MARITIME TRAINING
DYNAMIC POSITIONING
CENTER CROATIA
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COURSE: Dynamic Positioning – Induction/Basic ( Student Handouts – 1st version)
Split, 2008.
MARITIME TRAINING
DYNAMIC POSITIONING
CENTER CROATIA
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CONTENTS Objectives 1.
PRINCIPLES OF DP
6 – 13
1.1.
Dynamic Positioning - Introduction
6–8
1.2.
Application of Dynamic Positioning on various types of vessel
8–9
1.3.
Six freedoms of movement of a vessel
9 – 12
1.4.
Movements that are controlled under DP and monitored movements
2.
ELEMENTS OF A DYNAMIC POSITIONING SYSTEM
14 – 23
2.1.
Aids to manoeuvring commonly fitted to DP vessels
14 – 14
2.2.
Control system associated with a DP system
14 – 17
2.3.
Power requirements of a DP vessel system, typical power supply installations
17 – 17
2.4.
Position Reference systems commonly associated with DP installations
18 – 19
2.5.
Sensor systems associated with DP installations
19 – 20
2.6.
Requirements for redundancy within a DP system; methods by which redundancy is obtained within a DP system, relating to the IMO Equipment Classes
20 - 22
2.7.
Mathematical modelling of vessel behaviour characteristics, the advantages and limitations of this technique
13 – 13
23 – 23
3.
PRACTICAL OPERATION OF A DP SYSTEM
3.1.
Various controls, instruments and displays incorporated into the DP bridge console and computer cabinets.
24 – 26
3.2.
Correct procedure for setting-up the DP system in both the Manual and Automatic modes.
26 – 26
3.3.
Various modes of DP operation, e.g. Manual control, Semi-automatic control, čAutomatic control, various specialist functions (e.g. Follow-target, Follow-Sub, Track Follow, Auto-approach, Weathervane, Riser Angle mode).
27 – 36
3.4.
Station-keeping, position and heading change manoeuvres, using both automatic and manual DP facilities
36 - 38
3.5.
Setting up a pre-defined Autotrack given turn-point co-ordinates, vessel velocity and heading profiles. Initiating the Autotrack facility and monitor the vessel’s progress along the track.
38 - 41
3.6.
System switch-on, loading procedure and re-loading procedures
41 - 41
3.7.
Concept of Centre of Rotation, and the provision of Alternative Centres of Rotation
41 - 41
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4.
POSITION REFERENCE SYSTEMS
42 – 67
4.1.
Operation of a Hydro-acoustic position reference (HPR) system
43 – 44
4.2.
The principles of position definition using the various forms of HPR system (e.g. Ultra-short, Super-short, Long baseline and Multi-user principles).
44 - 47
4.3.
The use of the various types of acoustic Beacon, Transponder, Responder and seabed array used in conjunction with an HPR system
48 – 48
4.4.
The display and configuration of the various elements in 4.3, and the acquisition of HPR as a position-reference for DP operations
48 – 49
4.5.
Advantages and limitations of HPR as a position-reference for DP
49 – 49
4.6.
Principles and operation of the Artemis position-reference system
50 – 51
4.7.
The advantages and limitations of the Artemis position-reference system.
51 – 52
4.8.
Taut-wire position-reference system
52 - 53
4.9.
The procedure for deployment and recovery of the taut wire system
53 - 53
4.10.
Display of taut-wire reference data in the DP system. Principle of positionreference using the taut-wire system.
54 - 55
4.11.
The advantages and limitations of the taut-wire position reference systems
55 - 55
4.12.
Principles of the Differential GPS system
55 - 57
4.13.
The operation of a modern differential corrections network
57 - 57
4.14.
The sources of error and inaccuracy associated with the DGPS system, effects on the quality of positioning
57 - 58
4.15.
Available quality data associated with the DGPS system
58 - 58
4.16.
Advantages and limitations of the DGPS system compared with other PRS
59 - 59
4.17.
The principles used in Relative GPS systems
60 - 60
4.18.
The principles of position reference using optical laser-based systems
61 - 61
4.19.
The method of setting-up a laser-based system to provide position information
61 - 62
4.20.
Advantages and limitations associated with the optical laser PRS
62 - 62
4.21.
Relative accuracy and reliability of the aforementioned PRS, methods used to apply weighting and pooling when more than one PRS is acquired
63 - 63
4.22.
Other PRS which may be used in conjunction with a DP system
64 - 67
4.23.
The principle of Inertial Navigation, the methods of using INS to enhance existing PRS performance
67 - 67
5.
ENVIRONMENT SENSORS AND ANCILLARY EQUIPMENT
68 - 72
5.1.
Means of obtaining Vertical Reference for input into a DP system. The importance of the provision of vertical reference.
69 - 69
5.2.
The function of gyro compasses and their redundancy within a DP system
69 - 70
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5.3.
Provision of wind sensors within the DP system
70 - 71
5.4.
Wind Feed-Forward facility, and its importance within the DP system
71 - 71
5.5.
The limitations of wind sensor inputs, and the consequences of deselecting the wind sensor input
71 - 71
5.6.
Interpreting messages provided on the DP system displays and on the printer
72 - 72
5.7.
The alarms and warnings associated with catastrophic failure, i.e. position and/or heading Dropout
72 – 72
5.8.
Corrective actions to accept and remedy any alarm or warning condition
6.
POWER GENERATION AND SUPPLY, AND PROPULSION SYSTEMS
73 - 81
6.1.
The power generation and distribution arrangements in a typical dieselelectric DP vessel, with particular reference to system redundancy and Equipment Class
74 - 75
6.2.
The power supply and distribution arrangements in a typical non-dieselelectric DP vessel
75 - 75
6.3.
The power requirements of DP vessels, and the concept of “available power”
75 - 76
6.4.
Typical Power Management system as installed in a DP vessel
76 - 76
6.5.
The provision of Uninterruptible Power Supply systems to the DP system, with particular reference to power shortages, failures and system redundancy
77 - 78
6.6.
Various types of propulsion system commonly installed in DP-capable vessels
78 – 80
6.7.
Evaluation of fixed-pitch propellers compared with controllable-pitch propellers
81 – 81
6.8.
Operational characteristics and possible failure modes of the different types of propulsion systems
7.
OPERATIONS USING DP
7.1.
Procedures to be followed when approaching a worksite and transferring from conventional navigation to DP control
72 - 72
81 - 81 82 – 112 82 - 83
7.2.
The need for completing pre-DP and other checklists prior to and during DP operations
83 - 84
7.3.
The need for keeping logbook records of all DP operations, failures, incidents and repairs, including details of operation and maintenance of all position reference systems
84 - 84
The need for effective communications during the conduct of DP operations
85 - 87
7.4.
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7.5.
The watch hand-over procedure, completion of the appropriate checklist.
88 - 88
7.6.
Worksite diagrams using UTM co-ordinates, and plan DP operations using this diagram
88 - 90
7.7.
Plan for emergency and contingency situations and procedures.
90 - 90
7.8.
Interpretation of ERNs, Capability diagrams, Online Capability Plots, “Footprint” plots and other data relating to the capability of the vessel under a variety of environmental conditions
90 - 91
7.9.
Various documents containing statutory requirements and guidance relating to DP operations
91 - 91
7.10.
Equipment Classes and their application(with reference to the IMO guidelines for DP vessels)
91 - 92
7.11.
Various Classification Society notations (with reference to system and vessel redundancy and to the Equipment Classes)
92 - 93
7.12.
The arrangements made for the conduct of DP operations in specialist vessels:
93 - 93
7.12.1.
Diving and underwater support vessels
93 – 97
7.12.2.
Drill ships (with special reference to the Riser Angle mode of operation)
97 – 99
7.12.3.
Cable lay and repair vessels
100–101
7.12.4.
Pipe lay vessels
101-104
7.12.5.
Rock dumping and dredging vessels
105-106
7.12.6.
Shuttle tanker and FPSO operations
107-110
7.12.7.
Accommodation and “flotel” units
111-111
7.12.8.
Crane barges and construction vessels
111-111
7.13.
Hazards associated with DP operations conducted in areas of shallow water and strong tidal conditions. Hazards associated with operations in very deep water.
111-112
Appendix A
DP Incidents, The IMCA Database 1990-99
113–133
Appendix B
Kongsberg Simrad exercises
134-141
Appendix C
Alstom exercises
142-149
Appendix D
Nautical institute training programme
150-154
Abbervations
155-156
References
157
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OBJECTIVES Upon completion of the course the trainee should be able to: • Understand the Principles of a Dynamic Positioning System. • Set up and operate DP Equipment and Position Reference Systems. • Recognise the various alarms and wamings. •
Relate to DP Control Systems, the various Subsystems i.e.; Power Plant, Manoeuvring Facility, Position References and communications.
•
Relate to DP Operations; the limited conditions presented by Wind, Seas Current / tides and vessel Movement. (Capability plots and footprints.)
• Practice the setting up and monitoring of D.P. Systems.
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1.THE PRINCIPLES OF DYNAMIC POSITIONING 1.1.
Dynamic Positioning – Introduction
Dynamic positioning (DP) is a rapidly maturing technology, having been born necessity as a result of the increasing demands of the rapidly expanding oil and gas exploration industry in the 1960s and early 1970s. The demands of the offshore oil and gas industry have brought about a whole new set of requirements. Further to this the more recent moves into deeper water and hars-enviroment locations, together with requirement to consider more enviroment-friendly methods, has brought about the great development in the area of Dynamic Positioning techniques and technology. Dynamic positioning has changed a lot since 1960s. From being designed for test drilling and laying of pipelaines, DP is now being used for different types of operations, ranging from geological assignments, via military ones, to cruise ship manoeuvring in lagoons. The basic principles from 1961 are the same, but the explosive development within data has led to a similar development in DP systems, bot when it comes to operating the equipment and the technology itself. During the 1990s there was a rapid increase in the number of vessels with dynamic positioning systems. Many of these vessels have been designed for DP and integrated control of engines and thrusters, but there are also a large number of conversions and upgrades. The situation is market-driven and relies on operational efficiency which, in turn, places a high reliability requirement on equipment, operators and vessel managers. The dynamic Positioning (DP) is a method of positioning marine vessels accurately within predefined limits using a combination of computers, position reference systems and thrusters, various types of vessels make use of this to either maintain a fixed position or to follow a predetermined track or work plan. Virtually any type of shape of seagoing vessel could utilise a DP system. The basic purpose of dynamic positioning of a vessel is the automatic control of the vessel position and heading. Dynamic positioning may be defined as "A system that automatically controls a vessel to maintain her position and heading exclusively by means of active thrust". A DP-capable vessel must have a combination of power, manoeuvrability, navigational ability and computer control in order to provide reliable positioning ability. This forms an integrated system including such elements as the vessel's power plant, propulsion and thrusters, navigational systems, gyro compasses and control computers, while not forgetting the human element. There are other methods for vessel station keeping. These include spread and fixed moorings or combination of each. Each system has advatages and disadvantages.
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Figure1. - Schematic diagram of a DP system
DP Advantages: •
Vessel is fully self-propelled; no tugs are required at any stage of the operation.
•
Setting-up on location is quick and esay.
•
Vessel is very manoeuvrable.
•
Rapid respons to weather changes is possible (weather vane).
•
Rapid response to change since the requirements of the operation.
•
Versatility within system (i.e. track-follow, ROV-follow and other specialist functions).
•
Ability to work in any water dept.
•
Can complete short tasks more quickly, thus more economically.
•
Avoidance of risk of damaging seabed hardware from mooring lines and anchors.
•
Avoidance of cross-mooring with other vessels or fixed platforms.
•
Can move to next location rapidly (also avoid bad wether).
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DP Disadvantages: •
High capex and opex.
•
Can fail to keep position due to equipment failure.
•
Higher day rates then comparable moored systems.
•
Higher fuel consumption.
•
Thrusters are hazards for divers and ROVs.
•
Can lose position in extreme weather or in shallow waters and strong tides.
•
Position control is active and relies on human operator (as well as equipment).
•
Requires more personnel to operate and maintain equipment.
1.2. Application of Dynamic Positioning on various types of vessel The first vessel to fulfil the accepted definition of DP was the "Eureka", of 1961, designed and engineered by Howard Shatto. This vessel was fitted with an analogue control system of very basic type, interfaced with a taut wire reference. Equipped with steerable thrusters fore and aft in addition to her main propulsion, this vessel was of about 450 tons displacement and length 130 feet. By the late 1970s, DP had become a well established technique. In 1980 the number of DP capable vessels totalled about 65, while by 1985 the number had increased to about 150. Currently (2002) it stands at over 1,000 and is still expanding. It is interesting to note the diversity of vessel types and functions using DP, and the way that, during the past twenty years, this has encompassed many functions unrelated to the offshore oil and gas industries. A list of activities executed by DP vessels would include the following: •
Coring,
•
Exploration drilling (core sampling),
•
Production drilling,
•
Diver support ,
•
Pipelay (rigid and flexible pipe),
•
Cable lay and repair,
•
Multi-role,
•
Accommodation or "flotel" services,
•
Hydrographic survey,
•
Pre or post operational survey,
•
Wreck survey, salvage and removal,
•
Dredging,
•
Rockdumping (pipeline protection),
•
Subsea installation, 8
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•
Lifting (topsides and subsea),
•
Well stimulation and workover,
•
platform supply,
•
Shuttle tanker offtake,
•
Floating production (with or without storage),
•
Heavy lift cargo transport,
•
Passenger cruises,
•
Mine countermeasures,
•
Oceanographical research,
•
Seabed mining. DP is also used in;
•
Rocket launch platform positioning.
•
Repair/maintenance support to military vessels.
•
Ship-to-ship transfer.
•
Manoeuvring conventional vessels.
DP systems have become more sophisticated and complicated, as well as more reliable. Computer technology has developed rapidly and some vessels have been upgraded twice with new DP control systems. Position reference systems and other peripherals are also improving and redundancy is provided on all vessels designed to conduct higer-risk operations. 1.3. Six freedoms of movement of a vessel DP systems are computerised systems that allow the vessel to be controlled in heading and position accurately, to within a few metres or degrees, using a system of computers, sensors, and thrusters, utilising active thrust. The system measures the other motions, pitch, roll and heave. The forces acting on the vessel are the enviromental forces, including wind, current and waves, and task dependent forces such as cable pipe, anchors, tow ropes, fire monitor reaction. It is important to realise that enviroment forces are very variable. •
Enviromental Forces (wind, sea current and waves)
The wind forces can be defined by three components. The wind speed varies as a function of height above sea level, but above 3-5 metres to the height of the vessel, the change is small. The forces acting on the vessel are very dependent on the superstructure shape (the part of the vessel above the water line), and the wind direction relative to the vessel.
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The sea current can be caused by the slope of the seabed, tidal or storm surges along coastline, outflows from rivers. It can also be wind driven. It can be caused by the effect of heating and cooling and salinity. The effect is only a few knots, and usually slow variation over hours and days. The effect of current on the vessel is a characteristic of vessel shape. Waves are also described as sea state. A fully developed sea is the maximum wave size generated by a given wind. It takes many hours to build up and die down. The significant wave height is the mean of the 1/3 higest wave. •
Countforces
Moving from one to point to another or remaining stationary, requires lots of countforces device to produce a controlled combination of forces. Traditional devices included oars, sails, anchors, paddle wheels, propelers and rudders. Static Positoning Systems - These gain their countforces from anchors alone. They are also called multipoint mooring systems, and can be used station keeping or moving very slowly. By changing the anchor line lenghts and hence the forces, limited control of the vessel is possible. The alternative of moving the anchors from an elastic pattern and the vessel will take up a position in the middle of the pattern, where the forces balance, The use of anchors is depth dependent, with the cost increasing in proportion to the depth.
Figure 2. Static Positioning with Anchors Alone
Dynamic Positioning Systems - These use combination of thrusters, propellers and rudders. There are four types of thrusters: •
Propelles – provide thrust in the fore/aft direction.
•
Tunnel thrusters – provide thrust in the port/starboard direction.
•
Azimuthing thrusters – provide thrust in a 360°ar c.
•
Propellers and rudders – provide thrust forward, some side thrust and thrust straight astern.
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Thrusters can be used for roles such as station keeping through to complex track following. They are not depth dependent. The thruster array must provide independent control of surge, sway and yaw.
Figure 3. Thrusters
There are two configurations of anchors and thrusters that differ in how the anchors are connected to the vessel. In the simple configuration, the anchors are connected directly to the extremities of the vessel. The thruster is then used in combination with the anchors to increase their capability. The second configuration is turret moored. Here the anchors are attached to a turret about which the vessel can rotate. The thrusters are now used mainly to control the vessel heading with a secondary task of reducing anchor loadings.
Figure 4. Anchors and Thrusters
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In steady to strong winds, the vessel will align itself to the direction of the wind, usually called weathervaning. However, in light winds, the vessel will wander and oscilate about turret, which can be problem if the vessel is connected to a shuttle tanker for instance. The thrusters can be used to damp out any oscilation in the heading, and provide a steady heading. For combined application either a tunnel thruster or an azimuthing thruster is used. For maximum effectiveness the thrusters should be as far as possible away from the turret. A free floating body will translate (move fore and aft and port and starboard) and rotate due to forces acting upon it. In turn if there is to be control of the vessel position and heading, the vessel needs countforces and moments to control its motion. The vessel can move in three planes. For the purpose of DP systems we are interested in controlling the vessel in the horizontal plane. However, it is necessary to sense vessel motion, in other planes, and to monitor the wind, to be able to make corrections to PME and sensor readings.
Figure 5. Vessel movements
Axis of movement
Positive direction
Coordinate System
Use in DP
Surge
Forward
+-X
Position Control
Sway
Starboard
+-Y
Yaw
Clockwise (seen fom above)
+-N
Heave
Upwards
+-N
Pitch
Bow Down
Roll
Stbd Down
Heading Control
Compensation for acoustic beacon and radio aerial / same taut wire
Table 1. Vessel movement terms
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The axis of movement are traditional names for vessel's motion. The direction is navigation term which identifies the direction of the motion. The coordinate system is the way that the navigation term is described to the computer. The DP control system uses these coordinates.
1.4. Movements that are controlled under DP and monitored movements As stated earlier dynamic positioning is concerned with the automatic control of surge, sway and yaw. Surge and sway, of course, comprise the position of the vessel, while yaw is defined by the vessel heading.
Figure 6. Vessel movements controlled under DP
Both of these are controlled about desired or "setpoint" values input by the operator, i.e. position setpoint, and heading setpoint. Position and heading must be measured in order to obtain the error from the required value. Position is measured by one or more of a range of position references, while heading information is provided from one or more gyrocompasses. The difference between the setpoint and the feedback is the error or offset, and the DP system operates to minimize these errors. The vessel must be able to control position and heading within acceptable limits in the face of a variety of external forces. If these forces are measured directly, the control computers can apply immediate compensation. A good example of this is compensation for wind forces, where a continuous measurement is available from windsensors. Other examples include plough cable tension in a vessel laying cable, and fire monitor forces in a vessel engaged in firefighting. In these cases, forces are generated which, if unknown, would disturb the station keeping if unknown. Sensors connected to the cable tensioners, and the fire monitors allow direct feedback of these "external" forces to the DP control system and allow compensation to be ordered from the thruster before an excursion develops. 13
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2. ELEMENTS OF A DYNAMIC POSITIONING SYSTEM 2.1. Aids to manoeuvring commonly fitted to DP vessels To keep the floating object in place by means of propulsion systems requires relatively precise manoeuvring ability, as this is impossible to do manually a number of aids which enable automatization are fitted on DP vessels. These aids are known as elements of dynamic positioning and they include computer systems, control consoles, position reference systems, power systems and various propulsion systems.
2.2. Control system associated with a DP system A DP system is usually a combination of a position control system and a heading control system. A position control system uses the vessel's position measurment equipment and operator commands as inputs. The control system then provides commands to thrusters to maintain the position of the vessel at the desired location. This is a feedback control system. A heading control system uses the vessel's compass as the input to maintain the heading of the vessel in response to the enviromental elements (forces) and operator commands. DP-control system means all control components and systems, hardware and software necessary to dynamically position the vessel. The DP-control system consists of the following: •
Computer system/joystick system,
•
Sensor system,
•
Display system (operator panels),
•
Position reference system, and
•
Associated cabling and cable routeing.
A DP control system requires data at a rate once per second to achieve high accuracy. Some DP operations require better than 3m relative accuracy. This implies that navigational feedback is available providing higher accuracy than this. In general the DP-control system should be arranged in a DP-control station where the operator has a good view of the vessel's exterior limits and the surrounding area. Early DP control systems did not have the capability to learn from the past performance other than by the integral terms of the controller. Modern systems are able to improve station keeping performance by using a Karman filter, which provides a model of recent performance to improve present performance.
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Figure 7. DP Control systems
It is possible to divide DP control into separate functions: •
Measure the deviation of the vessel its target position and estimate/calculate the forces needed restore the vessel to the required position.
•
Measure the environmental forces acting on the vessel and estimate/calculate forces needed to counteract their effect.
The control system usually reilies on the first function, but makes use of the second, particularly when dealing with wind gusts. The basic control action can be summarised as: •
Measure the vessel's deviation from its target position and set heading.
•
Calculate the deviation in X, Y and N axes.
•
Calculate the required counteracting forces in the X, Y and N axes.
•
Transform the counteracting forces into commands to the individual thrusters. 15
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To do this we need: •
Sensors to give position reference with respect to a given location.
•
Sensor for measuring vessel heading.
•
Something to calculate the commands to the counterforce devices and to implement the commands.
For simple loop feedback control system a change of a sensed condition causes an action to counteract the change. The effect of the change is again sensed and so on. The main feature is to have some damping in the loop to reduce oscillations in the control. The feedback control of a vessel is complex because of the nature of the displacing force, the sensing systems and the vessel characteristica. The control system therefore incorporates a model of the vessel.
Figure 8. Vessel Control System Schematic
The control system consists of the following components: •
Model Ship - This is as accurate a description as possible of the vessel's response to any external forces. The model should be subjected to the same forces that effect the real vessel: thrusters, wind, and waves, currents, anchors, other external forces such as cable/pipe tensions.
•
State Gains - These are the factors that determine the tonnes thrust from the speed and position errors.
•
Thruster Allocation - This is a set of equations which take the total thrust demand, expressed in X, Y, N coordinates, to be applied by the vessel's thrusters and converts it into individual thrusts matched to the available thrusters and their characteristics.
•
Actual Thrusters - These are the available working thrusters.
•
Thruster Model - This model takes the individual thruster demands and calculates the total thrust exerted on the vessel.
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•
Pool - This combines the various estimates of the vessel position, and creates a best estimate of position.
•
Kalman Gains - The factors, which can vary between 0 and 1, determine if the model or estimated position is to be given preference. A value of 0.5 would provide equal weight.
•
Wind speed and Direction - The wind speed and direction are converted into the estimated wind forces on the vessel.
2.3. Power requirements of a DP vessel system, typical power supply installations Power system means all components and systems necessary to supply the DPsystem with power. The power system includes: •
prime movers with necessary auxiliary systems including piping,
•
generators,
•
switchboards, and
•
distributing system (cabling and cable routeing).
Vital to the safety of any DP operation is the continuity of the power supply. The power plant must always be considered to be an integral part of the vessel's DP system. Any interruption in the supply of power can have knock-on effects on the positioning capability of the vessel. DP vessels are particularly vulnerable to blackout or part-blackout situations.
Figure 9. Typical power distribution diagram
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2.4. Position Reference systems commonly associated with DP installations A variety of position reference systems is used by DP systems. The most common are: differential global positioning (DGPS), taut wires, hydroacoustics (HPR), and line-of-sight laser or microwave systems. The reliability of position references is a major consideration. Each has advantages and disadvantages, so that a combination is essential for high reliability.
Figure 10. Position reference systems
Position information from position-reference systems may be received by the DP system in many forms. In addition, the type of co-ordinate system used may be cartesian or geodetic. The DP control system is able to handle information based on either co-ordinate system. A Cartesian, or local, co-ordinate system is based upon a flat-surface twodimensional measurement of the North/South (X) and East/West (Y) distances from a locally defined reference origin. This reference origin will be taken from one of the position reference systems (e.g. HPR transponder, fanbeam reflector, taut wire depressor weight location). This type of co-ordinate reference system is purely local, or relative, not absolute or earthfixed.
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Figure 11. Local reference co-ordinates
For the DP system to handle earth-referenced type of data it is necessary to configure the DP system to accept geodetic data, or global references, such as GPS. A DGPS system, provides co-ordinates in terms of latitude and longitude referenced to the WGS84 datum. Most offshore operations are conducted using UTM (Universal Transverse Mercator) as the chart or worksite diagram projection. This reduces the positional co-ordinates into Northings and Eastings in metres.
2.5. Sensor systems associated with DP installations Vessel sensors should at least measure vessel heading, vessel motions, and wind speed and direction. The vessel sensors are: •
Gyrocompas for heading,
•
Vertical Reference Unit (VRU) for vessel attitude, roll and pitch,
•
Anemometer for wind speed and direction.
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Other environmental sensors may be fitted, such as current meters, tension meters. There is a force acting on the vessel that no sensor can calculate. This force can be defined as the resultant of all other forces acting on the vessel apart from wind. The possible components of this force are numerous. It will also contain any errors in measurement, or unmeasured forces acting on the vessel. Possible components are surface current , subsea current ,waves swell, effect of drag by attached equipment such as pipe or riser, effect of current on riser, workboats tied up to vessel, wind (when wind sensors are deselected) and etc.
2.6. Requirements for redundancy within a DP system; methods by which redundancy is obtained within a DP system, relating to the IMO Equipment Classes DP vessel design and operation is such that no single fault should cause a catastrophic failure. A catastrophic failure means that the failure would cause the vessel to move from her intended position causing risk to operations. As sated by MSC/Circ. 645 ANNEX IMO – Gudlines, redundancy means ability of a component or system to maintain or restore its function, when a single failure has occured. Redundancy can be achieved for instance by installation of multiple components, system or alternative means of performin a function. Redundancy of components for equipment class 2 is for all active components and for equipment class 3 for all components and physical separation of the components. For eguipment class 3, full redundancy may not always be possible (e.g., there may be a need for a single change-over system from the main computer system to the back-up computer system). Non-redundant connections between othervise redundant and separated systems may be accepted provided that it is documented to give clear safety advantages, and that their reliability can be demonstrated and documented to the satisfaction of the Administration. Such connections should be kept to the absolute minimum and made to fail to the safest condition. Failure in one system should in no case be transferred to the other redundant system. Redundant components and systems should be immediately available and with such capacity that the DP-operation can be continued for such a period that the work in progress can be terminated safely. The transfer to redundant component or system should be automatic as far as possible, and operator intervention should be kept to a minimum. The transfer should be smooth and within acceptable limitations of the operation.
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This is IMO Class 1. Loss of position may occur in the event of a single fault.
Figure 12. Simplex non-redundant control (ADP1or ADP12)
Du IMO Class 2. Loss of position should not occur from a single fault in an active component or system.
Figure 13. Duplex redundant control (ADP21or ADP22)
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Trip IMO Class 2, Loss of position should not occur from any single failure
Figure 14. Triple voting (ADP31 or ADP32)
lMO Class 3 (ADP21 orADP22 plus ADP11 orADP12) DP C acts as a stand alone simplex system. The exact requirements for Class III depend upon the classification society.
Figure 15. Stand alone simplex system
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2.7. Mathematical modelling of vessel behaviour characteristics, the advantages and limitations of this techniqe All modern DP control systems utilise a system of mathematical modelling in order to improve and optimise the positioning ability of the vessel. This involves the system fine-tunning itself to the conditions over a period of up to 30 minutes. During this initial period, the vessel positioning will not be as stable as may be observed later, once the settling period is complete. This is why a period of time is often required once the vessel has positioned on the worksite before commercing the operations. This precaution should not be neglected. The mathematical model of the vessel is a accurate as possible, but will never be 100% correct. To make it as accurate as possible, at a given time, continuous minor corrections are fed back into it. The ship model creates estimates of the vessel position, speed and current and wave forces. This data is compared with the required position of the vessel, input by the operator, the speed and any other forces and a thruster demand created. The result of the thrust is then fed back to update the model vessel. The use of a model vessel and Kalman Gains provides many advantages: • Signals from the sensors can be filtered to reduce noise and thruster activity. • Rogue data can be compared with model data and rejected. • The data from the different position reference systems can be combined while matching the characteristics of the individual reference system. • In the absence or loss of position or heading inputs, the vessel can remain under automatic control using predict of data based on the conditions of the previous few minutes. This is called Model Control or Dead Reckoning (DR). • Positioning can be maintained over a greater range of weather conditions, enabling the vessel to extend its operational window.
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3. PRACTICAL OPERATION OF A DP SYSTEM 3.1. Various controls, instruments and displays incorporated into the DP bridge console and computer cabinets The bridge console is the facility for the DPO to send and receive data. It is the location of all control input, buttons, switches, indicators, alarms and screens. In a well-designed vessel, position reference system control panels, thruster panels and communications are located close to the DP control consoles. The DP control console is not always located on the forward bridge - many vessels, including most offshore support vessels have the DP console located on the after bridge, facing aft. Shuttle tankers may have the DP system situated in the bow control station although most newbuild tankers incorporate the DP system on the bridge. Possibly the least satisfactory location for the DP console is in a compartment with no outside view. This is the case in a few older drilling rigs.The facilities for the operator vary from push-buttons and/or touch-screens to pull-down menus activated by roller balls and ‘enable’ buttons.
Figure 16. DP System outline
A DP operator needs to have awareness not only of the equipment, but the operations as well. There is no requirement for the DP operator to be a mariner, but bear in mind when moving between locations, and not in DP the non mariner cannot keep a watch.
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The Man Machine Interface (MMI) is an important feature, which enables efficient and safe operation of the system by helping the operator to make optimum operational decisions. During normal operation this reduces the risk of human error. Emphasis has been placed on ergonomics, logical operation, effective presentation of relevant information and userfriendliness. Dedicated buttons are provided on the operator panel for activation of main modes, reference systems, thrusters and other functions where indicator lights are of great importance for situation assessment. Frequently used functions are also initiated from dedicated panel buttons. The display is organised with four views simultaneously shown on the screen : •
Alarm view. Located just below the Menu bar.
•
Performance view. Located top-left.
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Working view. Located right.
•
Monitoring view. Located bottom-left.
Figure 17. Man Machine Interface
The processors operating the DP control software are generally known as the DP computers. The main distinction of concern to the DPO is the number of computers, their methods of operation, and the level of redundancy they provide. The computers may be installed in single, dual or triple configurations, depending upon the level of redundancy required. Modern systems communicate via an ethernet, or local area network (LAN), which may incorporate many other vessel control functions in addition to the DP.
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In all DP vessels, the DP control computers are dedicated specifically for the DP function, with no other tasks. A single-computer system, or ‘simplex’ DP control system provides no redundancy. A dual or two-computer system provides redundancy and auto-changeover if the online system fails. A triple or ‘triplex’ system provides an extra element of security and an opportunity for 2-out-of-3 voting.
3.2. Correct procedure for setting-up the DP system in both the Manual and Automatic modes Procedure for setting-up the DP system to the Manual Mode • The force demand comes from movement of joystick, and or heading control (if fitted). • The controls are linked to potentiometers that send control signal to the DP controllers that generate the thrusters commands. • If heading control is selected then heading priority will apply. • If no heading control is engaged thrust should be developed so as not to cause yawing forces. • High gain should give 100% thrusters forces, low or reduced gain should give 50% thrusters forces. • It may be possible to set up the joystick to automatically counter any calculated environmental forces. • It may be possible to set the joystick to act in a progressive mode rather than linear.
Procedure for setting-up the DP system to the Automatic or DP Mode •
The force demand for axis under automatic control is the sum of three different forces calculated individually, namely FEED FORWARD, DAMPING and GAIN.
•
Feed forward is the sum of wind and error compensation force.
•
With Damping the DP system calculates the vessels speed, and direction, then calculates and applies the forces necessary to stop the vessel.
•
Gain is not the same as the Joystick gain. It depends on the distance to the set point (heading or position).
•
The greater the separation between the set point and the vessel, the more force is applied.
•
High, low and medium gains are available they may be set in all axes or individually.
•
In high gain the vessel will deviate less from set points, but use more thrust.
•
Whether in manual or auto, the system will not apply 100% of thrusters out put, to use a thrusters to full capacity it is necessary to use an over-ride button (if fitted), or use the individual thrusters controls.
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3.3. Various modes of DP operation, e.g. Manual control, Semi-automatic control, Automatic control, various specialist functions ) e.g. Follow target, Follow-Sub, Track Follow, Auto approach, Weathervane, Riser Angle mode) All DP systems provide a combination of manual and automatic control. A minimum configuration is a single automatic system and a joystick based manual control. Depending on the vessel task, the automatic system may be required to be duplex or triplex. The follwing is a typical list of Operational Modes currently available. •
Joystick Manual Heading – The vessel is controled by the joystick and port/starboard movement and rotated by the turning control knob about its centre of rotation. This mode is used for totally manual vessel manoeuvring.
•
Joystick Auto-Heading – The vessel heading is automatically controlled. The joystick controls fore/aft and port/staroard movement. This mode can be used for close manoeuring.
•
DP – The vessel heading and position are both automatically maintained. This mode is used to maintain a fixed position in relation to a stationary target with a fixed heading.
•
Minimun Power/Wathervane – Maintains the heading of the vessel into the prevailing weather, while maintaining DP control.
Before an operational mode will work some requirements must be met. Sufficient thrusters are selected or “available to select” to support the mode. A gyrocompass is selected or “available to select”. A PME is selected or “available to select”. •
Joystick Manual Heading Mode (JSMH)
This mode allows single lever control of all selected thrusters. In this mode, the inputs to the system are provided by the operator alone.
Figure 18. Joystick Manual Heading
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Thrust can be applied to the vessel in fore/aft and port/starboard directions. The joystick controls the thrust on the vessel in the direction in which the joystick is pointing. The magnitude of the thrust is controlled by the amount the joystick is pushed forwards or backwards. The thrust can either move the vessel, or hold it stationary against the environmental forces. Heading is controlled by the turn control knob, which rotates the vessel about its centre of rotation, using the selected thrusters. •
Joystick Auto Heading Mode (JSAH)
JSAH mode allows single lever control of all selected thrusters. In this mode, the level and direction of thrust is provided by the operator, and the heading is controlled by the gyrocompass. Thrust can be applied to the vessel in fore/aft and port/starboard directions, while maintaining the operator set heading.
Figure 19. Joystick Auto Heading
The joystick controls the thrust on the vessel in the direction in vvhich the joystick is pointing. The magnitude of the thrust is controlled by the amount the joystick is pushed forwards or backwards. The thrust can either move the vessel, or hold it stationaryy against the environmental forces. The heading of the vessel is maintained at a set heading using the signal from a gyrocompass. The turning control knob is disabled.
•
Dynamic Positioning (DP)
DP mode maintains the vessel in a fixed position relative to a fixed reference point, while maintaining a fixed heading. In this mode, the vessel position is controlled by a PME and the heading controlled by a gyrocompass.
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Figure 20. Dynamic Positioning
The system receives the vessel's heading from the gyrocompass, and the vessel's position from a PME. When DP mode is selected, the current position and heading of the vessel are taken as the reference position and heading. The vessel's thrusters control the vessel to maintain the position and heading. The operator may change the position and heading of the vessel using the console display facilities (Change position and change heading).
Figure 21. Dynamic Positioning Inputs
•
Dynamic Positioning, Minimum Power
DP Minimum Power mode maintains the vessel's position relative to a fixed reference point, whilst minimising the vessels port/starboard thrusters demands resulting from the net weather forces on the vessel. This mode is also sometimes called Weathervaning. In this mode, the position of the vessel is controlled by a PME.
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Figure 22. Dynamic Positioning Minimum Power
The vessel's position is measured using a PME, and the thrusters are controlled to maintain the vessel at this position, as for DP mode. The vessel heading is then controlled so as to minimise the power used by the thrusters.
Figure 23. Dynamic Positioning Minimum Power Inputs
In this mode, the position of the vessel is controlled by a PME. The vessel's position is measured using a PME, and the thrusters are controlled to maintain the vessel at this position, as for DP mode. The vessel heading is then controlled so as to minimise the power used by the thrusters. The operator should be aware that should net weather change then heading will change to that required to minimize thruster use, there will be no input required by the operator, distance relative to a fixed object will change.
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•
Follow traget (Follow sub)
The purpose of ROV Follow is to maintain the vessel position relative to an underwater vehicle which is usually connected to the vessel by an umbilical providing it with services and a data link. There are two possible forms of this mode: •
Fixed Position Reference
The vessel is maintained in a fixed position and the ROV is allowed to move within a predefined area. If the ROV wanders outside the area, the vessel is moved to position the area so that the ROV is at its centre again. This form of the mode involves minimum vessel movement and is used when the ROV is moving over a limited area. The mode uses a PME and gyrocompass to control vessel position and heading, and an acoustic system to position the ROV relative to the vessel.
Figure 24. ROV Follow Mode Types
•
Fixed Distance
The vessel and the ROV move together maintaining a fixed horizontal (fixed seabed) distance apart between the vessel Centre of Rotation (COR), and the beacon on the ROV. In this mode, the vessel heading is controlled by a gyrocompass and the relative separation controlled by an acoustic PME. This form of the mode is used when the ROV is following a pipe or cable.
Figure 25. ROV Follow Inputs
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The relative separation of the vessel and ROV is measured with an acoustic transducer and transponder. With Fixed Position Reference, the vessel is maintained stationary using a PME such as Artemis or DGPS. The ROV is allowed to move around in a circular area with a radius equal to the reaction radius. The reaction radius is positioned at a constant heading on the offset radius. While the beacon or transponder on the ROV remains within the reaction radius, the vessel remains stationary. As soon as the transponder moves outside the area defined by the reaction radius, the vessel is moved so that the centre of the area is placed over the transponder.
Figure 26. Operation of Fixed Posidon Reference
•
Shutlle Tanker Pickup This is used for shuttle tankers forpicking up buoys.
Figure 27. Pickup with Various Field Types
Pickup mode positions the vessel bow at a specific point e.g. the offloading hose buoy, to enable the offloading hose (and hawser in an ALP field) to be easily lifted aboard the vessel. The mode enables the vessel to be positioned at a fixed point, without the heading pointing at the loading point, which is the case with the approach and loading modes. As an option, fixed heading can be selected in calm weather, or whenever preferred. 32
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•
Shuttle Tanker Approach
Approach mode takes the vessel from the outer perimeter of the controlled area surrounding the offloading point, to a position to either select Pickup or Loading mode, while maintaining a heading into the prevailing weather. In OLS and ALP, the vessel heads towards the loading base and the position setpoint moves around an unlimited are centred on the loading point. In FSU, the arc is limited to the stern of the FSU. There is also an option to select a fixed heading in calm weather or whenever preferred. After loading, Approach mode can be used to move down weather and leave the hose for the next tanker.
Figure 28. Approach with Various Field Types
For an ALP field, the vessel heading points to the end of the boom and the vessel always approaches with the boom to port side. •
Shuttle Tanker Loading
Loading mode positions and holds the vessel at a suitable position for offloading. The vessel moves on an arc, maintaining a heading towards the loading point and into the prevailing weather. Within an FSU, the arc is limited by the loading boundaries. There is also an option to select a fixed heading in calm weather, or whenever preferred.
Figure 29. Loading Mode with Various Field Types
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•
Shuttle Tanker Fixed Loading
Fixed Loading mode allows the vessel position to be offset from that determined by the heading. The mode is used in ALP and OLS fields to position the vessel so as not to drift into another structure. There is also an option to select a fixed heading in calm weather, or whenever prefered.
Figure 30. Fixed Loading Mode with Various Field Types
•
Riser Follow
Riser Follow mode, which is used in drilling vessels, controls the position of the vessel so as to maintain the Riser Angle close to zero.
Figure 31. Riser Follow
In Riser Follow mode, the system receives inclinometer and position signals from the drilling module. The system calculates the vessel position at which the riser angle will be zero, the zero angle position or ZAP. To avoid constant repositioning of
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the vessel, the riser angle is allowed to vary from the ZAP within a small Reaction Angle, similar to one of the ROV Follow modes. When the riser angle exceeds the Reaction Angle, the vessel is repositioned to again reduce the riser angle to zero. The reaction angle is actually translated by the system into a Reaction circle around the vessel control point. When the ZAP moves outside the reaction circle, the vessel's target position is moved towards the ZAP, and the new reaction circle drawn around it. The vessel moves towards the new target position to again reduce the riser angle. •
Heading Control for Anchor Moored Vessels
To increase the life of the anchors on an anchor moored vessel, such as an FPSO, the vessel thrusters can be used to control the vessel heading and reduce the anchor tensions.
Figure 32. Heading Control and Anchor Mooring
The simplest anchor mode provides monitoring of the anchor tensions and vessel parameters. Three other modes provide various methods of reducing the anchor tensions. • Manual Assist - The operator controls the vessel in fore/aft movement using the joystick, and rotates the vessel using the turning control knob. This mode is used for rough manoeuvring. • Auto Assist - In this mode, the system controls the thrusters to compensate for the effect of the net environmental force on the anchors. • Damped Assist - This mode also provides auto assist but in addition the vessel fore/aft vessel movement is damped. •
Simulation
This is a facility rather than a mode, in that it can simulate the operation of any mode. Its purpose is to provide operators with the opportunity to be trained on the system and to familiarise themselves with the system operation while using only the operator's console.
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Simulation can only be selected if the vessel's thrusters are not under automatic control. When the system is in simulation mode, it allows the operator to set the external environment such as wind, vessel heading, provide PME readings etc. With all the inputs selected, the vessel behaves as if it is controlled at sea. •
Model Control
Model Control is a mode that is automatically entered if there is a failure of all the vessel's reference systems. Model Control allows the vessel to be controlled for a period of time using the conditions prevailing at the time of failure. Model Control will allow the vessel to be bought under manual control in a safe and orderly manner. Model Control can be useful for periods of 1 to 10 minutes or longer, depending on the stability of the environmental conditions and other external factors.
3.4. Station-keeping, position and heading change manoeuvres, using both automatic and manual DP facilities There are other methods for vessel station keeping. These include spread and fixed moorings or combinations of each. Jack-ups fix their position by lowering legs to penetrate the sea bed. Vessels using moorings or legs may also occasionally have DP control systems to assist the setting-up on position and, in the case of a moored unit, to reduce mooring line tension. Each system has advantages and disadvantages.
Figure 33.Sstation Keeping Methods
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DP Advantages: •
Vessel is fully self-propelled; no tugs are required at any stage of the operation.
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Setting-up on location is quick and easy.
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Vessel is very manoeuvrable.
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Rapid response to weather changes is possible (weather vane).
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Rapid response to changes in the requirements of the operation.
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Versatility within system (i.e. track-follow, ROV-follow and other specialist functions).
•
Ability to work in any water depth.
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Can complete short tasks more quickly, thus more economically.
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Avoidance of risk of damaging seabed hardware from mooring lines and anchors.
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Avoidance of cross-mooring with other vessels or fixed platforms.
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Can move to new location rapidly (also avoid bad weather).
DP Disadvantages: •
High capex and opex .
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Can fail to keep position due to equipment failure.
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Higher day rates than comparable moored systems.
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Higher fuel consumption.
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Thrusters are hazards for divers and ROVs.
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Can lose position in extreme weather or in shallow waters and strong tides.
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Position control is active and relies on human operator (as well as equipment).
•
Requires more personnel to operate and maintain equipment.
From the above, it can be seen that DP will not always be the most economic solution. While vessels using moorings have a number of advantages, increasingly DP is the best option for many operations because the seabed is cluttered with pipelines and other hardware, so laying anchors has a high risk of damage to pipelines or wellheads. The option to moor to a platform rather than the seabed is also less frequent, because support vessels have become larger and platforms are not designed for the loads that can be placed in the mooring lines. Nevertheless, there is a risk that a DP vessel makes contact with a platform. During the 1990s there was a rapid increase in the number of vessels with dynamic positioning systems. Many of these vessels have been designed for DP and integrated control of engines and thrusters, but there are also a large number of conversions and upgrades. The situation is market-driven and relies on operational efficiency which, in turn, places a high reliability requirement on equipment, operators and vessel managers.
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In addition to maintaining station and heading, DP may be used to achieve automatic change of position or heading, or both. The DP operator (DPO) may choose a new position using the control console facilities. The DPO may also choose the speed at which he wants the vessel to move. Similarly, the operator may input a new heading. The vessel will rotate to the new heading at the selected rate-of-turn, while maintaining station. Automatic changes of position and heading simultaneously are possible. Some DP vessels, such as dredgers, pipelay barges and cable lay vessels have a need to follow a pre-determined track. Others need to be able to weathervane about a specified spot. This is the mode used by shuttle tankers loading from an offshore loading terminal. Other vessels follow a moving target, such as a submersible vehicle (ROV), or a seabed vehicle. In these cases the vessel's position reference is the vehicle rather than a designated fixed location. 3.5. Setting-up a pre-defined Autotrack given turn-point co-ordinates, vessel velocity and heading profiles. Initiating the Autotrack facility and monitor the vessel's progress along track •
Auto Track
The purpose of Auto Track (or Track Follow) is to move vessel along a track defined by two of more waypoints. The vessel speed is usually slow in Auto Track. The modes uses a PME for position and a gyrocompass for heading.
Figure 34. Auto Track
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Auto Track, the track may follow a pipe cable, a plan for paying out a pipe or cable, or a survey path. The first stage in Auto Track is to set up a series of waypoints in the system. These can be either input manually by the operator, loaded from diskette or downloaded from survey system. In the second stage, the vessel automatically follows a target which moves along the track. In practice, there are several additional functions which make Auto Track mode more effective. The first refinement is that the vessel speed and heading between waypoints can be independently set. The next refinement is the control of the change of vessel direction when it reaches a waypoint. To provide a controlled change of direction, a radius is defined around the waypoint. When the vessel reaches this distance from the waypoint, its direction is gradually changed so that it enters the next leg of the track in the same direction as the track. Another refinement is to offset the vessel's actual track by a set amount, say 10 metres, from the track defined by the waypoints. This vessel offset is sometimes required in cable or pipelaying. Additional sophistication in the vesel track is also necessary when moving between legs of the track so as to lay the pipe or cable at the required point on the seabed. •
Follow track
The track is programmed, or loaded into the DP system. The vessel is set up in DP auto position. If necessary the vessel is moved into the vicinity of the first waypoint. Follow track (Auto-track) mode is selected. The vessel will start to follow the track as programmed. The vessel can be stopped on the track at anytime.
Programmable functions •
Speed The operator can specify a different speed for each leg, or a single speed for the whole track. It may be possible to set a speed that the vessel will move across track. •
Leg offset
This allows the operator to move the TRACK LEG to the left or the right. This may be in increments, or as the operator requires. Track offset left and right is connected to following the track forward or reverse. In some systems the leg offset changes as the vessel passes waypoint. When applying offsets at the start of a track ensure that the offset and the vessel are on the same side of the track. •
Heading The operator can specify a heading for each track leg or single heading for the whole track. A system selected heading may be available, this will automatically keep the ships head, where the least amount of power will be used. Bear in mind the DP
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system will change heading in this mode without input from operator, should environmental conditions change. It may also be able to select that the vessel, heads towards the next waypoint. On some systems the operator may be able to control heading manually. The operator may also be able to change heading control menu. •
Position moves Normal position move disabled. It is possible to offset track legs.
•
Track offset
It is also possible to offset the whole track. Geographic offsets the whole track a bearing and distance, to make an exact copy of the track. Parallel offsets each leg a set distance to the left or the right. The leg lengths will change using this strategy. •
Pipe lay functions
Move up functions are available that will move the vessel up a single length of pipe. Speed set points can come from cable lay computers. Monitoring of cable or pipe tensions. Automatic slow down in the event over over tension. •
Turn Radius
Generally used to alow vessel to round a waypoint without the need to slow down. Used during a heading change as vessel passes a way point. This may be set for each way point, or for the whole track. There may be an automatic function that uses the heading change speed as set for normal heading changes. This should used with caution, if rotation speed is set very low the vessel may start to change heading long befor vessel reaches the waypoint.
Figure 35. Turn radius Alstom
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Figure 36. Follow track turn radius Kongsberg method
3.6. System switch-on, loading procedure and re-loading procedures It is possible to follovv the track forwards or reverse. The vessel may be able to possible to specify the reverse or turn to port or starboard to head the opposite way down the track. Stop or Tracking allows the vessel to be stopped at any time. It may be percentage power used when stopping the vessel. It may be possible to get the vessel to back up to the position at which the stop command was given. Loading Tracks may be possible to save tracks to, and load tracks from disk or charting package. It may be possible to load tracks from a remote computer. There has been a case of a pipeline laid in the wrong place because difference in spheroid or projection between what the DP system uses, and. what the track was written in.
3.7. Concept of Centre of Rotation and provisions of Alternative Centres of Rotation In any DP situation, there will be a reference point within the vessel known as the Centre of Rotation (C of R). This is the spot actually being navigated and can be located at various points. In simple installations the C of R max be placed at the centre of gravity, while other vessels have the C of R located at a specific point, e.g. moonpool, cable lay sheave, „A“ frame position or drillship rotary table. Many vessels have more than one C of R available. The DPO selects from a menu exactly where in the vessel he wants the C of R.
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4. POSITION REFERENCE SYSTEMS Accurate, reliable and continuous position information is essential for dynamic positioning. A DP vessel should be able to maintain her position to within one or two metres of the "set-point" or desired location. Reliability is, of course, of vital importance, to operations where life and proprety may be put at extreme risk through incorrect position data. A vital part of any DP system is the provision of Position Reference. This is the main area where DP differs from traditional navigation. Traditional electronic navigation systems are often insufficienly accurate (e.g. Decca Navigator, Loran-C and Global Positioning System (GPS) without differential corrections). The navigation systems familiar to navigators are generally of limited value in DP work. DP systems are thus interfaced with Position Reference Systems (PRS) or Position Measuring Equipment (PME) providing greater levels of accuracy and stability. All DP vessels have position reference systems (PRS), sometimes referred to as position monitoring equipment of PME, independent of the vessel's normal navigation suite. Five types of PRS are in common use in DP vessels; •
Hydroacoustic Position Reference (HPR),
•
Taut Wire,
•
DGPS,
•
Laser-based systems (Fanbeam and CyScan) and
•
Artemis.
Position reference systems should be selected with due consideration to operational reguirements. For eguipment classes 2 and 3, at least three position reference systems should be installed and simultaneously available to the DP-control system during operation. When two or more position reference systems are reguired, they should not all be of the same type, but based on different principles and suitable for the operating conditions. The position reference systems should produce data with adequate accuracy for the intended DP-operation.The performance of position reference systems should be monitored and warnings provided when the signals from the position reference systems are either incorrect or substantially degraded. For equipment class 3, at least one of the position reference systems should be connected directly to the back-up control system and separated from the other position reference systems. For any operations requiring DP redundancy (equipment Class 2 or 3 operations) it is necessary to utilise three position references. Two PRSs are not adequate, because if one has failed, contradictory reference data provides an impass whereas three systems provide two-out-of-three voting to identify a rogue sensor. Where three PRSs are required, the DPO should choose systems that are different. This reduces the probability of common-mode failure, where one event may result in a loss of position.
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4.1. Operation of a Hydro-acoustic position Hydroacoustic Positioning Reference (HPR) is one of the most prevalent PRS use in conjuction with DP. Acoustic systems provide positioning with devices below the water using the propagation of sound through water in the same way as radio waves.There are three basic system types and a fourth which is a combination of two of the basic types. The four types are: •
Long Baseline (LBL) – accurate, but requires an array of seabed beacons.
•
Short Baseline (SBL) – now superseded.
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Ultra Short Baseline (USBL) – less accurate than LBL, uses one beacon.
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Long and Ultra Short Baseline – combine best of both.
Although the names of the systems suggest a continuum, each uses a different technique for the sound sources and detection system. Each has advantages and disadvantages which determine when and how each is used. HPR systems are manufactured by Nautronix, Sonardyne and Kongsberg Simrad. Underwater acoustics have many applications, one of which is the provision of position reference for DP purposes. Specific application for acoustic •
Drilling
•
ROVs
For drilling in deep water, a combination of USBL on the vessel and the LBL transducer on the BOP is used. In addition, the drill string has inclinometers which have both wired and acoustic coupling. Placing the transducer on the BOP and wiring it to vessel has several advantages: •
The transceiver is removed from vessel nosie.
•
Update rates are reduced to 2,5 sec at 2500m.
•
Lower power transonders can used, giving additional life.
•
EHF transponders with an accuracy of ±10 mm can be deployed.
For ROVs, towing, drill string or other mobile target, USBL is used to track in terms of range and bearing relative to vessel. Acoustic positioning is also used for tracking of underwater vehicles or equipment, the marking of underwater features or hardware and the control of subsea equipment by means of acoustic telemetry.
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Figure 37. Acoustic Basics
4.2. The principles of position definition using the various forms of HPR system (e.g. Ultra-short, Super-short, Long baseline and Multi-user principles) •
Long Baseline - the baseline is the distance between the beacons.
In deepwater locations, where the accuracy of the other types degrades, the long baseline (LBL) becomes more appropriate. LBL systems are in extensive use in drilling operations in deep water areas (>l 000m). LBL acoustic consist of a single transducer on the vessel, and an array of at least three transponders, which are separated by more than 500 metres. Typically the array will form a pentagon (5 transponders) on the seabed, with the drillship at the centre above. One transducer upon the vessel interrogates the transponder array, but instead of measuring range and angular information, ranges only are measured, because the baseline distances have already been calibrated (distances between transponders). Position reference is obtained from range-range geometry from the transponder locations. Calibration is done by allowing each transponder to interrogate all the others in the array, in turn. If, at the same time, the vessel has a DGPS or other geographically-referenced system, then the transponder array may also be geographically calibrated. Accuracy is of the order of a few metres. The distance for the vessel transducer to each transponder is measured by timing a signal from the transducer to the transponder and back again. A single transducer signal is sent and each transponder then replies with a different frequency signal. An acoustic signal around 10kHz is used with LBL. Three ranges can provide the vessel position: however, more ranges are usually provided for redundancy. The baseline for the transponders can be over 100% of water depth. The layout of transponder array and position of vessel above the array affects when interrogations can be made. Obviously, the effect of multiple acoustic pulses being received affects the data rate. Interrogation is complete when all return pulses are
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received. At 4000m, the effective data rate can be over 10secs. Multiple interrogations are easier if the vessel is near the centre of the array.
Figure 38. Long Baseline System
•
Short Baseline System - the baseline is the distance between the hydrophones 15m.
SBL uses a single transponder an array of transducers mounted under the vessel hull. The term acoustic beacon is ussualy used because it sends out a series of pulses, rather than responding to an input. Similary, the transducer are sometimes called hydrophones as all they need to do is listen. The baseline for this technique is the separation of the transducers along the vessel bottom. Again, it is a range system but now it needs compensation for vessel motion, which is provided by the VRU. The beacon on the seabed emits short bursts of acoustic energy with known periodicity and frequency. The time of arrival of a single pulse at three or more transducers is measured. Detecting the required sound from the background noise requires hydrophones which reduce noise effects. The minimum distance between hydrophones is 15m.
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SLB can be used up to about 1000m. The positioning of the hydrophones on the bottom of the vessel should try to keep them away from sources of aeration (thruster). An alternative design uses phase comparison on the beacon signal. This is a similar time of arrival, but hydrophones need to be only 10 cms apart. Therefore, only one hydrophone assembly is needed, and the VRU can be put in hydrophone assembly.
Figure 39. Short Baseline Acoustic
•
Ultra Short Baseline
USBL or Super Short Baseline SSBL was introduced in 1993. The technique used in phase comparison with many receiving units positioned around the transducer assembly. Position is calculated from the measurment of range and angles.The time of the round trip is used to calculate the range. Small differences in time of arrival translate into direction, mesaured in time-phase differences which are mesaured in two axes to calculate the slant angle. As the technique requires angle measurment, vessel motion correction is required from a VRU. An accuracy of better than 0,2% of slant range is possible with VRU correction.
Figure 40. USBL Slant Range and Angles
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USBL is the most used PME acoustic system. It can be used for both fixed position and tracking applications. A transducer array can handle up to ten stationary or mobile transponders by using different interrogation and reply frequencies. Frequencies used range between 19 KHz and 36KHz. The vessel coordinates with respect to the transponder are calculated from R, Øx and Øy. Care is required int he adjustment and calibration oft he transducer. •
Ultra or Super Short Baseline Acoustic System
The principle of position measurement involves communication at hydroacoustic frequencies between a hull-mounted transducer and one or more seabed-located transponders. The ultra or super short baseline (SSBL) principle means that the measurement of the solid angle at the transducer is over a very short baseline (the transducer head). An interrrogating pulse is transmitted from the transducer. This pulse is received by the transponder on the seabed, which is triggered to reply. The transmitted reply is received at the transducer. The transmit/receive time delay is proportional to the slant and range. So range and direction are determined. The angles and range define the position of the ship relative to that of the transponder. The measured angles must be compensated for values of roll and pitch.
Figure 41. SSBL Principles
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4.3.
The use of the various types of acoustic Beacon, Transponder, Responder and seabed array used in conjunction with an HPR system
Acoustic systems use tranducers which transmit and receive the acoustic signal and transponders which receive the acoustic signal and retransmit it. Acoustic systems are effected by the depth of the water, salinity, temperature and frequency of the source. Currently, transponders are available which produce an acoustic signal at a depth of several thousand metres, which can be reliably detected at the surface. When considering system accuracy, the constant components can be eliminated, whereas the random components (acoustic nosie and acoustic attenuation) only reduced. Air is major source of attenuation and thrusters a main noise source. Coordinate calculation must take account of agliment of transponder array with the vessel coordinates and the pitch and roll compenstaion provided by the VRU. In comparasion to light or radio, acoustic signals travel very slowly. The typical speed of sound in water is 1485 metres/second. Therefore at depths of 4000 metres, the signal will take approximately five seconds to return, which is well beyond the normal PME position update rate of under one second. In selecting an acoustic system, the following factors should be considered: •
Water depth.
•
Accuracy.
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Area of coverage.
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Data rate.
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Operating convenience.
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Reliability.
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Cost of ownership.
The vessel must deploy at least one battery-powered transponder. They can be deployed by downline from the vessel, by an ROV or simply dropped overboard. Lavering can cause errors, especially when the horizontal displacement from the vessel is large. All use frequencies in the 20-30 kHz band. Some transponders are compatible with more than one supplier's equipment.
4.4. The display and configuration of the various elements in 4.3, and the acquisition of HPR as a position-reference for DP operations With any PRS, some allowances must be made. The most obvious is the x/y offset between sensor (in this case the transducer) and the fixed reference point in the vessel, known as the "Centre of Rotation". Another compensation must be made for the discrepancy between the geodetic vertical and local (ship-attached) vertical. Since HPR is heavily dependent upon the accurate mesaurment of his angle, it is
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important that the atitude (roll and pitch) of the vessel be monitored accurately at all times. This is function of a vertical reference sensor or unit (VRS or VRU) fitted specificaly for that purpose. 4.5. Advantages and limitations of HPR as a a position reference system All acoustic systems suffer from a number of problems. Any source of noise in the water will reduce the efficiency of the system, while the accuracy may be compromised by temperature layers, inversions and the resulting refraction. The working range of the system may also be limited, especially in shallower waters. Nevertheless, HPR is extensively used offshore, not only for DP purposes, but also for marking and location of underwater hardware, ROVs, etc. •
Long Baseline
The accuracy of acoustic systems is very dependent upon the depth of water and so generalised figures are of little use. However, LBL is more accurate than either SBL or USBL. It also has advantages that the technique used with LBL does not require a VRU for angle compensation for vessel motion. The main disadvantages of LBL are that deploying and calibrating the array expensive. •
Short Baseline
Thus the accuracy can be better than the ultra or super short baseline type of system and work with one transponder or beacon, but it still relies on vessel motion corrections. Some vessels have as many as eight hull penetrations for tubes or poles on which the hydrophones are deployed. Advantages •
Independence of a fixed station
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The system is under the ships control
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Beacons can be pre-deployed, and left at work site
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Can be deployed so that there is no physical link to seabed.
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Reasonable accuracy
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Lots of different uses
Disadvantages •
Suffers from interference from, refraction, noise, and absorbtion.
•
Affected by multipath.
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Some systems have beacons that operate on the same frequencies, and will interfere with each other.
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There are reports that equipment made by different manufacturers interferes with other systems.
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Batteries run out.
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Beacons need to be deployed.
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4.6. Principles and operation of the Artemis position-reference for DP The two main radio system are: • •
Artemis and Differential Global Positioning System (DGPS)
Artemis is a microwave system operating between a fixed and mobile station, which provides range and bearing dana relative to the fixed station. Differential Global Positioning System (DGPS) is USA based system development of the GPS satellite system which uses signals from several satellites, a signal from fixed reference station and a receiver on the vessel to provide the vessel position. Glonnass (Russia) is an alternative satellite system to DGPS. Other radio system includes Syledis, Argo, Trisponder, Microfix The Artemis range and bearing system was introduced in 1989. It is still used for offshore loading and close in operations. Artemis consists of two units working at microwave frequencies. One unit, the „mobile“ is located aboard the vessel, while the other unit, the "fixed" station, is located usually on a nearby platform. On modern systems the fixed and mobile units are interchangeable with programming. Each unit carries a double slotted waveguide antenna and, when working, a microwave link is maintained between the two stations. The antennae are servo controlled to track each other against vessel movement, thus allowinga continuous microwave link to be maintained. An interrogation signal originates at the mobile (vessel) unit, and is detected by the fixed station. The mobile receives the reply. The data passed between the two stations provides separate data on range and bearing between the fix and mobile stations. This data enables the vessel position to be calculated. The range between the fix and mobile stations is determined by measuring the time lapse between a signal leaving the mobile station, transisting the fixed station and returning to the mobile station. Bearing is measured directly at the fixed station end, with the bearing data coded into the reply. The fix station transmits the bearing from true north (or a set reference) that the fix station antennae has turned in order to point at the centre of the mobile antennae. The measurment is made by a precision shaft encodes coupled to the fix antennae shaft that measures the angle the antennae has to move to lock with the mobile station. Maximum range for DP operations is between five to 10 kilometres, although longer ranges have been achieved at the cost of positional accuracy. Artemis is useful where a relative position may be required, e.g. reference to a moving point. A shuttle tanker may need a reference from a FPSO, which may itself be slowly oscillating in position. Artemis is an accurate system, but is subject to line-of-sight breaks, antennae roll movement and heat (infra-red) interference.
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Figure 42. The principle of the Artemis microwave position reference system
Artemis has a range of 10 metres to 30 Kms and full 360° coverage around the fix station. At short ranges, the fixed antennae can be replaced by a beacon. It is then necessary to measure the bearing at the mobile station which introduces compass errors and limits the useful range to about 200m. Artemis operates at 9.2 GHz and is therefore unaffected by rain, gog or haze. It dose, however, require an unobstructed line of sight. Its use is limited to areas where fix stations or beacons have been installed. Artemis is used for positioning applications such as support vessels, in and offshore positioning and surveying. Beacon operation is used between Offshore Loading terminals (OLT) and shuttle tankeres.
4.7. The advantages and limitations of the Artemis position-reference system Advantages • Relatively long range (up to 30km). • High accuracy. • Possibility to geographically reference the system. • Very convenient inside the 500m zone, or any other controlled area.
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Disadvantages • Requires a fixed location nearby to set up the fixed station. • Unit needs to be calibrated and configured, this needs experience and skill. • Special unit needed for hazardous areas. • Assistance required from platform personnel. • Can be interfered with by platform personnel. • Can suffer interference from heat haze, or precipitation. • Signal lost with line of sight interference. • 3cm radar interferes with Artemis. • Vulnerable to power supply problems at fixed end. 4.8. Taut-wire position reference system Tautwire systems differ from all other PRS in that they are chiefly mechanical in nature. A taut wire is a useful position reference, particularly when the vessel may spend long periods in a static location and the water depth is limited. A typical tautwire system consists of an A-frame or davit assembly located on deck. The commonest consists of a crane assembly on deck, usually mounted at the side of the vessel and a depressor weight on a wire lowered by a constant-tension winch. A Taut Wire system measures the variation in the position of a fixed point on the vessel relative to a fixed point on the seabed. The two points are joined by a constantly tensioned wire, and it is the variation in the angle of the wire which is mesaured. The Taut wire davit can be installed at any convention point at the side, stern or bow of the vessel. The closer it is to the centre of rotation of the vessel, the less will be the effect of pitch and roll. This will reduce the wear on the tension control mechanism. A major consideration is choosing a position which enables the maximumm in board angle before the wire touches the vessell. The Taut Wire is installed on the vessel either parallel or at right angles to the axis of the vessel. The control station must be positioned so as to provide the operator with clear view of the sinker weight when it is being lowered and lifted past the vessel. Typical accuracy of a Taut wire is ± 2% of the water depth, up to 500 metres. As depth and/or angle becomes geater, the catenary effect increasees, causing the accuracy to decrease due to the effect of currents and tides. Typically, the maximum angle allowable is ± 30° in either plane. A service working range is ± 15°. Deployment and rettrieval of the sinker weight can be a problem in heavy sea conditions, as can dragging of the weight.
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Figure 43. Taut wire
4.9. The procedure for deployment and recovery of the taut wire system •
Before use the system will need to be switched on and allowed to warm up, the includes cooling water on some systems.
•
The system will also need any fastenings relasing.
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An idea of water depth is useful.
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When on location the weight is raised clear of its holder.
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The boom is then lowered, or swung out.
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Lower the weight to the seabed.
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Switch to constant tension, or „mooring“, this function may be automatic.
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Advise DP control, who can then select the taut wire into the system.
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The sensor then measures the fore, and aft, and port and starboard angles.
•
And use these signals to maintain position or move the vessel.
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4.10. Display of taut-wire reference data in the DP system. Principle of position-reference using the taut-wire system Variations on the taut wire theme;
Figure 44. Moon Pool Taut Wire
The Moon Pool Taut Wire is mounted inboard with a depressor weight deployed through the bottom of the vessel through a small moon pool or wet well. The gimbal head is incorporated into an elevator unit that is lowered from the stowage level down to the keel level, from where the weight and wire are lowered. Movement compensation is provided by hydraulic accumulator, and positioning data is obtained and processed in the same way as in the Light Weight Taut Wire system. With a Moon Pool Taut Wire the gimbal head is that much closer to the seabed than with a deck mounted unit, so a shorter length of wire is used. However as the wire cannot touch the vessels side the range is often greater than that of a deck mounted unit.
Figure 45. Surface Taut Wire
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The wire is passed across to the platform adjacent and secured. No boom is needed, instead The Surface Taut Wire gives position reference relative to a fixed structure (platform, buoy, etc.). The sensor is located atop a short vertical tower. The principles are the same as for the vertical taut wire systems. The range for a Surface Taut Wire System is around 50 m. For longer distances the accuracy would be reduced due to the curvature of the wire. There is also something called a gangway reference system, the pivot at the inboard end of the gangway is used to give a relative bearing, and the sliding section of the gangway is used to measure change in range. This reference is lost if the gangway is emergency disconnected.
4.11. The advantages and limitations of the taut-wire position reference systems Of necessity, a taut wire system is limited in many ways. Maximum range is dependent upon water depth, as is system accuracy. The system is limited to a rated maximum depth of typically 300-400m. The system is limited to providing a relative position for a vessel in a fixed location. The system may be influenced by strong currents. If the vessel has to shift location then the tautwire weight must be reset. Nevertheless, the tautwire is a popular, accurate and frequently used system. Taut Wire systems are reliable and rugged. They are excellent for maintaining position for long periods of operation. Set up and operation are rapid and simple, in moderate weather conditions.
4.12. Principles of Differential GPS system There are 2 main systems the American Global positioning System (GPS), and The Russian GLONASS system. These are military systems that are made available to civilian users. GPS is a satellite based passive ranging navigation system which provides latitude, longitude and altitude data anywhere in the world. GPS consists of 21 operational satellites, with three spare satelittes, in six orbits of 20 200 Kms. Each satellite takes 12 hours to orbit the world. This pattern of satellite means that 4 satellites are always in view from any point on the earth's surface. Differential GPS (DGPS) is an extension of the GPS of satellite navigation. Although GPS is now widespread use, the accuracy is still deliberately degraded by the US Department of Defence means of the Selective Availability (SA) applied the satellite data. This means that the commen GPS receiver may provide a positional accuracy 10 – 100 m. Military specification receivers, make use of the P-code, may achieve an accuracy of 1-5 m but these systems are not commercially available the present time.
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Figure 46. GPS Satellite Orbits
DP users may avail themselves of a number DGPS services provided by commercial organisations. Such an organisation will maintain a network of shorebased reference stations continually observing the positional effects, or errors resulting from the Selective Availability. These errors are then communicated to the vessel by means of a suitable radio link.
Figure 47. Differential GPS
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The ship may then obtain positional data with an accuracy of typically 1-3 m. This then becomes a useful input to the DP, although there are a number of pitfalls of which the DPO must be aware. These include the effects of loss of the data link providing the differential corrections, resulting in an immediate degradation in accuracy. Despite the problems, DGPS is now regarded as one of the most useful and versatile of PRS for use with DP. This defines a range of messages covering Differential GPS corrections and rate of change, reference station parameters etc. 4.13. The operation of a modern differential corrections network Most DGPS services accept multiple differential inputs obtained from an array of reference stations widely separated. Generally, network DGPS systems provide greater stability and accuracy, and remove more of the ionospferic error than obtainable from a single reference station. Network systems are more comprehensively monitored at the Hub, or control stations, where user information or warning data may be generated and sent out. The choice of which link to hire or purchase must be made based on the vessel's expected work areas. If a vessel is expected to be working near fixed platforms, a local HF connection can be best. For floating production, storage and offloading (FPSO) vessels, a local UHF link and relative GPS solution can be the best arrangement. The accuracy obtainable from DGPS systems is in the area of 13m dependent upon the distances to the reference stations, ionospheric conditions, and the constellation of satellites available. DGPS tends to be less reliable in close proximity to large structures (i.e. platforms) due to interference to satellite and differential signals. DGPS perfomance near the magnetic equator has suffered due to scintiilation (sun spot activity causing ionospheric disturbances).
4.14. The sources of error and inaccuracy associated with the DGPS system, effects on the quality of positioning Sources of error •
Multi-path - SV's signals bounce off nearby objects, and are also received, these cause a degradation of accuracy, and if severe enough make signals unusable. This is sometimes called long path interference. If the extra signals are bouncing off an object onboard you may need to move the antenna. If they are bouncing off an object outside the ship, moving position, or changing heading may reduce the effect.
•
Troposphere and Ionosphere errors - The SV signal is refracted (bent) as it travels through the earth's atmosphere. The bending depends on ionisation, temperature, pressure, and humidity. This cannot be measured everywhere so it is assumed. If the actual conditions are different, the signal will follow a different path to the projected path and you have got an error.
•
Geometry - This is position line theory. If the position lines cross at right angles and there is an error, the area of uncertainty will be smaller than if the position lines cross at a small angle.
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•
Clock errors - While the clocks used are very accurate, they are not perfect, any timing error can result in a position error.
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Ephemeris (orbital errors) - These can occur when the SV does not follow the path projected by Master control.
•
Finger trouble - This has not happened but has been predicted; the idea is that one day an operative will make some kind of programming error that will not be spotted by quality control., and will causing a position error.
•
Scintillation - This is extreme localised bending of the SV signal caused by sunspot activity. The path followed by the signals from the SV's is so different from that projected it can make the signal unusable. It can even affect differential corrections as the rate of ionisation at the correcting station and the user is so different the corrections are invalid.
•
Differential Satellite systems - Due to military concerns when GPS was first introduced, the signals were deliberately degraded, by dithering; this was a random rapidly varying timing error, called Selective Availability (SA). This reduced the accuracy of GPS to about 30 metres which was not good enough for DP. Commercial companies set up a system to correct for SA, and while SA was removed in 2001 Differential is still used as it helps improve reliability and correct for other sources of error.
4.15. Available quality data associated with the DGPS system The Differential GPS improves the accuracy of the SPS to 1 - 5 metres. The GPS satellites all transmit two frequencies, L1 and L2. The L1 frequency is modulated with Precise code (P code) and coarse acquisition code (C/A code). The L2 frequency is only modulated with P code. The critical feature is timing the satellite transmissions. To do this, each satellite has a caesium frequency standard on board. The positioning receivers on the vessel have 8 or more receive channels to track and decode the C/A code from the L1 frequency of 8 or more satellites. The GPS receiver uses two observations to enable it to position itself, a pseudo range and the carrier frequency. The Pseudo Range is the signal travel time between the satellite and the vessel receiver, converted into metres. The travel time is obtained by decoding the C/A code received from each satellite and calculating a time offset. The range is called a pseudo range because it is contaminated by errors in the cheper less accurate receiver clock. But as the error in the clock is the same for each satellite and as long as four satellites are in view, the error can be derived from quadratic equations. Four ranges are require as there are four variables to be found: x, y, z, and the clock error. The position of the satellite is provided in the navigation message part of the C/A code (a sophisticated set of Ground Monitoring Stations track the satellites and return position dana to the satellites using WGS84 geodetic system).
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4.16. Advantages and limitations of the DGPS system compared with other PRS Advantages •
Once set up it is easy to use.
•
Signals monitored, and problems promulgated.
•
Water depth (shallow or deep) not a concern.
•
No physical links to anything.
•
No need to reset within normal operational moves.
•
Helps to correct for all sources of error.
•
Some systems will give indication of multi-path.
•
Dual frequency systems available, which are more reliable and can, reduce the effects of scintillation.
Disadvantages •
Systems can be affected by multi-path.
•
May need to set for different areas of operations, trial points, reference stations, height aiding etc.
•
Can be affected by sunspot activity especially single frequency systems.
•
Corrections can be blocked line of sight.
•
If operating close to large tall structures SV's can be blocked reducing the number of satellites available, to the extent that fixes become unusable.
•
Correction links can be knocked out by microwave interference.
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Corrections become less relevant as you move away from reference stations (you would not operate in the Pacific using corrections for Aberdeen).
•
Corrections need to be up to date, before SA was removed this was about 30 seconds. Now SA is gone this is not so relevant.
•
Must have 3 or 4 satellites for a fix.
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Cannot use satellites that are too high or too low.
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System interfaces vary, some are quite complex, others are just a black box with the operator having no insight into processing.
•
Systems can be jammed.a 1 watt jammer could effectively jam a coastal port).
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4.17. The principles used in Relative GPS systems DGPS provides accurate position for a fixed position. Some vessel operation require accurate positioning between moving vessels. Such a situation is a shuttle tanker and FPSO which weathervanes. Typically the stern of the FPSO describes a figure of eight which the bow of the suttle tanker must follow. The FPSO may be turret-moored, so it can wathervane. The stern of the FPSO describes the arc of a circle, as well as surge sway and yaw motions, providing a complex positioning problem for the shuttle tanker. The FPSO uses a standard DGPS to monitor its position. The shuttle tanker receives GPS data on its own receiver and receives GPS data from FPSO over a UHF link. The shuttle tanker then compares the two positions and derives a range and bearing which is led to the DP system. The UHF link also provides various telegrams from the FPSO. The UHF aerial is provided with offset compensation by the VRU.
Figure 48. Relative GPS
For the measurement of relative position by GPS, differential corrections are not needed, as the errors induced are the same for the shuttle tanker as they are for the FPSO. A DARPS transmitter on the FPSO sends the received GPS dana to the UHF receiver aboard the shuttle tanker. A computer aboard the shuttle tanker then calculates a range/bearing from the FPSO's stern, which is put in to the DP control system as position reference int he same way as Artemis.
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4.18. The principles of position reference using optical laser-based systems There are two major systems in operational use Cyscan and Fanbeam, they both give a relative range and bearing, to a reflector, or reflectors. The operating station consists the laser device that transmits and receives the laser pulse and the reflector (either tube, prismatic type, or combined type). The systems have multiple uses, it can be used to provide fixing information for a DP vessel, it can be used to track targets relative to a vessel e.g. a seismic boat can track the relative position of tail buoys, or a pipe-laying barge can track the position of anchor handling vessels relative to the barge. Systems may track single or multiple targets.
4.19. The method of setting-up a laser based system to provide information •
CyScan
CyScan is a short range laser based high precision positioning and tracking system. It consists of a stabilised rotating laser and three or more reflective targets positioned on the fixed vessel or structure. The reflective targets are fixed at defined spacing along a baseline. Vessels can be uniquely identified by altering the spacing between the targets.
Figure 49. CyScan Positioning System
A pulse of light is fired at a reflective target and the round trip timed to provide the range measurement. At the instant the pulse is returned, an optical encoder on the shaft is triggered to provide the angle. The laser unit can be mounted anywhere on the vessel, but is normally placed above the bridge. It can easily be lifted and repositioned.
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The rotating laser head is placed on a 2 axes stabilised platform which provides compensation for pitch and roll. The system is set up for a particular vessel with the number of reflectors and their separation. The system receives range fixes and pattern of pulses. The use of three or more reflectors provides a high degree of redundancy and enables erroneous reflections to be discarded. The system is therefore not affected by objects getting in the way or direct sun. • Fanbeam The Fanbeam system is an alternative short range laser based positioning and tracking system. The system consists of a vessel borne laser unit and a reflector, providing range and bearing. The reflector can be fixed onshore or on a fixed structure. Although range-range and range and bearing modes are available, currently only the range and bearing model is used for DP. The practical useful range for DP is around 200-250 metres. The laser unit must be aligned to the vessel axis, to which ali bearings are referred. A VRU is needed for compensation for pitch and roll. The system consists of an array of lasers, which emit a vertical fan of light, mounted in a unit which can rotate up to 360°. Many returns are processed and average values provided. A reflector using reflective tape can be used up to 100m. Above this range, a retro prism reflector is used. At longer ranges, multiple retro prism arrays are needed.
Figure 50. Fan Beam
4.20. Advantages and limitations associated with the optical laser PRS Optical laser is fast becoming a very popular PRS for DP purposes. It is accurate and simple to set up and use, but has limited range, while range and efficiency is impaired by poor visibility, rain or simply from dirt on the lens. It is also vulnerable to line-of-sight breaks.
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4.21. Relative accuracy and reliability of the aforementioned PRS, methods used to apply weighting and pooling when more than one PRS is acquired DP systems depend upon being able to position the vessel in a manner appropriate to its role. So, a drilling platform will need PMEs to maintain it in a stationary position, whereas a shuttle tanker will need PMEs to be able to position it relative to a structure or vessel. The accuracy of PMEs depends on their role and the other PMEs with which they are used. The reliability of PMEs is usually handled by presuming that PMEs will fail and therefore providing redundancy both in similar and alternative PMEs. There are many different PME systems used for position reference with DP systems. The selection of PMEs for a vessel is based on the role of the vessel and the characteristics of the PME. It is possible to have a DP system supported by just one PME but for reliability, two or more PMEs are usually used. PME TYPE
RANGE
MAX DEPTH
ACCURACY
GEOGRAFICAL RANGE
Taut Wire
25% of water depth
500 m
2% of water depth
Worldwide
Radio
30km
N/A
± 1m
Limited to beacon availability
GPS
Unlimited
N/A
±3m
Worldwide
Hydro Acoustic
5 x water depth 4 000 m
1-2% of water depth
Worldwide
Laser
250 m (Useful range for DP)
< 0,5 m
Needs fixed target
N/A
Table 2. Relative accuracy and reliability of the PRS
Where several PME position references are available, their values can be pooled in several ways. The simplest form of pooling is to use the average value. A more sophisticated method is to discard any readings which fall outside a window placed around the average position. A further sophistication is to place weightings on each PME for creating the mean value. The pooling of PMEs complements the individual PME checks for signal reliability, which may cause the PME to be deselected. When several PMEs are available, a voting system can be used to pool the position values, weighting the values as appropriate. In certain weather conditions, the reference position from the PMEs may vary rapidly or erratically. To avoid unnecessary thruster demands, the operator can alter the vessel response to PME inputs by adjusting the Kalman filter in the control system.
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4.22. Other PRS which may be used in conjunction with a DP system •
Syledis
Syledis is a proprietary UHF radio positioning system developed by Sercel of France. It relies on a network of shore based transponders or becons to provide positioning over defined areas. Many areas exist all over the world which have Syledis cover. It is a propagation time measurment system. Typical positioning is up to 100 Kms with an accuracy of 1 metre within line of sight. The accuracy depends upon the beacon height, atmospheric conditions and the network geometry relative to the vessel. There are two types of Syledis, RangeRange Mode and Hyperbolic Mode. •
Hyperbolic or Passive Mode
With hyperbolic mode, the vessel receives a pair of signals from fixed stations. The signals are place din synchronisation by a master beacon, The pair of signals define a hyperbola upon which the vessel lies. With three hyperbola defined, the vessel can be positioned. There is obviously no limit to the number of vessels that can use this mode.
Figure 51. Syledis
•
Microfix
Microfix is a short range, 50 Km, microwave positioning and survey system. Arrays of transponders are placed in fixed locations or on platforms. The system uses range-range mode interrogation. It has a multiuser capability of up to 16 users in an array. Accuracy is about 1 metres. As with all microwave systems, they are limited to line of sight and atpospheric conditions.
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•
Trisponder
Trisponder is similar to Microfix, but offers both microwave anf UHF capability. The microwave version offers a single beacon interrogation providing range and bearing. For line of sight microwave, the accuracy is 1 metre. •
Argo
Argo is an HF multiuser positioning system which provides cover with an array of fixed and mobile beacons. The array is controlled by a fixedmaster station which provides the synchronisation pulse. As expected with HF radio. The range varies between day and night, varying between 300 and 700 Kms. Accuracy is about 5 metres. •
Other Satellite Systems
In 1996 the Russian equivalent of GPS, GLONASS became available in the West. GLONAS also uses a pseudo range system, but is different in its use of frequency (FDMA as opposed to CDMA) and it its choice of geodetic system PZ90 as opposed to WGS84). Translation between the systems is computationally possible and combined receivers are available. Real Time Kinematic GPS (RTK GPS) is a differential system which uses the pseudo range and carrier phases to improved accuracy to within 5cms. The system is processor intensive, but with improvements in computer speed, acceptable update delays are becoming possible. •
The GLONASS system
GLONASS (the Global Navigation Sateilite System) is the Russian counterpart to the American GPS, being similar in design and operation. The system was initiated with the first sateilite launches in 1982, and by 1996, 24 operationaal satellites were in orbit. However, this number has not been maintained and the number available has, at times, been unadequate for good positioning. The principles and practice of position determination with GLONASS are identical to that of GPS, using pseudo-range mesaurement from time and ephemeris data transmitted from the satellites. The higher orbital inclination of GLONASS satellites (65˚), compared to the GPS constellation (55°), results in better satellit e availability in higher latitudes. The limited sateilite availability precludes the use of GLONASS as a continuous position reference for DP. A number of combined GPS/GLONASS receivers are available. These have the effect of increasing the number of usable satellites within view of the observer. •
DARPS (Differential and Relative Positioning)
In its most basic form only 2 GPS receivers are needed, usually between something like a FPSO , and a shuttle tanker. The FPSO broadcasts its GPS fix over UHF to the shuttle tanker, which then calculates a range and bearing to the FPSO. As the 2 are relatively close
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together if there are any errors they should be common and the relative range and bearing should be accurate. •
Dual Frequency Systems
If both L1 and the L2 frequencies are used it is possible to calculate actual ray bending. At the Reference station both signals are received, by measuring the difference in rate of bending of the two signals the actual bending at the reference station can be calculated. This can then be applied to the SV signals to remove the effect, and then the fix is calculated and residual errors calculated, and transmitted. At the user end both frequencies are received, the signal bending at the user is calculated and its effect removed. The corrections from the reference station are applied to give a fix that has no ionospheric-tropospheric-scintillation errors.
Figure 52. Single and dual frequency with scintillation
•
Carrier Phase Difference System
40KM range needs a local reference station which the user sets up, gives position, Heading, VRU, and height of tide information. •
Error Segmentation
The errors are divided in 2 part those at the satellite and those at the user. The user errors are calculated using Dual frequency technology, the satellite errors (clock and orbit) are measured at land based reference stations and transmitted to the user. The satellite errors are valid worldwide. The local errors are calculated so with this combination of correction, the user GPS signal can be corrected for anywhere in the world.
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•
Galileo
A European stand alone GPS system called Galileo has just been given the go ahead, with multiple frequencies, and better atomic clocks, it is expected to exceed GPS standards. This is expected to be in operation between 2008-2010. •
GPS - INS Combination
A combination of ships inertial navigation system (SINS) and GPS, should the GPS dropout the SINS component can be used to provide a reference signal, that will hopefully not degrade before either GPS is regained or the operation is safely terminated.
4.23. The principle of Inertial Navigation, the methods of using INS to enhance existing PRS performance There are three parts to the process of positioning a vessel: •
Locate the vessel relative to a knovvn position in X, Y coordinates (a Cartesian system presuming a flat surface).
• Define the global position of the known position in an accepted coordinate system (a geodetic system which makes allowance for the shape of the world). • Relate the coordinate system to the coordinate system used to define the required vessel position (ali maps which define positions are based on specified geodetic systems). If a vessel position need only be relative to another object, say a vessel/ROV relationship then X, Y coordinates are sufficient. If the positioning system provides a global reference, such as DGPS, then the conversion of the X,Y values is not necessary. The Geodetic System will be based on one of two main coordinate systems to define the vessel position: • Latitude and Longitude This positions a vessel in degrees north/south and east/west. It is defined in relationship to a reference datum. The reference datum describes the shape of the earth either globally or in an area of the world. Most used datums are WGS84 (used for DGPS), WGS72, European 1980, Bermuda etc. Selection of the wrong datum can cause errors of 100s of metres. •
UTM Grid
The Universal Transverse Mercator grid is based on northings and eastings in metres from a set point. It divides the world into 60 strips each 6° of longitude wide. It presumes the strips to be part of a horizontal cylinder, which makes the grid rectilinear. With 60 zones, the distortion in each zone is minimal. Zone 1 covers 180° to 174° West, based on a central point of 177°. The North Sea is covered by Zone 31. There are separate sets of north and south zones. By a clever use of offsets, the easting and northings in the southern hemisphere increase as the vessel moves north. Another adjustment is required to translate the Grid North of the central meridian of the UTM zone with True North 67
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5. ENVIRONMENT SENSORS AND ANCILLARY EQUIPMENT Vessel Sensors should at least measure vessel heading, vessel motions, and wind speed and direction.Other environmental sensors may be fitted, such as current meters, tension meters, but these are usually informational sensors and their output may not be fed into the DP. There is a force acting on the vessel that no sensor can calculate. This force can be defined as the resultant of all other forces acting on the vessel apart from wind. The possible components of this force are numerous. It will also contain any errors in measurement, or unmeasured forces acting on the vessel. Possible components: •
Surface current ,
•
Subsea current,
•
Waves,
•
Swell,
•
Effect of drag by attached equipment such as pipe or riser,
•
Effect of current on riser,
•
Workboats tied up to vessel,
•
Wind (when wind sensors are deselected),
•
Error in AWC calculation,
•
Unidentified Forces, such as forces in a cable or pipeline being ploughed in
•
Thruster errors.
When an equipment class 2 or 3 DP-control system is fully dependent on correct signals from vessel sensors, than these signals should be based on three systems serving the same purpose (i.e. this will result in at least three gyro compasses being installed). Sensors for the same purpose, connected to redundant systems should be arranged independently so that failure of one will not affect the others. When class 2 and 3 is fully dependent on correct sensor readings, then 3 systems should be installed. For equipment class 3, one of each type of sensors should be connected directly to the back-up control system and separated by A.60 class division from the other sensors.
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5.1. Means of obtaining Vertical Reference for input into a DP system. The importance of the provision of vertical reference The latest version The Motion Reference Unit (MRU) measures pitch, roll and heave. Although a DP system does not control a vessel in the pitch, roll and heave axes, pitch and roll must be measured to provide accurate compensation for some position measurement equipment. The VRU on the vessel determines the difference between the "local" vertical and reference plane of vessel. VRU signals are used for position holding rather than transit. The compensation values of pitch and roll are used for: •
SBL and USBL acoustic,
•
Inclinometer for slope of taut wire,
•
Inclinometer for slope of riser,
•
Compensation for aerials.
Heave is calculated by the double integration of the vertical acceleration of the unit. Heave is not needed for DP operation, but it is often useful for other purposes, e.g. advice to helicopters. A typical VRU provides heave readings in the range +10m with an accuracy of 5cm or 5%, and pitch and roll readings to ±30° down to accuracy 0.1°.
Figure 53. Vertical Reference Unit
5.2. The function of gyro compasses and their redundancy within a DP system The most critical sensor for positioning will normally be the gyro, because the heading measurement is used to determine the position, for heading control and is needed to perform coordinate transforms. The gyrocompass is a pendulous suspend gyroscope which gravity controlled and damped. Gyrocompasses work over the range of 80N to 80S.
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North speed, east speed, north acceleration, east accelerating gimbal all have automatic compensation with speed input. The normal startup cycle of a gyrocompass is 6 hrs. However, slew controls can override the automatic starting cycle after 5 mins. The vessel speed compensation is set to the vessel's average speed for the duration of the voyage.
Figure 54. Gyrocompass
If there is more than one gyro, either one will selected as a preferred gyro, or an average is used. If there are only two gyros, and one starts to drift, the system can only report a gyro difference. The operator has to decide which one is correct, this may be difficult. If three gyros are available select them all, this can allow the DP to vote, two out of three and a drifting or failing gyro can be voted off even if it is the preferred gyro.
5.3. Provision of wind sensors within the DP system •
Anemometer
An anemometer is a device for measuring both the speed and direction of the wind. Wind is a major disturbing element on the vessel. The wind speed and direction are used to improve position control by modifying thruster demands.
Figure 55. Anemometer
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They have two main purposes: to assist in weathervaning for large stationary vessels such as shuttle tankers or production platforms, and to make allowance for gusty wind conditions. Separate sensors are provided for wind direction and wind speed. Wind speed sensor can operate at wind speeds up to 60 metres/sec, and gusts up to 100 metres/sec. The lower threshold is around 1 metres/sec. The accuracy is +0.3 metres/sec. Wind direction sensor the accuracy is ±3°. Care must be taken in installing the anemometers to avoid wind shadow from the vessel superstructure and spars. An anemometer should be at least 10 diameters from any spar or mast.
5.4. Wind Feed-Forward facility, and its importance within the DP system When a DP system is being designed for a particular vessel, the programmer will calculate the effect of wind direction and speed on that vessel taking into account variables such as draft and affected hull area. This calculation, called the Aerodynamic Model,- allows for the wind effect through 360 degrees as it will exert a greater offsetting force acting on the beam of a ship for example than it would acting from the bow. The amount of thrust required to maintain position with the wind from any direction and any speed (up to design limit) is then programmed into the system and the DP system will automatically and instantaneously use that amount of thrust to compensate for wind force. This is known as Automatic Wind Compensation (AWC) or Wind Feed Forward. Sudden changes of wind direction or speed are instantly compensated for by AWC commanding appropriate thrust, ensuring that position loss is minimised. It can be seen from this why it is important that the system receive the most accurate wind information possible.
5.5. The limitations of wind sensor inputs, and the consequences of deselecting the wind sensor input Wind Sensors are very important in that the DP treats wind as being one of the major forces affecting the vessels position keeping (position keeping meaning both heading and position). Care must be exercised in both positioning them on the vessel and also in selecting which one to use in the system. Ideally the anemometers should be sited where they are totally unobstructed e.g. on top of the derrick on a drilling rig. On many types of vessel it is impossible to find an unobstructed position and the anemometers may have to be sited at all four corners of a barge for example. If this is the case the operator must be aware at all times of wind direction and select the anemometer which is reading the true wind speed and direction. There are also times when the wind sensors are being masked e.g. alongside the lee side of a platform or they give a false reading e.g. down draft from a helicopter. The operator should again be aware of anything which may give a false reading and take appropriate action when required. A typical example is downdraft from a helicopter. When there is doubt regarding the accuracy of wind information from the sensors, they should be deselected from the system. Then reselected once the situation has been sorted out.
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5.6. Interpreting messages provided on the DP system displays and on the printer Environmental forces are never constant. Wind, current and swell should be monitored continuously as should their effects on position keeping. Electronic monitoring methods, such as wind sensors and resultant force vectors provide the DP control system with inputs, but these methods should be supported by visual monitoring and forecasting. Preventative measures may require the vessel to cease operations during these periods and move off to a safe location.
5.7. The alarms and warnings associated with catastrophic failure, i.e. position and/or heading Dropout Status lights should be provided in the operation control rooms, if necessary repeated onto working areas, ECR, and supervisor and Masters cabin. They should be manually activated from, and repeated in, the DP control room. Status lights should be checked for operation prior to commencing operations. Operations should not commence to switching on the green light. Steady green light to indicate vessel under full DP control, normal operational status, operations may commence. Yellow light to indicate the DP operating system is in a degraded status, operations may be stopped operations should be prepared to stop, divers move to a place of safety. Flashing red light to indicate DP emergency, operations to be stopped and equipment personnel recovered. A distinctive alarm should sound in the saturation control room, air diving control area, the Master's cabin, Operations Superintendent's cabin (if applicable) and the senior Diving Supervisor's cabin in conjunction with the flashing red light. Provision of a means of cancelling the audio and flashing functions of the signals from the receiving positions when they have been noted should be made.
5.8. Corrective actions to accept and remedy any alarm or warning condition DP control location requires the DP watchkeeper to be in attendance at the DP control console at all times the vessel is operating in DP mode. DP Alert Level Responses; •
Green – Normal. No action. Operations progress.
•
Yellow – Degraded. This may mean cessation of all supply operations, movements of the vessel away from the installation to a safe position, or to take manual control, for example in case of connected hose operatins. Red – Emergency. Take whatever action necessary to prevent human injury, avoid collision, make the vessel safe, avoid enviromental pollution and structural damage.
•
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6. POWER GENERATION AND SUPPLY, AND PROPULSION SYSTEMS Power is defined as the motive system that drives the thrusters and provides electrical supply to the remainder of the equipment. In some vessels these power supplies may be separate. Individual engines could power the thrusters with a normal generator supplying power to the rest of the vessel. The DP system may have a dedicated power system i.e. dedicated engine room for DP or it may share the power available with the normal vessel requirements.. On a drilling vessel the generated power would be shared between DP, Drilling drives and vessel Hotel load, (lighting, air conditioning, galley etc). The DP capability of the vessel is provided by her thrusters. In general, three main types of thruster are fitted in DP vessels; main propellers, tunnel thrusters and azimuth thrusters. Main propellers, either single or twin screw are provided in a similar fashion to conventional vessels. In DP vessels where such main propulsion forms part of the DP system, propellers may be controllable pitch (cp) running at constant rpm or variable speed. DC motors or frequency-converter systems enable variable speed to be used with fixed-pitch propellers. Main propellers are usually accompanied by conventional rudders and steering gear. Normally, the DP installation will include control and feedback of the rudder(s). Some DP vessels are fitted with modern hi-lift high efficiency rudders which enhance the vessel’s transverse thrust aft.
Figure 56. Typical Propulsion System Layouts
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In addition to main propellers, a DP must have well-positioned thrusters to control position. Typically, a conventional monohull-type DP vessel will have six thrusters; three at the bow and three aft. Forward thrusters tend to be tunnel thrusters, operating athwartships. Two or three tunnel thrusters are usually fitted in the bow. Stern tunnel thrusters are common, operating together but controlled individually, as are azimuth or compass thrusters aft. Azimuth thrusters project beneath the bottom of the vessel and can be rotated to provide thrust in any direction. Propeller drive is usually by bevel gearing from above. The whole unit may in some cases be retractable into the hull. Azimuth thrusters have the advantage that they can provide thrust in any direction and are often used as main propulsion in lieu of conventional propellers. A podded thruster is also a type of azimuth thruster, but in this case the motor and shaft are enclosed and rotate with the thrusters below the hull. Ship rings provide the power from the vessel to the rotating pod containing the drive motor or motors.
6.1. The power generation and distribution arrangements in a typical diesel-electric DP vessel, with particular reference to system redundancy and Equipment Class The need is a secure power supply with backup. This concerns the number, locations and utility support systems of the diesel generators. In a diesel electric installation, a number of generators provide power to a switchboard on a "power station" basis. Typically the voltage generated in a diesel-electric installation is high-tension, e.g. 6 kV or 6.6kV. Main and auxiliary switchboards run at 440V or 240V with power fed from the switchboard via transformers. The generators are driven by diesel engines, each of which should be provided with independent services such as fuel, cooling and lubrication. Failure of one generator will leave a number of others on-line, and normal margins of working should ensure that loss of one generator does not result in an emergency status. The number of generators running can be changed to match the power requirements. Beside the direct driven propellers all power generated on board has to go through a switchboard. In order to provide redundancy there are at least two main switchboards and one emergency switchboard. Different equipment is connected to either one of the main switchboards and normally in a way so as to provide greatest redundancy, e.g. Bow thruster 1 on switchboard No. 1 (BUS 1) and Bow thruster 2 on switchboard No. 2 (BUS 2). For each equipment there is an automatic circuit breaker protecting the switchboard if there is a shortcircuit somewhere in the system. Between the two main switchboards BUS 1 and BUS 2 there is a circuit breaker switch, (the bus tie) which enables the two switchboards to be linked together. In DP operations this Tie switch is normally open (Not switched), and the ship can still have power on one switchboard if there is a problem on the other. Some new systems are designed to operate with bus tie closed, this can give greater flexibility, there is the risk that a "dead short" can cause a black out. The FMEA should give the operating position of the bus tie for operating classes.
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For operations under Equipment Classes 2 and 3, the level of redundancy required is such that the power available for position keeping should be sufficient to maintain position subsequent to worst case switchboard failure, i.e. the loss of one complete section of switchboard and the generators supplying it. Vessels of Equipment Class 2 may have busbar sections connected by bus tie breakers, but these breakers must separate automatically upon overload or short circuit failure within one section. Vessels of Equipment Class 3 must operate with bus tie breakers open, with each section of bus-bar isolated from the remainder.
6.2. The power supply and distribution arrangements in a typical nondiesel-electric DP vessel As stated by IMO MSC/cirs. 645 Dynamic positioning system (DP-system) means the complete installation necessary for dynamically positioning a vessel comprising the following sub-systems: •
Power system,
•
Thruster system, and
•
DP-control system.
Power system means all components and systems necessary to supply the DPsystem with power. The power system includes: •
Prime movers with necessary auxiliary systems including piping,
•
Generators,
•
Switchboards, and
•
Distributing system (cabling and cable routeing).
As we can see IMO does not relate this specifically to diesel-electric but to all power systems in general, thus requirements are for non-diesel electric DP vessel are the same provided that the safety requirements of such system comply with the IMO requirements. Some DP vessels comprise part diesel direct-drive thrusters and part diesel electric plant and motor-driven thrusters. A vessel may have twin screws as main propulsion driven direct by diesel engines and bow and stern thrusters electrically driven, taking power from shaft alternators coupled to the main diesels or from separate diesel generator sets.
6.3. The power requirements of DP vessels, and the concept of “available power” In general, there should be sufficient power available to provide current demand with a spinning reserve equivalent to one generator. This protects against critical situations arising as a result of the loss of one generator. In cases where one or more generators are stopped and on stand-by, "auto start" facilities are provided to automatically start and bring on-line those
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generators at some pre-set limit of available power. Alarms are provided within the DP system when the 80% power limits are reached or exceeded. Central to the concept of safe operation and redundancy is the monitoring of available power.
6.4. Typical Power Management system as installed in a DP vessel The primary function of a power management system is to ensure continuity of electrical supply under all operating conditions. As a secondary function, it should also ensure rapid blackout recovery in the event that it fails in its primary function for whatever reason. Power management systems are intended to ensure that critical power shortages or blackout situations are avoided. A simple power management arrangement is a form of Blackout Prevention, ensuring that circuits are tripped off the switchboard under overload conditions. More complex systems contain a number of levels of load shedding. At a predetermined load value "start-blocking" will be initiated on large motors. This is to ensure that a blackout is not inadvertently tripped by the action of starting a motor when there are sufficient reserves. The starting current on a motor is far in excess of its full-loading running current. Load shedding will occur as power reserves dwindle. Circuits will be dropped off the board in reverse order of importance.
Figure 57. Emergency generators
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6.5. The provision of Uninterruptible Power Supply systems to the DP system, with particular reference to power shortages, failures and system redundancy The DP system (computers, bridge console, position and environment reference systems) requires a stabilised power supply to avoid voltage spikes or transients. It is also required to provide backup battery power for the system during the time of blackout. For Equipment Class 2 and 3 systems, the power supply must be fully redundant with no single point failures. The battery backup supply must have a minimum duration of 30 minutes (Classification Society requirement). UPS facility is provided by a major power system providing UPS function to a number of operating areas of the vessel. Thus critical operating capability relating to drill floor or pipelay facilities, as well as the Dynamic positioning, are provided from a large capacity UPS system. In a simple UPS the power supply is fed into a charging rectifier, turning the ships AC into DC. This DC, in addition to supplying an inverter, also acts to charge the batteries. In the event of loss of power from the ships supply, these batteries will power the system for approximately 30 mins. (the batteries only supply the DP system electronics - not the thrusters). Also the batteries do not power the taut wire winch). A more reliable system is to provide two complete UPS, each powering half of the redundant DP system. Each UPS is, in itself, redundant with a battery back-up, but in the case of a UPS failure the remaining UPS will maintain DP function to the vessel. Obviously during a total blackout the vessel is no longer under DP control as there is no power available to the thrusters but the purpose of the UPS is to keep "information" updated during the power loss so that when power becomes available it is possible to bring the DP system back on line without running up gyro compasses or restarting computers etc.
Figure 58. Uninterruptible Power Supply (UPS)
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The DP control system is protected against a mains power failure by the inclusion of an uninterruptible power supply (UPS). This system provides a stabilised power supply that is not affected by short-term interruptions or fluctuations of the ship’s AC power supply. It supplies the computers, control consoles, displays, alarms and reference systems. In the event of an interruption to the ship's main AC supply, batteries will supply power to all of these systems for a minimum of 30 minutes
6.6. Various types of propulsion system commonly installed in DPcapable vessels
Figure 59. Various types of propulsion system
Tunnels – Tunnels need to be as deep as possible, and should be at least one and a half times the diameter of the thrusters below the water line. Thrust is fixed in direction, usually only transverse. The longer the tunnel gives the less effective the thrusters. Advantages •
Known technology.
•
Equal thrust both ways.
•
Controllable pitch give precise control.
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Disadvantages • As speed increases thrusters less effective, at 4 knots thrusters may be only 50% effective. • Have moving parts outside the vessel; repairs may need a dry dock. • Maintenance more than fixed pitch. • May need guard to stop garbage going through thrusters. • CPP units require calibrating
Azipods - Fixed pitch may not reverse, if they do may not be to 100%. Need fixed modes, or may have to azimuth round constantly. Advantages •
Less moving parts less complex, easier and less expensive to maintain.
•
Moving parts more accessible, less likely to need dry dock.
•
Directional thrust, can be more efficient.
•
More efficient, so fuel savings.
•
Can be used on passage or for station keeping.
•
In biased modes prime movers can be loaded to operate, at less damaging loads.
Disadvantages •
New technology steers like an outboard, requires more thought and familiarisation.
•
Different considerations when manoeuvring, e.g. the best way to stop is to slew them outboard so they are blowing out and the wall of water acts like a brake. This means they are readily available to apply thrust. Need to be familiarised.
•
In light conditions constant azimuthing can cause alarms, and vessel may hunt. May require less efficient fixed modes to be used.
•
When changing between modes there may be loss of position.
•
Slewing at high speed can cause damage. Azimuth thrusters - Can be fixed or retracting. May be fixed or controllable pitch.
Advantages • Deeper than tunnels. • If retracting no extra drag, or draft increase. • If fixed less complex. • Directional thrust can be more efficient. • Different modes are available.
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• •
If set up correctly vessel can operate in Stealth mode, with main engines off when at stand by thus saving fuel (mainly supply boats). Maintenance cans are possible to save on dry dock during repairs.
Disadvantages • Need barred zones to stop thrusters blowing over each other. • Wash can interfere with diving, or Rov operations. • If flow is reversed, they are less efficient, need to check thrust curves, and ensure that you are operating the correct way. • If fixed, then draft is increased. • If retracting they are more complex. • CPP units require calibrating. Samuel White Gill jets (made by Elliots) - Not widely used in DP. Advantages •
Easy to retrofit.
•
With own prime mover, good for redundancy.
•
No draft increase.
•
Directional thrust.
Disadvantages •
Not as efficient as other types. CPP Propellers - Fixed thrust fore and aft.
Advantages •
Known technology.
•
Great variation in types available.
•
With nozzles and correct rudder type, transverse thrust is possible.
Disadvantages • Complex moving parts outside the vessel, if damaged may require dry dock to carry out repairs. • Maintenance costs higher than fixed pitch units. • Less effective astern (not because of blade , but because of disturbed flows). • Nozzles and specialised rudders cost extra, and require extra setting up. • CPP units need calibrating.
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6.7. Evaluation of fixed-pitch propellers compared with controllable-pitch propellers Propellers are traditional main vessel propulsion method. They may be either single or twin configuration. Control of the thrust is provided in two ways. •
Fixed pitch propeller - the thrust is controlled by varying the rotational speed of the propeller.
•
Controllable pitch propeller – the thrust is controlled by varying the pitch of the propeller and keeping the speed constant. A variation of CPP varies both pitch and speed using a variable speed drive to gain better efficiency.
Controllable pitch propellers have a variaty of methods to vary the pitch of the blades. These can be fairly complex and are therefore laible to fail at sometime. Care must be taken to ascertain the failure mode of the propeller. Propellers provide thrust in both directions but due shape of the blades and to the effect of the hull diamount of thrust in the revers direction is only 40 – 60 % of that available in the forward direction.
6.8. Operational characteristics and possible failure modes of the different types of propulsion systems Thrusters are complex, and as such are vulnerable to a variety of failures. The variety of failure modes is dependent upon the type of unit, i.e. a fixed pitch propeller has fewer failure modes than a fixed pitch azimuth thruster. More failure possibilities are associated with controllable pitch units, and many of these failures are only repairable in dry dock. This is expensive, and thruster repairs cannot be properly tested out until the vessel is again afloat. Any propeller is vulnerable to fouling by ropes and underwater objects. Particular damage may be caused to seals. Any leaking seal will result in contamination of gearbox or actuator oil, which will further result in mechanical damage and failure. Typically, a controllable pitch azimuth thruster has six seals ; one on each blade, one on each shaft, and one on the hull mounting. The DPO must take every precaution to ensure that his propellers remain clear. A fouled propeller or thruster immediately degrades or destroys the vessel's position-keeping capability. If a thruster fails, the failure may be a pitch/revs freeze, or "fail-as-set", or it may fail to any pitch/revs combination. It is important that the operator stops the thruster immediately, as a runaway thruster can very quickly destabilise the vessel positioning capability. Many propellers, particularly controllable pitch, have a "fail-safe" failure mode, usually relating to a loss-of-control-hydraulics situation. This fail-safe mode should be zero pitch, but it is important to realise that other failure modes may leave the propeller failed to residual full pitch.
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7. OPERATIONS USING DP With any DP vessel operation, comprehensive planning is essential. The operational requirements of the task in hand must be thoroughly discussed with the client, and a detailed plan of the preferred sequence of events compiled. The plan must include the approach to the worksite and set-up, together with the positional requirements of the task itself. At all stages there must be adequate contingency plans made. DP operators (DPOs) must be familiar with the details of the worksite and of the tasks planned. In many operations the vessel is simply providing a working platform for a project team, but it is essential that the key DP personnel are familiar with the detail of the operation and the possible hazards.
7.1. Procedures to be followed when approaching a worksite and transferring from conventional navigation to DP control In cases where the vessel is to start operations inside the 500m zone of an installation, permission to enter must be obtained from the appropriate authority on the installation. The vessel should be manoeuvred on manual control, preferably in DP joystick - AUTO HEADING (YAW) control, to a safe position before setting up in DP. This will enable environmental memory build up to begin and reduce initial thruster demand once full DP is selected. When choosing an approach course and a safe position, it will be necessary to take account of the environmental forces and to be satisfied that, as far as is reasonably practicable, the vessel keeps on the lee side of the installation in a 'blowoff' position. At a safe distance from the installation or nearest obstruction the vessel will set up in DP "Auto" control. The DP system should be allowed to stabilise for approximately 30 minutes. During this period the DP Operators will constantly monitor all available information from the computers. Only after this 30-minute period and when the DP Operators are satisfied that the DP System has adequately settled down will final steps be taken to make a close approach to the installation. At all times during the vessel's DP operations, the vessel will be required to operate within the limits of her capabilities. This effectively means that the calculated capability plots of the vessel's performance provide the basis for the operating parameters. The results from the capability plots are theoretical. The vessel's operating parameters may be amended from time to time on the basis of data acquired from DP Footprint Plots. When in open water the choice of vessel heading and position will be determined largely by operational requirements. For some vessels, transfer of control must be made from the navigation bridge to the DP console in another location.
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The vessel will change over well clear of any obstructions, usually outside the 500m zone, and complete a DP checklist. Items to be checked or tested include main engine/thruster control functions, communications (external VHF/internal) radar and navigation aids, gyrocompasses and steering systems. In addition, checks are made on specialist operational items associated with the work. These checks involve the key DP personnel on the bridge and in the engine control room. Thrusters and main propellers must be "proved" by taking manual control and trying each thruster each way, checking response and feedback. Once transfer is complete the watchkeeper may turn his attention to the DP control system.
7.2. The need for completing pre-DP and other checklists prior to and during DP operations Checklists are an essential and accepted feature of most DP operations. It is essential that checklists are treated as an aid to memory and not as a complete substitute for ‘thinking’. It is very easy for one person in a hurry to fill out a checklist without checking many of the items contained therein. Checklists need updating from time to time, as new important points are found and equipment is modified or updated. Checklists are usually controlled documents within the shipowner’s quality assurance system, where alterations may be seen as a ‘non-conformance’ and change takes too long. Typical checklists to be maintained by the watchkeeping DPO include: •
Pre-DP checklist,
•
Pre-operational checklist,
•
Watch hand-over checklist,
•
Periodic DP checklist and
•
MCR checklist.
All DP vessels will have pre-operational and watch keeping check lists. They are shorter versions of the location check lists and completion of the location check list will be adequate if the work is to commence immediately. The pre-operation or watch keeping check list then serves as a final check and typically form part of the permit to work system of the vessel or the installation. The pre-operation check lists should cover the present status of equipment and form the basis of a final check prior to the commencement of an operation. When completed these checks form part of the permission to dive/ROV document for ROV and/or survey operations. Separate check lists are sometimes used but there is no requirement for them. For some DP operations, further checks are executed in the final working position. A settling period of about thirty minutes is allowed, ensuring that the DP control system has time to build the mathematical model. During this time the bridge watchkeepers should complete the pre-operational checklist, and verify that preoperational checklists are complete at other locations, such as the engine control room.
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The bridge team must be aware of the significant change in status that may occur once the go-ahead (green light) is given for the operation to commence. Once the ‘green light’ is given, the contingency plan may change, because it must allow for the vessel to maintain position and heading adequately to reach a safe situation.
7.3. The need for keeping logbook records of all DP operations, failures, incidents and repairs, including details of operation and maintenance of all position reference systems An essential part of the DP documentation is the DP Log Book which is designed to provide a continuous record of all activities which are of importance to and associated with the DP operation of the vessel. As well as providing evidence of the systematic and structured manner in which DP operations are being carried out, the DP log book can prove to be of vital importance in the event of a DP incident or accident. In this respect the importance of the DP Log book is that the events are recorded as they happen, without reflection or modification and that they are accurately timed. The DP Log Book should be completed by the DP operator on watch. Entries in the log book should include but not limited to the following: • Selection and de-selection of references. • Crane lifts and movements. • Vessel movements. • Helicopter movements. • Entry to installation 500m zone. • Thruster selection and changes. • Deteriorating weather conditions. • Changes in DP operation status. • ROV movements. • Communications between vessels and installation connected to the operation. • Changes in vessel heading. • Changes in vessel rotation point. • Time of vessel in and off DP. • Any unusual event that may effect DP operations. The DP Log Book is used for continuous recording of events and occurrences; therefore it should be constantly available to the DP operators. The DP Log Books should be retained onboard the vessel and any copies retained by the operator or owner ashore.
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7.4. The need for effective communications during the conduct of DP operations In the interests of ensuring that planned DP operations are understood by all interested parties it is necessary to ensure that the vessel takes a positive role in providing adequate information to all nearby vessel and installations. In particular the DP operators are required to maintain clear and efficient lines of communication with the nearby installation. This is most conveniently achieved by constant radio contact. It will be necessary to ensure that there is as little conflict as possible between the vessel's operations and operations on the installation. Occasionally installations enter periods of radio silence. It is essential to ensure that effective communications procedures are established to cover such periods. Adequate periods of notice should be provided by the installation. Procedures should be established at the project planning stage in consultation with vessel interests. Good communications between all operational departments is essential for a safe and proper operation. As a minimum requirement, direct communications should be provided between: DP Console and • Dive Control • Engine Control Room • Ships Crane • Masters Cabin • Senior Diving Supervisor • ROV Dive control and • Bell And Diver • Life Support Control • Bell Handling Winch ROV
All voice communication systems should be provided with 100% redundancy either through duplication or via an alternative system e.g. (VHF, telephone or talkback). These systems should be situated in locations in close proximity to the operational equipment & personnel for whom they are provided. Communications between the Dive Control and DP Control should be regular and used to inform each other about any change to operational status.
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Typical information should include but not be limited to the following: DP Control to Dive Control • Operational Status • Intention to reposition Reference equipment(Taut-wires/ Transponders) • Intention to move vessel • Ship movements or helicopter operations • Any change to weather conditions • Intention to handle down-lines of any description • Early warning of observed dangers ( Risk of Collision) Dive Control to DP Control •
Bell Status.
•
Diver Status.
•
Use of equipment likely to interfere with DP Reference systems.
Handling of any down-lines At all times any communication between DP and Dive control should be clear and concise. .All communications should be acknowledged - this is of particular importance before any actions are taken which could endanger either the Diver or vessel.
Typical Communication Requirements between DP and ROV: ROV Supervisor: • Gives distance and direction of any required move to DP operator. • Reports to DP operator "ready to move". • Confirms "in position". DP Operator: •
Repeats distance and direction and asks for permission to commence move.
•
Confirms "move started" to ROV Supervisor.
•
Reports to ROV Supervisor when movement is accomplished, and ROV can
•
continue work (in position).
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Type of Information The following lists outlines which type of information shall be exchanged between Dive/Survey Control and DP Operator. ROV to DP Operator: •
ROV Status.
•
Intended excursions.
•
Type of DP mode required.
•
Type and number of transponder/responder in use.
•
Any situation which could develop into an emergency.
DP Operator to Dive/Survey Control: •
Intention to move vessel.
•
Any change in operational status.
•
Background information on causes of changes in operational status.
•
Any forecast or actual significant changes in weather.
•
Ship movements in the vicinity.
•
Intention to handle down lines of any description, including repositioning tautwire weight.
The following list indicates the type of information required by the DP Operator about activities on board the vessel: •
Intention to perform and notification of completion on, any electrical or mechanical system maintenance or modification which could directly affect on-line DP equipment, or make standby equipment unavailable. This may result in the vessel having to be placed in a position of safety and to recover the Divers/ ROV.
•
Intention to lower into the water any crane wires.
•
Intention to start and stop the use of radio or radar equipment which may affect the DP system.
•
Intention to handle equipment which may affect the trim of the vessel.
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7.5. The watch hand-over procedure, completion of the appropriate checklist There are many different DP vessels and DP operations. Some tasks require the vessel to maintain a static or relatively static position for days or even months on end (drillships, flotels). Other vessels will be continually manoeuvring in order to execute their work. Irrespective of the work the Watchkeeping principles are similar and some general watchkeeping procedures are included here. Some Class 1 vessels operate with one DPO on watch, but the majority of DP operations are carried out with two operators manning the bridge. On some vessels, one DPO mans the DP desk exclusively, while the other watchkeeper carries out all other bridge functions. These two individuals then swap roles every hour. The watch relief arrangement should allow staggered watch change-over such that there are never two fresh DPOs taking over at the same time. Wherever possible. Watch handovers should take place when the vessel is in a steady state and where the vessel is settled in position.When taking over the watch, DPOs must familiarise themselves with certain aspects of the management of the vessel at that time. The list of information that the bridge team must acquire at this time includes (but is not limited to) the following: •
Position and heading of the vessel.
•
Status and recent performance of the DP system and its peripherals.
•
Details of Position Reference Systems in use and their performance.
•
Availability of further PRS on failure of the above.
•
Level of redundancy.
•
Status of the operation in hand. Planned changes/progress for the coming watch.
•
Details and status of any operational elements (e.g. if the vessel is a DSV and diving operations are underway, then the status, position, depth of the diving bell or basket, the number of divers in the water, their umbilical lengths and expected return times, also details of their operational task).
•
Weather conditions and forecasts.
•
Communications, on-board and external.
•
Traffic in the area. Any planned traffic movements that may affect the vessel and her operation or positioning.
•
Any planned helicopter operations.
7.6. Worksite diagrams using UTM co-ordinates, and plan DP operations using this diagram A Geodetic co-ordinate system in widespread use is UTM, or Universal Transverse Mercator. This is a flat-surface, square-grid projection defined by a UTM zone number, and a Northing and Easting distance from the zero point of the zone. Some position reference systems, such as DGPS, may put out positions in UTM co88
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ordinates. The Universal Transverse Mercator (UTM) projection is used extensively for survey and other offshore work. UTM is a cylindrical projection with the axis of the cylinder coincident with the plane of the equator; the line of contact between the cylinder and the sphere is thus a meridian.
Figure 60. - UTM
Obviously a single cylindrical projection of this type cannot be used to chart the whole terrestrial surface. The useful scope of the projection consists of a zone 6° of longitude in width, centred upon the contact or "Central" meridian. Within this zone distortions are minimal. Zones are identified by a number. The numbering scheme is based upon Zone 1 being the area between the 180º meridian and Longitude 174º West, with the central meridian at 177º W. Successive zones are numbered in an easterly direction, with the North Sea generally being covered by Zone 31 ranging from the Greenwich Meridian to 6º E, with the Central Meridian at 3º E. There are sixty zones in total. Within a particular zone, the Northings and Eastings (in metres) are arranged to increase in a Northward and an Eastward direction, respectively, irrespective of position upon the globe. For Northings the datum is the equator, with Northern hemisphere Northings having a value of zero on the equator, and increasing northwards. For the Southern hemisphere, a false Northing of 10,000,000 is added to 89
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the (negative) values. This resolves the problem of requiring positive values increasing Northwards throughout. A false Easting of 500,000 is established on the central meridian, with Easting values increasing in an easterly direction. This allows the whole zone to be covered by positive Easting values.
7.7. Plan for emergency and contingency situations and procedures It is important that the planning of the worksite approach includes assessment of the various options for reaching a safe situation in foreseeable situations and hazards. One contingency will be for a power or thrust capability shortage caused by partial blackout or thruster failure. Other possibilities include failure of computer systems or position-reference systems, causing a drive-off. The vessel should be able to reach a safe situation, which might require exit from the worksite, often the worst-case single-point failure.The DPO will make good use of plans and worksite diagrams provided by the client, either in paper or in electronic form. These drawings are likely to be prepared in UTM projection and co-ordinates.
7.8. Interpretation of ERNs, Capability diagrams, Online Capability Plots, “Footprint” plots and other data relating to the capability of the vessel under a variety of environmental conditions Capability plot – provides an indication of a vessel's DP station keeping ability expressed in a common format. Owners should recognise the value of DP capability plots. Specifications for capability plots are provided in IMCA M 140, "Specification for DP Capability Plots". The purpose of the DP capability plots is to determine by calculation, based on assumed propulsion power, the position keeping ability of the vessel in fully intact and, in certain degraded conditions and, in various environmental conditions. The DP capability plots should be used in the risk assessment process used to determine the safe working limits at offshore installations. Owners should also recognise that recent developments have resulted in DP capability plots being made available on-line as an added facility in the DP control system. Owners should be avvare that such on-line information is based on theoretical calculation of assumed propulsion/thruster power and may not necessarily represent the vessel's actual DP capability. DP capability plots should be treated with caution and their results should be assessed for validity against the observed performance of the vessel as measured in the DP footprint plots. DP capability plots do not show vessel excursions when in DP. They show the likely enviromental limits within which a DP vessel will return to the target position when an excursion takes place caused by external enviromental forces. This can be intact and in degraded conditions, including, for equipment class 2 and 3 vessels, after worst case failure. 90
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DP footprint plot – a plot designed to record the observed movement of the DP vessel from its desired target location over a period of time. Owners should ensure that DP footprint plots are completed for each of their vessels. DP footprint plots are used to measure the actual position-keeping performance of the vessel in intact and degraded conditions, and in various environmental conditions. It is prudent to complete footprint plots at the time of annual trials and whenever opportunities arise. DP footprint plots serve two main purposes. They show the vessel's excursions in relation to the selected target position, thereby the tightness of the position keeping circle. They are also valuable in assessing the validity of the DP capability plots. Where there are differences between the measured footprint plot and the theoretical capability plot, owners should ensure that the results of the footprint plot take precedence over the capability plot. Where the results are significantly different from the capability plots then owners should consider investigating the reason and (if appropriate) modifying the capability plots.
7.9. Various documents containing statutory requirements and guidance relating to DP operations International Maritime Organization (IMO) MSC/Circ.645 – Guidelines for vessels with Dynamic Positioning Systems is the principal internationally accepted reference on which the rules and guidelines of other authorities and organisations, including classification. Its provides an international standard for dynamic positioning systems on all types of new vessel built after 1st July 1994. The Nautical Institute – The official guide to The Nautical Institute training standards. The International Marine Contractors Association (IMCA) – members are selfregulating through the adoption of IMCA guidelines as appropriate. They commit to act as responsible members by following relevant guidelines and being willing to be audited against comliance with them by their clients. Regulatios and guidelines from maritime and other authorities, operational handbooks.
7.10. Equipment Classes and their application (with reference to the IMO guidelines for DP vessels) A DP-system consists of components and systems acting together to achieve sufficiently reliable position keeping capability. The necessary reliability is determined by the consequence of a loss of position keeping capability.The larger the consequence, the more reliable the DP-system should be. To achieve this philosophy the requirements have been grouped into three equipment classes. The eguipment class of the vessel required for a particular operation should be agreed between the owner of the vessel and the customer based on a risk analysis of the conseguence of a loss of position. Else, the Administration or coastal State may decide the eguipment class for the particular operation. 91
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The equipment classes are defined by their worst case failure modes as follows: •
For equipment class 1, loss of position may occur in the event of a single fault.
•
For equipment class 2, a loss of position is not to occur in the event of a single fault in any active component or system. Normally static components will not be considered to fail where adequate protection from damage is demonstrated, and reliability is to the satisfaction of the Administration. Single failure criteria include; any active component or system (generators, thrusters, switchboards, remote controlled valves, etc), and normally static component (cables, pipes, manual valves, etc.) which is not properly documented with respect to protection and reliability.
•
For equipment class 3, a single failure includes: items listed above for class 2, and any normally static component is assumed to fail; all components in any one watertight compartment, from fire or flooding; all components in any one fire subdivision, from fire or flooding, including cables, where special provisions apply under section 3.5. of MSC Circ.645.
In addition, for equipment classes 2 and 3,a single inadvertent act should be considered as a single fault if such an act is reasonably probable.
7.11. Various Classification Society notations (with reference to system and vessel redundancy and to the Equipment Classes) The main classification societies have used the IMO principles of equipment class and redundancy requirements as the basis for their own DP rules. Classification society rules differ and evolve and none is a direct copy of MSC Circ.645. The following table lists the class notations and corresponding equipment classes for Lloyd’s Register, DnV and ABS: Description
IMO Equipment Class
Manual position control and automatic heading control under specified maximum environmental conditions
Corresponding Class Notations LR
DnV
ABS
DP(CM)
DNV-T
DPS-0
Automatic and manual position and heading control under specified maximum environmental conditions
Class 1
DP(AM)
DNV-AUT DNV-AUTS
DPS-1
Automatic and manual position and heading control under specified maximum environmental conditions, during and following any single fault excluding loss of a compartment. (Two independent computer systems).
Class 2
DP(AA)
DNV-AUTR
DPS-2
Automatic and manual position and heading control under specified maximum environmental conditions, during and following any single fault including loss of a compartment due to fire or flood. (At least two independent computer systems with a separate backup system separated by A60 class division).
Class 3
DP(AAA)
DNV-AUTRO
DPS-3
Table 3. Various Classification Society Notations
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Owners should ensure that their DP OSVs possess and maintain an appropriate DP class notation issued by a classification society. In cases where the DP system is integrated with other control systems, such as vessel management, thruster controls and position reference systems, this might be reflected in the classification society notation.
7.12. The arrangements made for the conduct of DP operations in specialist vessels There are many different PME systems used for position reference with DP systems. The selection of PMEs for a vessel is based on the role of the vessel and the characteristics of the PME. It is possible to have a DP system supported by just one PME but for reliability, two or more PMEs are usually used.
7.12.1. Diving and underwater support vessels Many DP vessels are designed specifically for supporting divers (DP DSVs). Other vessels have a multi-role function, including diver support. The variety of work that may be conducted by a diver is almost endless: carrying out inspection or survey work, installation and configuration of equipment, monitoring of an operation, or recovery of lost or abandoned equipment. Much of the work hitherto conducted by diver is increasingly carried out by ROVs (remotely operated vehicles - unmanned submersible vehicles) but there are still tasks which cannot be completed remotely, and which require human intervention. Divers may be deployed in a number of ways. Up to 50m depth the technique is referred to as "air diving", as the breathing mixture is compressed air. The divers are deployed in a steel basket, or by means of a wet-bell or mini-bell. Upon recovery, the basket or bell is recovered direct to the surface and the divers will enter a decompression chamber for a controlled return to atmospheric pressure. This is necessary to avoid the onset of decompression illness, or "the bends", which is a major hazard for divers. In the air-diving range (0-50m) divers (and ROVs) are exposed to a number of hazards. Chief amongst these is the proximity of running propellers and thrusters. In non-DP diving operations the vessel is moored and propellers are all stopped. With propellers running, the divers and ROV risk the danger of death or injury from being drawn into propellers. In addition to this problem, there are further problems relating to water turbulence caused by propellers, reduction in visibility and increased noise levels. All of these problems must carefully be considered by the planning team and must be the subject of a risk-assessment prior to the commencement of the operation. The foregoing paragraph summarises very briefly the problems of operating DP vessels in areas of shallow water and strong tides. Many further sources of hazard exist in these areas and the DPOs must be familiar with them. Consideration of problems associated with DP operations in shallow water and strong tides forms an important part of shore-based DP courses. 93
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Beyond 50m depth, divers will be deployed using a diving bell, forming part of a saturation diving life-support complex. Divers remain at the pressure of the working depth for up to 28 days, shuttling back and forth in the bell between the worksite and the saturation chambers in the vessel. Decompression times at depths of hundreds of metres are measured in days, not hours. The breathing mixture is a helium-oxygen mix (heliox) resulting in the divers' characteristic "donald duck" voices. A diving bell may well be used instead of a basket in depths of less than 50m, as this represents a greater level of safety. Also, of course, the problems relating to shallow water will also apply to operations where diving is taking place at a shallow depth in deep water, e.g. inspection of structure on a jacket at the —28m level, on a platform standing in 175m of water. Obviously, divers in the water are particularly vulnerable to vessel problems, particularly positioning problems. The diver's only way back to the surface is via the bell or basket If the vessel has a run-off, then the diver will be dragged on the end of his umbilical and this may well cause death or injury. The working location of the diver itself has a great bearing on his level of safety. If the diver is working in open water, close to the bell, then his return to the bell only takes a few minutes. If, however, the diver is working inside an enclosed sea bed structure, or habitat his return time may be twenty minutes or more. During this period the vessel must maintain position, irrespective of anything else.
Figure 61 - Diving techniques
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In waters greater than about 450m in depth, the only way human divers may be deployed is by means of Atmospheric Diving Suits (ADS). These are pressurised diving suits containing the diver in a one atmosphere environment. A commonly used commercial ADS is the "Newt-Suit". A DP support vessel may operate two ADS instead of saturation diving. The other alternative for operations in deep water is the deep-water ROV. One big advantage of using ROVs instead of real live divers is the lower level of redundancy required for non-man-rated operations underwater. Nevertheless, many modern vessels so equipped feature full Class 2 redundancy in consideration of the financial penalties associated with a system failure. The hazards of diving from vessels with rotating thrusters and propellers are obvious. One vital requirement of any diving set-up, from a DP vessel, is that the amount of umbilical the diver may be given, measured from the tending point (basket or bell) must be at least 5m less than the distance to the nearest thruster. This is to ensure that the diver cannot be drawn into a thruster or propeller.
Figure 62. - Umbilical length restrictions
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•
Survey and ROV Support
Support vessels of this type may perform a multitude of tasks from hydrographic survey, wreck investigation, underwater recovery, site survey, installation inspection and maintenance. Although the task itself may be relatively non-hazardous, the location itself may have hazards, especially if in close proximity to a platform structure.
Figure 63. - ROV Tether Management System (TMS)
An ROV may be deployed direct from a gantry or ‘A’ frame at the side or stern of the vessel, or from a tether management system (TMS) incorporating a cage or garage. If deployment is directly overside then great care must be taken to ensure that the umbilical does not foul the thrusters or propellers. The DP control system of the support vessel can be put into a ‘follow sub’ or ‘follow target’ mode for this work, where the acoustic transponder on the vehicle becomes the position reference.
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Figure 64. - Follow Target
7.12.2. Drill ships (with special reference to the Riser Angle mode of operation) In recent years, drilling operations have been extended into waters of ever greater depth. This has necessitated an increase in the number of DP-capable drilling facilities. The latest drillships are rated to work in water up to 10,000 feet (3,000m) depth. Many DP rigs are of the semi-submersible configuration, but the latest generation are very large monohulls. In deep water it is not sufficient simply to position the rig directly over the wellhead. Compensation must be made for tidal flow, in that the all-important measurement is that of riser/stack angle which must be maintained within the specified limits. This is the angle between the riser (containing the drill string) and the wellhead or Lower Marine Riser Package (LMRP). It is vital that this angle remains close to zero. With a tidal stream the riser will "bow", necessitating the vessel moving in an uptide direction to accommodate this riser angle.
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A drilling rig using DP for these functions will usually use dual DGPS and Long Baseline acoustic position references, as other references are not usually available in deep water. The centre of rotation used by the DP control system is the centre of the drill floor rotary table, which for both monohulls or semi-submersible rigs is usually in the centre of the vessel. For drilling operations, it is important for the vessel to keep over the well, such that the riser connecting the vessel to the well is practically vertical. The profile of the riser is, however, determined by current forces and tension, as well as by vessel position. The parameter that is continuously monitored is the lower main riser angle. If this exceeds 3°, action needs to be taken so tha t it does not get worse and force an unwanted disconnection. For each well or location, the rig will have well-specific operational guidelines (WSOG), which determine when alerts are to be given and what action is appropriate. Watch circles might be used and set which are distances that represent angles at the lower end of the riser.
Figure 65. - Deepwater drilling - the Riser Angle Mode
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Some DP control systems have a function known as ‘riser angle mode’. When selected, the DP continues with a geographical position reference, but moves to reduce the riser angle. The reference for positioning is the angle of the riser at the stack, using sensors attached to the riser and the lower marine riser package (LMRP). These sensors may be electrical inclinometers, hard-wired to the rig up the riser or a Differential Inclinometer Transponder assembly, sending angular and positional information acoustically via the HPR system interfaced to the DP.The DP system aboard the rig will have special display pages showing Riser angle offsets as part of a Position Plot display page. DP rigs are currently configured to operate in water depths of up to 3000m. In these water depths the most reliable form of position reference is DGPS. Two or three separate and distinct DGPS systems provide redundancy, provided that different differential correction links are used. Further position-reference is obtained via deep water Long Baseline acoustic systems.
Figure 66. DP drilling in riser-angle mode
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7.12.3. Cable lay and repair vessels The advent of fibre-optics in international communication cables has led to the requirement for greater precision in vessel positioning. Fibre-optic cables have very specific minimum - bend-radius (MBR) and loading limitations. They are more fragile than traditional cables If these are exceeded then the cable may be damaged. Most modern and new cableships are being fitted with a DP capability as standard. The facility is particularly useful when conducting cable repair operations or shore-end connections in shallow waters. Cable lay operations may involve a seabed crawler vehicle or a towed plough system. In both these cases the configuration of the DP system may be matched to the requirements of the operation, especially where the cable must be laid and buried to a specific depth in one operation.
Figure 67. - Cable lay methods
For cable lay operations within coastal waters or other shallow-water areas, it is often necessary to bury the cable in order to prevent damage from fishing gear. When a plough is used, it is towed by the ship, in a similar manner to a tractor towing an agricultural plough across a field. This reduces the power available for station keeping. The phase of the operation where the DP capability proves most useful is the shore-end tie-in. This is where the vessel comes to the end of the lay, a short distance from "the beach", to complete the connection. This involves the vessel 100
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keeping a fixed location, close to the shore, in shallow water, where strong tides may also stream.
Figure 68. Cable lay methods
7.12.4. Pipe lay vessels A number of DP pipelay vessels are currently in operation worldwide. Dispensing completely with anchors and moorings, these vessels are able to conduct pipelay more quickly and efficiently than the pipelay barges. Three methods of pipelay are in use;
•
•
S-lay,
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Reel-lay and
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J-lay.
Seabed Tractors and Trenchers
A seabed tractor or trencher may be configured to lay and bury a cable. These vehicles are tracked crawlers, built to be controlled from the vessel, with operators ‘driving’ the unit as if they were on board. These units usually move slowly, 101
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depending on soil conditions. In some cases an ROV is deployed independently, to record progress and performance. Trenchers for pipeline burial are much larger and heavier. The trencher is lowered onto the seabed over the pipeline and the DP control system can set the centre-of-rotation of the trencher. Associated with the business of laying pipelines is the need to protect them from damage. DP-capable vessels are used here also. A pipeline may be trench buried by use of a specialist seabed crawler vehicle. This vehicle will be deployed by an A-frame over the stern of the trenching vessel and will follow the pipeline, excavating a trench of the required depth. This will be done using ploughshare and water jetting. Once a trench has been established, the vehicle will be recovered and re-configured for a back-fill or cover operation. The DP vessel will use a specialist track-follow or vehicle-follow function to maintain station on the trencher. The vessel may also use a specialist position-reference system such as Trimcube or Smartwire, allowing position relative to the vehicle to be monitored.
Figure 69. - Trenching Operation
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•
Pipelay Operations Many pipelay operations are conducted by DP lay barges.
Figure 70. - Pipelay methods
•
S-Lay Operations
In a typical S-lay barge, the pipe is constructed in a linear pipe fabrication facility called the "Firing Line" in which a number of stages of welding take place. Each operation is conducted at a "station". Further stations conduct X-ray and NDT testing on the welded joints, anti-corrosion coating, and weight-coating if necessary. At intervals, the DPO initiates a move ahead a distance equivalent to the joint-length. Once the move ahead has been completed, the firing-line operations continue. It is essential that tension is maintained on the pipeline. At the back end of the firing line, the pipe is held by a number of pipe tensioners, or caterpillar tracks clamping the pipe. The tensioners control the movement of the pipe, maintaining a set tension on the pipe string. The pipe is supported aft of the firing line by the "stinger", which is an open lattice gantry extending beyond the stern of the vessel, sloping downwards. Tension on the pipe is needed to prevent pipe damage from 103
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buckling. The set tension is to ensure a smooth catenary to the touchdown point on the seabed. If tension is lost, then damage will occur at the touchdown area. Pipe tension values are communicated to the DP system which is continually providing thrust commands to maintain tension, position and heading. Pipelay operations are particularly dependent upon environmental conditions. The vessel must be able to cope effectively with the tides, sea state and wind conditions from most directions, because it is not possible to allow the vessel to weathervane. •
J-Lay Operations
In J-lay operations the pipe is constructed in a long narrow factory called the "firing-line" at deck level. Pipe is fabricated, welded, coated and inspected at a number of stations spaced at 12m intervals along the firing-line. The pipe is controlled by caterpillar-track pipe tensioners that feed it down the "stinger". The stinger is a hefty ramp at the stern supporting the pipe in the overbend area. The pipe is supported by its own tension only in the span between the end of the stinger and the sea bed touchdown point, or the "sagbend" zone. The DP system must allow the vessel precision positioning on a fixed heading, maintaining pipe tension and moving the vessel ahead an exact 12m on demand. These moves may occur every four minutes. Faster working may be achieved if double-joints are worked, with the vessel moving 24m each time. Pipe tension is fed back into the DP from sensors on the tensioners and must be maintained within specification tonnages. In deeper water, S-lay is not feasible and J-lay is common. In J-lay operations, the stinger is configured as a tower, angled between the vertical, and up to 20 degrees from the vertical. Pipe lengths are pre-jointed into triple or quadruple joints before being raised to the vertical for welding onto the pipestring. •
Reel-Lay Operations
This type of operation varies from those described in that the pipestring is prefabricated in one length at a shore-based factory. The vessel loads the pipeline straight from the factory, spooling it onto a reel or into a carousel. The vessel can transit to site with the pipe to lay it by feeding it off the reel/carousel via straighteners and tensioners, either singly or as a bundle. The pipe is laid by passing it from the carousel onto the lay-ramp, thence down the stinger. In very deep water the only suitable method is J-lay. Here the stinger is mounted close to vertical. The pipe is fabricated into triple-joint lengths, which are turned to the vertical at the stinger. Large forces are induced at the stinger due to the heavy weights of pipe involved and these forces must be countered by the vessel's DP capability. Pipelay vessels routinely conduct complicated evolutions using DP. The operations to commence and complete pipelay, conduct an in-water tie-in or to lay down the end of the pipe if necessary, all involve precision positioning.
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7.12.5. Rock dumping and dredging vessels •
Rockdumping operations
A small number of vessels are configured for rock dumping, to provide protection for underwater elements. This may be an alternative to trenching for a pipeline, or the rock dump may be for the purposes of erosion rectification for platform foundations and the like. Rock dumping vessels are usually mini-bulk carriers, specially fitted for automatic discharge into a hopper adjacent to the fallpipe tower. The fallpipe system is deployed over the side of the vessel from the handling tower. At the lower end of the fallpipe is an ROV that is able to direct the delivered rock accurately onto the target corridor. All of these vessels that work in the offshore industry are fitted with DP systems, because good track speed control, and hence uniform, economic rock distribution, is possible. The vessel uses Autotrack facilities to follow accurately the required line at a precise velocity. The commonest need for rock dumping is to provide protection to untrenched pipelines.
Figure 71. - Rockdumping
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A commonly used feature is the ‘auto-track’ function of the DP control system, which enables the vessel to track accurately along a line defined from the preset waypoints of an earlier pipeline survey. This type of vessel is also used to provide protection against tidal scour or erosion, which occurs in high tidal stream areas. The sediment around the legs of a Jack-up drilling rig, for example, can become eroded to the point where the rig becomes unstable. •
Dredging Operations
Many dredging operations are utilising the advantages of DP. Whether the dredging operation is for the purposes of harbour/channel maintenance, or for the recovery of aggregate materials, the precision available from the use of DP makes it an attractive method of operation. A trailing suction dredger may follow a predetermined track with the reference point being located upon the draghead, i.e. the draghead is the element being positioned rather than the vessel. The DP system is configured to receive and compensate for measured draghead forces determined from suitably located sensors.
Figure 72. - Autotrack or Track Follow
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7.12.6. Shuttle tanker and FPSO operations Broadly speaking, shuttle tanker operations may be divided into three groups; • Systems with hawser moorings, • Hawserless systems and • Submerged Turret Loading (STL) systems. A further grouping consists of those vessels configured to load directly from Floating Production, Storage and Offtake vessels (FPSO) installations. An increasing number of offshore oilfields must conduct export via tanker. In many cases the distance to the beach is too great to warrant the construction of a pipeline, otherwise the reservoir reserves may only support a limited production period. In such cases the tanker may moor to an Offshore Loading Terminal (OLT) and conduct loading by means of a bow manifold. In many areas, the exposed location of the OLT means that mooring is not possible due to the environmental loads that may be imposed on the OLT structure. It is these areas that need DPcapable tankers. DP shuttle tankers operate on a position-circle/ weathervaning principle. The vessel will position with her bows touching an imaginary circle, centred on the OLT. The vessel is continuously weathervaning, or actively seeking a minimum-power heading and adjusting her position to keep the OLT ahead. This allows the vessel's bow manifold to remain within specific maximum and minimum distances of the OLT reference point, ensuring that there is no risk of damage to the loading hose. The OLT avoids having major environmental loads imposed upon it and the DP system ensures that the vessel's position and heading are maintained in all but the most severe weather. The latest systems for these vessels also allow the shuttle tanker to operate in fixed heading and/or fixed position if necessary. Tankers built with this functionality are fitted with a conventional DP system configured to handle this weathervane ability. Typically, two or three tunnel thrusters are fitted at the bow, one aft and single or twin-screw main propellers. The DP system may be installed on the main bridge, aft, or in the bow house. Position references used may include DGPS, HPR Fanbeam and Artemis systems. A variation upon this theme is the STL system, in which the loading is carried out from a circular conical subsea turret. This turret is anchored at a depth below keel level and carries the loading hose. The tanker has a docking cone built into the bottom structure, forward. The vessel manoeuvres over the turret, picking up a messenger line. The turret is located by means of acoustic beacons. The turret is hauled up into the docking cone and locked. Once this is complete, the vessel weathervanes around the turret location maintaining position and heading using DP. At the present time two fields are configured for STL operations. A further variation is the FPSO tandem loading arrangement. A FPSO unit is usually a ship-shaped vessel moored to a turret arrangement The FPSO produces into her own tank storage and at frequent intervals must off-load cargo into a shuttle 107
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tanker. The tanker will normally position astern of the FPSO and load through a bow manifold. Positioning strategy is as for OLT or STL arrangements, with the added complication that the reference point for positioning may be slowly moving; the FPSO may be weathervaning or controlling the loading. Position reference for the offtake tanker-will usually be a combination of Artemis and relative GPS (the DARPS system). Both these PRS are relative in nature, as the offtake tanker is positioning in relation to a mobile point. •
Offtake Tanker and FPSO Operations
Tankers intended to load at Offshore Loading Terminals (OLTs) will be fitted with systems very similar to those in any other DP-capable vessel, but configured specifically for the offshore loading function. The installations which support offtake tanker operations vary from field to field. Typical installations are Spar buoys, which are large floating tower structures moored by a spread of mooring lines. Spar buoys usually carry a rotating turntable at the top to handle vessel moorings and hose handling equipment.
Figure 73. - OLT configurations
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A UKOLS facility has a loading hose connected to a mid-water buoy. The buoy is positively buoyant and is moored at a fixed depth, above a gravity-based housing or pipeline end manifold (PLEM). Vessels using this facility have no need for a mooring hawser; the only connection to the buoy is the hose. A more recent development is the submerged turret loading (STL) system, where the loading connections are located in a subsea buoy. The buoy is moored above the PLEM at a depth greater than the draught of the offtake vessel. The STL is mated into a docking port built into the forebody of the vessel, and carries the flowline connections to the vessel. Once locked into position, the vessel is able to weathervane using the swivel through the centre of the STL. A development of the STL is used for production.
Figure 74. - Shuttle tanker
•
FPSO Unit Operation
Floating Production, Storage and Offtake units are becoming common in many parts of the world. Many FPSOs are able to weathervane around the turret and maintain heading into the weather. 109
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Figure 75. - FPSO/shuttle tanker offtake arrangement
Most FPSOs utilise offtake tankers for export of oil, and these tankers are usually DP-capable. With any FPSO/offtake tanker operation, the tanker will experience more positioning problems than when loading from an ALP. The offtake vessel keeps position within a circle defined by the length of the loading hose. The reference position is the hose terminal point on the stern of the FPSO. The mooring and positioning system in the FPSO allows a degree of movement, especially in deep water, so the FPSO may be continually weathervaning, so that the shuttle tanker reference point will be moving. The shuttle tanker can try to follow this movement or position absolutely to pre-set limits. In FPSO offtake operations, a relative position reference is essential. One such position reference is the relative GPS (DARPS) system, yielding position information reduced to range/bearing data from the FPSO terminal location. Another position reference is Artemis, with the fixed station located on the FPSO and the mobile station located on the tanker. The prime consideration is the clearance distance from the FPSO so that the collision risk is minimised.
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7.12.7. Accommodation and “flotel” units During periods of construction, reconstruction or repair of offshore installations, there is a need for a variety of facilities. Sometimes these facilities simply consist of accommodation for extra workmen. In other cases, the facilities required are more complex. A simple flotel barge may be positioned close to a platform providing accommodation facilities. This barge may be connected to the platform by means of a gangway or, in adverse weather conditions, passage must be by helicopter. These barges are usually of semi-submersible configuration with a DP capability. Such vessels may also carry more extensive facilities than just accommodation, able to conduct diving and ROV work, carry out fabrication, assembly or repair work in workshops, conduct crane operations and have a major role in emergency intervention (firefighting, evacuation, medical, etc). Such a vessel may well be a semi-permanent support facility for one field or a group of adjacent fields.
7.12.8. Crane barges and construction vessels Crane barges are employed all over the world in construction and decommissioning operations relating to the oil and gas industries, and also in civil construction projects. They are also used in salvage and wreck removal operations. A number of heavy-lift vessels routinely use DP to good advantage. The number of facilities capable of lifting in excess of 4 000 tonnes is increasing. There also exist a number of vessels of lesser crane capacity but greater utility. The ability to do away with the necessity of laying an eight-point anchor spread considerably reduces the time required to complete a particular lifting operation. Many crane barge and construction vessels are DP-capable - the larger ones generally to IMO equipment Class 3. The major advantage of DP to these vessels is the ability to complete a task in a very short time span, because the time needed to lay and recover moorings is saved, as is the risk of the moorings damaging nearby pipelines and structures.
7.13. Hazards associated with DP operations conducted in areas of shallow water and strong tidal conditions. Hazards associated with operations in very deep water. A hazard is something with the potential to cause harm. In the case of DP vessels there is a need to differentiate between minor and major hazards and the consequences of causing serious harm and non-serious harm. A decision must be made to establish the criteria of equating the degree of seriousness of potential harm to a few or many people. It is obvious that the most serious hazards are those that could cause serious injury or loss of life, and in particular to five persons or more, or divers when working underwater.
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Shallow water operations does present problems for a DP Operation, however if the operation is well planned and the factors mentioned below are taken into account then work can be carried out safely. Shallow water can mean stronger currents and tides affecting the operation in ways: •
The thrusters will have to work harder and therefore create more "noise".
•
The vessels excursion pattern may be larger.
•
Due to the lag in the model position keeping stability may deteriorate.
The two position reference systems that will be affected are the HPR and the Taut Wire. The wire itself can be bowed by strong current, but it is the possibility of fouling from seaweed etc. That would cause a position error. Further the short wire lenght means that the vessel's movement is severely limited especially as the wire comes towards the keel. The HPR similary is limited by shalow water.
Figure 76. Shallow water operations
The transducer protrudes 3 -4 meters below the hull and the transponder ia a couple of metres above the seabed. Thus the vertical separation is reduced hence limiting the range. Combined with thruster noise the HPR can become unreliable. Should the vessel move then it is possible for both these systems to go "out of range" very quikly. When planning shallow water operations it is necessary to have at least one surface position reference system, preferably 2. Surface position reference systems are likely to remain unaffected by water depth.
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APPENDIX A - DP INCIDENTS
Figure 95/04 COMMENTS This incident has been explained as a typical soliton effect that can catch an operator unaware in the South China Sea (for more information see IMCA seminar November 1995). MAIN CAUSE Soliton
SECONDARY CAUSE Thruster failure
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Figure 95/09 COMMENTS Low pitch pressure alarm on the port hydraulic pump came up and the port thruster tripped on low pressure. The engineer went to investigate and witnessed a large oil spray. On returning to the ECR the starboard pitch pump was shut down by mistake. The loss of pressure had been caused by a nipple coming out of the valve block, possibly due to a partly stripped thread. MAIN CAUSE Operator error
SECONDARY CAUSE Thruster fault (hydraulic)
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Figure 95/09 COMMENTS Low pitch pressure alarm on the port hydraulic pump came up and the port thruster tripped on low pressure. The engineer went to investigate and witnessed a large oil spray. On returning to the ECR the starboard pitch pump was shut down by mistake. The loss of pressure had been caused by a nipple coming out of the valve block, possibly due to a partly stripped thread. MAIN CAUSE Operator error
SECONDARY CAUSE Thruster fault (hydraulic)
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Figure 95/02 COMMENTS The DPO had little chance of stopping contact with the platform when the powerful thruster gave full power 135 degrees from the requested direction. The vessel was off hire for 3 days while starboard azimuth thruster was stripped down, and a foreign body was found. Presumably the DP alarms included a thruster fault alarm and the starboard azimuth thruster should have been stopped more quickly and then the collision could have been avoide. MAIN CAUSE Thruster fault (hydraulic)
SECONDARY CAUSE Operator error
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Figure 96/18 COMMENTS There will also be a loss of position when changing heading a large amount quickly especially if the vessel has heading priority. It should never be necessary to go into manual control to out perform the DP software unless the software is poor or not designed for the operation being performed. In addition on this vessel the azimuth thrusters do not assist the, astern until the main propeller reached 100% pitch. The vessel is not optimal for working stern to rough weather. MAIN CAUSE Operator error
SECONDARY CAUSE Poor design (Software)
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Figure 12/92 COMMENTS The DGPS reference was interfaced to the DP system via a navigation/survey computer which configured the DGS into a pseudo-Artemis signal. When the taut wire was raised this pseudo-Artemis signal became the sole reference. At the same time the navigation computer failed to receive adequate data from the DGPS system, and continued to output the last pseudo-Artemis signal to the DP computer. MAIN CAUSE DGPS software fault
SECONDARY CAUSE Insufficient commissioning/testing/QA
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Figure 96/14 COMMENTS The vessel was operating with open bus ties, inadequate power on line and with power limit warnings on bus 1 and bus 2. The thrusters were poorly set up so that the demanded thrust was either not met or exceeded. There was heavy pitching and No.2 bow thrust tripped on overload (Amps) before any pitch reduction was possible (DP unaware of overload because of poor set up). Failure of the other two thrusters was a consequence of the failure of No. 2. It was not possible to restart them until an azimuth thruster had been tripped because starting was inhibited when high power was being used by other thrusters 85%. MAIN CAUSE Thruster Electrical Protection / Control
SECONDARY CAUSE Unadequate testing / commissioning / QA 119
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Figure 21/92 COMMENTS During operations the wind increased considerably and the decision ws taken to move the vessel to the optimum heading. This required the vessel to move astern and change the heading 30 degrees. During one of the moves the vessel failed to reach the "marked position". It eventually became necessary to go into manual control to complete the move. Subsequent tests of software revealed the program was configured such that the estimated sea current was only updated when the vessel reached the "marked position". MAIN CAUSE DP control fault (software)
SECONDARY CAUSE Insufficient commissioning / QA
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Figure 96/10 COMMENTS There were alarms for A/B difference, network serial interface timeout and then thruster feedback for thrusters 1, 4 and 5. No final explanation is available but clearly the ADP702 crashed and the vessel lost position until manual control was selected.
MAIN CAUSE Computer Fault
SECONDARY CAUSE Poor design
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Figure 96/08 COMMENTS The man on the transponder winch line was asked to “stand by” to “skip beacon” but failed to hear the stand by part of the message. MAIN CAUSE Operator error
SECONDARY CAUSE Poor Procedures
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Figure 96/04
COMMENTS There had been tests carried out to prove that the arrangement of DGPS and HPR back up was satisfactory and in the 71m of water the transponder had to be within 50m for it to be a good position reference. It became outside this range. MAIN CAUSE Operator error
SECONDARY CAUSE Poor Procedures
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Figure 5/92
COMMENTS The vessel was working down weather of the rig and drifted away from the rig and mooring lines during the blackout. The divers and equipment were back onboard twelve minutes after the blackout. The cause of the blackout was the operation of the interlocks for shore power to the 440V switchboard which tripped the high voItage/440V transformers. Any vessel with a shore connection and interlocks should make sure that these are isolated during DP operations, so that a fault or a single act of mal-operation cannot cause a DP blackout. MAIN CAUSE Electrical fault
SECONDARY CAUSE Poor design
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Figure 8/92 COMMENTS When the vessel was initially set up on DP, trials were carried out using Artemis only as Syledis was not available. The vessel was already working when Syledis became available and so DP trials on Syledis only were not carried out. The vessel was moving in open waters with only one position reference and one untested backup. Once the vessel commenced the planned movement the Artemis signal was lost leaving Syledis as the only reference system available. The Syledis system was not updating position, which was the cause of the drive off. Artemis signals were restored, and vessel went back in full auto DP on Artemis. MAIN CAUSE Operator Error
SECONDARY CAUSE Poor Procedures
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Figure 96/34 COMMENTS The master discovered after the incident that the differential signal form Inmarsat A had been lost but the other DGPS used for survey was using an HF diff signal. There was no alarm or rejection of DGPS when the diff signal was lost and the survey team forgot to inform the bridge. This failure illustrates the weakness of DGPS supplied by the survey team as pseudo Artemis. MAIN CAUSE DGPS Failure (diff signal)
SECONDARY CAUSE Operator Error
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Figure 14/92 COMMENTS After reloading the backup computer the operator pressed the restart pushbutton, which caused the online A computer to stop with the subsequent loss of DP control. The vessel's position was maintained using the manual controls, whilst both A and B computers were again reload. No investigation was undertaken into this incident and it is likely that the operator stopped the A computer by mistake. MAIN CAUSE Operator error
SECONDARY CAUSE Poor Procedures
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Figure 17/92 COMMENTS The wind squall lasted approximately 3 minutes and was accompanied by a 70 degree change in wind direction. Since the thrusters were in fixed positions, they could not supply sufficient thrust to counter the vessels movement. The maximum position loss was 9 metres, after which the vessel started to regain position. Had the thrusters been in free azimuth mode before the squall, it is likely that the position loss would have been much less and not necessitated the amber alert. The large wind change would have had a major impact on the vessel model and a stabilisation period should have been carried out. Diving resumed after three minutes. MAIN CAUSE Wind Squall
SECONDARY CAUSE Operator error (fixed azimuth mode) 128
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Figure 26/92 COMMENTS It was reported that the vessel initially lost position and heading because of a wind gust and a heavy swell on the beam. In attempting to regain position the bow thruster wen to 100% starboard thrust, and shortly afterwards the centre tunnel thruster tripped out. The thruster tripped on overload when restarted. It was subsequently found that the setting of zero pitch on the centre tunnel thruster was out, and the thruster motor overloaded when driven 100% starboard. The pitch setting was adjusted and after testing driving recommenced. MAIN CAUSE Thruster fault (electrical)
SECONDARY CAUSE Insufficient maintenance 129
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Figure 22/92 COMMENTS No more information is available about this incident. Loss of one position reference should not cause a loss of position. Of course there can be a small shift of position because the remaining position reference would have 100% weighting and before it may have had a low weighting. Discussions had taken place between the two vessels prior to this incident, regarding transponders channels to be used.
MAIN CAUSE Operator error
SECONDARY CAUSE Poor Procedures
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Figure 32/92 COMMENTS To clean the 660V switchboard the centre bus-tie was opened. Opening the bus-tie breaker caused the emergency generator to start. When this was discovered, an engineer went to stop the emergency generator, but as the door to generator room slammed shut the emergency switchboard supplies to the bridge and diving switchboard tripped. The DP operators did not know why there was a loss of power to the UPS and gyros, and initiated a "Red Alert" whilst the situation was brought under control. The vessel maintained position while the divers were retrieved. There appears to have been little communication between the engineers and DP operator, and certainly switchboard cleaning should not have been undertaken while diving was in progress. MAIN CAUSE Operator error
SECONDARY CAUSE
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Figure 96/51 COMMENTS
The differential corrections were thought to be independent, one Inmarsat A the other Inmarsat B, thus avoiding a potential single point failure. After this failure it was found that both were transmitted from the same dish in Eik Norway, and failure of the dish caused loss of both DGPS. This failure happened.
MAIN CAUSE DGPS Fault
SECONDARY CAUSE Insufficient testing / Commissioning / QA
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Figure 96/09 COMMENTS The vessel did not clearly establish the cause of this incident. A move to starboard was input and the vessel moved to port and continued to move. The alarm print out shows the diff signal was frequently being lost an hour earlier. We therefore think the most likely cause was DGPS fault or operator error or both. Once high thrust was used it is possible that the HPR was lost. There should have been three position references on line. MAIN CAUSE DGPS Failure (loss of diff)
SECONDARY CAUSE Operator error
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APPENDIX B – KONGSBERG SIMRAD EXERCISES
Practical Exercise No. 1 "Challenger" version 2.4 The purpose of this exercise is to use the SDP system in Auto Position mode, and to get familiarised with some of the different features available. •
• •
Use the default trainer set-up values N:
100
E:
500200
Wind :
10 m/s or 20 knots
340˚
Current :
0.5 m/s or 1 knot
310°
Draught:
8.50 m
DGPS, HPR and Artemis as position reference system. DGPS to be Reference Origin. Establish the vessel in Auto Position Mode with all thrusters running.
Insert: Limits POSITION:
WARNING
2m
ALARM
4m
HEADING:
WARNING
2˚
ALARM
3˚
Set: Heading:
043˚
There are two alternative methods, which? …………………………………………………………………………………………………...
Make a trace of the next movements by “click” for Control, and then Trace line
5 sec.
30 min.
Trend symbol
2 min.
30 min.
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Then: Bring the vessel 18 m in direction vessel relative 70˚. Then: Bring the vessel 18 m west. Then: Bring the vessel 14 m to true bearing 333˚. What is your Position?
N ……………..
E …………….
Your vessel is 112 m long and your rotation centre CG is 51 m from the bow. Change ROTATION CENTRE to the bow. What is your Position?
N ……………..
E …………….
Then: Change the REFERENCE ORIGIN to ARTEMIS. Use Artemis and HPR as position reference systems. What is your Position? Set: Heading: Speed:
N ……………..
E …………….
348˚ 0.6 m/s
Then: Take the vessel to a position where the bow is 70 m south of your HPR transponder.
Trends view: Enter: Trends from the Main display view. Configure three TrendPlot windows: Fig. 1 The Force Fig. 2 The Moment Fig. 3 Dev Pos What have maximum values been so far in this exercises? Thruster force: …………………... Thruster moment: ………………... Position deviation: ………………..
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Then: Change to SYSTEM SELECTED heading. What is the system selected heading? …………………………………………… When in position with a stabiliSed system, what is the load on the thrusters? Resulting force ……………. Tons Direction ………………………………. Rotation moment ………….. T*m
Rotate your vessel to the worst thinkable heading in current weather situation. When in position with a stabilized system, What is the load on the thrusters? Resulting force ……………. Tons Direction ………………………………. Rotation moment ………….. T*m
Then: Change your rotation point back to CG. Take the vessel 100m to north with speed of 1,5 m/s and with a heading of 015˚. Then: Use Low Gain, and observe the overshoot. Take the vessel 100m to straight fwd from present position. How many meters overshoot? …….
Then: Use Medium Gain, and observe the overshoot. Take the vessel 100m to straight fwd from present position. How many meters overshoot? …….
Then: Use High Gain, and observe the overshoot. Take the vessel 100m to straight fwd from present position. How many meters overshoot? ……. Change: Reference Origin to DGPS. Use DGPS and HPR as position reference systems. Change: Displays units/units
Speed to knots Envir. Speed to knots
When the vessel is in position with a stabilized system, set the vessel to 1.5 m/s. 136
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Then: Move the vessel to a position 75 m in a direction of 120˚. Set: Heading
006˚
Set: Heading limit to: 004˚ ALARM
002.5˚ WARNING
Then: Move to a new position: SUGE SWAY
- 16 m 26 m
What is your Position?
N ……………..
E …………….
Change: Position notation from Cartesian to Geodetic, and geographic presentation. Select DATUM WGS 84. What is your Position?
N ……………..
E …………….
Then: Bring the vessel to a new location, N 00˚00.1200’
E 003˚ 00.3500’
Set: Heading towards New Setpoint. After arrival on location, activate the ARTEMIS reference system. Then: After calibration OK, deselect the DGPS position reference system. Why does the position co-ordinates still show LAT-LON? ………………….
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Practical Exercise No. 2 "Challenger" version 2.4 The purpose of this exercise is to simulate the way a vessel would approach and execute an Autotrack. The exercise is divided into three parts : Planning, Set-up and Execution. Planning : Select trainer. Position N6482000 E500100 Set the position presentation to UTM co-ordinates. Make sure to be in Zone 31. Define the following track : use Heading 000° Speed 1,0
m/s
and Turn radius 10 m towards
Wpt 1 N 6482250 E 500200
use Heading 020° Speed 1,0 m/s and Turn radius 10 m towards
Wp2 N 6482400 E 500400
use Heading 045° Speed 1,0
m/s
and Turn radius 10 m towards
N 6482400 E 500600
use Heading 090° Speed 1,0
m/s
and Turn radius 10 m towards
Wp4 N 6482200 E 500750
use Heading 035° Speed 1,0
m/s
and Turn radius 10 m towards
Wp5 N 6481990 E 500600
use Heading 000° Speed 1,0
m/s
and Turn radius 10 m towards
Wp6 N 6481990 E 500400
use Heading 345° Speed 1,0
m/s
and Turn radius 10 m towards
W3
Wp7 N 6482150 E 500200 Wp8 N 6482250 E 500200 Check that the track looks correct.
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Set-Up: Use DGPS as Reference Origin. Bring the vessel into Auto position. Turn ON Trace Line and Trend symbols (running the Track takes app. 35 minutes). Activate Limits/Cross warning and alarm to suitable values. Go through all the settings in the dialog box named Track Settings. Use these settings, all other settings you have to decide yourself: Next waypoint 1 Low speed Forward direction Approach Track Waypoint Position Dropout Action = Stop Stop On Course = Stay with 100% Force Heading = System selected - Waypoint Table Along Speed Setpoint = Waypoint Table Turn Radius = Waypoint Table Execution : Start the Auto Track. When between Wpt 2 and Wpt 3 use the STOP buttons. How much force is used during stopping ? ……………….. When passed Wpt 3, input a Leg Offset of 15 metres. Vessel to move on the outer side of the track. Should the input be + or - ? …………………… When passed Wpt 5 press the YAW button (the YAW status lamp becomes unlit) and use the joystick to control the heading of the vessel.
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Practical Exercise No. 3 Follow Target "Challenger" version 2.4 In Display Units/Units set all SPEED values to knots. Also, set the position mode to UTM (Northern hemisphere, False Easting selected). Set the system to STANDBY and deselect all Position References. In TRAINER, set: • Northings to 1000 • Eastings to 500500 • Wind to 17 knots from 055° • Sea Current to 0.7 knots from 075° Enable Rudders, Gyro, VRS, Wind sensors and Thrusters. Press MANUAL, and enable DGPS1 as Reference Origin. Select AUTO POS. '-^ Move the vessel 300m to the South and 100m to the West. ' When the above move is complete, enable HPR on the panel. The mobile transponder B02 should appear approximately 50m off on the port side. While the vessel is stabilising, enter CONTROLLER/FOLLOW SUB and on the dialog box set the Reaction Radius to 12m Press FOLLOW TARGET. A dialog box will open - Select Transponder No. Accept MOB_B02, which should appear in the textbox. The Reaction Circle should now be visible on screen surrounding the position of the Mobile transponder. This transponder marks the position of the ROV. Select 75m range on the PosPlot. Using the panel Joystick, drive the Mobile transponder (ROV) SLOWLY. Observe the reaction of the vessel to the transponder movements. Note: The function of controlling the Mobile transponder using the Joystick is purely for demonstration purposes on the classroom SDP trainer units. While the ROV is moving, select successively High, Medium and Low Gain, and note the different thruster power used in each. Also, try changing the Reaction Radius to 5m., then to 25 m. (Controller - Follow Sub). Reverting back to Low Gain and 12m Reaction Radius, try increasing the speed of the Mobile transponder, noting the effects. When all the above complete, return the joystick to the neutral position, allow the vessel to stabilise in position, and select AUTO POS. 140
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Practical Exercise No. 4 Shuttle Tanker "Britannia" version 2.4 In Display Units/Position, select UTM, Northern hemisphere, and False Easting. Also in Display Units, set ali Speed selections to knots. In Trainer, enter the following values: • Northing 6793600 • Easting 403825 • Wind 15 knots from 015o • Sea Current 0.8 knots from 030° In System/Select Buoy, select Myrica SPM. See the panel layoutfor the "Britannia " shuttle tanker. This layout dijfers from the actual panel in the Thruster conjiguration, and in the System Mode buttons. On the DP panel, Enable sensors and thrusters. Press MANUAL and select DGPS1 as Reference Origin. Press AUTO POS. Press DGPS2 and wait for a steady lamp. Press Change Heading and select System Select Heading. Observe the vveathervane heading comnianded. Press APPROACH. Observe the Wvane Dev Wvane and Numeric Wvane views. Place the Dev Wvane display in the Performance area,, and the Numeric Wvane display in the Monitoring area. Set the main display area to PosPlot, 75Om range, vvith Mode set to True, Position Setpoint. When the vessel's heading has stabilised, press CHANGE POSITION and set the Speed to 0.8 knot. Select the OFF LOAD tab. Change the Setpoint Radius to 300m. While the vessel is moving ahead, set the ALARM LIMITS to 2m/3m for position, and 2°/3° for heading. Also activate the Wvane limits a t 5m fore and aft. Continue to reduce the Setpoint radius and speed values until the vessel is on a setpoint circle of 35m. Once on this circle, press Wvane on the panel. The vessel is now on location for the loading operation. Note: the Conn and Load buttons are not used in this configuration. These are used for operations v/ith a submerged rurret. THEN: In Trainer dialog box Change the direction of the wind and observe how the vessel reacts.
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APPENDIX C – ALSTOM EXERCISES Practical Exercise No. 1 The purpose of this exercise is to use the a DP system in Auto Position mode, and to get familiarised with some of the different features available. USE SET UP SHEET TO SET UP DP FIRST •
• •
Use the simulator set-up values. N:
60 00N
E:
002 00E
Wind:
10 m/s 340 0
Current :
0.5 m/s 310°
Draught :
7.0m
Deploy acoustic beacon A (Simulation data tab) first Deploy 2 DGPS note that DGPS becomes RO (The position will change) POSITION: HEADING: Heading:
ALARM ALARM
4m 3°
WARNING 2m
043 0
There are 4 alternative methods, which? .......................................... Bring the vessel 10 m to starboard. Bring the vessel 18 m in direction 113. Bring the vessel 18 m west. How many methods are there of changing position? ………………………… Re-centre the display to put vessel back in the middle of the screen. On Simulation page (mimic index) simulation data PME Toggle the acoustic beacon noise to High Check PME weightings and repeatability Weight increases or decreases ………………………… Repeatability increases or decreases ………………….. Toggle beacon noise to low What is your Position?
N …………….. 142
E …………….
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Change ROTATION CENTRE (COR) to the bow.
What is your Position?
N ……………..
E …………….
Change the REFERENCE ORIGIN (RO) to Acoustic beacon on the coordinate setup page. What is your Position?
N ……………..
E …………….
Has the vessel moved? Heading : Speed :
348˚ 0.6 m/s
Take the vessel to a position where the bow is 70 m south of your acoustic beacon. Real time trends view : Trends from the "Mimic Index" - "Standard Mimics" or bottom tool bar Configure three TrendPlot windows : Fig. 1. Thrust demand X Fig. 2. Thrust demand N Fig. 3. Position error X What have maximum values been so far in this exercise ? Thruster demand X Thruster demand N Position error X
....................... ....................... ………………..
Change to MINIMUM POWER heading. What is the system selected heading ? …………………. When the heading has settled what is the load on the thrusters ?
Resulting force ……………. Tons Direction ………………………………. Rotation moment ………….. T*m
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Rotate your vessel to the worst thinkable heading in current weather situation. Can you change the heading? When the heading has settled what is the load on the thrusters ?
Resulting force ……………. Tons Direction ………………………………. Rotation moment ………….. T*m
Change your rotation point back to CG Take the vessel 100m to north with a speed of 1,5 m/s and with a heading of 015˚ Use Low Gain, and observe the overshoot Take the vessel 100m straight fwd from present position. While you are moving toggle the radial error display How many meters overshoot? Use Medium Gain, and observe the overshoot Take the vessel 100m straight fwd from present position. How many meters overshoot?
Reference Origin to DGPS Use DGPS as position reference system. Turn HPR off Display units/units Speed to knots (Mimic index -Options) When the vessel is in position with a steady heading, set the vessel speed to 1.5 m/s
Heading:
0060
Position limit to:
002m
WARNING. 004 m
ALARM
Which 2 pages show pitch and roll ………………………………………. Which page shows the generators running ……………………………… Which page shows thrusters set point feed-back ………………………… Which page shows PME voting ……………………………………………..
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Move to a new position : SURGE -16 m. SWAY 25 m. What is your Position?
N ……………..
E …………….
SURGE -16 m. SWAY 25 m. What is your Position?
N ……………..
E …………….
Under Mimic index-Options toggle Position notation between Grid and Geographic presentation, end up in Geographic What is your Position?
N ……………..
Bring the vessel to a new location, N 60° 00" 7"'
E …………….
E002° 00" 21"'
After arrival on location, activate the HPR reference system. After calibration OK, deselect the DGPS position reference system. Why does the position co-ordinates still show LAT-LON ? ……………
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Practical Exercise No. 2 Cable ship 2 The purpose of this exercise is to simulate the way a vessel would approach and execute an Auto-track. The exercise is divided into three parts: Planning, Set-up and Execution. USE SET UP SHEET TO SET UP THE DP Planning: Set Trainer position prior to going into simulator mode Select trainer. Position Lat 58 28 43.08N long 003 00 06.17E Wind 10 knots 015, wave 0.5m 025, current 1 knot 070 Set the position presentation to UTM co-ordinates. Make sure to be in Zone 31. Define the following track on the TRACK FOLLOW page : Ensure entry option = E,N absolute Add 8 waypoints using waypoint control menu Wpt1 E 500200 N 6482250 Wp2 E 500400 N 6482400
Line,
Heading ABS 020˚ Speed 1.0 m/s
W3
Line,
Heading ABS 090˚ Speed 1.5 m/s
E 500600 N 6482400
Wp4 E500700 N 6482200
ArcC,
Heading INC+0°Spe ed 1.7 m/s, radius 150 m
Wp5 E500600 N6481990
Line,
Heading ABS 000° Speed 1,0 m/s
Wp6 E500400 N6481990
Arc A,
Heading Inc +10° Speed 1.3 m/s , radius 500 m
Wp7 E500200 N6482150
Line
Heading AB S 315° Speed 1.1 m/s
Wp8 E500200 N 6482250
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Set-Up Use DGPS as Reference Origin. Bring the vessel into DP mode. Turn ON Vessel trail, and display track under screen options Check that the track looks correct. Activate Limits warning and alarm to suitable values. Go through all the settings on the track follow page Use these settings, all other settings you have to decide yourself: Next waypoint Slider velocity /WP preset Forward / Backwards Continuous / increment Heading Intro shift / to WP
=2 = WP presets = Forward = Continuous = Follow WP presets = To WP
Execution : Go to first WP Modes select Follow track When between Wpt 2 and Wpt 3 use the TRACKING button to stop the vessel. How much force is used during stopping? ………………. Start the vessel by pressing the stopped button When you restart the vesselchange heading preset to follow course When passed Wpt 4, input a Track offset of 15 metres. Vessel to move on the outer side of the track. Should the input be left or right? ………………….. After passing WP 5 change heading presets to WP presets After you pass WP 6 change heading to 300
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Practical Exercise No. 3 Follow Target Set up DP as per Start up sheet In Modes menu/ Options set values to knots In SIMULATION set: Latitude to 60 degrees north Longitude to 02 degrees east Wind to 17 knots from 055° Sea Current to 0.5 knots from 075° Enable 2 DGPS. In screen options ensure ROV trail is on. PME display ensure 13 is showing position fixes. On simulation set ROV "free" to "attached" Set offsets to stb 50m Fwd 0m Set recovered to deployed Enter ROV speed and direction Under modes menu select Follow ROV Under alarm settings set the Reaction Radius to 12m. Set "stopped" to "moving" Drive the ROV slowly Observe the reaction of the vessel to the transponder movements. If the vessel is not keeping up (Observe "to destination" distance) what can you do? 1. 2. 3. 4. While the ROV is moving, select successively High, Medium and Low Gain, and note the different thruster power used in each. Also, try changing the Reaction Radius to 5m., then to 25 m. (Alarm settings - Top menu). Reverting back to Low Gain and 12m Reaction Radius, try increasing the speed of the Mobile transponder, noting the effects. Slow the ROV down and then move the vessel 10m to starboard Change the vessels heading 90 degrees to port Stop the ROV is the ship in the same relative position?
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Practical Exercise No. 4 The purpose of this exercise is to use the a DP system in Autotrack using a plough to lay cable. USE SET UP SHEET TO SET UP DP •
Use the simulator set-up values.
N:
60 00N
E:
002 00E
Wind: Current: Draught:
3 m/s 340 ° 0.5 m/s 310° 5.5
•
Use 2 DGPS
Set heading
000
Set up follow track using the pointer option, the track does not have to be precise reasonably close will do WP 1 WP2 WP3 WP4 WP5 WP6
present position 500m 000 500m 030 500m 060 500m 090 500m 120
then then then then
Use line for ali waypoints; Slider or track speed to 2 m/s, next waypoint 2 Under screen option ensure ROV trail is on PME display display beacon 12 fixes On Simulation page select simulation tab, select plough tab Set "brake off' to "brake on" Set "free" to "attached" Set "recovered" to "deployed" Change to follo\v track Then try and use track shift (under change position menu) to keep PLOUGH on track If heading does not match track heading use change heading function, or use follow course option NB THE PLOUGH HAS A FLXED 400M LAY-BACK
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APPENDIX D - NAUTICAL INSTITUTE TRAINING PROGRAM The Nautical Institute training program has been officially acknowledged as a recommended course of training by Governments, Industry and the I.M.O. The program, like any that is constantly seeking to keep pace with developments is continually under review. The differentiation between Class 1 and 2 & 3 vessels is explained in the flow chart on overleaf.
The training program To receive a DP Operators Certificate from the Nautical Institute the operator must have completed the following program: •
Attend and satisfactorily complete a 4 day basic (Induction) course.
•
Undergo seagoing DP familiarisation - 30 days.
• •
Attend and satisfactorily complete the Advanced (Simulator) course. Satisfactorily complete six months supervised D.P. operations.
DP Operator Log Book Entries A
Preliminary Shore Induction Course
B
Seagoing DP familiarisation (30 days)
C
Seagoing Familiarisation Watch keeping Log
D
DP Simulator Course
E
DP Watch keeping Experience 6 months
F
Suitability of officer to undertake full watch Keeping responsibility on board a DP vessel
D.P. Watch keeping experience
It is recommended that even after the issuing of the DP Operators Certificate a full record of DP experience is maintained. This is best achieved by obtaining the IMCA blue logbook.
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DP OPERATOR'S CERTIFICATE
NOTES •
The required DP time may be reduced by attendance at intensive simulator training in dynamic positioning, offshore loading courses or completion of part or the entire DP CAP programme. In any case these reductions may not add up to more than 50% of the required DP time or replace the seagoing DP familiarisation or the final 30 days of DP time. Further details of this available from the Institute.
•
All DP seatime, courses and other elements of the programme must have been completed within the past 5 years. 151
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NOTES ON COMPLETING THE NAUTICAL INSTITUTE DP OPERATOR TRAINING LOGBOOK General the scheme of training involves six elements: •
The induction/basic course.
•
30 days familiarisation at sea during which time the familiarisation tasks should be completed.
•
The simulator/advanced course.
•
Six months' or more recorded sea service on a DP vessel depending upon the level of certificate sought.
•
Assessment by the Master of suitability to undertake unsupervised DP watchkeeping.
•
Issue of a certificate.
Logbooks are issued to all persons attending the Induction/Basic course. In exceptional circumstances they may also available from The Nautical Institute at £30 each plus postage and packaging. The logbook is your property, it will contain the record that will enable you to obtain your DP certificate so look after it carefully and make sure all the sections are accurately and properly completed. On receipt enter your name, address and date of birth if this has not already be done by the training centre. This identifies the logbook as yours and contains your address, including the country and any zip or postcode, for us to return it to you with your certificate. If you change address during your time working towards the certificate please paste the new address over the previous one. It is surprising how many logbooks are submitted vvith no return address or a previous address. When attending a course ensure that the details are entered in the logbook and this is stamped and signed by the centre on satisfactory completion. When entering dates in the logbook please use the format day/month/year. Make sure the logbook is signed during the sea phases and that all periods of service have both a date joined and left. This has always been understood to be provided that between these dates the vessel carries out significant DP operations, and thus use of the DP system. This would not include operations where the vessel maintains its position through means such as a position mooring system or connection to a STL loading buoy unless the DP system is extensively used in approaching and setting up on location. In this section (DP vvatchkeeping experience) the entry in the column entitled "Rank" should indicate that the person fulfilled an operational DP role e.g. DP Master, 152
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DPO, etc. Unfortunately entries such as Master, Chief Officer, etc. do not indicate that that person had an operational DP role on board. These, and other ranks, may be combined with DPO in this entry e.g. Chief Officer/DPO. Make sure at the end of the training period the Master completes Section F (The statement of officer to undertake full watchkeeping responsibility on board a DP vessel). Section F should be signed by the Master who should also enter his/her own DP certificate number if held. This signature and the ship's stamp should correspond to an entry in Section E. If the Master is the holder of the logbook he/she should have this section signed by a DPO on board who should also enter his/her own DP certificate number if held. If the Master finds himself/herself in the position vvhere he/she is unable to have a DPO sign this section, he/she should sign this section himself/herself a company marine manager who is aware of the Master's DP abilities should authenticate it. It may be necessary to have an additional section F completed if you are applying for a full certificate or an upgrade and the section F has been previously completed for a DP Class 1 vessel. Please note that during the verification process the signature of the Master and date in Section F will be checked with those entered in section E of the logbook. If you are the Master you should have this section countersigned by an appropriate person such as a DPO on board or a marine superintendent who knows you DP ability. They should state their position on this page. It is also useful if the Master or other person signing section F enters their own DP certificate number if they have one. When the logbook is complete it should be sent to The Nautical Institute, 202 Lambeth Road, London, SE1 7LQ, United Kingdom. There may be a charge if the logbook is incomplete and has to be returned or additional documents have to be submitted. It is suggested that a copy is made of the logbook before sending in case it goes missing in transit. The Institute will verify the following information; •
signed evidence of seatime,
•
stamped evidence of courses attended,
•
verification by the Master in section F and corresponding signature in section E,
•
types of equipment and class of vessels,
•
DP systems in use and
•
Certificate of competency 153
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Extra vetting is carried out by random verification that the data is correct by contacting companies, training centres, masters and individuals. If all the data is correct the Institute will issue a numbered and dated certificate, record the information in the logbook and normally aim to return the logbook wrth the certificate within two weeks of receipt by first class mail or airmail overseas. If you vvish to have these documents returned by courier you should forvvard payment of £10 with the logbook. You should follovv the procedure above if you are forwarding logbook for an upgrade. For this you should also include the original limited certificate and an additional section F signed by the Master of a DP Class 2 or 3 vessel on which you have served. Should you have any questions about the DP certificates the staff at the Institute will be happy to answer these. They can be contacted by email at
[email protected] or
[email protected], by telephone on +44 20 7928 1351 or by fax on +44 20 7401
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ABBREVIATIONS Alstom Artemis C/A Code CG COS C/R – COR DARPS DGPS DoD DP DPO DR DSV ERS FMEA FPV FPSO FSU GPS HPR INS IMCA IMO MMI LAN LBL LTW OLT P-Code PM PME PMS PPS PRS ROV SA SBL SSBL STL Surge Sway
DP system manufacturer A microvvave position reference system The Coarse Acquisition code used with the GPS system Centre of Gravity Common Operator Station Centre of Rotation or Alternative Rotation Point Differential Absolute and Relative Positioning System, a DGPSrelated PRS used by shuttle tankers and FPVs Differential GPS The US Department of Defence Dynamic Positioning Dynamic Positioning Operator Dead Reckoning Dive Support Vessel Environment Reference Sensor Failure Modes and Effects Analysis Floating Production Vesse Floating Production storage offloading Floating storage unit The Global Positioning System of satellite navigation Hydroacoustic Position Reference Inertial navigation system The International Marine Contractors Association, an amalgamation of the DPVOA and AODC, these two bodies merged in 1995. The International Maritime Organisation The Man Machine Interface Local Area Netvvork Long Baseline HPR Lightweight Taut Wire system Offshore Loading Terminal The precision code transmitted within the GPS system, currently restricted to use by approved military users. Position Mooring Position measuring equipment (GPS, HPR etc) Power management system The Precise Positioning Service from the GPS system, only available to approved military users Position Reference System Remotely Operated Vehicle, usually unmanned submersible Selective Availability, the means by which non-approved users are denied access to the P-code within the GPS system Short Baseline HPR Super-short Baseline Submerged Turret Loading Vessel movement in the fore and aft direction Vessel movement in the transverse direction 155
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UPS USVL UTM VRS VRU WGS84 Yaw
Uninterruptible Power Supply Ultra Short Baseline The Universal Transverse Mercator projection and co-ordinate system Vertical Reference Sensor Vertical Reference Unit The world Godetic Spheroid upon which the GPS system is based Vessel rotation about the vertical axis; change of heading
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REFERENCES • ALSTOM, Guide to Dynamic Positioning of Vessels, Alstom Power Conversion Ltd, 2000. • Bray, D. J., Dynamic Positioning Operator Training, The official guide to The Nautical Institute training standards, The Nautical Institute, 1999. • IMCA, Introduction to Dynamic PositioningThe Training and Experience ofkey DP Personnel, Issue 1, Revision 1, January 1996. • IMCA, Training and Experience of key DP Personnel, Issue 1, Revision 1, January 1996. • IMCA, Guidance for The Initial and Refresher Familiarisation of Vessel Crews, December 2000. • IMCA, International Guidelines for The Safe Operation of Dynamically Positioned Offshore Supply Vessels, March 2006. • IMCA, DP Incidents, The IMCA Database 1990-99. • IMO, Guidelines for vessels with dynamic positioning systems, Annex MSC/Circ. 645, June 1994. • IMO, Guidelines for vessels with dynamic positioning systems (DP) operator training , July 2006. MSC/Circ. 645, • IMO, Guidelines for dynamic positioning system (DP) operator training , July 2006. MSC.1/Circ. 738/Rev.1, • Singapore Maritime Academy, DP Operator Manual
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