Cable Laying Vessels: a Review of the Classification, Statutory and Station Keeping Aspects

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Dynamic Positioning Committee

Marine Technology Society P

DYNAMIC POSITIONING CONFERENCE September 18-19, 2001

VERIFICATION, TESTING AND TRIALS

Cable Laying Vessels: A Review Of The Classification, Statutory And Station Keeping Aspects

Andrew McKinven LR Americas Inc.

Andrew McKinven, Lloyd’s Register Cable laying vessels Return to Session Directory

Cable laying vessels: A review of the classification, statutory and station keeping aspects

INTRODUCTION Section 4 focuses on the more popular, yet Section 1 of the paper provides an overview more onerous, LR DP(AA) notation (IMO of the development of cable laying vessels Class 2 equivalent), identifying the design from their earliest designs to those of the requirements for the proprietary DP control present day; particular reference is made to system and the ship's engineering systems. four Maersk vessels which recently entered Section 5 builds upon previous sections, into Lloyd’s Register (LR) Class. This discussing the requirement for a Failure section continues with a review of the Mode and Effects Analysis (FMEA) to be differences between modern cable laying conducted and the subsequent functional vessels and general cargo vessels before testing during sea trials. The final section considering how cable layers differ as a ship provides an overview of the key issues type. Section 2 addresses the relationship presented within the paper; particular between vessel operational / design emphasis is given to the relationship characteristics and the applicable between vessel operational characteristics / Classification and Statutory requirements. design and the relevant Statutory and The paper moves on in Section 3 to consider Classification requirements. the use of dynamic positioning during cable laying and the proprietary DP systems which are available to facilitate these operations.

Figure No. 1: HMTS Monarch

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1.0 BACKGROUND Figure No. 1 illustrates the almost A good example of an earlier design of cable symmetric arrangement of the ship: sheaves; laying vessel was His Majesty's Telegraph paying out gear; and dynamometer located (HMTS) Monarch (Figure No. 1) fwd. and aft. Much of the equipment e.g. constructed in the early 1940s at the Swan cable gear would have been steam driven – Hunters shipyard for the British Post Office. almost all vessel loads / systems are now At the time of build, it was the largest cable either completely or partially dependent layer in the world (8,056 gross tons), with upon some form of electrical power supply. the capability to lay and repair submarine Of course, the older vessels would not have telegraph cables, utilising a total cable had any form of DP capability. The capacity of 2,000 miles. Historically, a introduction of the first LR Rules for distinction was made between cable layers Dynamic Positioning in 1982, does however and cable repairers – purely based upon size. provide us with an indication as to when Most cable repair ships could lay vessels started to develop dynamic approximately 500 miles of cable when positioning capabilities. With respect to the repairing a cable fault, whereas cable laying structure of the vessel, it can be seen that the ships could lay up to 2,000 miles of new accommodation and wheelhouse decks are cable and also, if required, perform cable located above the cable tanks – introducing repairing functions on existing cables. the possibility of racking. In contrast with modern cable laying vessels, deck loads The complement of the HMTS Monarch would have been small in comparison. The comprised the master, thirty officers and one recent boom in the telecommunications hundred and seven men - laying and sector has resulted in a sharp increase in the repairing cables was a very labour intensive number of newbuild cable laying vessels and activity. conversions from other

Figure No. 2: Maersk Recorder (Courtesy of Maersk Shipping)

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suitable ship types. Over 28% of the world’s • additional generating capacity to fleet of cable laying vessels are classed by supply the large electric loads, e.g. Lloyd’s Register (113 ships – 746,269 GT). thrusters, which are necessary for Ongoing newbuilding projects include a dynamic positioning; vessel for Asean Pacific at the Jurong • dynamic positioning controller – the shipyard in Singapore, with the possibility use of position referencing systems of a second. Four newbuild cable laying and the controlled use of active vessels (constructed for Maersk in thrust help to ensure that cable Germany) entered into Lloyd’s Register tension is not exceeded and that the Class between the end of 2000 and the intended cable route is followed to beginning of 2001, the Maersk Recorder within precise limits. (Figure No. 2) was the first in the series. The DP(AA) notation (IMO Class 2) is Perhaps the first feature for comparison increasingly becoming the notation of between old and new cable laying vessels is choice for DP vessels. The various DP the cable tank capacity. Modern vessels configurations will be discussed later in have a capacity of up to 5,000 kilometres; Section 3, illustrations are provided in this is due, in part, to the fact that old Figure No. 4. It is apparent from Figure 2 submarine telegraph cables were bulkier and the above list that the deck loads have than today’s fibre optic cables, but also due increased dramatically. Therefore, not only to the increased number and size of cable will the deck plating and supporting tanks. structure have to be selected accordingly but the effects of the large open spaces (cable The following features are typical for a tanks) on the vessel's racking strength will modern cable laying vessel: have to be carefully considered. Having • A-frame for deploying the plough developed an appreciation as to the design (ensuring that in shallow waters the of modern cable laying vessels, we can now cable is dug into a trench); examine the differences which exist between • cranes for general loading / cable laying vessels and their nearest cousin: unloading and moving the cable general cargo ships. The primary differences tank hatch covers; are operational, and by implication design. • winches: o large linear winch for controlling Operation the cable, From an operations point of view, it is o towing winch for controlling the necessary that cable laying ships carry a plough; large number of special personnel, in o capstan winch for retrieving the addition to the crew, performing specialised cable. activities: laying and repairing (joining) • containers on deck to house the cables, testing cables and controlling the cable repeater boxes; plough, far more than would be found on • purpose built rooms for joining any normal general cargo ship. The cables, testing cables and implications of this will be examined in controlling the plough; Section 2. • one set of sheaves: pick-up/laying of the cable is from the stern only; • less crew – far less labour intensive (Maersk Recorder had a complement of 60 (14 crew and 46 special personnel);

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Design whilst facilitating operations specific to that With respect to the general structure, the ship type. In the event that a cable laying major difference relates to the large cable vessel (500 gross tonnage and above) were tank spaces which raise potential problems to carry more than 12 special personnel concerning not only racking strength, but (personnel required for particular operations: also localised strength issues at the double cable loading / laying / repairing / testing / bottom, and with respect to the vessel's ploughing) then it can be categorised as a global longitudinal strength. As for the special purpose ship - provided the National engineering design, where the vessel is to be Authority decide to invoke the IMO SPS assigned the DP(AA) notation, engineering Code. However, the SPS Code has, as yet, systems are to be tolerant to a single fault in only been adopted by a few National any active component. For this to be Administrations. demonstrated, it is required that an FMEA be performed. The FMEA should be The SPS Code does not treat special conducted at a stage which allows the results personnel as passengers; they are expected to be taken into account when finalising the to be able bodied, knowledgeable of the ship vessel’s design. A more detailed account of layout and to have received some training in the operational and design aspects particular safety procedures. Therefore, the ships upon to cable laying vessels will be discussed in which they are carried need not be Section 2, along with a summary of considered or treated as passenger ships. It is notations often associated with cable laying worth noting that a special purpose ship can vessels. carry up to 50 special personnel before having to consider: (i) the stability effects of flooding the 2.0 OPERATIONAL/DESIGN machinery space (as a result of damage CHARACTERISTICS vs. impact); or (ii) the SOLAS fire protection and life CLASSIFICATION AND saving appliances requirements for STATUTORY REQUIREMENTS passenger ships. This section discusses in more detail how the operational and design characteristics of At present, some cable layers comply with a cable laying vessel influence the the Offshore Supply Boat Code; the damage applicable Classification and Statutory stability requirements of which are less requirements. onerous. In the event of heavy weather, there is the potential for significant Statutory quantities of sea water to enter a cable In general, a cable laying vessel would be laying vessel via penetrations in the weather subject to the normal cargo ship deck - this would adversely affect the requirements of SOLAS, provided it was loadline (reserve buoyancy of the ship). carrying not more than twelve passengers. Suitably rated and positioned cranes are The main concern from SOLAS would be therefore necessary for moving the hatch the potential effects upon the vessel’s covers into place. Furthermore, the cable stability from flooding of one of the large laying procedure should cater for such an cable tanks, either as a result of impact event, e.g. the cable may be cut and buoyed damage or due to downflooding from the off and the hatches associated with each deck openings in heavy weather. However, deck penetration closed / sealed, preventing the SPS (Special Purpose Ship) Code is the ingress of water. The number of often considered more appropriate. It penetrations of the weather deck should be provides an international standard of safety minimised and capable of being made for the ship and its personnel, which is weathertight, such that the loadline aspects equivalent to that provided by SOLAS, are not adversely affected, and without

Dynamic Positioning Conference September 18-19, 2001 Page 4 of 10 Andrew McKinven, Lloyd’s Register Cable laying vessels Return to Session Directory placing any restrictions on the vessel’s deck and the main deck. With respect to operations (allowing cables to pass). It is longitudinal strength, due to the laying of common practice to have large hatch cable, the vessel’s deadweight will reduce coamings to prevent sea water / spray and the hull form will change from entering the tanks, deck scuppers to allow ‘sagging’ (when fully loaded), to ‘hogging’ sea water to be drained effectively and even (when unloaded). However, the variation in a cable tank bilge system. However, these deadweight can be catered for with reference matters fall outside the statutory loadline to the on-board loading instrument, and by aspects. performing ballasting, ensuring that the vessel’s longitudinal strength is not Structural exceeded. It is a Class requirement that all It is generally accepted that the size of the cargo ships (incl. cable laying vessels) of cable tank tends to be maximised, resulting length greater than 65m have an approved in minimal amounts of room for supporting loading instrument on-board to calculate structure at the side of the cable tanks. Cases bending moments and shear forces. A more have been experienced where the cable tank economic way to respond to the recent has not been integrated into the ship market demand for cable laying vessels has structure, hence not providing structural been to convert existing vessels. Vessels support. More specific issues relating to suitable for conversion tend to have a large racking, localised and global strength are flat aft. end, e.g. trawlers and offshore discussed in more detail below. Cable supply boats, in such cases cable tanks are laying vessels may be considered in many constructed on the existing deck. ways to have similarities with ro-ro ships. Alternatively, ro-ro ships with their existing The combination of large open spaces (cable large open internal spaces are a good option. tanks), the restrictions on transverse Whilst presenting a more timely and structure (transverse bulkheads) and heavy economical solution, conversions may loads (deckhouse superstructure, deck however prove to be problematic depending cranes and winches) above, may result in the upon the amount of steelwork removed to need for a comprehensive finite element accommodate the cable tanks and depending study to determine the racking strength of upon the inherent strength of the vessel. For the ship’s structure. This is not a Rule example, the racking strength of the vessel requirement, however, a finite element may be adversely affected. It is easier with analysis can be as straightforward as the newbuild constructions to design around standard direct structural calculations. potential structural problems. Localised strengthening may be required in the double bottom (due to the weight of a Engineering fully loaded cable tank). Double bottom In many respects the single fault philosophy strength needs to be investigated in a similar that we associate with the FMEA has been way to that of bulk carriers (taking account inherent within the classification Rule of hydrostatic pressure on outside and load requirements for many years, particularly on inside), to determine the location of any with respect to essential services. All LR additional structure. The spacing of class vessels (not small ships) are to have transverse and longitudinal members is to split bus bars, with barrier protection. ensure a uniform distribution of load under Essential services that are required to be the tank. Cable laying vessels may be too duplicated are to be supplied from opposite short to have any significant global strength sides of these bus bars. Their cables are to concerns. More specifically, they seldom be separated throughout their length as have the torsional problems associated with widely as practicable without the use of container ships, due to their intact decks. common feeders, transformers, converters, The Maersk ships mentioned earlier had four protective devices or control panels and intact decks: tank top; tween deck; platform circuits. So that any single fault will not

Dynamic Positioning Conference September 18-19, 2001 Page 5 of 10 Andrew McKinven, Lloyd’s Register Cable laying vessels Return to Session Directory cause the loss of both essential services. (i) a machinery fault has occurred; Whether a service is considered to be (ii) when it is being attended to; and essential will depend upon the ship type, its (iii) when it has been rectified. design and configuration of engineering It would be considered normal for a cable systems. The LR Ship Rules provide a list of layer to operate in UMS whilst also in DP, services which may be considered to be as the cable laying operation is not essential, these are expanded upon by the considered safety critical, unlike that of a Rules for DP Ships to include the following: diving support vessel. There are a series of • electrically driven thruster units; Class notation which relate to dynamic • generator and thruster control equipment; positioning. The two most popular DP • reference systems; and notations for cable laying vessels are • environmental sensors. DP(AM) and DP(AA), these relate to IMO Therefore, in addition to compliance with classes 1 and 2. LR’s equivalent to IMO the existing single fault requirements Class 3 is the DP(AAA) notation, which is regarding loss of essential services, by far the most onerous notation for if the vessel is to be assigned the DP(AA) dynamic positioning. notation, the engineering systems are to be configured such that a single fault in any DP is now considered necessary for cable active component will not result in the loss laying in order to ensure that a constant and of position. This will be discussed at length acceptable level of tension is placed upon in Section 4. the cable during laying / repairing (joining). It also assists with charting the position of Class Notations the cable on the sea bed. With a notation It would be remiss to discuss Classification such as DP(AA), the ship engineering without mentioning some of the notations systems and the DP control system will be which are common to cable laying vessels. tolerant to a single fault - minimising For navigation safety, these include interruptions to the cable laying operation Navigation Arrangements for Periodic One and hence reducing downtime. New Man Watch (NAV1) and Integrated Bridge constructions of cable laying vessels show a System (IBS), on the machinery side the definite preference for the DP(AA) notation. most common is the Unattended Machinery The main feature of a ship assigned a Spaces (UMS) notation. It is interesting to DP(AA) notation is that engineering systems note that such notations either require (machinery / piping / electrical and control) certain type of failure to be designed out or which are essential for dynamic positioning require mitigating measures to be in place to are to be configured such that a single fault control failures. For example, in the case of in any active component will not cause the the IBS notation, it is required that in the loss of position. Furthermore, the dynamic event of a single failure of an operator positioning control system for a DP(AA) interface, all functions of the Integrated vessel will, in the event of a controller fault, Bridge System are to remain available. In automatically changeover to the second addition integrated display and control controller without interruption. Hence, the functions are to be of an ergonomic design, owner / operator can be confident that whilst hence reducing the likelihood of human operating in DP the vessel will be tolerant to error arising or failures escalating due to the majority of likely single faults. It is a misinterpretation. The UMS notation Class requirement that this be verified by incorporates requirements which manage, to way of an FMEA on the ship systems an extent, the escalation of a failure. For (ideally providing an input to the design of example, machinery alarms are to be relayed the vessel) and verified by functional testing to the bridge such that the navigation officer during sea trials to the satisfaction of the of the watch is made aware when: attending LR Surveyor.

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3.0 DYNAMIC POSITIONING signal, the controller produces an output AND CABLE LAYING signal to the thruster units to reduce the error Dynamic positioning is a computer assisted to zero. The system allocates the optimum manoeuvring system that keeps a ship on amount of thrust to whichever thruster units station exclusively by means of active are available. In the context of cable laying thrust. It is the role of the DP system to keep operations, there are different uses for the the vessel within an area of operation as various reference systems: defined by set points for heading and (i) DGPS is the main reference system in position specified by the operator. These use, especially for surface laying over long limits will be specified in relation to the distances, deep water makes the use of other mode of operation. The ability to stay in reference systems difficult; position will be heavily dependent upon the (ii) acoustic reference systems tend to be number, rating and position of the thrust used when ploughing, mainly to indicate the units. The environmental conditions will subsea position of the plough and the burial also play a significant role. A DP system position of the cable; controls only three of the six modes of (iii) taut wire and DGPS are used for motion: surge, sway and yaw, by controlling maintaining the vessel in a fixed position the pitch and/or rpm of the thrust units whilst cable joining / repairing in against the prevailing wind, sea current and shallow water, or for landing shore ends, wave forces, see Figure No. 3. when the cable is pulled ashore from the vessel. As would be expected, the The dynamic positioning system works on configuration and redundancy of equipment the basis of input signals from position / components in proprietary DP control reference systems (indicating actual systems is indeed focused towards meeting position), these are fed to the controller the requirements of IMO classes 1-3 and where they are compared with the desired hence the requirements for the LR DP position. Based upon the resulting error notations. Figure No. 4 provides examples of the various configurations available on the market.

Figure No. 3: Modes of Motion, Surge, Sway and Yaw (Courtesy of Kongsberg-Simrad)

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Figure No. 4: Proprietary DP Control Systems (Courtesy of Kongsberg-Simrad)

Perhaps the most noticeable difference is the and without any adverse affect upon the level of redundancy offered by the system ship’s station keeping performance. Further components: the controller, position components of a DP(AA) installation which referencing units and environmental sensors. require redundant units (three of each) The performance of the control system include: position reference systems; gyro however depends upon the following: compasses and vertical reference units. (i) response to variations in environmental forces acting upon the vessel; There are specific requirements for (ii) ability to filter out unwanted effects engineering systems in a DP(AA) ship. This (first order wave forces and high frequency includes cables, pipes and other components wind components); and essential for the correct functioning of the (iii) the available thruster power (transverse DP system, located and protected such that and azimuth thrusters and CP propellers). the risk of fire and mechanical damage is minimised. Generation and distribution arrangements are to be such that no single 4.0 DESIGN REQUIREMENTS fault will result in loss of more than 50% of FOR THE DP(AA) NOTATION the generating capacity, or of any duplicated The standard DP(AA) control system has essential services. two automatic controllers, these are arranged to operate independently, such that a failure Engineering systems (power, control and in one will not affect operation of the other. thruster systems and other systems In the event of a failure of the working necessary for dynamic positioning) are to be controller, the standby unit takes over provided and configured such that a fault in automatically without manual intervention, any active component or system will not result in the loss of position. This is to be

Dynamic Positioning Conference September 18-19, 2001 Page 8 of 10 Andrew McKinven, Lloyd’s Register Cable laying vessels Return to Session Directory verified by means of an Failure Mode and It is the objective of the test schedule to Effects Analysis (FMEA), assessing introduce single failures into the system; key components such as: prime movers; failures tend to result in the loss of a generators; switchgear; gearing; pumps; consumer's ability to perform a specific fans; thrusters; valves (where power 'function'. As indicated in Section 1, the actuated) and by functional tests performed dependence upon an electric supply for during sea trials. The FMEA will be covered consumer power and control result in the in more detail in Section 5. majority of tests consisting of the disconnection of circuit breakers, fuses or switches. 5.0 FAILURE MODE AND EFFECTS ANALYSIS The Failure Mode and Effects Analysis SECTION 6: DISCUSSION (FMEA) is an established technique for Cable laying vessels may initially be examining the potential failure modes considered as cargo vessels, however, as associated with a system and investigating their operational characteristics and design the likely failure effects. The structure of the become more specialised the associated FMEA lends itself to the systematic and Classification and Statutory requirements rigorous examination of engineering become more onerous, to the extent that the systems. Whenever an FMEA is conducted, cable layer is far removed from the general it is essential that it is performed by cargo vessel. The differences become more qualified and experienced individuals in apparent when a cable laying vessel is accordance with an International Standard, assigned the DP(AA) notation, due to the e.g. IEC 60812, utilising worksheets to careful consideration required for the record the assessment. engineering design: duplication of essential services, segregation of supplies and most The worksheet in Figure No. 5 includes the importantly, consideration of the effects of standard content of an FMEA, but also allows component failure. The production of an for an assessment of criticality. The criticality FMEA and the subsequent functional tests analysis, whilst not required by Class, is one provides a high degree of assurance that method by which critical failure modes can be these matters have been carefully considered identified and ranked, hence providing the basis and suitable measures incorporated into the for the functional test schedule, which is vessel’s design. performed during sea trials. The test schedule should be considered as an extension of the written FMEA, based upon the design aspects which are identified as weak / sensitive to the introduction of a single failure, or where the failure effects have a degree of uncertainty. The strength of the test schedule lies in its ability to reveal (during operational conditions) the dynamic nature of a system’s response to failures, either demonstrating that the response is consistent with expectations and acceptable, or, unacceptable, hence providing the opportunity for design modifications to mitigate the unwanted failure effects. Whilst it is important that the test schedule covers the key failure modes, it is equally important that the sequence of failure modes is given due consideration, such that the true causes and effects are revealed.

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Equipment Ident. Failure Failure Failure effect Failure Alternative Failure Criticality Name Function No. Mode Cause Local End Detection Provisions Probability Level Remarks

Figure No. 5: Failure Mode, Effects and Criticality Analysis (FMECA) Worksheet

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