DP Operator's Handbook
byii
DP Operator's Handbook
By Captain David Bray FNI
David Bray has asserted his right under the Copyright, Designs and Patents Act of 1988 to be identified as the author of this work.
Published by The Nautical Institute
202 Lambeth Road, London SE1 7LQ, England Tel: +44 (0)20 7928 1351 Fax: +44 (0)20 7401 2817 Web:
www.nautinst.org First edition published 2008 Reissued 2010 Copyright ©The Nautical Institute 2008 © David Bray 2008
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form, by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, except for the quotation of brief passages in review. Although great care has been taken with the writing of the book and production of the volume, neither The Nautical Institute nor the author can accept any responsibility for errors and omissions or their consequences.
The book has been prepared to address the subject of dynamic positioning. This should not, however, be taken to mean that this document deals comprehensively with all of the concerns that will need to be addressed, or even, when a particular need is addressed, that this document sets out the only definitive view for all situations. The opinions expressed are those of the author only and are not necessarily to be taken as the polices or view of any organisation with which he has any connection.
Design and typesetting by Phil McAllister Design Printed in the UK byThanet Press Ltd.
ISBN 978 1 906915 15 5 THE NAUTICAL INSTITUTE
Foreword
D
ynamic Positioning as a working system was introduced in the 1970s. As its potential was realised, DP systems were applied to ever more critical operations involving divers and drilling.The UK Department of Energy and the Norwegian Maritime Directorate specified the need for training;The Nautical Institute assessed training needs and established a training programme. Captain David Bray from Lowestoft with Jim Simpson from Aberdeen in the UK and Rune Mellum in Kongsberg, Norway, played a leading role in converting the training specification into simulator courses that became the international standard.
As the range of DP applications becomes ever more specialised, so the training must meet these specialist needs. Captain Bray recognised the need for a multi-disciplinary committee to examine DP specialisation and it is pleasing to see the progress being made as DP matures into a new phase.
Currently there are some 55 DP training centres worldwide; The Nautical Institute has issued over 1,200 DP certificates a year since 2005, and numbers continue to increase. Captain Bray has devoted countless hours to the development of effective training methods which he has generously shared with colleagues. He has accredited numerous DP centres and contributed to the DP Training Forum. I would like to acknowledge his dedication and outstanding professional commitment. In our discussions over the years we concluded that, with so many applications, it might be useful to provide a handbook for DP operators (DPOs) to aid easy transfer from one ship to another which could serve as a handy reference book to be used as a refresher, a source of good management and operational practices and a guide for organising onboard training particularly for the benefit of understudies.
For example, communications has prominence in a number of sections and it is a maxim of all cooperative operational systems to keep lines of communication open at all times, always seek to explain intentions to those who need to know. By keeping lines of communication open and being open to the comments of people engaged in the operation, most problems will be overcome. Captain Bray has now retired. On behalf of everybody he has helped and trained over the past 20 years and for the wisdom of this handbook, he deserves our special appreciation and thanks.
Julian Parker
iv
Acknowledgements
M
any individuals and organisations have either directly or indirectly contributedto this book. Inevitably, a work of this type is a digest of existing knowledge, experience and previously published material.
Thanks are particularly due to my long-time colleague at Lowestoft College in the UK, Richard Lodge, whose help and support have been invaluable, both in the compilation of this book and in the running of training within the DP unit.
For information supplied over the years, I am indebted to the various manufacturers of DP and peripheral systems, in particular Kongsberg Maritime, Converteam, Nautronix, Sonardyne, Guidance Limited and MDL.
Thanks to the following for supplying illustrations: Heerema Marine Contractors, R Clarke, Teekay Shipping and Gary Ritchie FNI.
I must also thank Julian Parker at The Nautical Institute, not only for the opportunity of compiling this volume, but for all the help and encouragement over more than 20 years in the business of DP training, accreditation and competence.
David Bray
2008
Contents
Chapter 1 Introduction to dynamic positioning ... 1
Chapter 2 Dynamic positioning principles and systems ... 3
Chapter 3 Redundancy and equipment class ... 21
Chapter 4 DP vessel operations ... 31
Chapter 5 Operational planning and watchkeeping ... 47
Chapter 6 Position referencing ... 57
Chapter 7 Propulsion and thrusters ... 87
Chapter 8 Power plant ... 93
Chapter 9 Operator training and human factors ... 98
Appendix 1 Guidance to Masters of DP vessels in assessing the competency of DPO candidates ... 107
Appendix 2 References page... 109
Appendix 3 Glossary of Terms and List of Abbreviations... 111
Chapter 1 1 Introduction to dynamic positioning
Chapter 1
Introduction to dynamic positioning
D
ynamic positioning (DP) is now an established technique in the world of offshore- related and specialised shipping. The earliest, very rudimentary DP systems were in use in the 1960s in deep water drillships, but the technology did not come of age until the late 1970s and early 1980s when DP systems were being fitted to new vessels in large numbers. From these beginnings, there are now in the order of 3,000 DP-capable vessels extant, with more being built.It is more than 45 years since the first, fully automatic dynamically positioned vessel entered service. After all, the first DP-capable vessel entered service only 34 years after the launch of the last ocean-going cargo-carrying square-riggers. It may be thought that DP is now a fully mature technology, but a visit to any of the conferences regularly held on the subject of DP will soon dispel that myth! The technology is a very rapidly changing one, in every area (propulsion, control systems, position-reference, operational, etc) and it is sometimes quite a challenge to stay current. For the vessel operators and DPOs this is even more of a challenge, as, perforce dynamic positioning operators (DPOs) may find themselves working on a limited range of vessels without the most up-to-date equipment.
The technology related to DP has changed and developed very quickly, and the learning curve has been very steep. The number of personnel involved in DP operations has continually increased and there is now a huge base of expertise and experience. Nevertheless, a large number of personnel are new to DP techniques, and training is a major requirement within the DP vessel industry. The Nautical Institute has been involved with DPO training and qualification from the beginning: its scheme for the training of DPOs is accepted worldwide. Shore-based training centres all over the world provide elements of training approved and accredited to a common high standard.
This handbook is not intended to be a comprehensive treatise on the subject of DP. It is aimed at those perhaps entering the DP scenario from more conventional vessels. It does not attempt to replace the system handbooks supplied by the system manufacturers, nor does it go into great detail on individual manufacturers' systems. It attempts to provide
DP Operator's Handbook
an overview of the topic, leading the reader into further study and discussion. It also highlights some of the pitfalls and problems that may be experienced, while references to further reading are provided to allow the subject to be studied further if needed.
4 Chapter 2
DP Operator's Handbook
Operator (DPO). Measured values of position and heading are continually fed back into the controllers, while the difference between Set Point and feedback is termed error or deviation. The controller computers continually adjust thrust commands to reduce these errors to (or maintain them at) zero.
All modern complex control systems, such as DP, use mathematical modelling techniques as part of their control functionality. The system contains a mathematical model, or description of the vessel's dynamics. This is used to continually predict future vessel positions, headings and velocities. This data is continually compared with the corresponding measured values, allowing the computation of corrective thrust commands. A DP system is an example of an automatic Closed-loop control function.
The mathematical model contains static data on the vessel parameters, but is also an adaptive feature. The analogy is that of steering a vessel by hand. Although skilled at steering, a helmsman will take five minutes or so to get the feel of the vessel after taking the wheel. The DP system does the same, the mathematical model taking up to 30 minutes to fully adapt to the present environment and vessel configuration. Subsequent to this period, the system will continually adapt to changes in the vessel or environment, just as a helmsman will adapt his steering to changing sea states. This 30-minute model build period or settling period must be allowed for in the setting-up procedure; until the model is fully complete the vessel may oscillate in position and/or heading. Controlling computers will, in modern vessels, be configured as part of a fully integrated local-area network covering all vessel control and monitoring functions and facilities.The controller facility may be provided by one processor operating alone, or by an array of
Dynamic positioning principles and systems
two, three or maybe more, in order to provide a level of redundancy. If two processors are provided, then one is on-line while the other acts as a back-up. If three are installed, then there exists the possibility of voting or triple-modular redundancy: one unit on- line and two back-ups. All critical computations are thus triplicated and compared; any discrepancy allows automatic indication and rejection of the errant unit.
2.2 DP system components and layout
As mentioned above, DP is best described as an integration of a number of functions.
Central are the controlling computers or processors; these communicate with all other parts of the system via the vessel network.
The system is controlled and operated using the DP console, or desk containing operational controls, buttons, screens and manual joystick. This console should be located in a position affording a good view of the surrounding sea area, and is usually on the bridge or pilot-house. Most modern systems function under a version of Microsoft Windows - a familiar environment to the PC-literate DPO. A vitally important part of the DP system is the DPO. All DPOs must be fully competent to conduct DP operations. Discussion on the subject of human factors is included in Chapter 9 of this handbook.
In order to be able to control any variable function, in engineering terms, it is necessary that the variable be accurately measured. It is thus necessary that the controllers be
6 Chapter 2
I DP Operator's Handbook
provided with accurate and reliable data on vessel position and heading. DP systems are thus interfaced with gyro compasses and a variety of position reference systems (PRS).
Gyro compasses are standard pieces of ships'equipment, and in DP vessels are fitted in duplicate or triplicate dependent upon the level of redundancy desired. It is common to find three compasses installed to fulfil the requirements of equipment Classes 2 and 3 (see Chapter 3 of this handbook covering system redundancy). A more recent development is the fibre-optic compass. This device provides a full compass facility from a solid-state (no moving parts) unit.
Position-reference systems (PRS) provide feedback data on vessel position. In the DP scenario, there needs to be a greater level of precision than that needed in conventional navigation. Vessel positioning can never be more accurate than the precision of the PRS, and typical positioning accuracy of a DP vessel is within 1 to 2 metres. Position-reference is therefore required to be in the area of 1 metre or better.
Position-reference systems are independent systems interfaced into the DP controllers. They can be satellite-based (DGPS), optical laser systems (Fanbeam, CyScan), microwave- based (Artemis, RADius and RadaScan), underwater acoustic systems, or mechanical (Taut Wire). DP systems are enabled to receive and pool data from two or more PRS, determining a best-fix position from all monitored data. The greater the number of PRS in simultaneous use, the greater is the precision of this best-fix, and the lesser is the impact of the loss of any one unit. A detailed consideration of the topic of position-reference systems is given in Chapter 6 of this handbook.
The DP system is interfaced to a variety of sensors and other peripheral equipment. This is detailed further in paragraph 2.4 of this chapter.
The vessel is ultimately under the control of her propulsion units - propellers, rudders and thrusters. It is therefore necessary that all these are interfaced into the DP system. Propulsion commands are sent in respect of pitch, rpm, azimuth and rudder angle, while feedback from all units is continually monitored. The DPO must continually monitor the Set Point and feedback values for each propulsion unit, as, although a discrepancy should generate the appropriate warnings and alarms, a failed thruster does not necessarily trigger an alarm. This is because, if conditions are stable, the commands may not vary by much. A thruster might have failed as-set, ie it is outputting a constant fixed thrust value and not reacting to commands. No alarms are generated as the failure thrust value is very close to the command. This failure mode will only be picked up by the diligent DPO! Further information on propulsion systems is contained within Chapter 7 of this handbook.
Every vessel needs power, and as such, the power supply is very much part of the DP system. It must be remembered that a power problem has an immediate knock-on effect to the DP system and vessel capability. Most DP-capable vessels are diesel-electric,
Dynamic positioning principles and systems
thus the diesels, alternators, switchboards, cabling, propulsion motors and power- management system are all part of the DP system. Furthermore, diesel upstream systems must all be regarded as part of the DP system; cooling, lube, control and fuel systems. A water-contaminated fuel day-tank may cause the stop of one or more diesels, resulting in an immediate power shortage. The vessel may not have sufficient power for all the thrusters and propellers, thus a position loss may occur. Further notes on the topic of power plant are contained in Chapter 8 of this handbook. It is not the intention here to describe in detail the operation of any particular make or model of DP system, facility or function. It is vitally important that DPOs fully familiarise themselves with all functions, facilities and procedures appropriate to their system, and that all operational handbooks and manuals are studied in depth. What follows is an outline of the most common functions available.
2.3 DP system functions
The main function of DP is to enable the vessel to maintain a fixed position and heading, and, of course, to make changes to that position and/or heading in a controlled manner.
To that end, facilities are provided to allow DPOs to select a new position. This may be defined as a range/bearing from the present position, or in global co-ordinates such as
8 Chapter 2
DP Operator's Handbook
UTM Northings and Eastings, or Lat/Long. The vessel may then be moved to that new position at a specific speed. Likewise, the heading of the vessel can be adjusted to a new value at the desired rate-of-turn. A further heading-related facility is the optimum heading, or weathervane function, in which the vessel's minimum power heading is continually calculated in relation to the combined wind and current vectors, and the vessel 'hunts' this continuaily changing value. This facility may be of value in some situations, but not in others.
2.3.1 System gain
Gain is the relationship between the vessel's positioning situation and the power used. Most DP systems have three gain settings; low, medium and high. In general, low gain is used in calmer weather conditions, or where positional precision is not a prime concern. Low gain settings may reduce fuel consumption. In a high gain setting, proportionally more power is used in order to maintain a tighter position. This is more useful in position- critical operations or where the weather is more severe. Another function is known as relaxed gain in which the vessel's set-point position is expanded into a disc of specific radius (selected by the operator). Within this area the vessel is allowed greater freedom of movement, power only being increased when the vessel approaches or passes the limit of the area.
In modern DP systems, further adjustment may be made to the positioning ability to facilitate conservation of power and fuel. The latest generation of system from Kongsberg exhibits a 'green DP' function. In this function, complex predictive calculations allow the damping of positioning with the minimum of fuel expenditure. With any of the facilities mentioned above, it is vital that DPOs thoroughly familiarise themselves with
Dynamic positioning principles and systems
the function in order to avoid unwanted surprises! A gain setting is also applied to the joystick control, with a variety of alternative control selections.
2.3.2 The follow-target facility
Many systems are configured with a variety of this facility. The vessels making greatest use of this might be ROV support vessels and vessels operating Trencher units. These vessels have a need to maintain position relative to a moving target rather than to a geographical location. A common method of conducting such operations is a method often called 'dog-on-a-lead'. With this method, the DP system is configured to use one single PRS, which is an acoustic beacon located on the ROV or trencher. The DP system 'thinks' the vessel is stationary as the beacon is a fixed entity, whereas the vessel is actually trying to maintain position relative to a moving target. This procedure cannot be recommended as there are a number of problems. Often, the agility of the vehicle is greater than that of the vessel, which struggles to keep up. The DP system generates
10 Chapter 2
DP Operator's Handbook
a false current within the model. Also, the ROV pilot has direct hands-on control of the vessel's movements rather than the DPO.
To avert these problems, the system manufacturers provide a specific follow target or follow-sub mode of operation. In follow target mode the ROV acoustic beacon is designated mobile, thus the DP system does not include it in the PRS pool. Instead, that beacon is designated the follow beacon. The DP system must be configured with other PRS (eg DGPS) in the normal manner. A geographically-fixed circle (radius defined by the DPO) is placed around the ROV, becoming the ROVs operational area. ROV (with beacon) can move freely with the vessel on a fixed location. If the ROV breaks out of the circle, the DP reacts by adjusting the vessel position by an amount equal to the radius in the appropriate direction, and generating a new circle. This continues as many times as necessary. The DP system avoids generating a false current value, while the DPO retains full positional control of the vessel.
Another way in which the follow-target facility may be used is where the vessel must be positioned relative to a slowly moving target such as an FPSO. As the FPSO is anchored and weathervaning, the offtake tanker loading in tandem must match this movement. In this case the operational area consists of a target 'box' located on the FPSOs stern reference point. The tanker carries a mythical 'bowsprit', the end of which must be maintained within the target box. Provided the bowsprit end point remains within the box, the tanker maintains a fixed position. This position is adjusted as and when the bowsprit end breaks out of the box as the FPSO moves. A similar facility may be used by a DP supply vessel working the FPSO.
2.3.3 The autotrack function
The autotrack facility allows a vessel to track slowly along a predefined line, itself defined by waypoints. This facility may be useful in vessels conducting cable- or pipe- lay operations, dredging, rockdumping or survey operations. Whatever the type of operation, a comprehensive plan of the track will be compiled, with full details of vessel speed and heading on each leg of the track, and points where the tracking may need to be temporarily suspended. A numbered listing of the track waypoints and their co- ordinates is compiled, with due attention to such details as chart datum and co-ordinate frame in use. The track file may be compiled by the surveyor, away from the bridge, and imported into the DP system. If this is the case, then all parameters of the imported file must be carefully checked.
The DP system autotrack function is accessed via a complex menu system. The DPO can enter data manually from his track table or plan using a facility called 'track table' or 'track editor'; this facility will allow entry or deletion of waypoints, insertion of new waypoints at any point, entry of values of vessel heading and speed on each leg. Once a track has been compiled (often considerably in advance of the operation), it can be saved and
Dynamic positioning principles and systems
recalled at a future time.
A great variety of autotrack settings and configurations is available; DPOs must familiarise themselves with all the permutations available. They must also know the exact reaction of the vessel to each of the menu choices. The DPO specifies how the vessel will handle a waypoint passing (does the vessel stop on the waypoint, or does she proceed around a radiused turn, and does she slow down on the turn?) and any track offset. Offset facilities may be useful for making small adjustments to the track placement while the tracking is underway, and a variety of alternative offset strategies is available.
A further variation on the autotrack theme, is the 'move-up' function. If operating a pipelay vessel, it may be necessary to move the vessel forward a distance equal to one or two pipe-joints (12m or 24m) at frequent intervals. Having compiled the autotrack, the vessel can be operated in the move-up mode, with a single move of the required distance initiated by a button-push. The commonest mode of autotrack operation is the low speed option. Here, vessel heading is under the full control of the operator - the vessel does not necessarily sail along the track in a conventional bow-first manner. The vessel heading may be adjusted to give a lee for the operational elements, or the vessel may maintain a weathervane or minimum-power heading during the tracking. Vessel speed is limited to 3 or 4 knots in this configuration. An alternative option is'high speed' autotrack, or 'auto-sail', in which the full cruising speed of the vessel is available. In this configuration vessel heading is dictated by the system, the vessel navigating in a more conventional bow-first manner.
It is important that DPOs and all concerned are familiar and practised with the operation of the autotrack function. The commencement of an operation using autotrack is not
12 Chapter 2
DP Operator's Handbook
the time to investigate and practice the facility. A problem that has arisen in the past concerns a long East/West track which spans more than one UTM zone, and what happens when the zone border is crossed. Points such as this must be resolved ahead of time.
2.3.4 Riser angle mode
This function is vitally important in deep-water drilling operations, in which the critical factor is the management of riser angle. The riser is the pipe containing the drillstring, supported from the drillship and connected to the sea floor wellhead or 'stack' assembly. This connection is known as the lower flex-joint (LFJ) and conducts the rotating drillstring through to the well. It is vital that the riser be perpendicular to the stack, otherwise damage will occur to the drillstring and wellhead components. Riser angle is monitored via sensors located on the LFJ, and fed back to the pilothouse and the DP system. Typical angular criteria are 3° (yellow) and 7° (red). Up to 3° drilling proceeds normally. If riser angle reaches 3°, drilling stops and preparations are made for a riser
Dynamic positioning principles and systems
disconnect operation, which must be initiated if the riser angle reaches 7°. Riser angle is affected by tidal flow, the riser 'bows' down-tide. The DPO compensates for this by moving the vessel. In deep water there may be a complex tidal shear pattern, resulting in an irregular riser profile. In riser angle mode, the DP system receives feedback from the riser management system, and displays the 3° and 7° watch circles on-screen.
These circles will drift on-screen as tidal conditions change, and the DPO will adjust the vessel position and heading in order to maintain angles within limits. Thus, in this type of operation, the DPO is less concerned with the vessel's actual position but more responsive to the riser feedback.
2.3.5 Shuttle tanker functions
Some general information on shuttle taker operations is given in Chapter 4 of this handbook. Obviously, this is a specialist DP configuration, not found or required in other types of vessel, and many of the DP functions are also specialised. A shuttle tanker will always work in a weathervane mode; the power-to-weight ratio of these vessels precludes adopting any other heading. The tanker will be configured to load from one or more specific Offshore Loading Terminals (OLTs) which may be fixed tower structures, floating towers, submerged turrets or FPSOs.The position and other characteristics of the OLT are contained within a file in the DP system, accessed from a 'Select OLT' menu.
The vessel will approach from a down-wind or down-tide direction, keeping the OLT ahead, transferring into DP control at an appropriate point (usually outside the 500m zone). The DP system will have an 'auto-approach' function, allowing the DPO to progressively reduce the distance from the OLT. All the time the vessel is maintaining a weathervane, or minimum power heading. Once on the defined position circle, at the correct distance from the OLT for the loading phase, the DP is selected into the 'Loading' function. With the loading hose connected, the vessel maintains a fixed distance from the OLT while weathervaning around that point. Its position is continually adjusted to keep the OLT ahead and the vessel on the minimum-power heading. If loading is from a submerged turret, the position of the turret is monitored by underwater acoustic methods, as part of the vessel's hydroacoustic position reference (HPR). Special displays show the position relationship between the turret and the vessel.
The vessel's reference point will be the docking cone in the forepart of the vessel's bottom. Once the turret is recovered and locked into the cone, the vessel may simply stop all thrusters and propellers, and weathervane naturally, anchored to the cone. This is only possible if weather and tidal conditions are light to moderate. If more severe conditions are experienced, the DP system may be brought in use in a 'damping' mode.
Many offshore off-take operations are from FPSO installations. Here, the tanker maintains a position in tandem with the anchored FPSO, and must match her movements and heading. The FPSO is not normally DP-capable, but may have thruster control of heading,
14 Chapter 2
DP Operator's Handbook
within environmental limits. The approaching tanker may request the FPSO adopt a specific heading for the approach phase. The auto-approach facility will be used (see above) until the vessel is at the desired location relative to the FPSO stern reference point. The tanker's' loading' function will then allow a target-follow facility using a position box as described in 3.2 above. Set-point heading for the tanker is the FPSO heading, which must be monitored via telemetry in the tanker.
2.3.6 Auto-area mode
A further variation on the automatic control of positioning is 'Auto-area'. In this function, the DPO is able to specify a geographical area of any desired size, usually circular but may also be elliptical. This becomes a 'loiter'area, in which the vessel drifts with no thruster commands. When the vessel crosses the area boundary, power is gently applied, and the vessel is slowly restored to the centre of the area.
2.3.7 Other DP functions and facilities
It is not the intention here to provide a comprehensive description of all available DP functions, but an overview of common facilities may be useful.
Dynamic positioning principles and systems
2.3.8 External force compensation
Some vessels under DP control experience a variety of external forces which need to be compensated. An example of this is the pipe construction vessel, laying pipe using a Stinger assembly at the stern. The stinger is a rigid support ramp taking the pipe from the pipe deck into the water. From the end of the stinger to the pipe touch-down point on the sea floor the pipe is unsupported and must be kept under tension to maintain the correct catenary profile. To enable pipe tension to be maintained, the pipelay engine is programmed to pipe feed tension values back to the DP system. Thus the DP system and DPO are able to manage the thrust requirements enabling compensation. Other similar external force compensation facilities may be provided for cable plough operations, or on fire monitors.
If this compensation facility were not enabled, the external forces would be treated as false current.
2.3.9 Quick-current or fast learn
This facility provides a partial solution to the problem arising during rapid turn-of-tide situations. The tide or current value is deduced by the system, not measured directly. The accuracy of the current value is dependent upon the quality of the mathematical model.
If the tide turns quickly the model will lag behind, as time is required for new model data to build. This results in a deterioration of positioning ability. Modern DP systems incorporate a facility allowing a temporary acceleration of the model-build process, which may improve the positioning ability during this time. It must be emphasised that this facility is not perfect or foolproof, and there may still be some positional instability during such slack-water periods.
2.3.10 DP capability plot
DP operational handbooks provide capability diagrams which give an approximate indication of the capability of the vessel in a variety of weather conditions. The value of these diagrams is sometimes questionable, since the vessel and/or environmental conditions rarely match those tabulated in the diagrams. A better assessment of the capability is given by the online capability plot. This is a similar diagram calculated 'live' by the system on demand. DPOs must specify sea state and current values. They must also input a variety of degraded-status failure modes relating to power plant and propulsion.
Once set up, the facility calculates and displays a capability plot for that set of conditions. The operator must be aware of the limitations of this display, as it is still only a theoretical, computer simulation of the vessel's capability, and not obtained from real observation.
16 Chapter 2
DP Operator's Handbook
2.3.11 Drift-off calculation
This facility, when set up correctly, will show the drift-off profile related to an operator- specified failure mode, if, for example, a total blackout is selected, the display will show the predicted positions and headings of the vessel at specific time intervals (eg every minute) subsequent to the blackout.
2.3.12 Autopilot and transit mode
Frequently, the DP system incorporates autopilot functions. When the vessel is in transit, there will be a dedicated thruster configuration. As an example, a vessel with three azimuth thrusters aft may, in autopilot mode, have the two wing thrusters locked for forward thrust, while the centre thruster is enabled to steer with azimuth angle limited to 35 .
Dynamic positioning principles and systems
2.4 Sensors and peripheral equipment
Feedback is required in respect to the environment in which the vessel is operating.
The DP controllers need continuous and accurate data on vessel Roll and Pitch angular values. This data is provided by a motion reference unit, or MRU. An inertial motion sensor, this device outputs not only roll and pitch data, but also heave values, together with rate (velocity) data on each of the three measured values. In earlier installations this function was provided by a device called a vertical reference sensor or unit (VRS or VRU). Instantaneous roll and pitch data is required by the processors in order to correct data input from a variety of PRS which measure vessel position as an analogue of a vertical angle (taut wire systems and underwater acoustic systems). In these PRS, vertical angle is measured by a sensor that rolls and pitches with the ship; if no correction were applied, the processor would interpret this as a moving vessel. Roll and pitch data input allows these angles to be corrected or reduced to the true vertical.
The other element of the environment requiring measurement is the wind, and a number of transmitting anemometers are installed. Usually fitted in (at least) duplicate, windsensors may be of the traditional rotating-cup type, or may be of the more modern ultrasonic variety. These read data into the mathematical model allowing wind loads on vessel hull and superstructure to be computed and compensated for. However, there is a problem. Wind conditions can change rapidly and radically; gusting conditions will adversely affect the DP performance of the vessel. It is necessary that the system reacts rapidly to large changes in wind speed and direction. The mathematical mode! does not serve to best effect here, as there is a significant delay between changes and system reaction. This is because, as with most elements that change within the vessel model environment, the changes take place slowly (eg vessel displacement, tide etc). The model takes 5 to 30 minutes to update, and with most elements this is appropriate. However, gusting wind conditions can play havoc.
The DP system compensates for this problem with a function known as 'wind feed- forward', in which radical changes in wind speed and/or direction by-pass the mathematical model and generate compensating thrust directly. In some DP systems this is known as 'gust-thruster compensation'. It is important to realise that for this compensation to give satisfactory results, the windsensors must be reading representative values for the wind. The DPO must ensure that the windsensor selected, of the array available, is reading clear wind, unobstructed by any windshadow from structure.
The other environmental value needed is current, but it is not possible to obtain representative values for the current from any vessel-mounted sensor. Instead, current values are deduced or estimated. In effect, the current value shown on-screen is what forces remain when all known forces are accounted for. A continuous discrepancy between 'predicted position' within the mathematical model, and 'measured position' derived from the PRS indicates a current. The DPO must be aware that this 'current' value is only a deduced value and not a real measurement, and thus is subject to error. In fact,
18 Chapter 2
DP Operator's Handbook
any error within the DP system will be incorporated into the displayed current value. One DP system manufacturer does not call this vector 'current' at all - it is referred to as 'sea force' and displayed in tonnes. Errors within the system which may affect the accuracy of current display include erroneous thruster pitch feedback, or erroneous windsensor data, perhaps from a jammed wind-vane unit. In specialist-function vessels, other external sensors providing the processors with feedback data include measurement of external forces and measurement of riser- angle (in a drillship). External forces may take a number of forms. In a pipelay vessel, pipe tension must be maintained at pre-set values as the lay progresses. Pipe tension is monitored at the pipe tensioners, and fed directly into the DP system, allowing direct and immediate thrust compensation for tension and tension changes. In a cable-lay vessel towing a cable plough, hawser tension is monitored and fed back in a similar manner to pipe tension. A shuttle tanker tandem-loading from an FPSO unit ahead will monitor hawser tension and make similar compensation.
2.5 Some DP problem areas
As with any system, DP has its limitations. It is vitally important that the DPO is familiar with these limitations and problem areas. Some of them will be described here:
2.5.1 Operations in shallow water and strong tidal conditions
For a variety of reasons, DP vessels may not perform well in shallow water or strong tidal conditions. One of those reasons is the lack of efficiency of some position-reference systems in shallow waters. The PRS affected are taut wire systems, and hydroacoustic systems.The taut-wire system becomes of limited value in shallow water as a result of its limited horizontal scope, or range. The maximum allowable wire angle is typically 28° to the vertical, so if there are only a few metres under the keel, horizontal range is also only a few metres. Hydroacoustic systems will also suffer limitations in range capability due to considerations of ray-path angle; the flatter the path the greater the distortion and losses due to refraction. In shallow waters, acoustic systems will also suffer proportionally greater levels of thruster noise interference. In general, it is always wise to avoid the use of underwater PRS in shallow water.
If there are strong currents or tides (often the case in shallow water) the immediate effect is that the vessel will be using larger amounts of power and thrust in order to maintain position and heading. Consequently there will be smaller amounts of both in reserve, affecting the redundancy considerations, if the vessel is working harder to maintain position, she will undoubtedly be creating greater levels of noise and water
Dynamic positioning principles and systems
turbulence, with further detrimental effects on the performance of hydroacoustic position reference. As mentioned in 2.3.9 above, DP systems perform badly during periods of rapid turn of tide, and this is often a feature of areas where depths are limited and tidal conditions are severe. It must also be mentioned that many underwater operations are adversely affected by strong tidal conditions and shallow water, particularly diving and ROV operations. Any such operation must be the subject of a full risk-assessment process, with all hazards identified and assessed.
2.5.2 Operations in close proximity to fixed structure
Offshore support vessels of all types routinely work in close proximity to fixed platform structures. The main and immediate hazard is that of contact with the structure. DPOs must ensure that they have an immediate and direct view of the nearby structure. It is an axiom of DP work that a positioning problem will usually first be apparent to the Mk 1 eyeball before instruments and alarms start to react. This is, of course, always assuming that DPOs make use of their Mk 1 s! The nearby presence of a large fixed structure will have detrimental effects on the DGPS position reference. Satellite signals may be blocked by structure causing a reduction in satellite availability. There will be greater levels of multi-path reception caused by reflection of satellite signals from structure. Both of these mentioned effects result in degradation of DGPS performance. Further, the differential correction signal may be lost due to shadow from the platform structure. All of this adds up to a simple truth: DGPS cannot be regarded as a reliable PRS in close proximity to platform structures.
A further concern, when working in such close proximity, is the interference caused to windsensor readings. If a vessel is working close-in on the downwind side of a platform, the windsensor may be badly windshadowed, and not therefore delivering representative wind data. The vessel herself may be feeling the full effects of the wind blowing through the legs of the platform, thus the wind forces are not being properly monitored. The result may be a deterioration in positioning efficiency, and there is no real cure for this problem.
2.5.3 Operations in close proximity to other vessels
DP vessels are by their nature restricted in their ability to manoeuvre, and must always display the appropriate signals. Sometimes in the offshore theatre there will be a number of vessels working in close proximity to each other. A number of hazards exist, chief among which is the risk of collision. DPOs must always bear in mind that any vessel in the area, including his own, may suffer total power loss or blackout. Do they have a
20 Chapter 2
DP Operator's Handbook
contingency plan for this? That supply boat working the platform crane - if she suffers blackout, will she drift down on us? And is there any action we can take? If own vessel is position-constrained in a safety-critical task, then it might be necessary to suspend operations until a safer situation arises. Two DP vessels working in close proximity might generate problems also. Thruster wash from one vessel may disturb the positioning of the other, and vice-versa. This could lead to thruster-thruster interference and positional loss by one or both vessels. A nearby vessel may cause line-of-sight loss on laser or microwave based PRS. Other position- reference related problems might occur; if one vessel is using underwater acoustics, the thruster noise or aeration of the other vessel might block acoustic returns. All of these problems should be considered in the risk assessment for the task.
Chapter 3 21
Redundancy and equipment class
Chapter 3
Redundancy and equipment class
3.1 The need for redundancy
DP vessels undertake many tasks and operations, some of which are more safety- critical than others. In any operation the consequences of a DP failure must be taken into account. A DP failure may result in a simple positional excursion (a 'drift-off') or a powered drive-off. Both are regarded as catastrophic failures. The consequences of such a catastrophic failure can be categorised three ways; risk of death or injury to personnel, risk of damage to property or risk of pollution.
Redundancy arrangements within a system ensure that system function remains subsequent to the loss of any single element or subsystem. The function of redundancy is to prevent a catastrophic failure, ie a loss of position and/or heading. By the term 'redundancy' we mean the ability of the vessel to withstand the loss of any component within the DP system without losing position and/or heading. A more practical view of redundancy provision in a DP-capable vessel might be that redundancy arrangements allow the operation or task to be safely suspended, and the vessel to safely exit the worksite, still under the control of the DP system.
Often, especially in safety-critical operations, the task cannot safely be suspended without continued provision of DP ability. Most operations cannot be continued after the loss of DP ability, and in many operations, the DP ability is essential for the safe suspension of the task and vessel worksite exit. In effect, redundancy arrangements within the DP system provide a time-frame in which the operation can be safely suspended and the vessel to move to a position of safety after the failure of a critical component.
A number of methods for the provision of redundancy are available. The most common is to provide back-up or stand-by facilities, a good example here being the DP system control computers or processors. A redundant provision might be two processors, one 'on-line'and the other on 'hot stand-by'. This is a common arrangement in many vessels, a further requirement is a 'bumpless transfer' between units, ie if the on-line processor fails, the stand-by unit takes command automatically without any change in the vessel status, including position and heading. Subsequent to such an event, the vessel is still under DP
22 Chapter 3
DP Operator's Handbook
control and can safely move to a position of safety, albeit no longer redundant in the area of computers.
3.2 The I MO guidelines
The need for redundancy will be indicated by a variety of means. The Guidelines for Vessels with Dynamic Positioning Systems is published by the IMO and specifies three levels of redundancy known as equipment classes. Three equipment classes exist; the higher the class number, the greater the level of redundancy. Vessels may be described as compliant with equipment Class 1, 2 or 3. These guidelines, issued In 1994 (MSC Circ 645) have become accepted as a world industry standard. DP vessels will be designed for compliance with a particular equipment class, and will be issued with an appropriate notation by the classification society. Each classification society has class notations equivalent to the IMO classes.
The IMO guidelines introduce the concept of the single point failure. Redundant operation allows for the continuance of position and heading ability after any single point failure. The equipment classes are described in terms of the effects of worst-case single point failure. The definitions are 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. For equipment Class 3, a single failure includes: items as 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.
Thus, vessels of equipment Class 1 will not be fully redundant in every area. Vessels of equipment Class 2 have full redundancy in equipment and systems, while Class 3-designated vessels are capable of maintaining position and heading after the loss of all components in any watertight compartment or fire subdivision. In Class 3 a single-point failure might be the loss of one complete compartment. The choice of equipment class determines which vessel is chartered for any particular operation or contract. The greater the level of risk associated with the operation, the higher the equipment Class of the vessel tasked for that operation. The IMO guidelines state:
The equipment class of the vessel required for a particular operation should be agreed THE NAUTICAL
Redundancy and equipment class
between the owner of the vessel and the customer based upon a risk analysis of the consequence of a loss of position. Else the Administration or coastal state may decide the equipment class for the particular operation.
Thus the vessel owner and client must design the operation incorporating a full risk assessment process and safety case. The results of the risk analysis will indicate the consequences of a loss of position and/or heading, allowing a decision to be made as to the equipment class of vessel required for that operation. The more severe the consequences within the three categories (death/injury, damage, pollution) the higher the equipment class vessel deemed necessary.
Equipment class certification for the vessel may consist of a class certificate issued by the vessel's classification society, with the appropriate class notation, eg if a DP vessel is classed at Lloyd's Register of Shipping, the appropriate notations for Classes 1, 2 and 3 are respectively DP(AM), DP(AA), and DP(AAA). In addition to this the vessel may carry an FSVAD (flag state verification and acceptance document) issued by the IMO, again stating clearly the equipment class. Such FSVAD documents are generally issued by the classification society on behalf of the IMO.
Further guidance on the provision of redundancy is given in Guidelines for the Design and Operation of Dynamically Positioned Vessels, published in February 1999, by the International Marine Contractors Association (IMCA) as M 103. Other relevant documents from IMCA are M 161, a supplement to the above guidelines covering two-vessel operations, published in February 2001, and M 182, the International Guidelines for the safe operation of DP Offshore Supply Vessels, published in March 2006.
3.3 The provision of Redundancy
Hereafter follow a few comments upon the provision of redundancy at the various equipment class levels in respect of the major elements of the DP system, ie controllers, power systems, thrusters and propellers, position, heading and environment reference systems.
3.3.1 Redundancy in controllers
Computers are the heart and brain of any DP system. For Class 1 a single controlling computer is adequate, but for Class 2, two parallel identical computers are installed. Each is running independently in parallel, receiving the same feedback data and performing the same computations. One is 'on-line' while the other is the back-up. Each continually monitors or 'watch-dogs' the other, such that, if the two units are not running identically, then an 'A-B difference' warning can be initiated. This is important, because if such an A-B difference exists then, effectively, redundancy has been lost. A requirement of this type
24 Chapter 3
DP Operator's Handbook
of installation is that of 'bumpless transfer' between computers, ie if on-line fails and the back-up takes over, the vessel's position, heading and status are unaffected.
A more extensive installation may incorporate triple modular redundancy (TMR), providing greater security, but not necessarily sufficient redundancy to qualify for Class 3. In such a triple system, three computers are provided, all running in parallel; one on- line and two as stand-by. The arrangement allows the application of voting logic, in which every calculation is compared with the corresponding outcomes from the other two processors. If a malfunction occurs, then it can be automatically detected with the errant unit isolated. Such a system is logically allied to triplicated sensors and position references, such that the voting ability applies to all systems. For equipment Class 2, there must be available three independent position references and three gyro compasses. In DP system terminology, a single-computer system is referred to as 'simplex', a Duplex system refers to those with dual computers, while a system as described above with three computers is referred to as 'triplex'.
The triplex system described above does not necessarily comply with the requirements for equipment Class 3. In a Class 3 vessel, there must be a separate control computer located in a separate compartment. The essential point of Class 3 operations is that of subdivision, or compartmentalisation. The IMO requirement for Class 3 is that a single point failure includes all components contained within any single compartment.
Redundancy and equipment class
FULLY REDUNDANT (CLASS 3) SYSTEM LAYOUT
Thus, for compliance with the requirements of Class 3, a separate, independent control computer must be located remotely from the main, fully redundant computer system. In practice this means a fully redundant, two-computer system installed on the main bridge, with an independent back-up DP system including computer, located at a remote location. A more extensive system may comprise a main Triplex system, backed up by a Simplex system remotely located.
3.3.2 Redundancy in Position and Heading Reference
For equipment Class 1, a minimum of two position references must be installed. This seems to be in excess of the generally minimum requirements for the (otherwise
non-26 Chapter 3
DP Operator's Handbook
Table 12 - Provision of redundancy
THE NAUTICAL INSTITUTE
SUBSYSTEM OR
COMPONENT MINIMUM REQUIREMEN TS FOR GROUP DESIGNATION
Class notations: IMO EQUIPMENT CLASS 1 2 3
DNV AUT AUTR AUTRO
LR DP (AM) DP (AA) DP (AAA)
ABS DPS-1 DPS-2 DPS-3
Power System Generators and prime movers
non-redundant redundant Redundant separate compartments Main switchboard 1 1 with
bus tie 2 with normally open bus ties in separate
compartments Bus tie breaker 0 1 2
Distribution system
non-redundant redundant Redundant separate compartments Power management no yes yes
Thrusters Arrangement of thrusters
non-redundant redundant Redundant separate compartments Control Auto control: number
of control computers
1 2 2+1 in alternative control station Manual control:
joystick with auto heading
yes yes yes
Single levers for each thruster
yes yes yes
Sensors Pos. reference
systems 2 3 3, including 1 in alternative control station. External sensors wind 2 2 VRS 2 2 gyro 1 3 3 other 1 2 2 UPS 1 1 1 +1 in separate compartment Alternative
Redundancy and equipment class
redundant) Class 1. Operation with a single position-reference has a specific danger; that of PRS freeze. If the input data from a position reference freezes, ie the data simply continues incoming at fixed values, the common resut is that the vessel will drive-off location. Invariably, the vessel will be some small distance from her set-point position when the freeze occurs. The DP system will issue thrust commands to make up the position deviation. The position-reference will continue to give constant data indicating the vessel stationary; in reality she is driving steadily away from set-point. For heading reference, Class 1 is satisfied by a single gyro compass.
For Class 2 and 3, the equipment class requirement is for three independent position reference systems and three gyro compasses. This gives the voting capability referred to above, and allows automatic identification and rejection of an errant position reference or compass. Three is better than two in one major respect; if a function is duplicated, there is always the question 'which one has failed' when the two disagree. That question can only be answered by operator inspection and intervention. Triplication of gyro compasses and position-references provides a partial solution to this problem. It is still possible to defeat the voting.
The DPO must be vigilant to avoid the 'common-mode failure' syndrome. For example, a particular vessel may be using two independent DGPS and a fanbeam laser system for position reference. This in theory complies with the requirements for Class 2 or 3, but there is a major shortcoming to this arrangement. If both DGPS suffer a drift problem, because of the same root cause, then the voting is likely to cause automatic rejection of the Fanbeam - the only system at that moment providing correct position data.
It is always good advice to use three position references, all of which operate on different principles, thus reducing the exposure to common-mode failure conditions. Even so, it is still possible to defeat the system. A vessel might be using three position-references: taut-wire, hydroacoustic position reference, and DGPS. The operator has decided it would be most convenient if the underwater beacon which is the basis of the hydroacoustic system were secured to the taut-wire depressor weight. However, if the weight drags, the beacon goes with it, and the position-reference which is outvoted is the DGPS - again, the only good one at that moment.
Further good advice is to avoid the use of purely underwater position references, ie two taut wires and a hydroacoustic position reference is not a good solution. In rough sea conditions all three can become unreliable.
If working to Class 2 or 3, it is always advisable to consider deployment of four position references instead of three. If three references are in use and one fails, the vessel is immediately out-of-class, and may have to suspend operations until another reference is deployed and on-line. If, however, four references were in use, and one was lost, then the vessel is still operational.
28 Chapter 3
DP Operator's Handbook
For heading reference, gyro compasses are used, triplicated for Class 2 and 3. In the Class 3 vessel, one gyro must be remotely located to comply with the subdivision requirements.
3.3.3 Propulsion redundancy
For a diesel-electric vessel of Class 2 or 3, the worst-case single-point failure will be the loss of one complete busbar, one complete section of switchboard. This results in the loss of all propeller and thruster units that were powered from that bus. It is thus necessary to distribute power such that subsequent to worst-case failure, sufficient thrust capability remains to manoeuvre the vessel. At its simplest, if a vessel has three propellers at the bow, and three at the stern, powered from a split switchboard, then the loss of one bus leaves three thrusters intact, at least one at the bow and one at the stern. The intact thrust capability must be capable of effective manoeuvrability.
The DPO must be continually aware of the worst-case single point failure mode relative to propulsion. Modern DP systems are configured to give a consequence analysis against this eventuality.
Redundancy and equipment class
3.3.4 Consequence analysis
In respect of the power and propulsion configuration, DP systems are able to provide an on-line consequence analysis of worst-case single point failure mode. The IMO guidelines state that such a software programme must be running when operating in Class 2 or 3 states. Typically, the consequence analysis programme will repeat its calculation once per minute. Initially the programme scans the switchboard and propulsion configuration, and determines what the 'worst-case' mode is. It then simulates this failure mode, with an indication of the outcome. If the outcome indicates that the vessel will not hold position and/or heading, an alarm will be generated, such as Consequence analysis drift-off alarm.
If no such alarms or warnings are activated, the DPO can assume that power/propulsion redundancy is intact, although it is always wise to keep a watching brief on the power and thrust loads.
3.3.5 Power plant
This is not the place for a detailed discussion on the topic of machinery space redundancy, but a few brief comments may be appropriate.
The overall layout of DP vessels' power plants will be designed with redundancy in mind.
The diesel-electric configuration is particularly suited to the provision of redundancy. In this type of installation, a number of diesel generators deliver power to the switchboards, so the loss or non-availability of any one diesel is not catastrophic. For Class 3 operations, the machinery space and switchboard rooms will be subdivided, such that a fire will not result in total incapacity. Further, with a number of diesels, flexibility exists in the day- to-day configuration. The number of generators on the board can be varied to suit the loads, redundancy and contingency power existing.
Other systems in the machinery space requiring redundancy include the fuel and cooling system. Arrangements will be in place to prevent water contamination of the ready-use fuel tanks from causing all engines to stop. The cooling system may have an increased redundancy ability compared with that in a non-DP vessel. It must be said, however, that significant redundancy exists in conventional ships, as required by the classification society rules; it is not just DP vessels that have redundancy built-in!
The power distribution layout is designed for redundant operation. The configuration of switchboards, bus tie switches, thrusters and generators will comply with the equipment class requirements. For compliance with Class 3 the bus-tie switches must be open, such that each switchboard section operates in isolation, and a fault cannot transfer across the boards resulting in total blackout. For Class 2 operation, but-tie switches may be open or closed. With bus-tie switches closed there is greater flexibility in terms of power- sharing across the switchboard resulting in more economical operation. If a fault occurs on one bus, the appropriate bus-tie switches should trip, isolating the faulty section.
30 Chapter 3
DP Operator's Handbook
Nevertheless, the possibility exists of total blackout if the bus-tie switches themselves fail. All power systems should be under the control and monitoring of an efficient power management system. Low-voltage electrical systems such as computers, bridge consoles and DP peripherals should be power protected via UPS systems. Another design feature contributing to redundant ability is the separate routeing of cabling and pipelines.
3.3.6 Environment Reference
Equipment such as motion reference units (MRU) and windsensors is provided in duplicate to satisfy the requirements of Class 2 and 3. These devices provide data feedback to the DP system, and the loss of this data is usually of lesser impact than the loss of more critical elements. Simple duplication is considered adequate, although triplication is possible.
In general, DPOs must be very familiar with the redundancy arrangements of their vessels, in every area. They must be familiar with any weak points or shortcomings in the particular vessel's redundancy arrangements. They must question redundancy arrangements for modifications or newly installed equipment, for example. If new position-reference systems are installed, are the power supplies properly protected and redundant? The vessel needs to operate with one thruster stopped for operational reasons, has this affected the overall redundancy of the vessel manoeuvrability? What switchboard protection systems are in place and how regularly are they tested? How often are the UPS systems tested? This is the type of question the DPO should ask and obtain answers to.
DP vessel operations
Chapter 4
DP vessel operations
4.1 The range of applications for DP
When DP was in its infancy, the variety of vessels using the positioning ability was limited to deep-water drillships, but the technique rapidly became more widespread. DP soon became a standard feature in a number of vessel types, and the number of applications to which DP- capable vessels were being used increased rapidly. In the early days, DP capability was limited almost exclusively to those vessels engaged in the offshore oil and gas industries, in recent years however, many other areas of commercial and military shipping are using DP techniques. One reason for this is the tendency for modern vessels to feature a fully integrated vessel control system, combining all vessel monitoring and control functions. DP itself is best described as an integration of a variety of vessel functions (position and heading reference, propulsion, power, environment); so it is comparatively simple (and cheap) to include a DP ability at the design stage. All modern design platform supply vessels and anchor-handling tugs incorporate a DP capability in their design. A few years ago this would be an 'optional extra'. What is prohibitively expensive is the conversion of an existing non-DP vessel into a DP-capable vessel.
What follows is a description of a number of operations in which DP-capable vessels are regularly engaged, with some detail as to the practical application of DP.
4.2
FPSO and Tanker Offtake operations
Shuttle tanker operations may be divided into four groups; systems with hawser moorings, hawserless systems, and submerged turret loading (STL) systems. The fourth grouping consists of those vessels configured to load directly from FPSO installations (floating production, storage and offtake units).
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
32 Chapter 4
DP Operator's Handbook
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 DP-capable tankers.
DP shuttle tankers operate on a position-circle/weathervaning principle. The vessel will position with her bow touching an imaginary circle, centred upon 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.
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, two aft, and single or twin screw main propellers. The DP system may be installed on the main bridge, aft, or in the bow house. Modern shuttle tankers normally feature redundancy levels to Class 2, but many earlier vessels are to Class 1 only.
Position references used may include DGPS, HPR, a Laser system and Artemis. An important consideration here is the distinction between absolute position references, and relative ones. A tanker loading from an offshore terminal needs to maintain position relative to the terminal. If the terminal is mobile in any way, the tanker must match that movement. Many OLTs are floating anchored spar buoys which have motion
DP vessel operations
characteristics. Other terminals are FPSOs (see below) which are also anchored in a weathervane mode. In both cases, the offtake tanker needs position reference relative to the moving OLT. Laser-based systems and the Artemis system are both relative position references used in these circumstances.
The shuttle tanker will transfer into DP mode early in the approach, allowing a controlled hawser pickup. Once onto the position circle at the designated distance from the OLT, the hawser is latched and the hose connected. A diagram, specific to the OLT, shows the maximum environmental criteria for approach and hook-up, together with the position/ heading criteria for ESD (emergency shutdown and disconnection).
34 Chapter 4
DP Operator's Handbook
vessel operations
A variation upon this theme is the STL (submerged turret loading) 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. A number of fields are configured for STL operations.
A further variation is the FPSO tandem loading arrangement. A floating production, storage and offtake 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 tanker. The tanker will 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 will herself be weathervaning. Position reference for the offtake tanker will usually be a combination of Artemis, a Laser system and relative GPS (such as the DARPS system). Both these PRS are relative in nature, as the offtake tanker is positioning in relation to a mobile point.
Chapter 4 35 DP vessel operations
A variation upon this theme is the STL (submerged turret loading) 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. A number of fields are configured for STL operations.
A further variation is the FPSO tandem loading arrangement. A floating production, storage and offtake 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 tanker. The tanker will 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 will herself be weathervaning. Position reference for the offtake tanker will usually be a combination of Artemis, a Laser system and relative GPS (such as the DARPS system). Both these PRS are relative in nature, as the offtake tanker is positioning in relation to a mobile point.