing with water or oil and other additives to make mud. BARREL: A unit of volume measurement widely used in the petroleum

industry. 1 barrel (1 bbl) = 42 US gallons = 35 Imperial gallons approx. = 159 litres approx. 1 cubic metre = 6.2897 barrels. BEAM: The travsverse width of a vessel. BENTONITE: A type of grey clay that swells when wet and which is a major

component of drilling muds, providing the ability to hold solids in suspension. Like barite it is stored aboard rigs in fine bulk powder form in special ‘P’ tanks. Often called ‘gel’ because of its gelling property. BENT SUB: A bent tubular tool used in directional drilling that allows the

drill string to bend so as to build up the desired angle. B.H.A.: Abbreviation for bottom hole assembly, q.v. BILGE PUMPS: Pumps for expelling accumulated water from the bilge

spaces of rigs and other vessels. Usually remotely operated from the rig’s ballast control room. BILGES: Void spaces under machinery, etc. where waste water and oil can

accumtilate. BIT: The cutting device used to drill a well. Rotary bits are attached to teh

bottom of a drill string which is rotated mechanically. They have nozzles through which the circulating fluid is expelled at high velocity. There are many types of bit for different geological structures, each type being manufactured in a range of diameters. -..,

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Glossary of Marine Drilling terms BIT BALLING: The clogging of a bit’s component parts with drilled material. Balling sometimes happens when drilling soft clay, and it may be prevented by foam additives in the drilling fluid. BIT BREAKER: A heavy plate which fits in the rotary table opening and holds the drill bit firmly to enable it to be unscrewed from the bottom hole assembly. Bit breakers are made to fit individual types of bit. BIT SUB: A drilling tubular that has both ends internally, or female, threaded, so that the pin, or male, tool joints of both the bit and the drill collar above the bit can be fitted to it. BIT WEIGHT: The downward force applied to a bit when drilling. Abbreviated WOB (for ‘weight on bit’). BLIND RAMS: Rams in the blowout preventer whose ends fit tightly together in order to seal off the space below, thus preventing the passage of fluids upwards from the hole. Sometimes combined blind/shear rams are fitted. (See ‘shear rams’). BLOCK (CONCESSION): A sea area lessed off or licenced for drilling. The continental shelves of many countries are divided into regular-shaped blocks, the exploration and production rights in which are bid for by competing oil companies. In the North Sea a block is defined for numbering purposes as a unit ten minutes of latitude by twelve minutes of longitude. BLOCK GUIDE: A vertical steel trackway inside the derrick up and down which the travelling block moves during hoisting or lowering. BLOCK (MECHANICAL): An assembly of rope sheaves fixed in a frame. There are two main blocks in a drilling rig hoist: the crown block is fixed at the top of the derrick, and the travelling block, to which the hook is connected, is suspended from the crown block by the drilling line. ,

BLOOEY LINE: A gas diverter pipe leading overside that is fitted to a drilling unit when air drilling is in progress. A pilot flame is kept burning at its end to ignite and thus dispose of any gas encountered. BLOW-OUT: A sudden, violent and uncontrolled flow of oil, gas, drilling fluid or water from a well, occurring when the formation pressure exceeds the hydrostatic pressure of the column of drilling fluid.

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Glossary of Marine DriIhg terms BLOW-OUT PREVENTER: A device to control formation pressures in a

well by sealing the annulus around the drill pipe when pipe is suspended in the hole, or alternatively by sealing across the entire hole if no pipe is in it. Another type can shear the drill pipe passing through the preventer. Different types of preventer are assembled in a blow-out preventer (BOP) stack, q*v. BLOW-OUT PREVENTER DRILL: A training exercise regularly given to a

drill crew in the operation of well-control equipment and techniques. BLOW-OUT PREVENTER STACK: A series of blow-out preventers

arranged in a vertical tier and installed above the wellhead to control downhole pressures. Usually referred to on the rig as ‘the BOP’, or simply as ‘the stack’. The weight in air of the BOP stack may be nearly two hundred tons a&its cost may be of the order of several million dollars. The BOP stacks of floaters are placed on the sea bed, while those of jack-ups and fixed platforms are installed on the rig or platform. BOLLARD PULL: A measure of the towing or dragging capability of a ship,

especially a tug, anchor-handler or supply boat. Purpose-built anchor-handlers,might have a bollard pull of 150 tons. BOLL WEEVIL: An American term for an inexperienced oilfield hand, the

equivalent British term being a ‘green hand’. Sometimes also called a ‘bronc’. BOLSTER: A protective steel frame fitted round a. column of a semi-sub-

mersible just below a fairleader so that an anchor can be racked or ‘bolstered’~without damaging the column. BOND, or BONDED STORES: Duty-free goods such as cigarettes and

_~ tobacco~which are obtainable on some rigs when customs regulations permit. Offshore installations are generally ‘dry’, i.e. liquor is not permitted aboard them. B.O.l’,~: Abbreviation for a blow-out preventer, or the BOP stack, q.v. BOTTOM HOLE: The deepest part of a well being drilled. BOTTOM HOLE ASSEMBLY: The assembly of heavy drilling tools made

up in the lower part of the drill string to put weight on the bit and keep the drill ~pipe above in tension. In addition to the bit, the ‘BHA’ normally

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Glossary of Marine Drilling terms includes drill collars, stabilizers, reamers, heavy weight drill pipe and assorted other tools. BOTTOM HOLE PRESSURE: The pressure in a well at a point adjacent to the producing formation, as recorded on surface instruments. BOTTOM HOLE TEMPERATURE: The temperature in the hole at the bit, as recorded on surface instruments. BOTTOMS UP: The point in the circulation process when the drilling fluid has reached the shale shaker on its return from the bottom of the hole, announced by the ‘shaker hand’ as ‘returns at the shaker’. BOWL: The ring inside the rotary master bushing in which the slips are set. BOX: The internally-threaded ‘female’ end of a joint of drill pipe, into which the ‘male’ end (the ‘pin’) is inserted and torqued-up when making a connection. BOX GIRDER: A structural strengthening member of some types of rig, comprising a hollow box-section framework built around the perimeter of the main deck. It contains working spaces and storerooms. BRACES: Tubular girders acting like struts to support the main structural components of a semi-submersible rig. They may run either horizontally or diagonally. Athwartships-running braces may be called ‘cross-braces’. BRAKE: A device in the drawworks controlling movement of the hoist and its load. There are normally several types of brake fitted to a drawworks, including band, hydraulic and electric or electro-magnetic brakes. BREAKING-DOWN: The operation of unscrewing a string of drill pipe joint by joint and laying the joints down singly on the pipe rack, either when the well is completed or prior to running casing. Handling single joints of 31-foot pipe is easier and safer than handling ‘stands’ of three joints as is normally done when actually drilling. , BREAKING-OUT: The operation of unscrewing one stand of drill pipe from another when pulling out of the hole, using the break-out tong. The opposite of making-up. BREAK-OUT CATHEAD: The cathead on the furthest side of the drawworks from the doghouse.

Glossary of Marine Drilling, terms BREAK-OUT TONG: A large mechanical steel wrench suspended by a wire

from the derrick and used to loosen the ‘male’ (‘pin’) end of a’ joint of pipe from the ‘female’ (‘box’) end, when breaking a connection, and for backing up the lower joint when making a connection. Its arm is connected to the drawworks by a wire break-out line, and it is also tethered to a Sampson post. It is used in conjunction with the make-up tong. BRIDGE (DOWNHOLE): An obstruction in the hole usually caused by the

wall of the hole caving in or by the entry of a large boulder form the wall. BRIDGE (BETWEEN RIG & PLATFORM): A gangway between a plat-

form and a rig or other vessel moored close to it. Accommodation rigs are sometimes moored near to platforms under construction and a bridge is erected for personnel to cross by. BRIDGE PLUG: A short cylindrical tool which can be set inside casing to

provide a pressure-tight seal to isolate a zone. Bridge plugs are set when squeeze cementing, fracturing, plugging and abandoning or when testing an upper or lower zone. Afterwards they can be retrieved, or drilled through. BRING IN A WELL: The operation of completing an oil or gas well and brin-

gin it on line for production purposes. BROACHING: The term used when formation fluids break through to the

surface around or away from the casing, making it impossible to shut in the well. BUCK UP, TO: The American term used for tightening pipe to its correct

torque (‘torquing-up’). BUG BLOWER: A large fan used to keep insects away from the drill floor

where these are a nuisance. Used more on land rigs than offshore rigs. BULK HOSE: A stout but flexible rubber hose passed from a rig to a supply

boat to enable fuel, water, dry bulk powder or other fluids to be loaded or back-loaded. . BULK MUD: Drilling mud which may be brought out to a rig in ready-mixed

liquid form, as opposed to being mixed onboard from dry powder and liquids. BULLET PERFORATOR: A perforating gun that is lowered into a well and

fires steel bullets through the casing or liner. This is done during well testing ..,

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Glossary of Marine Drilling terms in order to provide access to the well for fluids lying in formations behind the casing. BUMPER JAR: A tubular tool incorporated in a drill string that can be compressed or tensioned to actuate a jarring action when the string is stuck and has to be released. Bumper jars are often used in fishing operations. BUMPER SUB: A type of tubular motion compensator incorporated in the drill string of some floaters to eliminate bit movement caused by the vessel’s heave. In addition to providing a slip joint in the drill string it can be used as a bumper jar to free a stuck string. It is about 30 feet long, has a stroke of about 5 feet and is sometimes known as a ‘slack joint’. BUMPING THE PLUG: The operationof pumping cement down inside teasing between two plugs and landing the top plug above the bottom plug, thus ensuring that the maximum volume of cement has been expelled into the annulus. BUNKERS: Engine fuel oil stored in special tanks aboard a mobile drilling unit and other vessels. BUOY: A floating device deployed by an anchor vessel to mark the position of a rig’s anchors, or to indicate the position of a location when the rig is approaching it. BUOYANCY: The upthrust of water on a vessel’s hull, or the amount of weight loss owing to immersion in water.

-cCABLE: A unit of nautical measurement, equivalent to 600 feet, 100 fathoms, 200 yards, 183 metres or approximately one tenth of a nautical mile. CAISSON: A vertical column, or leg, supporting the main deck of a semisubmersible rig. ’ CALCIUM CARBONATE: A drilling fluid weighting agent with a specific gravity of about 2.80. CALIPER LOG: A well logging device used~ to measure the diameter of the hole.

GlossWy of Marine Drilling terms CAMERA, TV: A remote-controlled television camera run down to a wellhead on guidelines to monitor sea-bed operations from a rig. CANTILEVER-TYPE JACK-UP: A jack-up drilling unit in which the dr& ling package can be moved on beams to a position overhanging the water at its side. CAP ROCK: Impermeable rock overlying an oil or gas reservoir that tends to prevent migration of fluids from the reservoir. CASED HOLE: A drilled hole in which steel casing has been set. CASING: Steel pipe set in the hole as drilling progresses to line its wall, preventing caving-in and to provide a passage to the surface for drilling fluid and for hydrocarbons if the well is proved productive. CASING CUTTER: A milling tool used to detach strings of casing from the hole when plugging and abandoning a well or when ‘patching’ damaged casing. Strings of casing that can not be cut are sometimes blasted free with explosives . CASING ELEVATOR: A handling device that is attached to a lugged collar around the end of a joint of casing when running it in the hole. The elevator is suspended from the hook/travelling block assembly by links. CASING HANDS: A team of specialists who are contracted to run well casing. They are not permanent members of a rig’s complement. CASING HANGERS: Devices inside a wellhead on which the top ends of the . different strings of casing are suspended. The widest casing fits on the lowest hangar. CASING PATCH: A device that can be lowered into the hole to repair dmaaged casing or to join two detached strings of casing. CASING PROGRAMME: The well operator’s plan for running each string . of casing to be used in the well. CASING PROTECTOR: A short threaded rubber or plastic ring fitted on the ends of joints of casing during transport and storage to protect the threads from damage and dirt. They are removed when teh casing is run. CASING SCRAPER: A downhole tool used to clean and smooth the inside of casing. ..

Glossary of Marine Drilling terms CASING SHOE: An alternative name for a guide shoe or float shoe, q.v. CASING STRING: The total length of casing run into a well. CASING TONG: A special power tong used for making up and breaking out joints of casing. CATCHING SAMPLES: The action of obtaining samples of the drilling fluid for analysis as it emerges from the hole through the flowline. CATENARY: The natural curve adopted by an anchor chain deployed from a rig to the seti bed. If there is insufficient catenary the anchor is liable to break out of the holding ground. Much of the anchor chain or wire’s length is therefore laid along the sea bed before it curves up to the rig. CATHEAD: A small spool-shaped drum on the side of the drawworks, around which one end of a catline is wound during general purpose lifting operations on the drill floor of some rigs. On modern floaters which are fitted with air tuggers the ‘automatic catheads’ fitted on the shafting near the catheads are used for pulling the make-up and break-out tong lines. CATLINE: A general purpose fibre rope which is reeved through a sheave in a high part of the derrick of some rigs, used for lifting heavy items such as drill pipe in and about the derrick. Its hauling part is taken to a cathead. It is usually replaced by air tuggers on modern offshore rigs. CATWALK: A narrow ramp leading from the pipe racks to the base of the V-door on the drill floor. Tubulars ready for use are laid here by the roustabouts and crane operator. Also a narrow walkway between a barge and a platform, or between two parts of a vessel. CAVE-IN: A collapse of the wall of the hole. A major cave-in, or ‘crater’, can occur during a blow-out. A minor cave-in is termed ‘sloughing’ (pronounced ‘sluffing’). CELLAR DECK: An enclosed area on the main deck below the drill floor. On a semi-submersible the cellar deck provides a sheltered working area around the moonpool, and the blowout preventer stack is housed there when not positioned at the wellhead. On a jack-up unit the BOP stack and wellhead are permanently housed in the cellar deck. Sometimes termed mezzanine deck. CEMENT: Portland cement in which casing is set after running. It is normally stored in powder form aboard a rig in silos called ‘P’ tanks.

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Glossary of Marine Drilling terms CEMENT ADDITIVES: Materials mixed with cement during a cement j,&

to alter its properties. Chemical accelerators and retarders and reducing solid materials are common additives.

weight

CEMENTER: The contracted hand who operates and maintains the cement

pumping equipment. CEMENT HEAD: A cap fitted on the upper end of a casing string or on the

top of a string of drill pipe on the drill floor that allows the introduction of cement under pressure during a ‘cement job’. CEMENT JOB: The operation of pumping cement down the hole to set cas-

ing inplace. A remedial cement job to fill up holes is called a ‘squeeze job’. CENTRALIZER: A device with bowed strips of metal running vertically

between two collars, fitted round a joint of casing to contact the wall of the hole and keep the casing centralized. This allows more even distribution of cement. CENTRE OF BUOYANCY: The point through which the resultant of the

forces of buoyancy on a floating vessel is said to act vertically upwards. It lies at the geometrical centre of the underwater shape. CENTRE OF GRAVITY: The point through which the resultant of the

forces of gravity on a rig (or any other body) is said to act vertically downwards, this force being equal to teh weight of the rig or body. CENTRIFUGE: A machine incorporated in most drilling fluid circulation

systems to remove fine solid particles from the mud after it has passed through the desander, desilter and mud cleaner. Sometimes called a barite recovery centrifuge, since this is what it does in effect. CHAIN LOCKER: A deep compartment aboard a rig or drill ship in which

anchor chain is housed when not deployed on the sea bed. There is a locker ofr each of the anchors, and usually they are in the corner columns of a semisubmersible or at the bow and stern of a drill ship. l

CHANGING RAMS: Rams in blowout preventers have to be changed when

drill pipe is going to be used that is of a diameter different to that previously used. On a semi-submersible or drillship this necessitates the stack being brought to the surface for the modification to be made by the sub-sea engineer.

Glossary of Marine Drilling terms CHARTER: The document of hire of a ship such as a supply boat, anchor

handler, standby vessel, etc. Charters may be for a single voyage or for a specified length of time. The equivalent document for the hire of a rig is a drilling contract. CHARTERER: The person or company who hires a vessel. The oil company

which is the well operator is usually the charterer of vessels used during the well programme. CHASER SYSTEM: A system involving a chasing collar and chaser pen-

nants which is commonly used by anchor handling vessels to assist in the retrieval of a rig’s anchors. See ‘chasing collar’. CHASING COLLAR: A large steel ring attached to a chaser pennant and

through which a rig’s anchor chain runs. During anchor setting operations the collar is manipulated by an anchor handling vessel to stretch out the chain. When retrieving anchors the anchor is ‘chased out’ from the rig by the boat working the collar along the chain until it eventually snags on the anchor, which can then be lifted. CHOKE: A device with a fixed or variable aperture installed in a flowline for

releasing the flow of well fluids under controlled pressure. CHOKE AND KILL MANIFOLD: A large assembly of pipes, valves and

chokes installed on the drill floor for controlling downhole formation pressures in emergencies such as kicks and blowouts. CHOKE LINE: A line connected to the BOP stack, used to direct and con-

trol the flow of well fluids from the annulus. On a semi-sub or drill ship the choke line runs up the marine riser to the choke and kill manifold on the drill floor. CHRISTMAS TREE: A high pressure assembly of valves, pipes and fittings

installed on a wellhead after completion of drilling to control the flow of oil and gas from the casing. CIRCULATING BOTTOMS UP: The oderation of pumping drilling fluid from the mud pits through the drill string to the bottom of the hole and back through the annulus to the rig while drilling is temporarily suspended. This is done to clear cuttings from the bottom of the hole. CIRCULATION: The passage of drilling fluid from the suction pit through

the mud pumps, stand pipe, rotary hose and swivel, into the drill string and

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Glossary of Marine Drihg terms

down to the bottom of the well where it emerges in jets through the bit. From there it returns to the pits via the annulus and mud cleaning equipment. CLASSIFICATION SOCIETY: An independent body whose purpose is to

assess vessels for the adequacy of their strength and seaworthiness. The British classification society is Lloyd’s Register, and its American equivalent is the American Bureau of Shipping. Most major maritime nations have their own societies, but ships are not necessarily classed by their own flag state’s society. COLLAR, DRILL: See drill collar. COLUMN: A vertical caisson or leg supporting a drilling platform. Semi-

submersibles may have four or more columns, usually cylindrical and containing storage, ballast, reserve buoyancy and working spaces. COLUMN-STABILIZED SEMI-SUBMERSIBLE: A semi-submersible ves-

sel the stability of which is governed by the level at which its columns are immersed in water, Another type of semi-submersible is a heavy lift vessel which looks like a conventional ship but which can submerge its mid-body. COME OUT OF THE HOLE, TO: To pull the drill pipe out of the drilled

hole. This is necessary when the bit needs changing, or before coring, logging, running casing, etc. It can take a drill crew several hours, and is.known as ‘tripping’. COMPANY MAN: The name generally used for the operator’s representa-

tive onboard a rig whose job is to supervise the well programme. The company man, in consultation with his colleagues in ‘town’, directs all downhole and associated operations and is on call 24 hours a day. He is often a selfemployed consultant. Most company men work a one-on, one-off system like the rig crew.

h ewe

COMPENSATOR, MOTION: See &x&ion compensator. COMPLETE A WELL, TO: To install in a well, having finished drilling ind

testing, downhole equipment such as a liner, tubing and packers, ready for production. COMPRESSOR: Machinery for compressing fluids. Rigs employ large vol-

umes of compressed air for machinery control purposes and the compressors are in constant use.

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Glossary of Marine Drilling terms CONDITIONING MUD: The treatment of drilling mud with special additives to give it certain desirable properties or to restore it to its full efficiency. Mud is often conditioned by the mud engineer and derrickman while circulating through the mud system. CONDIJCTOR CASING: A short string of large-diameter casing which is run by a floater to keep the top of the hole open and prevent sloughing, to serve as a base for wellhead equipment and to convey the drilling fluid up to the marine riser. Deep well casing programmes may call for two conductor casing strings: an outer, 30” conductor and an inner, 20” conductor. CONDUCTOR PIPE: A string of large-diameter pipe installed between the cellar deck of a bottom-supported drilling unit, such as a jack-up, and the hole. It serves as a conduit for drilling tools and the returning drilling fluid, and as a base for the BOP stack which is installed above it. CONNECTION, MAKING A: The operation of joining a new length of drill pipe to the drill string as drilling progresses, performed by the drill crew. CONTINENTAL SHELF: The part of the seabed skirting a continent or island that is’ an extension of the land mass and is relatively shallow compared with the sea bed beyond. Most maritime nations regulate the oil and gas activity on their continental shelves. CONTRACT DEPTH: The depth of hole which a drilling contractor has agreed to drill. CONTROL PANEL: The driller’s console in the dog house from where drilling machinery is controlled. CONTROL PANELS, BOP: Systems of controls that can be operated manually or remotely to direct hydraulic fluid to the BOPs. A valve panel is normally at the hydraulic power source, and other panels of controls are placed at convenient remote locations for actuating the valves at the primary control panel. There is usually a panel in the doghouse, in the toolpusher’s office and on deck near one of the lifeboats. , CONTROL POD: An underwater housing containing actuators and valves for the BOP hydraulics, fitted to the BOP stack and connected to the rig by a thick rubber hose enclosing numerous control lines. CONTROL ROOM: The compartment aboard a rig from where various vital functions are controlled, including ballasting and stability, power plant

300

Glossary of Marine Drilling terms

operation and fire and gas alarm monitoring. CONTROL ROOM OPERATOR: The watch-keeping crewman on a semi-

submersible who monitors and controls stability and ballast, fire and gas alarms, anchor tensions, engines, etc. Usually the CR0 is a certified merchant marine deck officer. CORE: A sample drilled out of the bottom of the well in a solid cylindrical

block and retrieved for examination and analysis. CORE ANALYSIS: An examination of a core to determine various proper-

ties of the formation from which it came, including porosity, permeability, fluid content, geological age, lithology and likely productivity. Cores obtained from rigs are sent ashore to core laboraties. CORE BARREL: A cylindrical barrel, made up in 30-foot lengths to a total

of 60,90 or 120 feet and fitted to a special coring bit, that is run at the bottom of the drill string in place of the normal bottom hole assembly in order to remove a core. CORING: The removal of a core by rotating the core barrel and coring bit at the bottom of the hole, or, in the case of sidewall coring, by the retrieval of several small barrels fired into the adjacent formation by a gun. CORING BIT: A bit with a wide central aperture surrounded by a ring of

cutting elements set into a matrix. As the bit is rotated, the cut core passes up through the middle of the bit into the core barrel above. Coring bits often employ diamond cutting elements. CORROSION CAP: A cap placed on a suspended well which may be reentered in the future. CRANES: Rigs employ long-jibbed pedestal cranes for general purpose

.

hoisting. Supplies such as drill pipe and containers have to be lifted off supply boats, casing has to be lifted from pipe racks to the drill floor and anchorhandling equipment is passed to boats by the cranes. They are in daily use on the rig at all times of the day and night. CRATER, TO: To cave in. In a violent blowout the surface around a well can

fall into a large hole blown in the sea bed by the force of escaping mud, gas, oil and water. This can cause a bottom-supported drilling unit to collapse and sink.

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Glossary of Marine Drilling terms CREW-BOAT: A small, fast vessel used in some smooth-water drilling locations such as the Arabian and US Gulfs to transport offshore rig crews from the shore. CREW-CHANGE: The relief of one crew by another after a period of offshore duty. Most rig crews in the North Sea work fourteen days offshore before being relieved, while on overseas locations 28 days is the norm. CROSS-BRACES: Large-diameter support pipes or struts running across the underside of some types of semi-submersible, bracing the columns apart. CROSS-OVER SUB: A tubular tool with box and pin threads of different diameters so that it can be used to provide the connection between two strings of different gauge pipe. CROWN BLOCK: The large block fixed at the top of the derrick from which the travelling block and hook are suspended by means of the drilling line. CUTTINGS: Fragments of rock which are chewed out of the formation by the bit and brought up to surface by the drilling fluid.

-DDAILY DRILLING REPORT: An IADC standard format log of daily drilling operations completed by the drillers at the end of their daily tour. DAY RATE: The agreed daily hire payment made by the well operator to the drilling contractor. An alternative type of payment is by footage drilled, but this is not commonly used offshore. DEADLINE: The fixed, or ‘standing’ part of the drilling line opposite the fastline. It runs from the crown block to a point on or near the drill floor deck, where it is held in tension by the deadline anchor. DEADLINE ANCHOR: A device fixed to the structure of the rig on or &ar the drill floor around which the deadline is wound and secured. DE-BALLASTING: The operation of pumping out seawater ballast from the ballast tanks of a vessel. Semi-submersibles are de-ballasted from drilling draft to transit draft before a rig-move, and from drilling draft to survival draft at the onset of very severe weather.

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Glossary of Marine Dri&ng, km DECK AREA: The area on a drilling platform available to carry

equipment

and stores. Some rigs have a much larger deck area than others and are therefore more attractive for certain operators’ purposes, e.g. for conversion to production units. DECK FOREMAN: The supervisor of a rig’s deck crew, commonly called,

the bosun. Usually a qualified able seaman. DECKLOAD CAPACITY: The weight of equipment and stores able to be

carried by a drilling rig. Some types are able to carry much more deckload than others, this being one of the advantages of a drill ship over a semi-submersible. DECOMPRESSION CHAMBER: A module in the diving spread in which a

diver can undergo decompression after a period ‘in saturation’. DEGASSER: A device for removing entrained gas from drilling fluid. It may

be a vacuum or may use fluid flow through an orifice to ~achieve separation of the gas. DERRICK: The tall girdered tower erected over the drill floor that supports

the hoist and the drill string and other tubulars that are run into the hole. Land rig derricks are usually portable, in which case they are called masts, whereas ‘derrick’ refers to the more permanent type of structure. A typical large semi-submersible’s derrick might be 160 feet high, with a 40-foot square base. DERRICKMAN: The drill crew hand whose work station is in the derrick

during tripping, when he physically manipulates the tope ends of pipe out of or into the finger board. He works from the monkeyboard wearing a safety harness. His other responsibilities include conditioning mud to the directions of the mud engineer, cleaning out the mud pits and maintaining the mud pumps. DESANDER: A device situated near the shale shaker for removing small sol-

ids from the returning drilling fluid. , DESILTER: A device for removing silt from the drilling fluid, situated near the desander. DEVELOPMENT DRILLING: A programme of drilling to exploit a field

that has been discovered by exploratory drilling. It is usually carried out from a fixed platform using some vertical and many deviated wells. ‘.”

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Glossary of Marine Drilling terms DEVELOPMENT WELL: A well drilled following the drilling of a discovery well to exploit an oil or gas reservoir. It is usually drilled from a fixed platform. DEVIATED HOLE: An alternative name for a direction hole, q.v. DEVIATION SURVEY: A survey carried out with downhole instruments to measure the angle and azimuth of deviation of the hole from the vertical. These surveys are usually either ‘magnetic surveys’ or, where steel casing has been set, ‘gyro surveys’. DIAMOND BIT: A drilling bit that has a matrix head into which small industrial diamonds are set. Special diamond bits are also used for coring. DIFFERENTIAL WALL STICKING: Sticking of the drill string to the wall of the hole when the permeability of the formation causes fluid filtration and a consequent low pressure zone which sucks the pipe into the side. DIRECTIONAL DRILLER: A specialist in the techniques of directional drilling who may be called out to a rig to advise the company man when the hole is deviated. DIRECTIONAL DRILLING: A drilling operation performed with special downhole tools to deviate the hole from the vertical, either where an obstruction has been encountered in the straight hole and has to be sidetracked, or where a number of wells spaced out in a field are drilled from one central location, which is usually a fixed platform. DIRECTIONAL HOLE: A well drilled at an angle from the vertical for one of a number of reasons. See ‘directional drilling’. DISCOVERY WELL: An exploratory well that produces evidence of oil or gas in commercial quantities. Wells that produce ‘shows’ of uncommercial hydrocarbons are termed ‘dry holes’ or ‘dusters’.

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DISPLACEMENT: The weight of water displaced by a vessel, i.e. the actual weight of the vessel in the water. Light displacement is the weight of the vessel as built but without any additional load, and load displacement is its weight when at its deepests permissible draft. DISPLACING THE HOLE: Forcing one drilling fluid down the hole to displace another, e.g. when changing the fluid from water base mud to oil base mud.

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Glossary of Marine Fhi&g terms DIVER: A technician trained to work underwater from an offshore m&alla-

tion. Drilling rigs sometimes carry a diving spread and a squad of divers who carry out repairs, maintenance and inspections on sub-sea equipment. Saturation divers. who work from a diving bell at great depths, are pressurised with a mixture of oxygen and helium and live in pressurised accommodation for periods of up to 15 days, at the end of which they are decompressed gradually in special decompression chambers. DIVERTER: A T-shaped pipe attached to the top of the marine riser that

closes the vertical passage and directs the flow of well fluids away from the rig floor and overside. DIVING BELL: A pressurised capsule in which divers can be transported to

their underwater working area. A bell is part of the diving spread carried by floaters when saturation diving is required by the operator. DIVING SPREAD: When divers are required onboard for the well prog-

ramme, their equipment, collectively termed the diving spread, is usually set up around a special small moonpool on the main deck of a rig. For saturation diving the spread generally includes a diving bell, a winch unit, a dive control cabin and chambers for living and for decompression, interconnected by narrow airtight passageways. DOG HOUSE: The small cabin in a corner of the drill floor in which the dril-

ler operates the drilling controls. DOPE: A zinc-based compound applied to the threads of drilling tubulars

before use to lubricate and protect them. DOUBLE: Two joints of drill pipe, casing or tubing connected together. DOUBLE BOTTOM: A false bottom in a vessel, under which fuel and water

may be stored in reserved compartments, or ballast water is held. DOWNHOLE DRILLING MOTOR: A tubular tool that converts the hyd-

raulic power in a stream of drilling fluid into rotary power, so as to turn a bit. Sometimes called a”mud motor’ or ‘turbo drill’. DOWNHOLE TOOLS: Any item of equipment for drilling, fishing, etc. that

can be run into the hole. DOWN-TIME: A period of operational delay during which the rig is techni-

cally off hire and not earning money from the operator. _

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Glossary of Marine Drilling terms DRAWWORKS: The large winch situated on the drill floor, which controls the movement of the hoist and around which the fastline part of the drilling line is wound. DRIFT: A device that can be run through the middle of tubular5 to check the unformity of their inside diameters. DRIFT INDICATOR: A downhole survey tool that measures the angle of inclination and the direction of travel of a bit. It may be known by various other names, such as ‘Totco’ or an inclinometer. DRILL BIT: See bit. DRILL COLLAR: A length of steel pipe much heavier than the drill pipe, several of which are placed at the bottom of the drill string just above the bit to add weight to the bit and stiffen the bottom hole assembly. Non-magnetic drill collars, sometimes called ‘monels’, are used when magnetic survey tools are to be run. DRILL CREW: The team comprising the driller, assistant driller, derrickman and floormen or roughnecks, who together perform the drilling operations on the drill floor and elsewhere on the rig. DRILLER: The supervisor of the drill crew who controls drilling operations from the dog house. DRILL FLOOR: The area beneath the derrick in the centre of which is the rotary table and from which drilling operations are conducted. Sometimes called the rig floor. DRILLING AHEAD: A period of normal drilling, when the bit is cutting into the formation. Also termed ‘making hole’. DRILLING BREAK: A sudden increase in the rate of penetration of the bit when it enters a zone of softer material in the formation. This may give advance warning of a kick. . DRILLING CONTRACT: The document of hire of a mobile offshore drilling unit, in which a drilling contractor agrees to drill a well for an operator under certain stipulated conditions. DRILLING CONTRACTOR: The operator of a drilling rig and the employer of its crew.

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Glossary of Marine Drilling terms DRILLING CAPACITY: The maximum depth for which a rig is designed

and equipped to drill. DRILLING DEPTH: The depth to which a drilling contractor has agreed to

drill a well for an operator. DRILLING DISPLACEMENT: The weight of water displaced by a floater at

its drilling draught, equal to the weight of the vessel and everything on it at that time. DRILLING DRAUGHT: The depth of water, measured from the keel

upwards to the waterline, at which a semi-submersible, drill ship or drilling barge floats in its drilling mode. For a large semi-submersible this may be 65 or 70 feet. DRILLING ENGINEER: A trained engineer who advises the company man

on technical aspects of the drilling operation. DRILLING FLUID: The fluid circulated down the well and back up to the rig

for a number of important purposes including the containment of formation pressure, the retrieval of cuttings, bit lubrication and cooling, plastering the wall of the well, providing ~a well data source. Usually referred to as ‘mud’, although air, gas and foam can also be used as a drilling fluid. DRILLING LINE: The stout wire rope reeved from the storage drum,

through the deadline anchor, up through the blocks of the hoist and down to the drawworks drum. DRILLING MUD: A liquid drilling fluid composed of colloidal clays mixed

with water or oil and a variety of chemical additives. Sea ‘drilling fluid’. DRILLING RIG: In offshore jargon, any vessel, her machinery and equip-

ment used for drilling a well. Strictly, the term should only apply to the drilling plant onboard, while the platform on which the rig stands should be referred to as the ‘barge’, ‘platform’, ‘unit’ or ‘vessel’. DRILL PIPE: Connected lengths of tubing, usually steel, on the end of

which the bottom hole assembly a’nd the drill bit are suspended. DRILL SHIP: A ship employed as a platform for a drilling rig. It may be pur-

pose-built or converted from another type of ship, and it may be moored with an anchor spread or dynamically positioned. DRILL STEM: The assembly of drill pipe that runs from the kelly to the top

end of the bottom hole assembly. .., 1A7

Glossary of Marine Drilling terms DRILL STEM TEST: A test, usually run over a period of several days, to determine whether commercial quantities of oil or gas are in the formations drilled through. Drilling may continue after the test period to explore deeper zones, or the well may be completed or plugged and abandoned, depending on the findings. DRILL STRING: The assembly of drill pipe and other tools that runs from the kelly down to the bit. DRILL WATER: Non-potable fresh water, usually used in large quantities for drilling operations, e.g. for mixing mud. Rigs have large holding tanks for storing drill water which is brought out by supply boat. DRY HOLE: A well in which no commercially significant evidence of hydrocarbons is found. DUSTER: A dry hole. DYNAMIC POSITIONING: An automatic station-keeping system for drilling and other vessels in which thrusters are activated on the commands of computers which monitor the vessel’s position in relation to fixed reference points on the sea bed or elsewhere.

-EEIGHT POINT MOORING SYSTEM: A mooring arrangement fitted to many semi-submersible rigs in which eight anchors are used. Two anchors can be deployed from each corner of the deck, each pair being winched by a common windlass. ELECTRIC WELL LOG: A recording by downhole wireline instruments of certain electrical characteristics of the formations drilled through. These are identified with their locations and depths and used for estimating the amount of well liquids they contain. ELECTRICAL WOtiK PERMIT: A written authority given by the OIM to an electrician to perform high voltage electrical repairs or maintenance. ELEVATOR: A latching device, attached to the hooMtravelling block assembly by two long links, which is placed around the end of a tubular joint when running into or pulling out of the hole. There are special elevators.for casing, drill pipe, drill collars and tubing.

Glossary of Marine Dri@ng b&Z EMERGENCY GENERATOR: An independent power plant housed sepa-

rately to the main power generation machinery that is set to start automatically on the failure of the main engines. Limited emergency power can be obtained from it. EXPLOITATION WELL: A name somtimes used for a development well,

q.v. EXPLORATION WELL: A well drilled to find a new reservoir of oil or gas.

It may be drilled in a totally new exploration area, in which case it is termed a ‘wildcat’ well, or it may be drilled to find a new producing formation in an existing field. -FFAIRLEADER or FAIRLEAD: A roller over which the direction of an

anchor chain or wire is changed. Semi-submersibles have fairleaders on their columns over which the anchor chains pass between the windlass and the anchor. FIELD: The area over an oil or gas reservoir on which drilling and associated

operations take place. There may be several separate reservoirs at different depths in the same field. Each field is given a name by its operator, e.g. the Forties field, the Brae field. FILLING THE HOLE: Pumping drilling fluid into the hole via the fill-up line

as drillpipe is pulled out, so that the fluid level in the hole is kep topped up. This is done to minimise the risk of blowout or caving. FILL-UP LINE: A line connected to the diverter at the top of the riser,

either via or below the flow line, to allow the pumping of drilling fluid into the hole while pulling the drill string from it. FILTER CAKE or FILTRATE CAKE: Mud solids deposited on the perme-

able wall of a hole by filtration of the drilling fluid. Also called mud cake. ‘> FINGER BOARD: A rack of ‘fingers’ insl;.lled approximately half way up the derrick into which the derrickman slots drill pipe and collars, etc. when tripping out of the hole. FIRE PUMP: A high-velocity sea water pump specially reserved for firefighting. -,_.

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Glossary of Marine Drilling terms FIRE DRILL: A crew fire-fighting exercise, routinely carried out at weekly or other intervals, often in conjunction with a boat drill. FISH: Any item that becomes accidentally lost or stuck down the hole and which must be retrieved before normal drilling can resume. FISHING: A downhole operation to retrieve a fish such as stuck pipe. FISHING TOOLS: Special devices for retrieving fishes from the hole. These include overshots, which are female-ended devices that fit over a fish, and spears, which are male-ended devices which penetrate the end. FLARE: A naked flame emitted from a flare boom burner during well-testing or when disposing of unusable gas from a completed well. FLARE BOOMS: Long booms attached to each side of a rig on the ends of which burners are fitted for burning well hydrocarbons emitted during welltesting. FLARING-OFF: The burning of any well hydrocarbons emitting from a flare boom. FLEX JOINT: An alternative name for a ball joint, q.v. Sometimes called the ‘riser flex joint’ or the ‘flexible riser joint’. FLOAT COLLAR: A cylindrical steel collar containing a non-return valve, inserted into the casing string one or two joints above the guide shoe or float shoe. It prevents drilling fluid from back-flowing upwards inside the casing when this is being run, thus allowing the casing to ‘float’ down and thereby decreasing the load on the hook. The non-return valve also allows the cement to be pumped down through the casing but prevents its return inside it. FLOAT SHOE: A short, heavy cylindrical steel section of casing with a rounded bottom and containing a non-return valve. It is attached to the bottom of the casing string and is used to ‘float’ the casing into the hole, reducing the strain on the rig. It functidns in the same way as a float collar (q.v.) but also guides the bottom of the easing in the same way that a guide shoe does. FLOORMAN: A member of the drill crew whose work station is on the drill floor and who performs most of the manual tasks in drill floor operations. Known usually as a ‘roughneck’ or, sometimes, as a ‘rotary helper’.

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Glossary of Marine, Drilling terms FLOW LINE: A line fitted to the diverter at the top of the marine riser

through which drilling fluid returning from the well flows to the shale shaker. Sometimes called the mud return line. FORMATION: A geological bed or deposit containing mostly the same min-

erals throughout. Each formation is known by a different identifying name in oilfields. FORMATION PRESSURE: The pressure in the hole exerted by formation

fluids when the well is shut in, recorded at the level of the formation. FORMATION TESTING: The testing of a formation to determine its poten-

tial productivity before installing production tubing at the bottom of the well. The usual methods of testing are a drill stem test and perforating. FOUNDATIONPILE: An alternative name for outer conductor casing, q.v.

-G’GAS: In a drilling context, the vapour state of well hydrocarbons. GAS ALARM: An alarm on the rig activated by gas. On a rig there might be

several gas alarms, e.g. for well gas, hydrogen sulphide (H2S), carbon dioxide (COz) and halon, the latter two being fire-extinguishing gases. GAS CAP: The free gas that lies above an oil reservoir in a formation. GAS-CUT MUD: Mud containing bubbles of formation gas, giving it a

characteristic fluffy texture. The gas is removed in the de-gasser. GEL: A gelatinous substance formed by some types of drilling fluid when

not circulating. This is desirable so that the cuttings can be held in suspension in the mud if circulation ceases for some reason. Bentonite is usually commonly known as ‘gel’ aboard a rig because of this property. GENERATOR: A machine coupled to a main engine fore generating electri-

cal energy. Each diesel engine on a rig is usually connected to an AC generator, and there is an independent emergency generator in ajddition. GEOLOGIST: The scientist often carried on a drilling unit whose ~job is to

obtain and interpret data concerning the geological strata drilled through. GEOLOGRAPH: A patented device which automatically records the rate of

penetration and depth during drilling operations.

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Glossary of Marine Drilling terms GERONIMO LINE: A speed-controlled escape line rigged from the monkey board to the drill floor on some rigs, down which the derrickman can slide in an emergency such as a blowout. GOING INTO THE HOLE: Running drilling equipment into the hole. GREEN HAND: A new member of a rig crew; a raw recruit to the offshore industry. Termed a ‘boll weevil’ or ‘weevil’ on American rigs. GUIDE BASE: A heavy steel frame placed on the sea bed to guide tools into the hole and serve as a foundation for other equipment such as the wellhead and the BOP stack. The ‘temporary guide base’ is run before the ‘permanent guide base’. GUIDE SHOE: A short, heavy, cylindrical steel section of casing which has a rounded bottom and is filled with concrete. It is fitted at the bottom of the casing string and prevents the casing from snagging on any projections in the hole as it is being run in. A hole through the shoe allows drilling fluid to pass upwards as it is being lowered and allows cement to pass out during the cementing operation. The concrete. is later drilled out. GUMBO: A sticky type of clay, sometimes encountered during drilling in certain areas, that tends to clog equipment. GUN PERFORATING: The firing of steel projectiles through casing or liner set through a producing zone, to allow the entry of formation fluids to the well. This is a process often used in well testing. GUSHER: An uncontrolled escape of oil from the hole, i.e. a blowout. The first oil gusher was from the famous Spindletop well.

-HH#: Hydrogen sulphide gas, q.v. HALON: A fire extinguishing gas used on rigs.

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HAND: Any individual amongst the personnel aboard a drilling unit. HANGER: A circular device inside the wellhead assembly on which a number of strings of casing of different diameters can be suspended.

312

Glossary of Marine Drilling terns HANGING OFF: The operation of landing the drill string in the wellhead

with a special tool and unlatching the lower marine riser package from the BOP stack so that the drilling rig can be moved quickly off location. This is an emergency measure that may have to be adopted, for example, when an iceberg is threatening to collide with a rig. HEAVE: The vertical motion of a vessel in a seaway. The ability of a semi-

submersible to continue to drill in bad weather is often determined by the amount of heave. HEAVE COMPENSATOR: Also called a motion compensator. A device

which allows the drill string to remain relativelymotionless while the vessel supporting it is heaving up and down. Heave compensation can be made either with bumper subs installed in the drill string, or by a device attached to the hoist in the derrick. HEAVE PERIOD: The time taken for one complete cycle of heaving motion

of a vessel. A typical heave period for one type of rig in fairly calm water might be 7 or 8 seconds, and in large swells 11 or 12 seconds. HEAVY WALL DRILL PIPE: A type of drill pipe with thicker and heavier

steel than normal that provides intermediate weight heavier than that of drill pipe but less than that of drill collars. Sometimes known as heavy-wate drill pipe and abbreviated HWDP. HEEL: An angle of inclination from the vertical assumed temporarily by a

vessel for some reason, e.g. wind or wave pressure. Small heel angles are sometimes intentionally given to drilling vessels in order to assist drill floor operations such as running in or pulling out of the hole, or draining mud pits. Not a permanent inclination, which is called a ‘list’. HELICOPTER LANDING OFFICER (HLO): A crewman appointed by the

OIM of a British rig to take certain specified responsibilities for the safe operation of helicopters and the safe conduct of personnel on and around the helideck. HELIDECK: The helicopter landing area on a rig. This is slightly cambered

for drainage purposes, is painted with non-slip material and is covered with tautly-stretched rope netting to aid the landing of wheeled helicopters in strong winds. A large ‘H’ in an aiming circle is painted on it and fire appliances are kept nearby. H.L.O.: Helicopter Landing Officer, q.v.

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Glossary of Marine Drilling terms HOLE: The usual name used on a rig for the well or wellbore. HOLE OPENER: A large-diameter bit used to make the initial entry into the

seabed when spudding in. On some types, drilling fluid circulation pressure swings swivelling cutter arms outward, increasing the bit diameter as it starts drilling. HOOK: A large hooked device with a closing mechanism that hangs from

the travelling block and from which the swivel and elevator links are suspended. Some types of hook are combined as aunit with the travelling block. HOOK LOAD: The weight suspended from the hook at any given moment.

When running the drill string into the hole the hook load will be more than when the bit is on the bottom and drilling, since some of the weight of the drill collars is allowed to compress the bit as it turns. HOPPER: A large funnel through which bulk.solids are fed into a pipe carry-

ing liquids during mixing operations. Hoppers are used for mixing cement and for mixing drilling mud. HOT WORK PERMIT: A permit obtained from the OIM to carry out

specified operations such as welding or burning on a rig after tests have been made for the presence of gas in the work area. H.W.D.P.: Heavy wall or heavy wate drill pipe, q.v. HYDROCARBONS: Organic compounds of hydrogen and carbon

molecules which may be gaseous or solid, making natural gas or oil. A name often used to describe oil and gas. HYDROGEN SULPHIDE: A type of toxic gas emitted by hydrocarbons and

occasionally encountered during drilling. Also known as sour gas because of its ‘rotten eggs’ smell. It is most dangerous when its presence cannot be smelt and specia breathing apparatus has to be available when it is likely to be met, necessitating the removal of all facial hair by rig crewman. HYDROSTATIC HEAD: The pressure exerted by the weight of a’column of

liquid at rest, considered in terms of its height.

-II.A.D.C.: The International Association of Drilling Contractors.

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Glossary of Marine Drilling terms I.D.: Internal diameter(e.g. of casing). INCLINOMETER: A downhole survey tool used to measure the angle and

direction of the bit’s inclination from the vertical. It may be known by vari= ous other names. INDEPENDENT: In the oil industry, an oil company that has interests only

in the sphere of oil production, as opposed to the oil majors who are involved in refining, transportation and marketing as well as production. There are numerous independents in the USA, but comparatively few in Britain and elsewhere. The well operator may be either an independent or a major. INNER CONDUCTOR: A string of casing, usually of 20” diameter, used in

deep wells which are begun with 30” outer conductor casing. INSTRUMENT TECHNICIAN: The specialist aboard a rig whose duties are

to maintain and repair the numerous measurement and control devices onboard. IRON ROUGHNECK: A trade name commonly used for a large drill floor

machine that is able to make or break drilling tubular connections using pneumatic or hydraulic power, replacing the usual tongs and dispensing with much of the manual involvement.

-JJACKET: A structure made from tubular pipe fixed to the sea-bed to sup-

port a platform. Production platform jackets are usually towed out to loca.tions and sunk into position. JACK-UP RIG: A self-elevating mobile offshore drilling platform. JAR: A downhole tubular tool that can release stored compressive or tensile

energy when desired so as to impart a shock to the drill string. Jars are often used in fishing operations and may be incorporated in a bottom hole assem/ bly as a precaution. , JET BIT: A drilling bit with nozzles through which drilling fluid is expelled

at high pressure. JET PERFORATING: A well testing operation in which a charge of high

explosives is used to burn a hole through the tubing or casing. An alternative method to gun perforating, q.v. ..,

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Glossary of Marine Drilling terms JETTING: A method of making hole for conductor casing in which a string of pipe, thfough which a powerful stream of fluidis pumped, is run inside the lower end of the conductor as it is lowered into the sea bed. JETTING SYSTEM: A high pressure water system fitted to the legs of a jack-up rig for use in freeing the legs from the sea-bed before jacking ‘up when leaving the location. JEWELLERY: Devices such as scratchers and centralisers that are attached to joints of casing before they are run. JOINT: A single length of drill pipe or other type of tubular. JUNK: Metal debris lost down the hole. This might consist of rock bit cones, reamer cutters, items dropped through the rotary, collapsed pipe, etc. JUNK A WELL, TO: To abandon a well because of difficulties in retrieving a fish from it, or because of the expense incurred in doing so. JUNK BASKET: A retrieving tool run in the hole during fishing operations or to remove milled or drilled plugs, cement, packers or other junk.

-KKELLY: A long steel pipe, usually with a hexagonal, or sometimes square cross iection, suspended from the swivel and connected to the drill pipe. It transmits torque from the rotary table to the drill string and is able to move vertically, permitting the gradual lowering of the bit. It is hollow, allowing the passage of drilling fluid for circulation purposes, has box and pin threaded ends and is most either 40,46 or 54 feet long. KELLY BUSHING: A sliding device, through which the kelly fits closely, that engages with the master bushing of the rotary table so that rotary torque can be transmitted to the kelly while simultaneously allowing the kelly to move up or down. KELLY COCK: A valve installed between the swivel and the kelly to relieve the swivel and rotary hose from fluid pressure when necessary. KELLY SPINNER: A pneumatically powered tool fitted at the top of a kelly to enable fast kelly cqnnections to be made and for rotating drill pipe slowly.

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Glossary of Marine Drilling terms KEYSEAT: A slot worn on the high side of a deviated hole when the rotating

drill pipe makes a sharp angle change. The keyseat prevents drill collars from passing through the hole when the string is pulled and has to be reamed out. KICK: An unexpected flow of formation fluids into the wellbore. KICK-OFF: The action of starting to deviate a hole from the vertical by

directional drilling techniques. KICK-OFF POINT: The depth at which the deviation of a hole is begun. KILL A WELL, TO: To prevent a threatened blowout from a well by subdu-

ing well pressure. Several alternative methods are available for the driller to use, in most cases involving the pumping of heavier than normal drilling fluid through the BOPs. KILL LINE: A high pressure line attached to the BOP stack through which

heavy drilling fluid can be pumped into the hole to kill a well. On a semi-submersible or a drill ship the kill line runs down the side of the marine riser. KIP: An American unit commonly used in the offshore drilling industry meaning a kilo-pound, or 100,000 pounds weight. Anchor tensions are frequently measured in kips. KOOMEY UNIT: A trade name often applied to a BOP control unit of any make. BOP hydraulic fluid is called ‘Koomey fluid’. -LLAP: A point in a cased hole where the top of a liner overlaps the bottom of the lowest casing string. LATCH ON: To attach elevators to a joint of pipe, or to connect any subsea

item of equipment to another. LAYING DOWN: The operation of unscrewing pipe into single joints and

laying them down on the pipe rack. This is done when changing to a different size of pipe or when the well is completed and the rig is about to make a transit to a new location. LEG: A vertical column or girdered structure on a drilling platform that sup-

ports its deck. Most semi-submersibles have fixed circular column-type legs, -.%

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Glossary of Marine Drilling terms

while jack-ups usually have open-girdered legs edged with teeth by which they are jacked up or down. LENGTH O.A.: The overall length of a vessel, meaning her extreme length. LIFEBOATS: Survival craft carried by all offshore drilling installations.

They are normally diesel-engined and totally enclosed. Some types are almost circular in shape. LIFEBOAT DRILL: A crew survival training exercise held at frequent inter-

vals, often in conjunction with a fire drill. LIFERAFTS: Automatically inflating rafts that are carried in addition to the

lifeboats. LIGHTSHIP WEIGHT: The displacement of a drilling vessel as built, with-

out her most of her equipment, stores, ballast, etc. LINER: A string of narrow casing which is set inside the bottom of the lowest

string of well casing and runs to the bottom of the well, usually serving as the ‘oil string’. Tubing can be installed in this to extract well fluids. LIST: A permanent inclination of a vessel, caused, for example, by bad

weight distribution or by a flooded compartment. LOCATION: The place at which a well is to be drilled, and usually referring

to the area immediately around it. A supply boat is said to be ‘on location’ when she is on station at or near the rig. The geographical position of drilling locations are measured very precisely by electronic equipment. LOG: A systematic and permanent record of data. On a drilling rig many dif-

ferent types of log are obtained including well logs, machinery logs, mud logs, etc. Some are continuous recordings, others obtained when required. LOGGER: A mud logger, q.v. LOGGING SHACK: A portable cabin in which mud logging instruments are

installed and the loggers work. LOST CIRCULATION: The loss of quantities of drilling fluid into a forma-

tion. This may be due to caverns, fissures or permeability. It is evidenced by lack of returns of drilling fluid and stopped by the pumping downhole of lost circulation materials, q.v.

Glossary of k&wine Drill&g tLOST CIRCULATION MATERIALS: Material such as walnut shells, mica,

olive stones, etc., added to drilling fluids or cement slurries to help, stem the loss of drilling fluid into a formation. LOST TIME ACCIDENT: An accident in which a period of working time is

lost by a rig crewman. LOWER MARINE RISER PACKAGE: An assembly comprising the flex or

ball joint, an annular blow-out preventer, hydraulic accumulators, sections of riser and the riser slip joint, all of which can be detached from the rest of the BOP stack in an emergency to allow the drilling unit to move off location whilst leaving the well secure. L.T.A.: A lost time accident, q.v.

MACHINE ROOM: The engine room, where the main diesel engines of a rig

are located. MAKE-UP: The act of screwing one joint of pipe, etc. onto another. MAKE-UP CATHEAD: The cathead on the doghouse side of the draw-

works. MAKE-UP CHAIN: The chain connecting the arm of the make-up tong with

the automatic cathead. It is tensioned to provide the torque for tightening a connection. MAKE-UP TONG: A large mechanical steel wrench suspended by a wire

from the derrick and used to tighten the ‘male’ (‘pin’) end of a joint of pipe when this has been stabbed and spun up inside the ‘female’ (‘box’) end of another pipe. It is also used to back-up the lower joint when a connection is being loosened by the break-out tong. The arm of the make-up tong is connected to the drawworks by a chain, and a snub-line tethers it to a Sampson post. A torque gauge is fitted to its end. MAKING A CONNECTION,: The operation of screwing a new joint of drill pipe onto the top end of the drill string. This is done when the kelly has travelled down through the rotary table almost as far as it will go. The kelly is often made up into the new joint while this is standing in the mousehole; and the joint is then stabbed into the drill string in the rotary.

Glossary of Marine Drilling terms MAKING A ROUND TRIP: The long operation of pulling the drill string

completely out of the hole for some reason, such as changing the bit, and then returning it to the hole. It can take eight hours or more in a deep well. MAKING HOLE: Rotating the drill bit and deepening the hole. Also called

‘drilling ahead’. MAN OVERBOARD DRILL: A recovery exercise, usually using a dummy

thrown overboard from a rig, performed periodically with the rig standby vessel to the operator’s requirements. MANIFEST: A list of cargo loaded or backloaded by supply boat or helicop-

ter. MANIFOLD: A control point in a piping system at which flow can be

directed in a number of ways. On the drill floor there is a choke manifold for the control of fluids returning from the well. See ‘choke and kill manifold’. MARINE CREW: The mariners aboard a rig who, under the direction of the

bargemaster, barge engineer or barge operator, attend to marine work such as the maintenance of lifesaving and fire appliances, and perform boat operations. CROs, radio operators and sometimes the medic are also marine crew. MARINE RISER: The large-diameter pipe connecting the BOPstack to the

drill floor of a semi-submersible or drill ship, through which the drill string passes to the well and through which returns of drilling fluid pass from the well to the rig. MARINE SUPERINTENDENT: The rig operator’s staff member responsi-

ble for the day-to-day maintenance of all marine equipment aboard the fleet’s units. MATERIALS CO-ORDINATOR: A person responsible for the movement

and supply of equipment and stores needed for a well programme. Some oil companies keep a materials co-ordinator aboard during the period of hire, while others leave this work to the company man. MATSMAN: A common abbreviation used for the materials co-ordinator. M.D.: Measured depth, q.v. MEASURED DEPTH: Measured depth, which is the total actual length of a

Glossary of Marine Drilling terms

drilled hole taking account of every deviation from the vertical. MEASUREMENT WHILE DRILLING: A technique of logging certain

information about downhole conditions through sensors in the bottom hole assembly. Information is then sent to measuring devices and a digital display on the drilling rig by means of pulses transmitted by telemetry through the mud. This requires rotary drilling to be stopped only for a short period, and does not interrupt drilling at all if a downhole motor is being used. It is often used in directional drilling to measure angles of inclination and direction. Abbreviated to MWD. MECHANIC or RIG MECHANIC: A crewman whose job is to maintain the

rig’s machinery in good running order. Many mechanics are former marine engineers, although aboard a rig they may be called upon to repair items of drilling machinery in addition to the main diesel engines. MEDIC: A qualified nurse who is on call 24 hours a day for treating minor

injuries. He usually doubles as a clerk. MEZZANINE DECK: A name sometimes used to describe the cellar deck

beneath the drill floor. MILL: A special tool with a rough, sharp and very hard cutting head used for

milling, q.v. Mills are made in many shapes and either fit on the end of a drill string or are incorporated within it like a reamer. MILLING: Using a mill to grind down metal debris in the hole, remove sec-

tions of casing when sidetracking, or for reaming out tight spots in the hole. MOBILE OFFSHORE DRILLING UNIT: A marine drilling platform which

us capable of being moved from one location to another. Semi-submersibles, j,ack-ups, drill ships and drilling barges all come into this category, whereas fixed platforms do not. Abbreviated MODU. M.O.D.U.: A mobile offshore drilling unit, q.v. MONEL: A type of non-magnetic drill collar.

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MONKEY BOARD: A narrow platform, like a diving board, from which the

derrickman manipulates the top ends of stands of drill pipe when making a trip. The board can move up or down to reach pipe of different heights. MOONPOOL: The void space cut in the deck of a semi-submersible, or

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Glossary of Marine Drilling terms inside a drillship, which is open to the water and through which sub-sea equipment is run. Commonly, if wrongly, also used to describe the area of the cellar deck immediately round the void. A small moonpool is also used for running diving equipment. MORNING REPORT: The company man’s report sent to ‘town’ at or about 0600 daily, detailing all rig operations during the last 24 hours. MOTORMAN: The mechanic’s assistant responsible for keeping the engine log, records of lubricants used, and for other minor engine room and associated work. A mechanic on an American rig is usually termed a ‘motorman’. MOUSEHOLE: A deep tube suspended below the drill floor from which a single joint of drill pipe can be taken when making a connection. The joint is made up to the kelly while in the mousehole, then pulled out, transferred to the rotary and made up to the drill string. This is called making a ‘mousehole connection’. MUD: Liquid drilling fluid circulated down the hole and back to the rig. MUD ADDITIVES: Chemicals mixed with mud to alter its chemical or physical properties. MUD BUCKET: A cylinder fitted round a connection on the drill floor to save mud from being spilled when the connection is broken. MUD CAKE: See filter cake. MUD CLEANER: A machine used in the drilling fluid treatment process to remove solids from returning mud and conserve barite and fluid. Desilter cones are mounted over a motorised shaker screen which removes drilled solids, while the barite and liquid component of the mud is returned to the circulating system. MUD ENGINEER: The contracted specialist who supervises the correct mixing and conditioning of mud, whiih is usually supplied by his company. Often called the ‘mud man’. MUD LABORATORY: The mud engineer’s ‘office’, in which he tests mud samples. MUD MAN: The commonly used term for the mud engineer. ..

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Glossary of Marine Drilling tern MUD LOGGER: A trained analyst who records well log data using machin-

ery installed in the logging shack. Loggers are sometimes qualified geologists. MUD LOGGING: A continuous logging process carried out by mud loggers

in which the presence of any hydrocarbons in the drilling fluid is recorded on a graph. Mud loggers constantly monitor this equipment in the logging shack. MUD PITS: Large holding tanks for drilling fluids, grouped together in the

mud room. MUD PUMP: A large reciprocating pump for forcing drilling fluid through the circulation system. There are usually two or three main mud pumps. MUD ROOM: A large internal space where mud pits and, frequently, mud pumps and other associated equipment are installed. MUD SCREEN: A screen on the shale shaker, through which mud is strained

under vibration to remove cuttings. MULTIPLE ZONE COMPLETION: A method used to complete a well so

that production can be obtained simultaneously from two or more formations at different depths.

-NNATURAL GAS: Hydrocarbons occurring naturally in a gaseous form in a well. Methane is the main constituent, but small percentages of several other

gases are present in natural gas, as well as contaminants such as hydrogen sulphide (sour gas) and water vapour. NIGHT PUSHER: The junior toolpusher, who no-maily works the night

tour from 18.00 hours to 06.00 hours. Sometimes called the ‘tourpusher’. NON-MAGNETIC DRILL COLLAR: A type of drill collar used in the bot-

tom hole assembly when magnetic survey tools are going to be run.‘Sometimes called a ‘monel’. -oO.D.: Outside diameter. Drilling tubulars are usually referred to by their

outside diameter, e.g. 5” drill pipe, 9%” drill collar. -..

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Glossary of Marine Drilling terms OFFHIRE SURVEY: An independent survey of a drilling rig made at the

end of a hire period when quantities of consumables remaining onboard are checked, as well as structural damage to the unit. OFFSHORE INSTALLATION MANAGER: The person appointed under

the Mineral Workings (Offshore Installations) Act 1971 to be legally responsible for the safety, health and welfare of all persons on or about a British offshore installation while drilling or production is in progress. OIL BASE MUD: Mud mixed using a base of diesel oil instead of water. It is

useful when drilling shales, deep high-temperature holes and salt formations, and in directional drilling and for freeing stuck pipe. OIL SAND: Sandstone containing oil in its pores. OIL ZONE: A formation which a hole has penetrated from which oil may be

produced. The oil zone usually lies under the gas zone and over the water zone in a reservoir. O.I.M.:, Offshore Installation Manager, q.v. ONHIRE SURVEY: An independent survey of a drilling rig made at the

start of a, hire period, to determine quantities of consumables onboard and the general condition of the unit. OPEN HOLE: A well in which no casing has yet been set. This is undesirable

if there is a likelihood of pressurised hydrocarbons entering it, so casing is set soon after each section of hole has been drilled. OPERATOR: The company to whom a rig is contracted to drill a well, and

which is either the sole financier ?f the project or the representative of a consortium of partner companies. The name ‘operator’ is also given to the person who operates a unit of machinery offshore, e,g, the wireline operator. OUTER CONDUCTOR: A short string of wide-gauge casing, usually 30”

bore, which is used as the foundation string in deep wells. Also called the . ‘surface conductor’. OVERSHOT: A ‘female’ ended downhole tool used in fishing operations for

lost or stuck pipe, etc., when it is lowered over the end of the fish to obtain a grip. Its male counterpart is a spear, q.v. OVERSIDE WORK PERMIT: A work permit issued by the OIM to a person

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Glossary of Marine Drilling terms

performing work outboard of the rig structure, when there is a possibility of falling into the sea. The standby vessel is called to close standby at such times and a lifevest and safety harness has to be worn by the permit holder.

-PP.A. SYSTEM: The public address system by which announcements are nor-

mally made aboard a rig. PACKER: A tubular sealing device that can be lowered into the casing, liner

or open hole and made to expand flexible rings at its circumference in order to isolate a section of the hole, e.g. for well testing purposes. Many different designs are made for a variety of uses. Packers generally have a hole through their stems for circulating drillig fluid or for running wireline tools, and they may have box and pin connections for the attachment of other tools. PAY SAND: The zone which is the target area of the drilling operation; alter-

natively a formation which is already producing. PELICAN HOOK: A device used by anchor handling vessels to secure a rig’s

anchor chain temporarily on the after deck while connections or disconnections of attachments are made. PENETRATION, RATE OF: The rate at which the drill bit is cutting through

the formation, expressed in feet per hour. Abbreviated to ‘ROP’. PENNANT: A length of strong wire rope with eyes at its ends for making

connections, used for a variety of purposes in anchor-handling. Many pennants of different lengths and load capacities are used in an anchor-handling operation. PERFORATE, TO: To fire steel projectiles through casing or liner, and its

surrounding cement, that has previously been set through a formation expected to produce. The gun firing the bullets is fired electrically from the surface. .

PERMANENT GUIDE BASE: A steel frame, usually square and with four

corner posts, that is lowered onto the temporary guide base following the running of the outer conductor casing to serve as the foundation for the wellhead and BOP stack. The guidelines run through the corner posts, and these are used to locate the BOP stack when it is run. PERMEABILITY: The ability of hydrocarbons to flow through the pores of

a rock.

Glossary of Marine Drilling terms PETROLEUM: Hydrocarbon oil or gas obtained from the earth’s subsurface rocks. PIGGY BACK: A back-up anchor used in tandem with a main anchor when the main anchor is unable to hold the required chain pre-tension on its own. Piggy backs are usually lighter than main anchors. PILOT HOUSE: The wheelhouse and navigation centre of a rig, usually found at the forward end near the centreline. PIN CONNECTOR: A pressure-sealed device used to connect the marine riser to the wellhead when drilling through large diameter conductor casing that prevents the use of a blow-out preventer stack. Pin connectors are used when shallow gas pockets might be encountered. PIPE: Oilfieid tubulars such as drill pipe, collars, casing and tubing. PIPE RACK: A row of spaced parallel beams on the deck of a rig, on which drilling tubulars are laid down for storage. PIPE SPINNER: A hydraulic wrench used in place of a spinning chain to spin up a connection prior to torquing-up. PITS: Large tanks in which drilling fluid is held prior to pumping down the well by the mud pumps. They are usually sited near the mud pumps. PITCH: The forward-and-backward oscillating motion of a vessel in a seaway, particularly in a head sea. PLATFORM: The name sometimes used to describe a mobile drilling unit on which a drilling rig is erected. Also the term used commonly offshore to describe a self-contained fixed production platform. PLUG: A device inserted into a drilled hole to block the passage of fluids. Plugs may be made of rubber, cement, or other substances, and some can be drilled out or retrieved when no longer required. . PLUG & ABANDON, TO: To seal the top of a well with a cement plug and abandon it. As much casing as possible is retrieved, together with the wellhead and sub-sea equipment, and the sea-bed is left clear of debris. Dry holes are usually plugged and abandoned. POD: A sub-sea container, mounted on the BOP stack, that houses control ._ 326

Glossary of Marine Drilling terms

valves and actuators for operating the BOP hydraulics. Two pods are usually fitted for full redundancy, each connected by a flexible hose to controls on the drillinge rig. PONTOON: A hull of a semi-submersible rig, at the lower end of its col-

umns, which provides buoyancy as well as space for ballast, fuel, water and drilling fluids. There may be two or more pontoons, depending on the rig’s design. P.O.O.H.: Abbreviation used for pulling out of the hole, which is the oper-

ation of withdrawing the entire drill string from the well bore for some purpose, such as changing the bit. POSSUM BELLY: The naine often given to the trip tank, which is a tall and

narrow cylindrical tank holding drilling fluid and used for determining whether formation fluids are entering the wellbore during the tripping operation, or whether drilling fluid is being lost into the formation. POWER SWIVEL: A type of drilling swivel that is turned by electric or

pneumatic power and replaces the rotary table, master bushing, kelly and circulating swivel. It is sometimes used for light drilling operations. Pm-LOAD: A load placed on a jack-up rig before it is jacked fully up to its

working air gap, to test the penetration of individual legs. With the rig at about 5 feet air gap, pre-load tanks in the hull are filled with sea water to achieve even total weight distribution on the legs. After a test period of several hours the air gap at each leg, which may have changed during this time, is adjusted so that the rig is standing level, after which it is jacked to the desired air gap. PRESSURE TEST: A test performed on an item of equipment that is

designed to withstand high pressures from fluids. BOPs and associated drilling fluid circulating equipment are frequently pressure tested, especially as the hole gets deeper. PRE-TENSION: The load which a rig’s anchor chain has been proved to

withstand without breaking the anchor out of the ground. Pre-tension prov‘. ing is camed out before slacking back to a working tension. PREVENTER: A blowout preventer, q.v, PRIME MOVER: One of the main engines which are the sour&. of power on

a rig.

327

Glossary of Marine Drilling terms PRODUCTION: The process of bringing hydrocarbons to the surface from

a sub-surface reservoir for onward transport to the refinery ashore. PRODUCTION PLATFORM: A platform, which may be either fixed or

floating, designed chiefly for production, although development drilling may take place from it. PRODUCTION WELL: A well from which oil or gas is produced. PROPULSION ROOMS: Compartments in the pontoons of a semi-submer-

sible rig in which propulsion machinery is housed. In a diesel-electric rig the diesel engines are on the main deck level and the electric motors that drive the propeller shafts are in the propulsion rooms. P.S.L.: Pounds,per square inch: the unit of pressure normally used in drilling. ‘P’ TANK: A silo in which powdered drilling material such as cement, barite

or bentonite is stored in bulk on a drilling unit. On a semi-submersible the ‘P’ tanks are usually in several of the columns. PULL OUT OF THE HOLE, TO: To withdraw the drill string completely

from the hole for some purpose, such as for changing the bit. PUMP ROOM: The compartment in a semi-submersible rig in which the bal-

last, bilge and other pumps are sited. It is usually in the pontoons. PUP JOINT: A short joint of any kind of tubular, such as drill pipe or marine

riser, used to make up a required total length in a string. -RRACKING ARM: A large hydraulically controlled telescopic arm used on

the drill floor for moving, guiding and stabilizing pipe of various diameters. RACK PIPE, TO: To stand pipe back on end in the setbacks inside the der.

rick as they are pulled out of the hole during tripping. RADIOACTIVE WELL LOG: A record of the radioactive characteristics of

a formation measured by wireline logging equipment that is lowered into the hole. RADIO ROOM: The communications centre aboard an offshore drilling

unit.

328

Glossary of Marine Dri&mg b&g

RAM: A closing and sealing device in a BOP stack. The rams are activated

by hydraulic pressure when a blow-out is threatened, and can be locked shut. RAT HOLE: A tube recessed below the drill floor, into which the kelly, kelly

cock, kelly bushing, kelly spinner and swivel are temporarily placed when ‘tripping’ is in progress. R.C.V.: Abbreviation for remote-controlled vehicle, which is a pilotless

submersible deployed from a rig to perform subsurface tasks. Usually known as an ROV, q.v. REAMER: A downhole tool sometimes included in the bottom hole assem-

bly, looking like a drill collar with short fins on which there are cutters. It is used for smoothing and enlarging the wall of the hole, stabilizing the bit, straightening the hole where doglegs occur, and in directional drilling. If the cutters revolve the tool is called a roller reamer. REAMING: The operation of enlarging the hole by re-drilling it with a reamer. RESERVOIR: A subsurface formation in which the pores of the rocks hold

hydrocarbons. REVERSE CIRCULATION: The circulation of the drilling fluid in the oppo-

site direction to the normal way, i.e. down the annulus around the drill pipe and up through the centre of the drill pipe. This is sometimes done to alleviate problems in the hole. RIG: Strictly speaking, the derrick and drilling equipment that are mounted

on a platform such as semi-submersible, drill ship, etc. In practice, however, the drilling unit itself is commonly referred to as ‘the rig’. RIG AIR: The rig’s compressed air supply, maintained by air compressors and used for many purposes around the rig. RIG DOWN, TO: To dismantle and secure certain items of equipment following completion of a well, when’preparations are made for the rig-move to the next location. RIG FLOOR: An alternative name often used for the drill floor. RIG SUPERINTENDENT: The drilling contractor’s staff member who is

responsible for managing the day-to-day operations of the drilling unit. He

329

Glossary ofMarine Drilling terms

usually works from the office in ‘town’ and maintains regular contact with the toolpusher or OIM on the rig, although some companies employ rig superintendents onboard their rigs. He may be called the ‘drilling superintendent’ in some companies. RIG UP, TO: To prepare a drilling unit and its equipment for the start of dril-

ling following a rig-move to a new location. R.I.H.: The usual abbreviation for running in the hole. RISER: The steel conduit connecting a floating drilling rig with the wel-

lhead, down which all drilling tools, casing, etc. are guided to the well and up which the drilling fluid returns from the well to the rig. It also serves as the running string for the BOP stack. Properly termed the ‘marine riser’. RISER ANGLE: The angle from the vertical made by the riser. This must be

within a certain tolerance to avoid damage to the riser. It is usually monitored by acoustic transponders which send signals to instruments on the rig. RISER FLEX JOINT: The ball joint, q.v. RISER TENSION: The amount of tcnsile,load, usually measured in pounds,

set on the riser tensioner wires by adjusting the tensioner air pressure. As the hole gets deeper and heavier mud is used, the tension is increased. RISER TENSIONER: The system of wires, sheaves and cylinders, operated

by pneumatics and hydraulics, that maintain a constant tension on the riser support wires to avoid the riser’s collapse. ROLL: The side-to-side oscillating motion of a vessel in a seaway, particu-

larly in a beam sea. Drill ships are more susceptible to rolling than semi-submersibles are. Two or three degrees of roll from the vertical would be considered unusually large on some types of semi-submersible, but different types have their own motion characteristics. ROTARY BUSHING: A circular steel cup-shaped lining that fits into therot-

ary table and into which the kelly bushing is inserted during drilling. When the kelly bushing is removed, slips can be wedged into the space between the rotary bushing and drill pipe running through the rotary. Also called the master bushing. ROTARY DRILLING: Drilling with a bit which is rotated while a force is

applied above it; the normal method of drilling an offshore well. The rotary

Glossary of Marine Drilling terms drive may be applied by the rotary table or by a downhole drilling motor. ROTARY HELPER: A name sometimes used (particularly on American rigs) for a roughneck or floorman. ROTARY HOSE: A flexible hose that connects the top end of the stand pipe with the gooseneck connection at the swivel, through which drilling fluid enters the drill string. ROTARY TABLE: The underdeck housing for the mechanism in the centre of the drill floor that drives the kelly and turns the drill string and bit. All downhole tools, casing, etc. are run through its opening. ROUGHNECK: A member of the drill crew who performs manual tasks on the drill floor, such as making and breaking connections, racking pipe back, etc. Also called a floorman or rotary helper. ROUND TRIP, MAKING A: Pulling the entire drill string out of the hole for some purpose, such as changing the bit, and then running it back in. This can take many hours in a deep well. ROUSTABOUT: A general-purpose hand who may be called upon to carry out virtually any manual task aboard the rig for either the drilling or marine departments, e.g. laying down drill pipe, standing in for roughnecks during meal breaks, preparing loads for boats, painting or washing down, etc. R.O.V.: Abbreviation for a remotely operated vehicle, which is a small remote-controlled submersible sometimes deployed for sub-sea tasks such as repairs or inspections to underwater equipment. The ROV often has mechanical arms which can perform most of the functions of a human diver. RUCKER WIRES: A name sometimes used for the riser tensioning wires. RUN CASING, TO: To lower steel casing into the well joint by joint to line it. This is usually done by contracted specialist casing hands. RUN INTO THE HOLE, TO: To lower the drill &ring into the hole. Usually abbreviated RIH. -sSACK: A measure of volume often used in the drilling industry for dry powders such as cement, barite and bentonite. A sack of cement measures one .. .

331

Glossary of Marine Drilling terms

cubic foot and weighs 94 lb. A sack of barite weighs about 100 lb. and there are 22.5 sacks of barite to a ton. There are 39 sacks of bentonite to a ton. Sacks is often abbreviated ‘sx’. SACK ROOM: The compartment where the bagged and drummed con-

stituents and additives required for mixing the drilling mud are stored, usually close by the mud pumps. SAFETY AWARD: An award is given by some drilling contractors to the

crews of their rigs in recognition of passing a certain number of days without a lost-time accident. SAFETY COMMITTEE: A committee formed on a rig for the purpose of

monitoring and improving the standard of safety awareness amongst its crew. SAFETY MEETING: A regular meeting held by sections of a rig crew to dis-

cuss aspects of safety in their work area or elsewhere onboard. The minutes of the safety meeting are then discussed by the safety committee at their own meeting. SAFETY OFFICER: A member of a rig crew appointed to be responsible for

convening safety meetings and for implementing improvements in rig safety. All British-registered vessels must have a safety officer amongst the crew. The safety officer is often the bargemaster. SAND LINE: A long, narrow diameter wire line kept on the drill floor for

downhole jobs such as retrieving the ‘Totco’ tool, q.v. SAND TRAPS: Tanks in the drilling fluid circulation system in which the

fluid is recovered after passing through the shale shaker. SATCOMS: A satellite communications system installed on a rig to enable

a speedier flow of messages to and from shore bases than by conventional radio-telephone or telex. Recognised by a large dome housing a dish aerial. SATELLITE WELL: A well drilled i:dependently of a platform by a mobile

unit but tied in to the platform foi prpduction purposes by a sea-bed pipeline. Most platform wells are directionally drilled form the platform. SATNAV: Satellite navigation equipment, used for positioning the rig on the

location and for navigation during ocean transits.

.*

332

Glossary

of

Marine Drilling terms

SCHLUMBERGER: The name given to the wireline logging

equipment

installed on a rig that is often owned by a company of the same name. Schiumberger was a French scientist who first developed the technique of electric wireline logging. SCRATCHER: A multi-pronged device fitted round a joint of casing to

make contact with the wall of the hole, to scrape of excess filter cake and to improve the chances of a good bond when cement is pumped into the annulus. SEMI-SUBMERSIBLE: A type of vessel the draught of which, by flooding

certain compartments, can be increased to submerge much of its structure, either to give a degree of stability not attainable in conventional monohull vessels, or for some other purpose, such as to float another vessel onto its deck. The hull or hulls may be designed to rest on the sea-bed in certain conditions, but most semi-submersible rigs drill whilst floating. See also ‘column stabilized semi-submersible’. SETBACKS: The areas inside the derrick adjacent to the V-door where drill

string tubulars are ‘set back’, or stood on end, when they are out of the hole temporarily. Their top ends are racked back between fingers. SETTING CASING: The operation of running casing and then fixing it in

place by pumping cement into the annulus between it and the surrounding wall of the hole. SHALE: The type of rock most frequently encountered during offshore dril-

ling, composed of small silt and clay particles. SHALE SHAKER: The vibrating screens across which the drilling fluid

returning from the hole is poured to strain off the liquid and deposit the solid particles. SHOCK ABSORBER: A tubular tool used in the drill string to absorb shocks

and vibration and thereby extend the life of the bit and minimise damage to the drill string. , SHOW: A trace of oil or gas found in cuttings, samples or a core recovered

from the well. SIDETRACK, TO: To divert the drill bit round an obstruction in the well,

such as stuck pipe. This is done using directional drilling techniques and tools such as a whipstock. . .#

333

Glossary of Marine Drilling terms SINGLE ZONE COMPLETION: A simply type of well completion in which packers do not separate producing zones. SKIDDING REAMS: Rails along which the drilling package of a cantilevertype jack-up rig into its drilling position. SKIDDING UNIT: A thrustin,g unit with hydraulically-operated rams that moves the drilling package of a cantilever-type jack-up rig along the skidding beams. SLANT DRILLING: A technique of drilling directionally from a jack-up rig in which the drilling derrick is tilted to an angle of about 30 degrees from the vertical. SLICKER SUIT: A suit of waterproof overalls worn by drill crews when making a ‘wet trip’ or when washing down. SLIP & CUT, TO: To move the drilling line over the sheaves of the hoist a certain distance and cut off a length of the line, so that the sheaves are not always in contact with the same parts of the line, with consequent risk of overstressing. SLIP JOINT: A telescopic joint inserted near the top of the marine riser to absorb the vertical motion of the drilling unit when heaving up and down in a seaway. SLIPS: Tapered steel wedges that are inserted between the rotary bowl and a tubular joint to grip the string temporarily, e.g. while it is disconnected from the hoist when making a connection. The wedges are hinged so that they effectively wrap around the tubular to provide a grip around its circumference. Different types of slips are used for drill pipe, collars and casing. SLUG: A quantity of heavy drilling fluid that is pumped into the top of a joint of drill pipe before it is disconnected so that it will not be full of fluid when the connection is broken. SLUG PIT: The small mud pit where heavy mud for slugging is k;pt. SLUSH PUMP: An alternative name for a mud pump. SLURRY: A semi-liquid mixture of cement powder and water that is pumped up into the annulus between the casing and the wall of the hole so that it can harden and fix the casing in place.

SNUB LINES: Wires connecting the arms~ of each set’of man& tongs to Sampson posts at either side of the rotary table, to arrest their tendancy to turn round when they are acting as the back-up tong. SPEAR: A fishing tool which‘stabs the end of a joint of pipe that has been

lost or stuck in the hole. It is the male counterpart to the overshot, which goes over the end of the fish. SPIDER: A circular steel frame which is positioned round the rotary opening

when the marine riser or other tubulars are being run to act in the same way as a set of slips. Each joint of riser is passed through the spider and is clamped tightly by arms or a hydraulic device while the next joint is connected. SPIDER BEAMS: Movable steel beams in the moonpool of a floater which

can be positioned beneath the BOP stack to support it during the attachment of riser or during overhaul or repairs. SPINNER HAWK: An alternative name for the pipe spinner, q.v. SPINNING CATHEAD: A name sometimes used for the make-up cathead.

SPINNING CHAIN or LINE: A chain used to start the operation of screwing two joints of drill pipe together on some rigs. SPOTTING: Pumping a slug of oil down through a stuck drill string and up

its annulus to the position where it is stuck against a formation. The oil then soaks into the filter cake,~ sometimes freeing the pipe. SPUD CANS: Tanks on the bottom ends of the legs of a jack-up rig which

‘spud’, or penetrate into the sea-bed at the start of the jacking-up operation. They are filled with sea water. SPUD IN, TO: To commence drilling a well with a hole opener.

,

SPUD MUD: Mud specially mixed for the spudding-in operation. It is clay-

based and normally contains a bentonite gelling agent to prevent sloughing in the conductor hole. STAB, TO: To insert the pin end of one pipe into the box end of another

when making a connection. STABBING BOARD: A movable platform 20 to 40 feet above the drill floor

335’

.

Glossary of Marine Drilling terms

on which a casing hand stands to guide one joint of casing into another in the rotary. STABILITY: The ability of a vessel to recover equilibrium when it is moved

from the upright by external or internal forces. The stability characteristics of semi-submersibles and monohulls are quite different but, both are measured in terms of transverse and longitudinal metacentric height. STABILIZER: A downhole tool used for stiffening the bottom hole assem-

bly and for keeping the bit central in the bottom of the hole and drilling vertically. It looks like a drill collar but has short fins that contact the wall of the hole. One or more stabilizers may be used in directional drilling when they are positioned to act as a fulcrum about which the assembly turns. STACK: The name commonly used offshore for the blow-out preventer

stack. STACK, TO: To lay-up a drilling unit between contracts, usually because of

lack of work. ‘Cold stacking’ is mothballing, when equipment is demobilized and the crew taken off. In ‘warm stacking’ a small maintenance crew keeps the rig ready for quick mobilization. STAND: Three or sometimes two joints of pipe screwed together for easier

and faster handling on the drill floor. STANDBY BOAT: A ship whose duty is to stand by near the location of a

manned offshore installation in case of an emergency necessitating evacuation, or in case of a man-overbaord situation or oil pollution. The older standby boats in use in the North Sea are mainly converted trawlers, but newer vessels are purpose-built. They carry fast inflatable rescue boats and radio equipment for liaising with helicopters and have emergency accommodatio~n for the entire rig complement. Under British regulations boats must remain within 5 miles of the installation. STAND PIPE: A tall, rigid pipe in the side of the derrick that carries drilling

fluid up from the mud pumps and feeds it into the rotary hose that 1; suspended between its top and the swivel. STEP-OUT WELL: A well drilled close to a discovery well but in an

unproven area, so that the boundaries of the producing formation can be determined. STOREMAN: The rig crewman who maintains and distributes the stock of .._,

336

Glossary of Marine Drilling terms

spare equipment and parts held onboard for the drilling and other departments. STORM AIR GAP: The distance between the sea surface and the underside

of a semi-submersible when it is floating at its storm or survival draught. STORM DRAUGHT: Survival draught, q.v. STRING UP, TO: To reeve the drilling line through the blocks of the hoist,

securing one end at the dead line anchor and winding the other end onto the drawworks drum. STUCK IN THE HOLE: The condition when drill pipe or some other tool or

casing, etc. is jammed in the hole after efforts have been made to pull it out. STUMP TEST: A pressure test performed on the BOP stack when it is on the

test stump in the cellar deck of a semi-submersible. SUB: A short adaptor with different-threaded ends used to connect two

items of downhole equipment which would otherwise not mate. Short for ‘sub-assembly’. SUBMERSIBLE: A type of drilling platform which is designed to be floated

to its location and then sunk so that its bottom rests on the sea-bed. SUB-SEA ENGINEER: The rig crew-man on a floater responsible for the

maintenance and running of the BOP stack and other items of sea-bed equipment such as guide bases, pin connector and wellhead. He needs particularly a knowledge of hydraulics. SUPPLY BOAT: A vessel which carries oilfield equipment from shore bases

to rigs on location. It may be designed solely for supply work, or may be multi-purpose, doubling as an anchor-handler and/or tug SURFACE LOCATION: The exact geographical position on which a drilling

unit is lo?ated. This might be different to the target location to which the drilling bit is directed, especially if the well is a directional well. SURVEY: A downhole examination normally carried out by wireline tools

to determine hole conditions such as bottom hole temperature and pressure or the direction and angle of inclination made by the hole. SURVIVAL DRAUGHT: The draught at which a semi-submersible rig is -.., aq7

Glossary of Marine Drilling terms

designed to be floated in severe weather conditions, such as a loo-knot wind. It is usually a compromise between having sufficient air gap and sufficient immersion of the pontoons and columns for adequate stability. SURVIVAL SUIT: A special suit of thermally protective overalls worn by all

helicopter passengers flying to offshore installations in cold-water areas such as the North Sea. SWABBING: The action of creating a suction at the sides of a hole, either

accidentally or on purpose. If the bit or drill string is pulled out too quickly, swabbing can occur beneath it which may induce well fluids out of the formations, creating a kick. SWAMP BARGE: A type of flat-bottomed barge used for supporting a dril-

ling rig and accommodation in swampy or shallow-water areas such as in West Africa and the US Gulf. SWIVEL: The device which hangs from the hook below the travelling block

that permits free rotation of the kelly whilst at the same time admitting drilling fluid to it from the rotary hose.

-TTAG LINE: A line attached to a load being lifted by a crane so that the load

can be safely manoeuvered when over the deck. TARGET LOCATION: The exact geographical position of the target to

which the drilling bit is directed as it drills. In a deviated or directional hole this might be different to the surface location of the drilling rig. T.D.: Total depth, q.v. TELESCOPIC JOINT: An alternative name for the riser slip joint, q.v. TEMPORARY GUIDE BASE: A heavy steel frame which is lowered to the

.

sea bed at the start of an offshore drilling operation to serve as a foundation ,for subsequently deployed sub-sea equipment, to guide tools into the hole, and to provide an anchorage for the guide lines down which those tools and equipment are run. Sometimes termed a ‘drilling template’. TENDER: A mobile barge which is moored alongside some types of fixed

drilling platform to carry the drilling fluid circulation system, engines, drill pipe, accommodation, etc.

338

Glossary of Marine Drilling terms TENDER, TO: To make a bid for a drilling contract. TENSIONER, RISER: See riser tensioner. TEST STUMP: A short post fitted to the cellar deck on which the BOP stack can be pressure tested before use. THREAD PROTECTOR: Caps screwed on the ends of tubulars such as drill pipe, casing, etc., to protect their threads. THRIBBLE: The American name for a ‘treble’, which is a stand of three single joints of drilling tubulars. THRIBBLE BOARD: An American name for the monkeyboard which projects from the derrick at the height of a thribble, or a stand of three connected joints. THROW THE CHAIN, TO: To throw the spinning chain up around the pin end of a joint of pipe, etc. to obtain a purchase on it after it has been stabbed into the box of another joint. The cathead then pulls the chain off the joint, tightening the connection. THRUSTERS: Propellers fitted on a vessel for manoeuvring purposes rather than for propulsion. In addition to their main propellers, many drill ships have thrusters which are operated by the dynamic positioning system, while semi-submersibles usually have thrusters at the after end of their pontoons. TIGHT HOLE: A security condition imposed by an operator when any information about the well operations is restricted in circulation. When the well is a tight hole it may mean that it holds promise of being productive, but this is never certain. TIGHT SPOT: A point in the hole at which the bit encounters difficulty in drilling or wiping. Tight spots are removed during a wiper trip, q.v. TONG LINE: A line attache’d to the handle of a tong for applying leverage when making a connection. *The back-up line connects the back-up (or make-up) tong to a fixed post, while the break-out line connects the breakout tong to a drawworks cathead. TONGS: Large steel wrenches suspended from the derrick by wires that are used to tighten or loosen connections of drilling tubulars. Two sets of tongs,

Glossary of Marine Drilling terms the back-up (or make-up) and break-out tongs, are used for drill pipe, and there may be sets of power tongs in addition. Special tongs are used for casing. TOOL JOINT: A short section of special steel pipe welded around each end of a joint of drill pipe to provide a means of connection and lifting. The tool joint on the lower end, the ‘pin’, has a male thread which is inserted into the female thread at the upper end, the ‘box’, of the joint below when making a connection. There are shoulders on the tool joints for the elevators to grip when lifting. TOOLPUSHER, SENIOR: The supervisor of the drilling department on a rig, responsible to the rig (or drilling) superintendent for the day-to-day operations on the unit and for carrying out the operator’s well programme as directed by the company man. The ‘pusher’ may also be the Offshore Installation Manager on a British rig, but this depends on company policy. On some American rigs the toolpusher may be known as the drilling foreman or the rig superintendent. TORQUE: The turning moment of a force applied to a shaft, e.g. a joint of drill pipe being made up to another. The applied force in pounds multiplied by the lever length in feet gives the torque in foot-pounds. TORQUE INDICATOR: A gauge attached to the end of the make-up tong arm and with a remote read-out in the doghouse, so that the correct make-up torque can be applied to the tong. TORQUE-UP, TO: To tighten a connection of two joints to the correct torque with the aid of tongs. TOTAL DEPTH: The final depth attained by a well at the completion of drilling. TOTAL VERTICAL DEPTH: See true vertical depth. TOTCO: A downhole survey tool run down the inside of the drill string to determini the angle and direction of the bit’s inclination from the vertical at any instant: A small compass card is punched on the tool’s arrival at the bit, and is read at the surface after the tool’s withdrawal by the sand line. TOUR: A work shift, usually of 12 hours duration on a rig. Pronounced ‘tower’ in the oil industry, in the American fashion.

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Glossary of Marine Drilling terms TOURPUSHER: An alternative name sometimes used for the junior or night toolpusher. Pronounced ‘towerpusher’. TOWMASTER: The person (usually a master mariner) who takes charge of a rig-move from one location to another when towing vessels are employed. He may be an employee of the drilling contractor or a specialist hired from a marine consultancy firm. TRANSIT: The passage of a rig from one location to another. TRANSIT DRAUGHT: The draught of water drawn by a rig during her trasnit. Normally this would be the lightest draught attainable, so that speed is maximised. The waterline of a semi-submersible at transit draught is usually in the area of the top of the pontoons, i.e. about 20 feet on a large unit. TRAVELLING BLOCK: The lower, movable, block of the hoist, which is suspended by the drilling line from the upper, or crown block. TREBLE: Three joints of drill pipe made up to make a stand of approximately 90 or 93 feet, for speed of handling. TRIM: Strictly, the difference between the rig’s forward draught and her after draught. Usually, however, taken to mean the angle the rig is lying at with respect to the waterline. TRIP GAS: Gas which comes up from the hole when a trip is being made. Although it is often enough to activate the gas ~alarms it is rarely very much in volume. TRIPPING: Making a round trip, q.v. TRUE VERTICAL DEPTH: The depth of a hole measured in a vertical line from the surface to the bit, without taking account of any angle of deviation. Measured depth (MD) may be considerably longer. TUBING: Narrow-bore pipe which is run do&n through casing into a liner to serve as a channel for oil or gas after well test’s have found evidence of hydrocarbons. TUBULARS: Any oilfield pipe, e.g. drill pipe, collars, casing or tubing. TUGGER, AIR: See air tugger. ..-

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Glossary of Marine Drilling terms TURBINE DRILLING: A method of rotary drilling in which the drill bit is

rotated by a downhole drilling motor, or ‘mud motor’, turbine or turbo-drill, just above it that is operated by the hydraulic force of the drilling fluid. Sometimes called ‘turbo-drilling’. TRUBO-DRILL: A type of combined drill bit and downhole drilling motor

used for turbine- or turbo-drilling. T.V.D.: The usual abbreviation for true vertical depth or total vertical

depth.

-uU.K.O.O.A.: The United Kingdom Offshore Operators Association, an

organisation representing the interests of a group of about 40 operators of offshore licences for production of oil and gas from the UK Continental Shelf. It provides a forum for discussion of technical and administrative matters and also consults with the UK government on these and other matters. UKOOA has many technical committees aimed at ensuring the safety of offshore workers and the environment. One of these prepares safety training guidelines. UMBILICAL: A flexible hose providing services to an underwater’installa-

tion or person. A diver’s umbilical connects him to a surface vessel ,or to a diving bell and contains pipes carrying air, hot water, gas, etc., while a pod umbilical carries the numerous control lines that actuate the BOP, hydraulics. ROVs are often operated by means of umbilicals. UNDERREAMER: A downhole tool with rock bit cones, on the ends of

pivoting arms that can expand from the sides of the tool to open up a previously drilled hole. This might be done to provide extra clearance for running casing so as to obtain adequate annular space for cementing, etc.

-vVARIABLE DECK LOAD: The weight of non-permanent equipment,

stores, ballast, etc. that a rig B able to carry when drilling. V.C.G.: The vertical centre of gravity of the rig, which must be at a certain distance below the metacentre for positive stability. V-DOOR: An opening in the drill floor windwall on the side opposite the

drawworks, from which a dragway ramp leads down to the pipe rack. Dril-

Glossary of Marine Drilling terms

ling equipment is dragged up the ramp and through the V-door to the drill floor. VENT: When loading bulk cargo into ‘P’ tanks, the lines are first blown

through by the supply boat’s pumps to establish that they are clear, the sight of dust venting from an open overside valve proving this to the bargemaster and supply boat captain. VENT LINE: A line fitted to the diverter at the top of the marine riser

through which gas can be vented safely to atmosphere. A deflector channels the gas to either the port or starboard side, whichever is downwind. VERTICAL WELL: An undeviated well. This is the type most frequently

drilled from a mobile unit, while directional or deviated wells are commonly drilled from fixed platforms.

-wWAITING ON CEMENT: A period of several hours that must elapse after

a cement job to allow the cement to set. No downhole work is done during this time. WAITING ON WEATHER: A period of downtime when drilling cannot

continue because of the sea state. High seas may cause excessive movement of the riser slip-joint, or of compensator or tensioner pistons. WALKAROUND: The platform approximately half way up the derrick, at

the height of the monkeyboard, which is normally enclosed by wind-resistant sheeting. WASH OUT, TO: To erode a metal object such as a drilling tubular joint or

a valve by the action of fluid pressure. Washing out of damaged tool joints may occur through, leakage. WASH OVER, TO: To run a fishing tool, usually with a mill-type edge, over

a fish

SO

that the tool can be rotated to cut the fish free.

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WATCH STANDER: The name used in some drilling companies for the

control room operator or the crewman whose job is to monitor the ballast. controls. WATER TABLE: The frame near the top of the derrick in which the crown

block is supported.

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Glossary of Marine Drilling terms WATERTIGHT DOOR: A door fitted with securing devices which make it

effectively watertight. Mobile rigs have many of these and they should normally be kept closed except for access. WATER TOWER: A retractable cylinder or frame containing deep well

pumps and pipes that is lowered from a jack-up rig into the sea so that suction for engine cooling water and the fire main can be obtained. WEAR BUSHING: A ring-shaped device fitted inside the wellhead to pro-

tect the top of the casing from abrasion by the drill bit as it enteres or leaves the well. WEATHER WINDOW: A period between spells of bad weather when a

weather-dependent operation such as a rig-move or mooring operation might be carried out. WEEVIL: An American name for a new hand or ‘green hand’ on a rig; short

for ‘boll weevil’. WEIGHT INDICATOR: A gauge in the doghouse which tells the driller the

weight suspended from, the travelling block and the weight on the bit. This information comes from a tension sensor at the deadline anchor. WELL: The completed hole made by the drill bit. WELLBORE: The hole made by the drill bit. Usually referred to as ‘the

hole’. WELL COMPLETION: The final phase of operations after total depth has

been reached, when the well is fitted with production equipment. WELLHEAD: A cylindrical device placed at the top of the hole by a floater

in which casing hangers are fitted and sealed and to which wellcontrol equipment can be attached during drilling and during subsequent production. The BOP stack and, later, the Christmas tree are attached to the wellhead. Jackup rigs and platforms install their wellheads on their decks. . WELL LOGGING: Any of various downhole methods used for the purpose of gaining information about formations drilled through. In addition to mud logging, which is continuous, these include electric, radioactive and sonic logging techniques. WELL STIMULATION: Any of several methods employed to increase flow

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Glossary of Marine Drill& terms of hydrocarbons from a well. These include acidizing and fracturing. WELL STIMULATION VESSEL: A specially-designed ship which can be brought to a location to carry out well stimulation. WET TRIP: A ‘trip’ when mud flows out of the joints of drill pipe every time a connection is broken. WHIPSTOCK: A downhole tool used to deflect a bit out of its original course and set it in a desired direction towards a predetermined target. They are used in directional drilling, in straightening crooked holes and in sidetracking junk left in a hole. WILDCAT: An exploration well drilled in an unproved area, far from any existing producing well. WILDCATTING: Drilling wildcat wells. This is more likely to be the employment of a rig when the oil price is high, since only about one wildcat in forty, on worldwide average, becomes a discovery well. WINDLASS: A large winch for winding anchor chain in or out. Sometimes called an anchor winch. WINDWALL: Sheeting erected round an exposed work area such as the drill floor or monkeyboard. WIPER TRIP: An operation to remove filter cake from the wall of the hole, made by running a rotating bit up and down the hole, going over any tight spots repeatedly until the wellbore is smooth. It is normally done during drilling and before logging, running casing, etc. to condition the hole. WIRELINE: A long, narrow wire wound on a storage drum on the drilling rig and used for well logging. A variety of devices for measuring downhole conditions can be attached to the wireline. Sometimes also the name given to the drilling line on some American rigs. W.O.C.: The normal abbreviation for ‘waiting on cement’, q.v. WORKING TENSION: The tension to which anchor chains are initially slackened off to following pre-tensioning tests. WORKOVER: An operation in which a rig is employed to restore or improve production from a completed well.

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Glossary of Marine Drilling terms WORK PERMIT: A written authorization to perform potentially hazardous

work such as welding, flame cutting, electrical or overside work, issued by the OIM or person ultimately responsible for safety. W.O.W.: Normal abbreviation for waiting on weather.

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