Deepwater Installation of Subsea

Proceedings of the 10th Offshore Symposium, February 20 2001, Houston, TX Texas Section of the Society of Naval Architects and Marine Engineers

DEEPWATER INSTALLATION OF SUBSEA HARDWARE Stephen J Rowe BMT Fluid Mechanics Limited Brian Mackenzie Offshore Technology Management Limited Richard Snell BP Limited ABSTRACT Offshore oil developments are now being planned in water depths of 2000m and greater. At these depths the technical challenges of installing the necessary subsea systems become increasingly severe. Conventional means of lowering and positioning heavy subsea equipment may not work in ultra deep water, and the industry needs assurance that adequately reliable and economic installation techniques and equipment will be available to give the necessary confidence to plan ultra deep water projects. It is important that any new techniques that are developed are within the feasible capability of existing construction vessels. The paper describes the work of a new research project which is seeking to identify and develop solutions to these deepwater installation problems, and to provide the industry with the assurance it needs.

INTRODUCTION

shallower developments. The risks associated with installation are also probably higher.

The oil industry has made a concerted effort to gain access to deepwater acreage, and anticipates that a Relatively benign conditions such as those found significant proportion of the non-OPEC production will offshore West Africa may not be easier for installation be from deepwater developments within 10 years. operations than the Gulf of Mexico because the persistent swell is likely to result in ideal conditions for vessel Deepwater developments are currently being motion resonance. Offshore Brazil high currents are likely pursued in 1300m off West Africa, 2000m in Brazil and to be a dominating factor, and in N W Europe, where shortly 2000m in the Gulf of Mexico - Figure 1. water depths up to 1000m are currently being explored, Licenses for potential future development extend to the harsh metocean conditions are likely to result in much ultra deepwater depths exceeding 2500m. The industry higher installation downtime. does not at present have the capability to install equipment on the seabed in this depth, other than by The scale of some of the potential deepwater fields is using a drilling semi-submersible. Project economics a factor which drives a development to include subsea would not normally make it feasible to use a deepwater systems, and focuses particular attention on assuring the capable drilling semi-submersible for an extensive successful execution of the installation. Unlike most Gulf construction program. of Mexico blocks the geographical size of the West African and some Brazilian blocks is very large, with the In deepwater fields the contribution of installation potential for several separate reservoirs and different activity to project cost and schedule is higher than for development areas within the one block. The ability to

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Proceedings of the 10th Offshore Symposium, February 20 2001, Houston, TX Texas Section of the Society of Naval Architects and Marine Engineers exploit multiple reservoirs through one surface facility the time from development to full acceptance of the will greatly improve the chances of them being capability by the operators. developed. The new Deep Water Installation of Subsea Hardware The development options available to the industry (“DISH”) project is setting out to address these issues, and comprise either subsea wells remotely or locally located to develop this capability in a collaborative manner. flowing back to a host surface facility, or surface wells DEEPWATER INSTALLATION ISSUES suspended from a floating facility. Separation at the seabed is used in only a limited number of fields. It has been seen that water depths for hydrocarbon For future developments there is an economic developments are going to increase considerably in the incentive to increase the amount of processing years to come. This will result in a number of technical challenges where existing methods and equipment will undertaken at the seabed in order to; (a) reduce the flow assurance problems associated either not work, or will be uneconomic to use. These with long distance tie-backs (which currently challenges therefore potentially constrain the ability to make accessing a number of remote middle install subsea hardware in deepwater. sized reservoirs from a single host very Whilst we are aware of some of these issues, we do difficult), not yet know which are the main cost drivers or ‘show (b) reduce the amount of produced water handled stoppers’ which should be focused on during the DISH through the surface facility. project. A focus on the cost drivers will help bring down A development with subsea wells tied back to a the cost of these deepwater operations, and make fields host may typically have 30 wells, and requires an economic which would not otherwise be developed. A extensive installation program of manifolds, control focus on the ‘show-stoppers’ may perhaps make umbilicals and jumper hoses linked to the host by developments technically possible that today are not flowlines. Excluding the main flowlines these are small possible. lift weight components of compact dimensions, but the The actual issues that the DISH project will large number of individual items requires a lengthy installation program. Future fields will need heavier, investigate in detail remain to be identified by the initial less compact, components for subsea separation (see phase of the project just starting. However, in the Figure 2), as well as a large number of manifolds, following mention is made of several that may be important. jumper hoses and control umbilicals. The challenges may be classified in the following At winch speeds of 20 m/minute deployment and general areas: 10 m/minute recovery each operation in 2500m water depth is likely to take a day, including time to rig up the lift at the surface, connect line from storage reels and to • Lifting and lowering technology - Those issues directly related to the weight of the loads to be position, place the load on the seabed and recover the lowered to the deep seabed, the dynamic responses line. that can augment these loads, and the capability of the lifting systems. Within the next few years early planning for the ultra deep fields will commence, and development • Load control and positioning - Issues related to placing the load in the desired location, at the correct decisions will be made based on the best understanding compass heading, and at a stable attitude on the of technically and commercially realistic installation seabed. capability. As the success of very large investments will effects and weather window be dependent on this installation it is likely that the • Metocean requirements - The influence of weather and other operators will require a high level of confidence in the metocean effects on the technology that can be used, proposed installation equipment and methods. the required weather windows, and the speed with which tasks must be accomplished in order to fit into Whilst each construction contractor has his specific these practical windows. vessels, there is considerable value in collaborative development of deployment equipment, techniques and In all this it must be recognized that considerable analytical tools which can be demonstrated to be effective. This not only reduces the cost to each strides have been made in deepwater field developments contractor of developing the capability, but also reduces in recent years, with very deep fields being developed, or

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Proceedings of the 10th Offshore Symposium, February 20 2001, Houston, TX Texas Section of the Society of Naval Architects and Marine Engineers under development, in the Gulf of Mexico, offshore Brazil and West Africa. However, some of the techniques used in these developments may not suitable for harsher ocean environments, or for greater depths.

solve the recovery problem if/when the subsea equipment needs to be returned to the surface for any reason. However, free fall installation might be possible for large assemblies, in the knowledge that recovery may be performed on individual modules or components.

Lifting and lowering technology Steel wire ropes with multi-fall lowering systems are very well understood and durable, but they are limited in their application to very deepwater. As the depth increases, the ratio of the weight of the cable to the weight of the payload becomes increasingly extreme [1]. At 3000m the weight of a 5” wire rope is about the same as its 170t payload At a depth of about 6000m the safe working load (SWL) of the steel wire rope is entirely used up by its self-weight, leaving zero payload capacity. There are also difficulties in manufacturing sufficiently long lengths of steel wire rope. Currently the single length manufacturing capability is 200 tonne weight or 2900m length of 5” wire with SWL 350 tonne. There are also problems with free rotation of the wire under load which can amount to 200-700deg rotation per m length. This can result in damaged wire or loss of the end termination, particularly when the tension is removed and the rotation tries to unwind. Even so-called non-rotating designs can still have significant problems with the very long lengths required for deepwater. Synthetic fiber rope lowering systems provide a potential answer to the self-weight problems, being neutrally buoyant. They have further attractive properties such as small allowable bend radii, and the ability to be repaired. They have been used in some applications, but to-date there is little track record, and there are potential problems related to stretch, creep, and the relatively low melting point. For large installations and repetitive tasks, there are important questions on the durability and life of synthetic rope and winch systems which need to be resolved.

Buoyancy units may also have an important role to play in reducing the static lifting line tensions. However buoyancy units for large subsea components to be installed in deepwater are not easy to design in such a way that they are manageable and economic. There may also be control and stability problems to solve, particularly as they increase the inertia and hydrodynamic loading of the system, and may therefore contribute to undesirable dynamic effects. There can be very significant dynamic effects when lowering heavy weights on long lines. The excitation caused by the motions of the surface vessel can be amplified with large oscillations and high dynamic tensile loads in the lifting line. Motions in the heave direction may be only lightly damped, and the virtual (or added) mass of the load can be very significant. For example, a suction anchor consisting of a flooded cylinder with closed top will have an added mass which is many times it’s weight in air due to the water trapped inside and entrained around it. When combined with dynamic magnification caused by oscillations it has been estimated that this has resulted in line tensions of 460 tonnes for a suction anchor with a weight in air of 44 tonnes. The shape of the item to be installed, which in turn determines the added mass, can therefore be crucial to this dynamic response, and to the ability to install it. In future it is likely that much greater attention will need to be paid in design to ensure that undesirable shapes with very large entrained masses of water are avoided wherever possible.

It can be shown that for lowering into deepwater there will nearly always be a depth at which a resonant response will occur. It is important that this resonant region can be passed through relatively quickly, and that it does not occur at full depth where careful control is required for placement of the payload on the seabed. Modelling methods have been developed to predict the behavior of Another interesting possibility for the lowering line these dynamic responses so that design and planning of are spoolable compliant tubulars (as used in composite the lowering operations can attempt to minimize and coiled tubing workstrings) [2]. These pipes can be avoid them. fabricated with embedded copper conductors and fiber optics, which might avoid the use of separate umbilicals Deepwater often means strong and complex currents and their associated handling problems. They can also which can affect the shape of the lifting line (forcing it be fabricated so as to be neutrally buoyant. into a lateral catenary shape), which in turn can influence the apparent axial stiffness of the line and its dynamic Free-fall installation systems have been suggested. properties - Figure 3. Vortex shedding (or ‘strumming’) This is clearly a non reversible process which does not can dramatically increase the drag load from the current,

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Proceedings of the 10th Offshore Symposium, February 20 2001, Houston, TX Texas Section of the Society of Naval Architects and Marine Engineers and result in even greater load offset and curvature in tubulars has already been noted above, but another interesting potential solution to this is the DeepTek Curly the lifting line. Wurly concept [3] which has been developed for Drum winches are not suitable for synthetic ropes. deepwater salvage operations. This system automatically Whilst they are simple, they have high inertia, and with winds the umbilical around the lifting wire (see Figure 5). long lengths and high tensions suffer from the line It would, however, be required to be developed for the becoming embedded in underlying layers. The line pull much greater weights, sizes, and depths required for also reduces as the number of wraps increases. Traction deepwater field installation. winches can be used for synthetics, but slippage is The success of the final touchdown operation, possible if the system design if poor. Design of the grooves which grip the rope is a critical issue. They whether achieved with a conventional hook or an have the advantage of constant line pull, but intelligent one, is also susceptible to the load’s interaction coordination is difficult for high speeds. They are also with the seabed. Deepwater soil conditions tend to be very soft, and so the deployment system must be capable mechanically more complex. of touchdown without causing immediate bearing capacity Load control and positioning failure underneath the installed hardware. This may in turn result in an unacceptable component orientation and Apart from the dynamic response, there are a connection problems. Recovery, once embedded, may be number of issues related to positioning the load in the difficult if the component was lowered at close to the required location on the seabed. In very deepwater, deployment system’s load capacity. Both hardware relatively small currents can introduce a very large foundation design and release system design play a role in offset between the surface ship and the load on the addressing this touchdown issue. However, with the seabed. There is likely to be a need for new ways of difficulties and costs associated with gathering deepwater controlling the position of the surface vessel and the soils data for detailed foundation design, more onus may location of the load using thrusters to compensate for need to be placed on the release mechanism design to the steady and dynamic effects of the current. Loads address the problem. that are lowered at great speed may also be subject to strong unstable lateral fluid forces which may cause the Once a load is placed on the seabed and released load to ‘flutter’ or ‘glide’ away from the desired there are further problems with controlling the location of position. The load also needs to be aligned on the the lowering system hook which will inevitably become correct compass heading. less controllable when the beneficial tension is removed. It must remain under control and be prevented from getting This indicates that some kind of dynamically entangled with the subsea equipment. positioned power pod is required to permit the load to be steered into the desired position (as already Metocean effects and weather window requirements developed by Bluewater - see Figure 4). Such a pod There is a natural tendency for subsea installation requires positional and rotational control, may provide powered lowering (down force), and hydraulic systems tasks in deeper water to take longer than the equivalent for load release together with instrumentation to tasks in shallow water. It will take longer to lower to the seabed, and longer to raise the lifting gear afterwards for monitor the status of the load. the next lift. It is likely to take longer to locate and Position reference is also a problem in great water position equipment on the seabed. Attempts to speed up depths, and conventional acoustic systems may not the lowering process raise the question as to whether the work. Communication with the surface may be load can actually be made to sink at higher speed without unreliable (due to long path lengths and vessel noise) gliding off laterally and losing lateral position, landing at a point distant from that required. This is another area and rather slow (4s round trip time at 4000m). where the shape of the load, and the hydrodynamic forces A certain amount of intelligence may therefore be acting on it, will be a very important part of ‘design for required at the load. Some have referred to this as an installation’. ‘intelligent crane hook’. Whether intelligent or not, it It may be that some of these task durations may will need a power umbilical, and this creates a further problem of controlling the umbilical and the lifting wire become untenable in relation to the available weather independently, and preventing them from becoming windows and metocean forecasting capabilities. entangled. The possibility of incorporating copper Consequently there will be strong pressure to find ways conductors and fiber optics inside spoolable composite of: doing things quicker, or doing them in such a way that

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Proceedings of the 10th Offshore Symposium, February 20 2001, Houston, TX Texas Section of the Society of Naval Architects and Marine Engineers they are less sensitive to the influences of winds waves years. This objective is to be achieved via a two-phased and currents. project. Phase 1, just starting now, will be an 8-month exploratory, problem definition study, resulting in the Summary of Issues for DISH Phase 1 targeting of key technological uncertainties and capability gaps. Phase 2 will then address these detailed challenges, A large number of deepwater installation technical over a period expected to be in the region of 16 months. issues have been touched on above. They can be Thus the whole project is expected to last about 2 years. summarized as follows: Phase 1 will review the industry’s state-of-the-art Lifting and lowering technology capabilities, and also the oil industry’s envisaged • Use of synthetic fiber ropes including, reliability, installation needs, over the next 10 years. The findings of durability and winch design issues. each will be used to populate a capabilities and • Likely applicability of spoolable tubular requirements database. composites as a lifting line option. • Applicability of free-fall installation processes. The state-of-the-art capability data will be gathered from the installation contractor participants, and the • Applicability of buoyancy systems. • Identification and validation of numerical installation contracting sector as a whole. Data will also be gathered from the specialist supply sector (for instance models for simulation of lift dynamics. • Motion compensation (e.g. is partial of specific deepwater deployment system components, or of lift line dynamic modelling software). compensation of any value?) • Hydrodynamic design of loads to ameliorate The installation requirements will be gathered from added mass and fluid loading issues, and to the oil company participants, with the specific aim of facilitate faster lowering. • Estimation of upper weight/depth limits for the reflecting the geographical spread and metocean-severity spread of their deepwater prospects. The data will also various potential lifting systems. seek to reflect a full range of development solutions and hardware components involved. This has implications for Load control and positioning • Powered, and perhaps ‘intelligent’, crane hooks. the positioning accuracy, number of lifts, component criticality, connectability requirements, life-of-field • Position reference systems. intervention requirements, and configuration of each component. Metocean effects and weather window requirements • • • •

Duration of deep installation tasks. Phase 1 will then combine and compare these two Practical available weather windows. aspects – capabilities and requirements - and identify Technology required to speed up these tasks. technical uncertainties and capability gaps. These will be Technology required to reduce metocean reviewed, ranked and prioritized during the course of a sensitivity of the tasks. specially facilitated “mid-flight” project workshop. This workshop will result in a key deliverable of Phase 1, Figure 6 from [2] also pictorially summarizes many where specific technical uncertainties are targeted, and of these issues. Most will be investigated to some extent plans developed to address them. in Phase 1 of the DISH project, and those considered most important ‘cost drivers’ or ‘show-stoppers’ will Phase 2 will address these targeted technical become the subject of detailed investigation in Phase 2. challenges in detail. Tasks performed during Phase 2 are likely to be of three distinct types: PROJECT PLAN AND DELIVERABLES A full description of the DISH project plan and • deliverables may be found in [4], but an outline of the project activities follows. The objective of the project is to build a common understanding of existing deepwater lifting/lowering technical limitations, and then to develop feasible, • globally applicable, solutions which meet the industry’s deepwater installation requirements over the next 10

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modelling tasks, which aim to develop, validate and apply appropriate numerical models describing a deployment system’s response to the environment, taking account of the mechanical properties of the lifted weight, line and vessel, and the effects of any motion compensation and control systems; engineering tasks, based on concept design studies, and resulting in functional specifications for pieces of hardware (but stopping short of detailed engineering design because these will be vessel specific);

Proceedings of the 10th Offshore Symposium, February 20 2001, Houston, TX Texas Section of the Society of Naval Architects and Marine Engineers •

ACKNOWLEDGEMENTS procedural studies, with the objective of assessing how subsea operations should be carried out, The authors wish to thank all those who have whether they are feasible and economic, identifying key areas of difficulty, and finding practical supported the launch of the DISH project. Particular thanks are due to Stewart Willis of Stolt Offshore, and Ian solutions. Edwards of Halliburton who provided much of the The deliverables of Phase 2 will vary, depending background information on existing deepwater lifting and on the nature of the task. However, likely deliverables lowering limitations given in this paper. are: •

REFERENCES

Engineering reports defining the nature of the task and its objectives, scenarios assumed, key parameters, results, technical developments arising and conclusions; Functional specifications enabling participants to develop deployment system components; Studies dealing with the feasibility and cost-impact of different deployment system options; Statements of what is still required in terms of hardware developments/software abilities to allow technical developments to reach prototype stage, and recommendations about actions and priorities for further work.

[1] Willis, S, A Contractor’s view of Lifting and Lowering in Deep Water, Presentation to DISH Project Seminar, London, 1st November 2000. • [2] Edwards, I, Lifting Technology Developments Required for Subsea Fields in 2000m and Beyond, • Presentation to DISH project Seminar, London, 1st November 2000. • [3] Fletcher B E, Curly Wurly Concept Analysis, Phase 1 Report, Technical Document 3072, SSC San Diego, May 1999. [4] Mackenzie, B, Deepwater Installation of Subsea Hardware (DISH), Proposal for Phase 1 of a Joint Industry Research Project, BMT Fluid Mechanics CONCLUSIONS Limited, and Offshore Technology Management Limited, The DISH project will benefit the offshore industry November 2000. because it will ensure that technical challenges for installation of systems in very deepwater are addressed in time to boost operator confidence in their deepwater development plans, and in time for complete engineering solutions to be developed in the contractor industry. The benefits of the project for operators will be access to enabling technology, confidence to progress deepwater development plans, and confidence to evaluate bids from, and award contracts to, installation contractors. It should also reduce lead times and development costs, and provide the opportunity to pilot new technology, and to influence future industry best practice. The dialogue with contractors and suppliers will represent an international focus for knowledge sharing by assembling participants’ knowledge and requirements for developments throughout the world’s deepwater prospects. For contractors and suppliers the benefits will be similar, but will include market intelligence and an enhanced understanding of operators’ plans and requirements, and the provision of a commercial focus for internal capability development. The dialogue with the operators should also represent an enhanced market opportunity.

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Figure 1 - Field Depth Trend - from [1].

Figure 2 - Subsea installation weight trends - from [1].

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Figure 3 - Effect of current on lifting line and load position - from [2].

Figure 4 - The Bluewater Powerpod.

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Figure 5 - The Curly Wurly System.

Figure 6 - Deepwater installation issues - from [2].

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Deepwater Installation of Subsea

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