Dynamic Positioning-Basic Student Handout

MARITIME TRAINING

CENTER CROATIA DYNAMIC POSITIONING ______________________________________________________________________________________________________

COURSE:

Dynamic Positioning Induction/Basic

( Student Handouts 1st version)

Split, 2008.

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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

13 13

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

23 23

3. PRACTICAL OPERATION OF A DP SYSTEM 24-41 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, Automatic 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 vessels 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 position-reference 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 de-selecting 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

72 - 72

6. POWER GENERATION AND SUPPLY, AND PROPULSION SYSTEMS 73 - 81

6.1. The power generation and distribution arrangements in a typical diesel-electric 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-diesel-electric 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

81 - 81

7. OPERATIONS USING DP 82 112 7.1. Procedures to be followed when approaching a worksite and transferring

from conventional navigation to DP control

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

7.4. The need for effective communications during the conduct of DP operations 85 - 87

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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 100101 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 113133 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,

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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 Sway Starboard + - Y

Position Control

Yaw Clockwise (seen fom above)

+ - N Heading Control

Heave Upwards + - N Pitch Bow Down Roll Stbd Down

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.

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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.

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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 DP-system 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 two-dimensional 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 earth-fixed.

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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)

Figure 13. Duplex redundant control (ADP21or ADP22)

Du IMO Class 2. Loss of position should not occur from a single fault in an active component or system.

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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

Trip IMO Class 2, Loss of position should not occur from any single failure

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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 user-friendliness.

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. 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.

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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. Setting-up on location is quick and easy. Vessel is very manoeuvrable. Rapid response to weather changes is possible (weather vane). Rapid response to changes in the requirements of the operation. Versatility within system (i.e. track-follow, ROV-follow and other specialist

functions). Ability to work in any water depth. 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 new location rapidly (also avoid bad weather).

DP Disadvantages: High capex and opex . Can fail to keep position due to equipment failure. Higher day rates than 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.

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. Ultra Short Baseline (USBL) less accurate than LBL, uses one beacon. 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 t

Dynamic Positioning-Basic Student Handout

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