LNG Ship Construction - 1
Download LNG Ship Construction - 1...
LNG Ship Construction
Jayan Peter Pillai MSc CEng FIMarEST MRINA MIBM
Braemar Engineering Fullbridge Mill Fullbridge, Maldon Essex, CM9 4LE www.braemar.com
Index Page 1. 2. 3. 4. 4.1 4.2 4.3 4.4 4.5 4.6 4.7 5. 5.1 5.2 5.3 6. 7. 8. 9. 10. 10.1 10.2 10.3 10.4 10.5 10.6 10.7 11. 12. 13. 14. 15.
Introduction……………………………………………………….. 2 History of LNG Shipment………………………………………. 3 Definition of Ship………………………………………………... 5 Basic Design of a Ship………………………………………….. 5 Ship Contracts…………………………………………………….. 6 Terminology……………………………………………………….. 7 Ship Types………………………………………………………… 9 Development of Ships……………………………………………. 10 LNG Carriers……………………………………………………… 13 Materials and Strength of Ships………………………………… 16 Ship Building Steels……………………………………………… 18 Gas Carrier Rules and Regulations……………………………… 19 SOLAS Convention ………………………………………………. 21 IGC Code………………………………………………………….. 22 STCW Convention………………………………………………… 26 Testing of Materials………………………………………………. 27 Stresses on Ships………………………………………………….. 29 Welding and Cutting……………………………………………… 37 Shipyard Layout……………………………………………………. 38 Design Information for Production……………………………….. 39 Assembly of Ship Structures………………………………………. 40 Shell Plating and Framing………………………………………... 46 Bulkhead and Pillars……………………………………………… 48 Decks, Hatches and Superstructures……………………………… 50 Fore End Structure…………………………………………………. 53 Aft End Structure………………………………………………….. 54 Tanker Construction……………………………………………….. 56 Launching…………………………………………………………… 56 Liquified Gas Carriers……………………………………………. 57 Sea Trials………………………………………………………….. 64 Ship Inspection……………………………………………………. 65 Dry Docking and Ship Repairs………………………………….. 67
Annex 1 Annex 2
References………………………………………………… Useful Websites…………………………………………..
Braemar Engineering (Wavespec Limited) is an engineering company specialising in the marine, offshore and land based gas industries. The Company has three main divisions: - Ship Design, Ship Construction and Ship Operation. - Offshore Dynamic Positioning Studies and Projects - Land Based LNG Import and Export Terminals, LNG Liquefaction Plants, Peak Shaving Plants, LNG Regasification Plants, LNG Fuelling Stations and LNG Distribution & Trucking. Braemar Engineering (Wavespec Limited) has offices in Maldon in the United Kingdom and in Houston, Texas. Braemar Engineering is part of the Braemar Technical Services Group. It is supported by a worldwide network of offices with over 380 technical staff of all disciplines. LNG Ships are expected to work the same route for its working life, which may exceed 40 years. Design for operation is the result. The key requirement of a new ship is that it can trade profitably, so economics is of prime importance in designing them. An owner requires a ship that will give the best possible returns for the owner’s initial investment and running costs. The final design should be arrived at taking into account not only present economic considerations, but also those likely to develop within the life of the ship.
History of LNG Shipment 3
LNG Carrier (Membrane Tanks)
LNG Carrier (MOSS Tanks) 4
Definition of “Ship”
A ship is a vessel of considerable size for deep water navigation. The Historic Ships Committee have designated a vessel below 40 tons and 40 feet in length as a boat. Submarines and Fishing Vessels are always known as boats, whatever their size. The Statutory Definition of a “ship” in the UK Merchant Shipping Act of 1995, is provided in s.313 “ Ship includes every description of vessel used in Navigation”.
Basic Design of a Ship
The main requirement of a new ship is that it can trade profitably. The owner requires a ship that will give the best possible returns for the initial investment and running costs. With LNG Ships, the vessel could work the same route for its working life of 40 years. With the aid of computers it is possible to make a study of a large number of design parameters, and hence design of a ship that is technically feasible and economically efficient. The design should take into consideration first cost, operating cost and future maintenance. The initial design of a ship goes through 3 stages: concept; preliminary and contract design. A concept design should, from the objectives, provide sufficient information for a basic techno-economic assessment of the alternatives to be made. Preliminary design refines and analyses the agreed concept design, fills out the arrangements and structure, and aims to optimize service performance. At this stage the builder should have sufficient information to tender. Contract design details the final arrangements and systems agreed with the owner and satisfies the building contract conditions. Post-contract design requires confirmation that the ship will meet all operational requirements, including safety requirements from regulators. It addresses design for production, where the structure, outfit, and systems are planned in detail to achieve a cost and time effective building cycle. The post contract design will also ideally consider the future maintainability of the ship in the arrangements of equipment and services. Information provided by design: - Dimensions - Displacement - Stability - Propulsive characteristics and hull form - Preliminary General Arrangements - Principal Structural details Each item of information may be considered in more detail. The dimensions of most ships are mainly influenced by the cargo carrying capacity of the ship and the draft. Increase in length produces higher longitudinal bending stresses. Breadth may be such as to provide adequate transverse stability. A minimum depth is controlled by the draft plus statutory freeboard. Increase in depth is preferred to increase in length. Draft is limited by area of operation.
Displacement is made up of lightweight plus deadweight. Lightweight is the weight of the ship as built. Deadweight is the difference between the lightweight and loaded displacement ( weight of cargo plus weight of fuel, stores, water ballast, fresh water, crew and baggage). In determining the dimensions, static stability is kept in mind, in order to ensure that this is sufficient in all possible conditions of loading. Beam and depth are the main influences. Statutory freeboard and sheer are important together with the weight distribution in arranging the vessel’s layout. Adequate propulsive performance will ensure that the vessel attains the required speeds. The hull form is such that it offers a minimum resistance to motion. Service Speed is the average speed at sea with normal service power and loading, under average weather conditions. Trial Speed is the average speed obtained using the maximum power over a measured course in calm weather with a clean hull and specified load condition. This speed may be a knot or so more than the service speed. Unless a hull form similar to that of a known performance vessel is used, a computer generated hull form and its predicted propulsive performance can be determined. Propulsive performance can be confirmed by subsequent tank testing of a model hull. This may lead to further beneficial modifications. The owner may specify their choice of propulsion plant. The general arrangement is prepared in cooperation with the owner. All LNG vessels are built to the requirements of a classification society. (Lloyds Register, American Bureau of Shipping, Bureau Veritas, Det Norske Veritas, RINA). Class have rules on structural scantlings. Computer programs can determine the minimum hull structural scantlings. Owners may specify thicknesses and materials in excess of IMO and Class requirements.
4.1 Ship Building Contracts Ship Owners may employ a firm of consultants to provide the preliminary design, prepare the tender specifications, evaluate tenders and oversee the construction on their behalf. The successful shipbuilder will prepare building specifications for approval by the owner. The technical specification will normally include:1. Brief description and essential qualities and characteristics of the ship. 2. Principal dimensions 3. Deadweight, cargo and tank capacities, etc. 4. Speed and power requirements. 5. Stability requirements. 6. Quality and standard of workmanship. 7. Survey and certificates 8. Accommodation details 9. Trial conditions 10. Equipment and fittings 11. Machinery Details, including electrical installation.
Typical Payment Schedule - 10% on signing contract - 10% on arrival of materials on site - 10% on keel laying - 20% on launching - 50% on delivery.
Aft Perpendicular (AP): A perpendicular drawn to the waterline at the point where the after side of the rudder post meets the summer load line. Where no rudder post is fitted it is taken as the centre line of the rudder stock. Forward Perpendicular (FP): A perpendicular drawn to the waterline at the point where the fore-side of the stem meets the summer load line. Length Between Perpendiculars (LBP): The length between the forward and aft perpendiculars measured along the summer load line. Amidships: A point midway between the after and forward perpendiculars. Length Overall (LOA): Length of vessel taken over all extremities. Lloyd’s Length : Used for obtaining scantlings if the vessel is classed with Lloyd’s Register. It is the same as length between perpendiculars except that it must not be less than 96% and need not be more than 97% of the extreme length on the summer load line. Register Length: Length of ship measured from the fore side of the head of the stem to the aft side of the head of the stern post. In the case of a ship not having a stern post, to the fore side of the rudder stock. If the ship does not have a stern post or a rudder stock, the aft terminal is taken to the aftermost part of the transom or stern of the ship. This length is the official length in the register of ships maintained by the flag state. IMO Length: is defined as 96% of the total length on a waterline at 85% of the least molded depth measured from the top of keel Molded dimensions are taken to the inside of plating on a metal ship. Base Line: A horizontal line drawn at the top of the keel plate. All vertical molded dimensions are measured relative to this line. Molded Beam: Measured at the midship section. This is the maximum molded breadth of the ship. Molded Draft: Measured from the base line to the summer load line at the midship – section. Molded Depth: Measured from the base line to the heel of the upper deck beam at the ship’s side amidships. Extreme Beam: The maximum beam taken over all extremities. Extreme Draft: Taken from the lowest point of keel to the summer load line. Draft marks represent extreme drafts. Extreme Depth: Depth of vessel at ship’s side from upper deck to lowest point of keel. Half Breadth: A ship’s hull is symmetrical about the longitudinal centre line , so half the beam or half breadth in any section if given.
Freeboard: The vertical distance measured at the ship’s side between the summer load line and the freeboard deck. Freeboard Deck: Normally the uppermost complete deck exposed to weather and sea, and has a permanent means of closing all openings, and below which all openings in the ship’s side have watertight closings. Sheer: A rise in the height of the deck (curvature or in a straight line) in the longitudinal direction. Measured as the height of deck at side at any point above the height of deck at side amidships. Camber (Round of beam): Curvature of decks in the transverse direction. Measured as the height of deck at centre above the height of deck at side. Straight line camber is often used on large ships to simplify construction. Rise of Floor (Deadrise): The rise of the bottom shell plating line above the base line. This rise is measured at the line of moulded beam. Large ships often have no rise of floor. Half Siding of Keel: The horizontal flat portion of the bottom shell measured to port or starboard of the ship’s longitudinal centre line. Useful to know when dry docking. Tumblehome: The inward curvature of the side shell above the summer load line. Not common on large modern ships. Flare: the outward curvature of the side shell above the waterline. At the fore end of the ship. Stem Rake: Inclination of the stem line from the vertical. Keel Rake: Inclination of the keel line from the horizontal. Parallel Middle Body: The length over which the midship section remains constant in area and shape. Entrance: The immersed body of the vessel forward of the parallel middle body. Run: The immersed body of the vessel aft of the parallel middle body. Gross Tonnage: is a measure of the enclosed internal volume of the vessel (originally computed as 100 cubic feet per ton). Suezmax: the largest tanker than can transit the current Suez Canal fully laden (150 000 dwt). Suez maximum breadth & draft limits are 75 and 20 metres. Aframax: (American Freight Rate Association) 80 000 to 115 000 dwt Panamax: Vessel with beam & length restrictions of 32.2 and 275 metres. Upto 70 000 dwt Handymax: Tankers of 35 000 to 45 000 dwt Capesize: Ships too large to transit the Panama Canal, so have to voyage around Cape Horn.
Liquid Cargo Ships: Oil Tankers, Liquefied Gas Carriers , Chemical Tankers Dry Cargo Ships: Tramps, Bulk Carriers, Cargo Liners, Container Vessels, Barge Carriers, Ro-Ro Ships, Refrigerated Cargo Ships, Timber Carriers, Livestock Carriers, Car Carriers Passenger Ships: Liners, Cruise Ships, Emigrant & Pilgrim Ships (STP’s), Cross-channel Ferries, Coastal Ferries, Harbour Ferries, Passenger Submarines High Speed Craft: Multi-hulls including Wave Piercers, Small Waterplane Area Twin-hull (SWATH), Surface Effect Ship (SES) & Hovercraft, Hydrofoil, Wing in ground effect craft (WIG) Off Shore Vessels: Supply Ships, Pipe Layers, Crane Barges, Semi-Submersible Drill Rigs, Drill Ships, Accommodation Barges, Production Platforms, Floating Storage Unit (FSU), Floating Production & Storage unit (FPSO), Fishing Vessels: Factory Ships, Trawlers, Purse Seiners
Harbour/ Ocean Work Craft: Tugs, Cable Layers, Dredgers, Salvage/buoy Vessels, Tenders, Pilot Craft, Floating Dry Docks, Floating Cranes, Lightships, Wind Farm Support Vessels Submersibles: Warships: Air Craft Carriers, Helicopter Carriers, Destroyers, Frigates, Petrol Boats, Nuclear Submarines, Submarines
Development of Ships
Floating Logs Canoes carved from Tree Trunks Wooden Sailing Ships Metal Hulled Ships Flush Deck Ship Three Island Ship type Combined Poop & Bridge Raised Quarter Deck Awning or Spar Deck Open Shelter Deck All Aft Cargo Ship Oil Tankers Chemical Tankers Liquefied Gas Carriers
Hull Form: Between 1940 and 1970 there was a steady increase in the speed of the dry cargo ship. A much finer hull is apparent in modern ships. Bulbous bow forms and open water sterns are used to advantage, and considerable flare may be seen in the bows of container ships to reduce wetness on deck. Oil Tankers Until 1990, the form of vessels specifically designed for the carriage of oil cargoes has not undergone a great deal of change since 1880. The growth in size from 1880 to 1945 was gradual, from 1 500 dwt to about 12 000 dwt, reached 20 000 in 1953 and 30 000 in 1959. By 1979 the largest ULCCs reached 564 763 dwt, 458.45 metres in length, 70.06 metres beam and 28.50 draft. The Seawise Giant ( Jahre Viking ) was the longest and had the greatest cargo carrying capacity, the Batilus had the deepest draft, and the Sea World had the widest beam.
ULCC Jahre Viking (ex-Seawise Giant) DWT 564 763 Length 458.45 Beam 68.8 m Draft 22.60 m Service speeds of tankers has increased since the 1940s from 12 knots to 17 knots and above. The service speed is related to the optimum economic operation of the tanker. Optimum size of the tanker is related to current market economics. The tanker fleet has grown enormously to meet expanding demand for oil. Structurally, one of the greatest developments has been in the use of welding, and oil tankers were amongst the first vessels to utilize the application of welding. Riveting is very labour intensive, and maintaining oil tight joints more difficult. Welding allows cheaper fabrication methods in ship building. The current trend is placing the machinery and propulsion plant aft. Most commercial ships have their accommodation and bridge aft. After the tanker Exxon Valdez ran aground in 1989 in the Prince William Sound in Alaska, double hull oil tankers were being built. Oil tankers now generally have a single pump space aft, and just forward of the machinery space, and specified slop tanks into which tank washings and oily residues are pumped. Tank cleaning may be accomplished by water driven rotating machines on smaller tankers, but for crude oil tankers of 20 000 dwt and above, the tank cleaning system uses crude oil washing.
ULCC Al Rekkah DWT 414 366 Length 366m Beam 70.06 m
Draft 22.60 m
In 1959, the Methane Pioneer , a 5 034 dwt vessel was the first to carry a large cargo of LNG. Today the largest LNG vessel can carry 266 000 cubic metres. ********************************************************************* ***** Liquefied Natural Gas LNG is made from natural gas, which is clean burning source of heat energy with many applications, including as fuel for power generation, industrial and home 13
heating, and as a chemical feedstock. Natural Gas is composed primarily of methane (typically 85 to 90%), but it also may contain ethane, propane and heavier hydrocarbons (butane, pentane, hexane, etc). Small quantities of nitrogen, oxygen, carbon dioxide and sulphur compounds are also found in most sources of natural gas. Natural gas is transported by pipeline to its consumers, but when the distance between source and consumption is great ( >1500 km by sea or >5000 km over land) then liquefaction of the gas to reduce its volume by a factor of 600 becomes economic. The gas in liquid form can then be carried economically by ships equipped with wellinsulated tanks made from special steel. Liquefied natural gas is formed when natural gas is cooled by the refrigeration process to temperatures of - 1630 C, at atmospheric pressure, and the gas condenses to a liquid. Before natural gas can be liquefied in this way, the impurities, including carbon dioxide, sulphur compounds, heavier hydrocarbons and water must be removed by various processes. If nitrogen is present in the natural gas at high levels, it may be removed at the end of the process as it condenses at an even lower temperature (-196 0C) than pure methane. The liquefaction process can be designed to purify the LNG to almost 100% methane, or leave in more ethane and some LPGs (propane and butane) to match the pipeline gas specifications in the receiving gas system or country. Most gas distribution systems specify limits in terms of the heating (calorific) value of the gas to maintain safe conditions in combustion equipment. The specifications are quite different in Japan (the largest LNG importer) and the USA, the major emerging LNG import market. LNG is about 47% as dense as water and is odourless, colourless, non-corrosive and non-toxic. When vaporized it burns only in concentrations of 5% to 15% when mixed with air. Neither LNG nor its vapour can explode in an unconfined environment. The LNG chain from field production to pipeline gas consumers across oceans is as follows:Field Gas Production - Condensate & LPG Recovery – Liquefaction & Storage as LNG – Shipping - Receive, Storage & Re-gasification - pipeline - Natural Gas distribution In French, Spanish, Portuguese, or Italian-speaking countries, the abbreviation GNL is used in place of LNG. How is LNG made ? LNG involves the purification, chilling and liquefaction of natural gas by various processes including refrigeration using hydrocarbon refrigerants. The first step is removal of carbon dioxide and other acid gases such as hydrogen sulphide by a recirculating amine process. This is a very common process in natural gas treatment plants producing pipeline gas and in petroleum refining and petrochemical plants. The carbon dioxide is normally vented to the atmosphere. If carbon dioxide is not removed, it would freeze solid in the heat exchangers involved in the liquefaction process and cause blockages. Water would do the same, so it is removed using molecular sieve driers as commonly used in natural gas treatment plants producing pipeline gas, Traces of mercury sometimes occur in natural gas and this is a potential problem for LNG Plant, as mercury corrodes aluminium which is used in some of the equipment. Mercury removal facilities are usually incorporated into LNG plants as a result.
The chilling of the gas to moderately low temperatures causes at first the condensation of heavy hydrocarbons, which might also freeze and cause blockages of equipment at lower temperatures. These components of the gas are removed in a “scrub” column along with some of the LPG (propane and butane) as the gas is cooled to about minus 35 degrees C. Chilling and then liquefaction is accomplished by a refrigeration process powered by a large compressor typically driven directly by a gas turbine or steam turbine. In principle, the refrigeration process is no different from that in a domestic refrigerator or air conditioner, but the difference in scale is immense. Cooling and condensation of the high pressure refrigerant gas is accomplished in air coolers or water cooled heat exchangers. When the high pressure liquid refrigerant is depressured through an expansion valve, the drop in temperature is used to extract heat energy from the process gas through a heat exchanger. Usually there are at least two main compressors. Usually there are at least two main compressors with multiple stage of heat exchangers for maximum efficiency in liquefying the methane and ethane in the purified natural gas to make LNG. How is LNG stored ? LNG is stored in shore tanks at both ends of its sea voyage to accumulate sufficient volumes for economic shipping. The tanks are of double walled construction with extremely efficient insulation between the walls. Large tanks tend to have a low aspect ratio (height to width) and are cylindrical in design with a domed roof. Storage pressures in these tanks are very low, less than 5 psig. The outer walls are made of reinforced concrete and are designed to safely contain the contents of the inner tank in the extremely unlikely event of it developing a leak. How is LNG kept cold ? The shore tank or ship tank’s insulation, as efficient as it may be, will not keep the LNG cold enough to remain as a liquid by itself. LNG is stored as a “boiling cryogen”, that is, it is a very cold liquid at its boiling point for the pressure at which it is being stored. Storage of LNG utilizes a phenomenon called “auto-refrigeration”, where the LNG stays at near constant temperature if kept at constant pressure. This constant temperature occurs as long as the LNG vapour boil off is allowed to leave the storage tank. The vapour is either removed and used as fuel or re-liquefied and returned to the tank. Is LNG safe ? It is important to remember that LNG is a form of energy and must be respected as such. Today LNG is transported and stored as safely as any other liquid fuel. Before the storage of cryogenic liquids was fully understood, there was a serious incident involving a LNG storage tank failure killing 128 people in Cleveland, Ohio in 1944. This incident virtually stopped all development of the LNG industry for 20 years. The race to the Moon led to a much better understanding of cryogenics and cryogenic storage with the expanded use of liquid hydrogen (- 2520 C) and liquid Oxygen (1820 C). LNG technology grew from the advancements developed by NASA for the space programme. Today the LNG industry maintains an excellent safety record by incorporating many years of experience and engineering solutions and safety codes into the design and operation of LNG liquefaction, storage and re-gasification plants around the world. Where does LNG come from ?
LNG primarily comes from areas where large gas discoveries have been made, such as Algeria, Australia, Brunei, Egypt, Equatorial Guinea, Indonesia, Libya, Malaysia, Nigeria, Norway, Oman, Qatar, Trinidad and the United Arab Emirates. Some LNG is produced in the US (Alaska) and Europe. LNG can be produced wherever natural gas is available. Hydraulic fracturing or “Fracking” is also used to produce natural gas. Fracking is the process of propagation of fractures in a rock layer by a pressurized fluid.
Materials and Strength of Ships
Basically a ship can be built of any material provided it can float on water and navigate safely from one port to another. However the most common materials used are steel, aluminium, wood, GRP and FRP. Steel is used for the hulls of all large ships, because it is strong, cheap and easily available. Steel alloys are more expensive but have some advantages over mild steel. Mild steel is not suitable for LNG tanks. At low temperatures mild steel becomes brittle and cracks. Aluminium and its alloys are 10 times more expensive than mild steel, lighter and not as strong. It is used for smaller craft and for the accommodation blocks of some passenger ships. Aluminium has a lower melting point than steel. Because of their cryogenic properties, some LNG tanks are made of aluminium. Glass Reinforced Plastic and Fibre Reinforced Plastic is used for smaller hulls. They are flammable and lose their strength properties above ambient temperature. Wood was the traditional material for building ships until the 20th century. Some small boats and Fishing Vessels are still made of Wood. Wood boat building is an ancient art and does require a lot of training and skill. Steel The production of all steels used for shipbuilding purposes starts with the smelting of iron ore and the making of pig-iron. The iron ore is smelted in a blast furnace, which is a large, slightly conical structure lines with a refractory material. Coke is used to 16
provide the heat for smelting, and limestone is also added. This makes the slag formed by the incombustible impurities in the iron ore fluid, so that it can be drawn off. Air for combustion is blown in through a ring of holes near the bottom., and coke, ore and limestone are charged into the top of the furnace in rotation. Molten metal is drawn off at intervals from a hole or spout at the bottom of the furnace, and run into molds formed in a bed of sand or into metal molds. The resultant pig-iron contains 92-97% iron, the remainder being carbon, silicon, manganese, sulphur and phosphorus. In the subsequent manufacture of steels the pig-iron is refined and the impurities are reduced. Steels are alloys of iron and carbon. The carbon percentage varying from 0.1% (mild steel) to about 1.8% in some hardened steels. Iron has a melting point of 15100 C. The four processes for producing steel are: 1. Open Hearth Process 2. Electric Furnaces 3. Oxygen Process 4. Bessemer Converter Process The Bessemer Converter Process is not used for shipbuilding steels. Chemical Additions to Steels: Chemical elements are added to steels during the above processes. They may be used to deoxidise the metal, remove impurities and bring them out into the slag, and to bring about the desired composition. The amount of deoxidizing elements added determines whether the steels are “rimmed steels” or “killed steels”. Rimmed steel is unsuitable for thicker plate, as it contains gas blow holes near the centre of the ingot. The term “killed steel” indicates that the metal has solidified in the ingot mold with little or no evolution of gas. Addition of sufficient quantities of deoxidising material (silicon or aluminium) has reduced the gas content. Steel of this type has a high degree of chemical homogeneity, and killed steels are superior to rimmed steels. Semi-killed steel has more gas content in the ingot. Heat Treatment of Steels: the properties of steels may be altered greatly by the heat treatment to which the steel is subjected. Heat treatment brings about a change in the mechanical properties, by modifying the steel’s structure. Annealing: This consists of heating the steel at a slow rate to a temperature of 850950 0 C, and then cooling it in the furnace at a very slow rate. Annealing relieves any internal stresses and softens the steel. Normalizing: This is carried out by heating the steel slowly, similar to annealing, and allowing it to cool in air. The faster cooling rate produces a harder, stronger steel, and also refines the grain size. Quenching: Steel is heated similar to annealing and normalizing, and then quenched in water or oil. The fast cooling rate produces a very hard structure with a higher tensile strength. Tempering: Quenched steels may be further heated to 6800 C, and some alloy steels are then cooled fairly rapidly by quenching in oil or water. This treatment is to relieve the severe internal stresses produced by the original hardening process and to make the material less brittle but retain the higher tensile stress. Stress Relieving: To relieve internal stresses the temperature of the steel is raised so that no structural change of the material occurs and then it may be slowly cooled.
Steel Sections A range of steel sections are rolled from ingots. Flat Bars, Offset Bulb Plate, Angle Bar, Tee Bulb, Channel Bar, Tee Bar are some of the main sections used in shipbuilding.
Ship Building Steels
Mild Steel containing 0.15 to 0.23% carbon (and a reasonably high manganese content) is generally used for hull construction. Both sulphur and phosphorus are kept to a minimum (less than 0.05%). Higher concentrations of both are detrimental to the welding properties of the steel. Cracks can occur during the rolling process if the sulphur content is high. Steel for a ship classed with Lloyd’s Register is produced by an approved manufacturer. Inspection and prescribed tests are carried out at the steel mill before dispatch. All certified materials are marked with the society’s brand, and other particulars as required by the rules. From 1959 major societies agreed to standardise their requirements. IACS graded steels are A, B, C, D & E. Grade A is ordinary mild steel to LR requirements and generally used in shipbuilding. Grade B is a better quality than Grade A, and specified where thicker plates are required in the more critical regions. Grade C, D and E possess increasing notch-tough characteristics. Grade C being to ABS requirements. Normal mild steel are not used for LNG tanks or barriers, as they turn brittle at low temperatures. High Tensile Steels have a higher strength than mild steel and could be used in the more highly stressed regions of large ships. It also allows reduction in thickness of deck, bottom shell and framing. They do cause larger deflections, rusting of thinner plate is an issue and have reduced fatigue life compared to mild steel. They are also susceptible to stress corrosion cracking. LNG ships with membrane tanks generally do not have these steels, as the flexing can damage the tanks. Corrosion Resistant Steels: (stainless steels) steels with alloying elements are too costly for normal ship building. They are used for the tanks carrying corrosive chemicals. They could be used for Liquefied Gas tank barriers due to their cryogenic properties. Steel Sandwich Panels: Proprietary manufactured steel sandwich panels are available for deck repairs and naval ship construction, where lighter weight is a bonus. They are not used on LNG ships. Steel Castings Stern Frames, Rudder Frames, Spectacle Frames for bossings, and other structural components may be produced as castings. Molten steel produced by the Open Hearth, Electric furnace or Oxygen Process, is poured into a carefully constructed mold and allowed to solidify to the shape required. After removal from the mold, heat treatment is required – annealing or normalising and tempering to reduce brittleness. Steel Forgings Forging is a method of shaping metal by heating it to a temperature where it becomes more or less plastic and then hammering or pressing it to the required form. Forgings are manufactured from killed steel made from the Open Hearth, Electric Furnace or Oxygen Process. Where possible the working of the metal is such that metal flow is in
the most favourable direction with regard to the mode of stressing in service. Large engine crankshafts are forged. Aluminium Aluminium is a relatively new discovery. Mainly because it was difficult without electricity to achieve the high temperatures, 2000 0 C, required to melt Bauxite. It was first used for small craft in 1891 and for experimental naval vessels in 1894. It has not been a significant material for ships until comparatively recently. Aluminium (2723 kg/m3) is lighter than steel(7840 kg/m3). With an aluminium structure 60% of the weight of a steel structure may be saved. Aluminium has a high resistance to corrosion and is non-magnetic. A major disadvantage of aluminium alloys is the higher initial and fabrication costs. Aluminium ( 660.30 C) also has a lower melting point than steel (about 1370 0 C). Aluminium is used for the construction of MOSS Spherical Tanks for the carriage of LNG. These tanks can be as large as 800 tonnes, 45 metre diameter, 32mm wall thickness and 160mm thickness at the equatorial ring. Because of the low melting point of aluminium, fire protection is more critical on ships built of aluminium. Fibre - Reinforced Composites (FRCs) Composite materials combine two or more elements with very different characteristics to provide a material with good structural capability. The fibre provides the strength and the matrix in which it is contained, usually a plastic, holds the fibre in place. The fibre can be arranged to provide directional strength so the composite can be tailored to very specific structural requirements. Composite technology is a very ancient art. Bricks and concrete are composites. For marine applications Glass fibre reinforced plastic (GRP) was first introduced in the 1950s. It is now the main material for small boats. Some boats are still operational after 50 years. GRP is generally light and durable. The major advantages of GRP/ FRP for small vessels include low weight, combined with high strength and stiffness. The disadvantages are: It is labour intensive, dependent on the skill level of workmanship, is flammable, loses its mechanical properties above ambient temperatures. ********************************************************
Gas Carrier Rules and Regulations
Regulations & Codes of Practice governing Gas Carriers It is IMO’s (International Maritime Organization) global responsibility to develop international standards and adopt conventions on many aspects of marine operations, including the carriage of liquefied gas. This responsibility is discharged through a number of Codes, Recommendations, Treaties and Guidelines. The main conventions that apply to all vessels, including Gas Carriers are: 1. International Conference on Load Lines, 1966 2. International Convention on Safety of Life at Sea (SOLAS), 1974 as amended. 3. International Convention on Marine Pollution Pollution 1973/78 (MARPOL)
4. International Convention on Training & Certification of Seafarers, 1978 (STCW), as amended in 1995 (STCW’95) Ships built after 1986, are required to comply with the International Gas Carrier Code (IGC), and its International Certificate of Fitness. This compliance is necessary to gain entry to a foreign port. Ships built before 1986 were recommended to comply with the IMO Gas Carrier Code (GC), and before 1976 with the Existing Gas Carrier Code (EGC), in addition to a Certificate of Fitness. Packaged gases carried on other ship types (Ro-RO’s or Container ships) are covered in the IMDG Code Class 2. The underpinning philosophy of the IGC Code is to relate the ship type to the hazards of each of the products covered by the Code, including cargoes transported under cryogenic or pressurised conditions. The code is based on established naval architectural and engineering principles, together with the best understanding available of product hazards. The code is a live document, as it reflects the continuing development as it reflects the continuing development of gas carrier technology. The Classification Societies Classification Societies are organisations that establish and apply technical standards in relation to the design, construction and survey of marine related facilities, including ships and offshore structures. The standards are issued as published rules. A Certificate of Class is issued when a vessel is designed and built to the appropriate rules of the society. Such a certificate does not imply, and should not be construed as, an express warranty of safety, fitness for purpose or seaworthiness of the ship. It only attests that the vessel is in compliance with the standards developed and published by the society issuing the classification certificate. Classification Societies are independent, self regulating and no commercial interests relating to ship design, ship building, ship ownership, ship operation, ship management, ship maintenance or repairs, insurance or chartering. In drawing their rules they consult with members of the industry who are considered expert in their field. Classification rules are developed to contribute to the structural strength and integrity of essential parts of the ship’s hull, appendages and the reliability and function of the propulsion and steering systems, power systems, power generation and auxiliary systems that maintain essential services on board. A Ship is maintained in class, provided that relevant rules have been complied with, and approved surveys carried out in accordance with its rules. Classification societies also maintain significant research departments that encourage innovative developments in the maritime industry. All classification societies act on behalf of Flag Administrations in carrying out statutory surveys and inspections. Inter- Industry Organisations All existing International Conventions, Codes and Regulations deal with the legislation of the ships. The right equipment and qualifications are recommended to achieve safe operations, but no operational guidance or recommendation is given. A number of Inter-Industry Organisations are involved in producing “Codes of Practice” that support the operator in complying with legislation. 1. ICS - International Chamber of Shipping.
2. OCIMF – Oil Companies International Marine Forum 3. SIGTTO – Society of International Gas Tanker and Terminal Operators International Chamber of Shipping (ICS) The ICS is made up and supported by the Shipowner’s Associations of the member countries. Their Head Quarters are based in London, England. The structure of the ICS is based on committees that deal with the various branches of maritime activity (Nautical, Construction, Tanker Safety, Operational Pollution, Liquefied Gas and Chemical Panels, etc.) Some of the codes of practice produced by the ICS for Gas Carriers are: - Tanker Safety Guide (Liquefied Gases) - International Safety Guide for Oil Tankers & Terminals (ISGOTT) - Ship to Ship Transfer Guide (Liquefied Gasses). Oil Companies International Marine Forum (OCIMF) Only Oil Companies can be members. Their primary aim is to foster safe and pollution free operations for all types of tankers at sea and in port. Some of their codes of practice specifically for Liquefied Gas Carriers are: - Safety Inspection Guidelines and Terminal Safety Check-list for Gas Carriers. - Standardisation of Manifolds for Refrigerated Liquefied Gas Carriers (LNG). - Standardisation of Manifolds for Refrigerated Liquefied Gas Carriers for Cargoes from 0 degrees to -104 degrees C. - Design and Construction Specifications for Marine Loading Arms. - Safety Guide for Terminals Handling Ships Carrying Liquefied Gasses in Bulk. Society of International Gas Tankers & Terminal Operators (SIGTTO) The majority of Gas Companies are members of SIGTTO, covering gas carrier owners as well as liquefied gas producers and users. The main SIGTTO publication is “Liquefied Gas Handling Principles on Ships & Terminals”. In 2005 SIGTTO published “LNG Shipping Suggested Competency Standards”. The standards were written using the STCW “competence based methodology” and are presented by SIGTTO as guidance to the industry. Ship Inspectorate Groups : SIRE and CDI Commercial inspectorate groups act on behalf of charterers. Ship Inspecting and Reporting (SIRE) are mainly involved with oil and Liquefied Gas Carriage. The Chemical Distribution Institute (CDI) deal with chemical tankers and those gas carriers carrying butadiene vinyl chloride and other chemicals. Their aim is to improve the standard of ships.
SOLAS International Convention on Safety of Life at Sea 1974 SOLAS and its amendments, main objective is to lay down the basic design and equipment requirements to ensure that all ships comply with basic safety standards. Chapter II-1 : Construction – Subdivision and Stability, Machinery and Electrical Installations. Chapter II-2 : Construction, Fire Protection, Fire Detection and Fire Extinction. 21
The fire main on gas carriers should be capable of at least 5 bar to adequately supply the water spray system. Chapter III : Life Saving Appliances Chapter IV : Radio communications Chapter V: Safety of Navigation Chapter VII : Carriage of Dangerous Goods Chapter IX : Management for the Safe Operation of Ships (ISM Code) Chapter XI-1 and XI-2 : Special Measures to Enhance Maritime Safety.
IGC The International Code for the Construction & Equipment of Ships Carrying Liquefied Gases in Bulk Chapter 1 - General Deals with:- Requirements for the specification of the Code - Date of entry 1st July 1986 - Definition of Hazards - Survey and Certification - Maintenance after Survey, so that the ship is maintained to conform with the provisions of the Code Chapter 2 - Ship Survival Capability & Location of Cargo Tanks For each cargo, the Code aims to achieve a high level of safety by specifying the criteria for ship survivability after damage and the tank location and cargo containment standards. Ships subject to the Code are classed as either Type 1G, 2G or 3G. The intact stability of gas carriers in all conditions, including damage stability, should be positive (GM 0.15m). The bottom and side damage that a gas carrier should be able to survive are as shown in the Figures below:
Chapter 3 - Ship Arrangements Outlines the segregation standards for the ship, covering:
Cargo Tanks Hold Spaces Cargo Piping Accommodation Compressor Rooms Motor Rooms
Chapter 4 - Cargo Containment Details the requirements for the cargo containment systems and types of tank: - Integral - Membrane - The independent tanks of Types A, B & C It includes specific details for design loads, structural analysis, allowable design stress, secondary barrier, thermal calculations, materials, construction and testing. All categories of tanks, except for independent type C, are generally restricted to a maximum vapour pressure of 0.25 bar, although type B may be allowed to hold slightly more pressure. For temperatures between -10 and -55 0 C - the hull may act as a secondary barrier For temperatures below -55 0 C - a secondary barrier is required. For temperatures below -100 C - suitable insulation around the tank is required. Integral tanks are usually limited to the carriage of cargoes that have a temperature of -100C or higher at atmospheric pressure. Chapter 5 - Process Pressure Vessels and Liquid and Pressure Piping System This chapter details the requirements for the design, construction and testing of cargo piping systems. Chapter 6 - Materials Gives the requirements for plates, sections, pipes, forgings, castings and elements used in the construction of cargo tanks, cargo and process piping, secondary barriers and the hull structure. Chapter 7 - Cargo Pressure / Temperature Control This chapter covers the equipment and arrangements necessary for containment when the cargo is carried at a pressure below that corresponding to the ambient temperature. It covers boil-off gas by either reliquefaction or by combustion in the ships boilers, engines and similar equipment. Only LNG can be used in the ship’s boilers or engines, as the vapour is lighter than air, and any leaks would rise out of the Engine Room. Chapter 8 - Cargo Tank Vent System The requirements are given for pressure relief systems serving cargo tanks, spaces surrounding cargo tanks and cargo piping. This includes the capacity and arrangements of relief valves. Unless the cargo tank is smaller than 20 m3, each cargo tank must be fitted with two pressure relief valves. Chapter 9 - Environmental Control Provides standards for the environmental control of : - Cargo tanks and piping systems - Interbarrier spaces and void spaces - Vapour spaces of loaded cargo tanks - Holds, when inerting is not required
The quality and control of the inert gas produced onboard is also covered in this chapter. Chapter 10 - Electrical Arrangements This chapter defines the hazardous zones created by the products and the class of electrical equipment, including instrumentation that is required in them. Requirements for cargo pump room and electric motor rooms are specifically addressed. Chapter 11 - Fire Protection and Fire Extinction This chapter details standards for structural fire protection, fire protection arrangements in cargo handling spaces, sea water extinguishing systems and water spray systems in the cargo tank area. A dry powder extinguishing system is specified for small flammable gas fires. Requirements for firemen’s protective clothing and outfits is included. Chapter 12 - Mechanical Ventilation in the Cargo Area Covers the spaces normally entered during cargo handling operations and the spaces that are not normally entered. Location of intakes and exhaust, number of air changes and materials of construction for fans are also covered. Chapter 13 - Instrumentation Requirements are given for cargo tank level indicators and alarms, pressure gauges and temperature indicators in cargo systems and vapour detection equipment. Chapter 14 - Personnel Protection This chapter discusses requirements for protective clothing, gas masks, breathing apparatus, first aid, resuscitation equipment, decontamination showers, eye wash facilities and personnel rescue equipment. Chapter 15 - Filling Limits for Cargo Tanks Limits are specified to prevent a tank becoming liquid full by thermal expansion after loading. The maximum limit is considered to be 98% (of the volume of the tank) liquid full at the reference temperature. Some administrations allow for greater tank volume when carrying LNG, with some ships permitted to fill to 99% volume. This limit takes account of the expected boil-off of vapours from the cargo tanks during the loaded voyage. Chapter 16 - Use of Cargo as fuel Describes the conditions under which LNG boil-off gas may be used as fuel. The necessary arrangements of piping, valves and gas detecting equipment for safe delivery of the gas to the machinery are specified, as are the ventilation systems to ensure that there is no accumulation of hazardous vapours. Chapter 17 - Special Requirements The code incorporates special requirements for cargoes that have unusual characteristics. Chapter 18 - Operating Requirements Deals with the operation of a gas carrier. It highlights the regulations in other chapters that are operational in nature and includes other requirements unique to gas ship operation. Chapter 19 - Summary of Minimum Requirements Links the product hazards to the ship design. In determining the applicable ship type requirements, products are categorised as high hazard, medium hazards or low hazard. MARPOL 73 - International Convention for the Prevention of Pollution from Ships Annex I Regulations for the Prevention of Pollution by Oil (2-10-1983) 25
Annex II Annex III Annex IV Annex V: Annex VI
Regulations for the Control of Pollution by Noxious Liquid Substances in Bulk (6-4-1987). Regulations for the Prevention of Pollution by Harmful Substances Carried by Sea in Packaged Form (1-1-1992) Regulations for the Prevention of Pollution by Sewage from ships. Regulations for the Prevention of Pollution by Garbage from Ships (31-12-1988) Controlling atmospheric pollution by the prevention of release of volatile organic compounds and NOX or SOX engine exhausts. (19-52005)
STCW International convention on Standards of Training, Certification & Watchkeeping for Seafarers, 1978 as amended 1995. The convention is a framework of agreed international standards of training, certification & watchkeeping for seafarers. It covers all of the qualifications required for Masters, Chief Engineers, Deck and Engine Officers, Radio Operators and Ratings. They contain special requirements for Masters, Officers and ratings on Oil Tankers (Regulation V/1), on Chemical Tankers (Regulation V/2) and Liquefied Gas Tankers (Regulation V/3) Regulation V/3 specifies: Officers and ratings who are to have specific duties and responsibilities in connection with cargo and cargo equipment on liquefied gas tankers, and who have not served on liquefied gas tankers as part of the regular complement, shall have completed an appropriate shore-based fire-fighting course. They will also have: -
An appropriate period of supervised shipboard service in order to acquire adequate knowledge of safe operational practices. An appropriate liquefied gas tanker familiarisation course which includes basic safety and pollution prevention precautions and procedures, the layout of different types of liquefied gas carriers, types of cargo, their hazards and their handling equipment, general operational sequence and liquefied gas tanker terminology. Masters, Chief Engineers, Chief Officers, Second Engineers and any person with the immediate responsibility for loading, discharging and care in the transit or handling of cargo, in addition to the provisions stated above shall have: Relevant experience appropriate to their duties on liquefied gas tankers. Completed a specialised training programme appropriate to their duties including liquefied gas tanker safety, fire safety measures and systems, pollution prevention and control, operational practice and obligations under applicable laws and regulations.
According to the Convention requirements, every officer who has completed a specialised level training course, must have his licence “endorsed” to serve either on an oil, chemical or liquefied gas carrier, depending on the nature of the cargo being carried. STCW 1995 was a revision to the existing Annex, no change was made to the 1978 convention. STCW 95 provided a complete, consolidated text of the STCW Convention, including its original articles, a revised Annex and the supporting STCW Code. Mandatory technical standards are contained in Part A of the STCW Code. Part B of the Code provides guidance to assist those involved in educating, training or assessing the competence of seafarers or those who are otherwise involved in applying STCW Convention provisions. **********************************************************
Testing of Materials
Metals are tested to ensure that their strength, ductility and toughness are suitable for the function they are required to perform. The Strength of the material is its ability to resist deformation. Yield Stress and Ultimate Tensile Strength measure the ability to resist forces on the structure. Hardness is the ability to resist abrasion. Hardness is usually measured on Rockwell or Brinell scale, based on test results. Ductility is the ability of a material to be deformed before it fails. Brittleness is the opposite of ductility and describes a material that fails under stress because it cannot deform. Softer metals, like aluminium are ductile. Hard materials like cast iron are strong but brittle. Toughness is the ability of a material to absorb energy. Stresses and strains are often referred to when comparing the strengths of various metals. Stress is a measure of the ability of a material to transmit a load, and the intensity of stress in the material, which is the load per unit area. Total strain is defined as the total deformation that a body undergoes when subjected to an applied load. Strain is the deformation per unit length or unit volume.
Stress is directly proportional to strain. Stress is equal to a constant, which is the slope of the straight line part of the graph. This constant is referred to as the Modulus of Elasticity (E). (for mild steel it is about 21 100 kg/mm2). The Yield Stress for a metal corresponds to the stress at the yield point, that is the point at which the metal no longer behaves elastically. Ultimate Tensile Stress is the maximum load to which the metal is subjected, divided by the original cross-sectional area. Beyond the yield point the metal behaves plastically. Proof Stress is obtained by setting off on the base some percentage of the strain, say 0.2%, and drawing a line parallel to the straight portion of the curve. The intersection of this line with the actual stress-strain curve marks the proof stress. Ship’s structure is designed for working stresses that are within the elastic range and much lower than the ultimate tensile strength of the material to allow a reasonable factor of safety. Classification Society Tests for Hull Materials Mild Steel and Higher Tensile Steel plates and sections built into a ship are produced at works approved by a classification society. During production an analysis of the material is required, and so are prescribed tests of the metal. Similar tests are also required for steel forgings and steel castings, in order to maintain an approved quality. Destructive Tensile and impact tests are also carried out. Tensile Test: A specimen of given dimensions are subject to an axial pull and a minimum specified yield stress, ultimate tensile stress, and elongation must be
obtained. The specimen has a gauge length 5.65 times the square root of the cross sectional area, which is equivalent to a gauge length 5 times the diameter Impact Test: The Charpy V-notch test or Charpy U-notch test is commonly specified . The impact test is to determine the toughness of the material, that is, its ability to withstand fracture under shock loading.
The specimen is placed on an anvil and the pendulum is allowed to swing so that the striker hits the specimen opposite the notch and fractures. Energy absorbed in fracturing the specimen is automatically recorded by the machine. Making allowance for friction, the energy absorbed in fracturing the specimen is the difference between the potential energy the pendulum possesses before being released, and that which it attains in swinging past the vertical after fracturing the specimen.
Stresses on Ships
Ships experience stresses when floating in still water and when underway at sea. -
Vertical Shear & Longitudinal Bending in Still Water Bending Moments in a seaway Longitudinal Shear Forces Bending Stresses Transverse Stresses (Racking & Torsion) Local Stresses (Panting, Pounding & other local stresses) 29
Brittle Fracture Fatigue Failures Buckling
Vertical Shear & Longitudinal Bending in Still Water For a homogeneous body of uniform cross section and weight that is floating in still water, at any section the weight and buoyancy forces are equal and opposite. There is no resultant force at a section, and the body will not be stressed or deformed. A ship floating in still water has an unevenly distributed weight owning to both cargo distribution and structural distribution. The buoyancy distribution is also non-uniform since the underwater sectional area is not constant along the length. Total weight and total buoyancy are balanced, but at each section there will be a resultant force or load, either in excess of buoyancy or excess of load. Since the vessel remains intact, there are vertical upward and downward forces tending to distort the vessel, which are referred to as vertical shearing forces, since they tend to shear the vertical material in the hull.
The ship shown will be loaded in a similar manner to the beam shown, and will tend to bend in a similar manner owing to the variation in vertical loading. A vessel bending in this manner is said to be hogging. If there is excess weight amidships the vessel is sagging. When sagging the deck will be in compression and the bottom shell in tension. Bending Moments in a seaway When a ship is in a seaway, the waves with their troughs and crests produce a greater variation in the buoyant forces, and therefore can increase the bending moment, vertical shear force, and stresses. In a seaway the overall effect is an increase of 30
bending moment from that in still water when the greater buoyancy variation is taken into account. Longitudinal Shear Forces When the vessel hogs and sags in still water and at sea, shear forces similar to the vertical shear forces will be present in the longitudinal plane. Vertical and longitudinal shear stresses are complementary and exist in conjunction with a change of bending moment between adjacent sections of the hull girder. The magnitude of the longitudinal shear force is greatest at the neutral axis and decreases towards the top and bottom of the girder. Bending Stresses From bending theory, the bending stress ( a ) at any point in a beam is given by: a = M/I x y about the
M = applied bending moment I = second moment of area of cross-section of beam neutral axis y = distance of point considered from
neutral axis When the beam bends, say in the case of hogging, it is in tension at the top and in compression at the bottom. Somewhere between the top and bottom is a line that is neither in tension or compression. This position is called the neutral axis. The neutral axis also contains the centre of gravity of the cross-section of the beam. The greater the value of the second moment of area, the less the bending stress will be. The second moment of area of the section varies as the depth squared and therefore a small increase in depth of section can be very beneficial in reducing the bending stress. The Ship as a Beam Ships bend like a beam does. The hull can be considered as a box-shaped girder for which the position of the neutral axis and second moment of area can be calculated. The deck and bottom shell form the flanges of the hull girder, and are far more important to longitudinal strength than the sides that form the web of the girder and carry the shear forces. The box shaped hull girder and a conventional “I girder” are compared below.
In a ship the neutral axis is generally nearer the bottom, since the bottom shell will be heavier than the deck, having to resist water pressure as well as bending stresses. In calculating the second moment of an area of the cross-section, all longitudinal material is of greatest importance and the further the material is from the neutral axis, the greater will be its second moment of area about the neutral axis. However, at greater distances from the neutral axis, the sectional modulus will be reduced and correspondingly the higher stress may occur in extreme hull girder plates such as the deck stringer, sheer strake, and bilge. These strakes of plating are generally heavier than other plating.
Bending stresses are greater over the middle portion of the length and it is owing to this variation that Lloyd’s Register give maximum scantlings over 40% of the length amidships. Other scantlings may taper towards the ends of the ship, apart from locally highly stressed regions where other forms of loading are encountered. Strength Deck The deck forming the uppermost flange of the main hull girder is often referred to as the strength deck. All continuous decks should be strength decks. Along the length of the ship (the top flange of the hull girder, ie the strength deck), may step from deck to deck where large superstructures are fitted or there is a natural break, for instance in way of a raised quarter deck. Larger superstructures tend to deform with the main hull and stresses of appreciable magnitude will occur in the structure. Early vessels fitted with large superstructure of light construction demonstrated this to their cost. Attempts to avoid fracture have been made by fitting expansion joints, which made the light structure discontinuous. These were not entirely successful and the expansion joint may itself form a stress concentration at the strength deck., which one would want to avoid. In modern construction the superstructure is usually made continuous and of such strength that its sectional modulus is equivalent to that which the strength deck would have if no superstructure were fitted. Transverse Stresses When a ship experiences transverse forces, these tend to change the shape of the vessel’s cross-sections and thereby introduce transverse stresses. These forces may be produced by hydrostatic loads and impact of seas or cargo and structural weights, both directly and as the result of reactions due to change of ship motion. Racking When a ship is rolling, the deck tends to move laterally relative to the bottom structure, and the shell on one side to move vertically relative to the other side. This deformation is known as racking. Transverse bulkheads primarily resist such transverse deformation. When transverse bulkheads are widely spaced, deep web frames and beams may be introduced to compensate. Torsion When a body is subject to a twisting moment (torque) , the body is said to be in torsion. Beam seas cause torsion to happen to ships. In most ships these torsional moments and stresses are negligible, but ships with extremely wide and long deck openings they are significant.
Panting Panting refers to a tendency for the shell plating to work in and out in a bellows like fashion, and is caused by the fluctuating pressures on the hull at the ends when the ship is in rough seas. These forces are most severe when the ship is running into waves and is pitching heavily, the large pressures occurring over a short time cycle. Pounding
When a ship is driven into head seas, severe local stresses occur in way of the forward bottom shell and framing. These pounding stresses are likely to be more severe in a lightly ballasted condition, and occur over an area of the bottom shell aft of the collision bulkhead. Additional stiffening is required in this area. Other Local Stresses Ship structural members are often subjected to high stresses in localized areas, and these have to be correctly designed. Examples are where various load-carrying members of the ship intersect, where longitudinal meet at transverse bulkheads, and at intersections of longitudinal and transverse bulkheads. Another highly stressed area occurs where there is a discontinuity of the hull girder at ends of deck house structures, also at hatch and other opening corners, and where there are sudden breaks in the bulwarks. Brittle Fracture When welding of ships began to overtake riveted ships in the 1940s, brittle fracture occurred more frequently. Welded joints provided metal continuity, and cracks propagated more rapidly that on riveted ships. Brittle fracture occurs when an otherwise elastic material fractures without any apparent sign or little evidence or material deformation prior to failure. Fracture occurs instantaneously with little warning and the vessel’s overall structure need not be subject to a high stress at the time. Mild steel is particularly prone to brittle fracture. The following factors influence the possibility of brittle fracture: 1. 2. 3. 4. 5.
A sharp notch is present in the structure from which the fracture initiates. Tensile stress is present. There is a temperature above which brittle fracture will not occur. The metallurgical properties of the steel plate. Thick plate is more prone.
Brittle fracture is distinguishable from ductile failure by the lack of deformation at the edge of the tear, and is bright granular in appearance. Brittle fracture is also distinguished by the apparent chevron markings, which aids location of the fracture initiation point since these tend to point in that direction. A ductile failure has a dull grey appearance. Sharp corners and notches where cracks may be initiated should be avoided in ship construction. Because ships are very large structures, faults in welds will occur, and a complete weld examination is time consuming and costly. Steel specified for hull construction should have goof “notch ductility” at the service temperatures, particularly where thick plate is used. Crack arrester was the practice of introducing a riveted seam in cargo ships to subdivide the vessel into welded substructures, so that any possible crack propagation was limited to the substructure. Such a crack arrester was usually specified in the sheerstrake/stringer plate area of large ships. With modern ships, strakes of higher notch toughness steel are required to be fitted in such areas. Lloyd’s Register require the mild steel sheerstrake and stringer plate at the strength deck, over the midship
portion of ships over 250 metres to be Grade D if less than 15mm thick, and Grade E if of greater thickness. Fatigue Failures Fatigue failure occurs very slowly and can take years to propagate. Fatigue failure occurs at low stresses that are applied to a structure repeatedly over a period of time. A fatigue crack, once initiated, may grow unnoticed until the load bearing member is reduced to a cross-sectional area that is insufficient to carry the applied load. Fatigue failures are associated with sharp notches or discontinuities in structures, and are especially prevalent at hard spots (regions of high rigidity in ship structures. With the growth in size of oil tankers, bulk carriers, and container ships, there has been increasing use of higher yield strength steels in their hull structures. The classification societies have subsequently places special emphasis on analysis of the fatigue performance of these larger structures, usually over a 25 year life cycle, as part of their approval process.
Buckling With the substantial increase in size of ships, greater attention has had to be given to the buckling strength of the stiffened plate panels constituting the shell. Buckling of a structural member loaded in compression may occur at a stress level that is substantially lower than the material’s yield stress. The load at which buckling occurs is a function of the structural member’s geometry and the material’s modulus of elasticity rather than the material’s strength. The most common example of buckling failure is the collapse of a pillar under a compressive load. A stiffened plate panel in compression will also have a critical buckling load, whose value depends on the plate thickness, unsupported dimensions, edge support conditions, and the material’s modulus of elasticity. 36
Monitoring Ship Stresses at Sea To enhance safety during shipboard operations, real-time motion and stress, monitoring information equipment can be supplied by classification societies such as Lloyd’s Register to a ship. This means fitting of strain gauges to the deck structure, an accelerometer, and a computer with software that displays ship stress and motion readings on the bridge. Where this equipment is fitted, the notation SEA(R) is assigned.
Welding and Cutting
The welding processes employed in shipbuilding are of the fusion welding type. Fusion welding is achieved by means of a heat source that is intense enough to melt the edges of the material to be joined. Gas Welding, Arc Welding, Laser Welding, Resistance Welding, Friction Stir Welding, all provide heat sources of sufficient intensity to achieve fusion welds. Steel plates and sections are mostly cut to shape in shipyards using gas cutting techniques. The introduction of plasma arc cutting machines has led to their widespread use in modern shipyards. Weld faults: Various faults may be observed in butt and fillet welds. These may be due to a number of factors: bad design, incorrect welding procedure, use of wrong materials, and bad workmanship. Some faults are illustrated below. The judgement of the seriousness of the fault rests with the weld inspector and surveyor. Where the weld is considered to be unacceptable, it will be cut out and rewelded. Non-destructive Testing (NDT): Non-destructive testing is required to enable the soundness of ship welds to be assessed. Some of the NDT methods used are: -
Visual Examination Dye Penetrant Magnetic Particle Radiographic (X-Rays) Ultrasonic
Ship Yard Layout
The Ship Building Yard will have an ergonomic flow of materials and production. 38
10. Design Information for Production In order to build a ship, it is necessary to develop the initial structural and arrangement design into information usable for production. Initial design is focused on the operation of the ship once completed. Production requires information in a form that relates to the manufacturing, assembly and construction processes. Traditionally this was done by creating a full size model of the structure, in the mold loft, from which dimensions could be lifted, and templates could be made. The relatively simple outfitting of early ships was carried out on the completed ship structure, with pipes and other connectors run where space was available. Computer modelling has replaced some of these traditional methods.
10.1 Assembly of Ship Structure Historically, ships were built on a slipway piece by piece. The transition from wood to iron and then to steel was somewhat similar in the construction process. During the 1940s a large number of merchant and war ships were built in a short period of time. Welding speeded the construction process. And mass production of sections was possible, prior to erection. Sections could be built away from the slipway and transported to be assembled and welded together. Welding and construction faults resulted because of the rapid building of ships that were needed during the Second World War. Since then, quality has improved in leaps and bounds, and larger and larger prefabricated sections were being built. By 1970 a 200 000 dwt tanker could be built and launched in 2 to 3 months. Outfitting then took place. On modern ships, considerable amount of the outfitting and pipework was incorporated in the sections and blocks, and married up when the blocks were welded together to form the ship. Computer Aided Design and Computer Aided Manufacturing techniques smartened up the whole process of designing and building the ship. A typical bulk carrier of 250 metres and 32 metres beam would require about 10 000 tons of steel. Assembly would take the following pattern:Minor Assembly: Brackets, intercostal floors or girders, bulwarks, two dimensional assemblies and a maximum size of 2 by 5 metres and