deepwater installation of subsea

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Proceedings of the 10 th Off shore Symposium, February 20 2001, Houston, TX

Texas Section of the Society of Naval Archi tects and Mari ne Engineers

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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

The oil industry has made a concerted effort to gain

access to deepwater acreage, and anticipates that a

significant proportion of the non-OPEC production will

be from deepwater developments within 10 years.

Deepwater developments are currently being

pursued in 1300m off West Africa, 2000m in Brazil and

shortly 2000m in the Gulf of Mexico - Figure 1.

Licenses for potential future development extend to

ultra deepwater depths exceeding 2500m. The industry

does not at present have the capability to install

equipment on the seabed in this depth, other than by

using a drilling semi-submersible. Project economics

would not normally make it feasible to use a deepwater

capable drilling semi-submersible for an extensive

construction program.

In deepwater fields the contribution of installation

activity to project cost and schedule is higher than for

shallower developments. The risks associated with

installation are also probably higher.

Relatively benign conditions such as those found

offshore West Africa may not be easier for installation

operations than the Gulf of Mexico because the persistent

swell is likely to result in ideal conditions for vessel

motion resonance. Offshore Brazil high currents are likely

to be a dominating factor, and in N W Europe, where

water depths up to 1000m are currently being explored,

the harsh metocean conditions are likely to result in much

higher installation downtime.

The scale of some of the potential deepwater fields is

a factor which drives a development to include subsea

systems, and focuses particular attention on assuring the

successful execution of the installation. Unlike most Gulf

of Mexico blocks the geographical size of the West

African and some Brazilian blocks is very large, with the

potential for several separate reservoirs and different

development areas within the one block. The ability to





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exploit multiple reservoirs through one surface facility

will greatly improve the chances of them being

developed.

The development options available to the industry

comprise either subsea wells remotely or locally located

flowing back to a host surface facility, or surface wellssuspended from a floating facility. Separation at the

seabed is used in only a limited number of fields.

For future developments there is an economic

incentive to increase the amount of processing

undertaken at the seabed in order to;

(a) reduce the flow assurance problems associated

with long distance tie-backs (which currently

make accessing a number of remote middle

sized reservoirs from a single host very

difficult),

(b) reduce the amount of produced water handled

through the surface facility.

A development with subsea wells tied back to a

host may typically have 30 wells, and requires an

extensive installation program of manifolds, control

umbilicals and jumper hoses linked to the host by

flowlines. Excluding the main flowlines these are small

lift weight components of compact dimensions, but the

large number of individual items requires a lengthy

installation program. Future fields will need heavier,

less compact, components for subsea separation (see

Figure 2), as well as a large number of manifolds,

jumper hoses and control umbilicals.

At winch speeds of 20 m/minute deployment and

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

position, place the load on the seabed and recover the

line.

Within the next few years early planning for the

ultra deep fields will commence, and development

decisions will be made based on the best understanding

of technically and commercially realistic installation

capability. As the success of very large investments will

be dependent on this installation it is likely that theoperators will require a high level of confidence in the

proposed installation equipment and methods.

Whilst each construction contractor has his specific

vessels, there is considerable value in collaborative

development of deployment equipment, techniques and

analytical tools which can be demonstrated to be

effective. This not only reduces the cost to each

contractor of developing the capability, but also reduces

the time from development to full acceptance of the

capability by the operators.

The new Deep Water Installation of Subsea Hardware

(“DISH”) project is setting out to address these issues, and

to develop this capability in a collaborative manner.

DEEPWATER INSTALLATION ISSUES

It has been seen that water depths for hydrocarbon

developments are going to increase considerably in the

years to come. This will result in a number of technical

challenges where existing methods and equipment will

either not work, or will be uneconomic to use. These

challenges therefore potentially constrain the ability to

install subsea hardware in deepwater.

Whilst we are aware of some of these issues, we do

not yet know which are the main cost drivers or ‘show

stoppers’ which should be focused on during the DISH

project. A focus on the cost drivers will help bring down

the cost of these deepwater operations, and make fields

economic which would not otherwise be developed. A

focus on the ‘show-stoppers’ may perhaps make

developments technically possible that today are not

possible.

The actual issues that the DISH project will

investigate in detail remain to be identified by the initial

phase of the project just starting. However, in the

following mention is made of several that may be

important.

The challenges may be classified in the following

general areas:

• Lifting and lowering technology - Those issues

directly related to the weight of the loads to be

lowered to the deep seabed, the dynamic responses

that can augment these loads, and the capability of the

lifting systems.

• Load control and positioning - Issues related to

placing the load in the desired location, at the correct

compass heading, and at a stable attitude on the

seabed.

•

Metocean effects and weather windowrequirements - The influence of weather and other

metocean effects on the technology that can be used,

the required weather windows, and the speed with

which tasks must be accomplished in order to fit into

these practical windows.

In all this it must be recognized that considerable

strides have been made in deepwater field developments

in recent years, with very deep fields being developed, or





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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.

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.

Another interesting possibility for the lowering line

are spoolable compliant tubulars (as used in composite

coiled tubing workstrings) [2]. These pipes can be

fabricated with embedded copper conductors and fiber

optics, which might avoid the use of separate umbilicals

and their associated handling problems. They can also

be fabricated so as to be neutrally buoyant.

Free-fall installation systems have been suggested.

This is clearly a non reversible process which does not

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.

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 tensileloads 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 thisdynamic 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

these dynamic responses so that design and planning of the lowering operations can attempt to minimize and

avoid them.

Deepwater often means strong and complex currents

which can affect the shape of the lifting line (forcing it

into a lateral catenary shape), which in turn can influence

the apparent axial stiffness of the line and its dynamic

properties - Figure 3. Vortex shedding (or ‘strumming’)

can dramatically increase the drag load from the current,





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and result in even greater load offset and curvature in

the lifting line.

Drum winches are not suitable for synthetic ropes.

Whilst they are simple, they have high inertia, and with

long lengths and high tensions suffer from the line

becoming embedded in underlying layers. The line pullalso reduces as the number of wraps increases. Traction

winches can be used for synthetics, but slippage is

possible if the system design if poor. Design of the

grooves which grip the rope is a critical issue. They

have the advantage of constant line pull, but

coordination is difficult for high speeds. They are also

mechanically more complex.

Load control and positioning

Apart from the dynamic response, there are a

number of issues related to positioning the load in the

required location on the seabed. In very deepwater,

relatively small currents can introduce a very large

offset between the surface ship and the load on the

seabed. There is likely to be a need for new ways of

controlling the position of the surface vessel and the

location of the load using thrusters to compensate for

the steady and dynamic effects of the current. Loads

that are lowered at great speed may also be subject to

strong unstable lateral fluid forces which may cause the

load to ‘flutter’ or ‘glide’ away from the desired

position. The load also needs to be aligned on the

correct compass heading.

This indicates that some kind of dynamically

positioned power pod is required to permit the load to

be steered into the desired position (as already

developed by Bluewater - see Figure 4). Such a pod

requires positional and rotational control, may provide

powered lowering (down force), and hydraulic systems

for load release together with instrumentation to

monitor the status of the load.

Position reference is also a problem in great water

depths, and conventional acoustic systems may not

work. Communication with the surface may be

unreliable (due to long path lengths and vessel noise)

and rather slow (4s round trip time at 4000m).

A certain amount of intelligence may therefore be

required at the load. Some have referred to this as an

‘intelligent crane hook’. Whether intelligent or not, it

will need a power umbilical, and this creates a further

problem of controlling the umbilical and the lifting wire

independently, and preventing them from becoming

entangled. The possibility of incorporating copper

conductors and fiber optics inside spoolable composite

tubulars has already been noted above, but another

interesting potential solution to this is the DeepTek Curly

Wurly concept [3] which has been developed for

deepwater salvage operations. This system automatically

winds the umbilical around the lifting wire (see Figure 5).

It would, however, be required to be developed for the

much greater weights, sizes, and depths required for deepwater field installation.

The success of the final touchdown operation,

whether achieved with a conventional hook or an

intelligent one, is also susceptible to the load’s interaction

with the seabed. Deepwater soil conditions tend to be

very soft, and so the deployment system must be capable

of touchdown without causing immediate bearing capacity

failure underneath the installed hardware. This may in

turn result in an unacceptable component orientation and

connection problems. Recovery, once embedded, may be

difficult if the component was lowered at close to the

deployment system’s load capacity. Both hardwarefoundation design and release system design play a role in

addressing this touchdown issue. However, with the

difficulties and costs associated with gathering deepwater

soils data for detailed foundation design, more onus may

need to be placed on the release mechanism design to

address the problem.

Once a load is placed on the seabed and released

there are further problems with controlling the location of

the lowering system hook which will inevitably become

less controllable when the beneficial tension is removed. It

must remain under control and be prevented from getting

entangled with the subsea equipment.

Metocean effects and weather window requirements

There is a natural tendency for subsea installation

tasks in deeper water to take longer than the equivalent

tasks in shallow water. It will take longer to lower to the

seabed, and longer to raise the lifting gear afterwards for

the next lift. It is likely to take longer to locate and

position equipment on the seabed. Attempts to speed up

the lowering process raise the question as to whether the

load can actually be made to sink at higher speed without

gliding off laterally and losing lateral position, landing at

a point distant from that required. This is another area

where the shape of the load, and the hydrodynamic forces

acting on it, will be a very important part of ‘design for

installation’.

It may be that some of these task durations may

become untenable in relation to the available weather

windows and metocean forecasting capabilities.

Consequently there will be strong pressure to find ways

of: doing things quicker, or doing them in such a way that





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they are less sensitive to the influences of winds waves

and currents.

Summary of Issues for DISH Phase 1

A large number of deepwater installation technical

issues have been touched on above. They can be

summarized as follows:

L if ting and loweri ng technology

• Use of synthetic fiber ropes including, reliability,

durability and winch design issues.

• Likely applicability of spoolable tubular

composites as a lifting line option.

• Applicability of free-fall installation processes.

• Applicability of buoyancy systems.

• Identification and validation of numerical

models for simulation of lift dynamics.

• Motion compensation (e.g. is partial

compensation of any value?)• Hydrodynamic design of loads to ameliorate

added mass and fluid loading issues, and to

facilitate faster lowering.

• Estimation of upper weight/depth limits for the

various potential lifting systems.

Load control and positi oning

• Powered, and perhaps ‘intelligent’, crane hooks.

• Position reference systems.

Metocean effects and weather wi ndow requi rements

• Duration of deep installation tasks.

• Practical available weather windows.• Technology required to speed up these tasks.

• Technology required to reduce metocean

sensitivity of the tasks.

Figure 6 from [2] also pictorially summarizes many

of these issues. Most will be investigated to some extent

in Phase 1 of the DISH project, and those considered

most important ‘cost drivers’ or ‘show-stoppers’ will

become the subject of detailed investigation in Phase 2.

PROJECT PLAN AND DELIVERABLES

A full description of the DISH project plan anddeliverables 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

years. This objective is to be achieved via a two-phased

project. Phase 1, just starting now, will be an 8-month

exploratory, problem definition study, resulting in the

targeting of key technological uncertainties and capability

gaps. Phase 2 will then address these detailed challenges,

over a period expected to be in the region of 16 months.

Thus the whole project is expected to last about 2 years.

Phase 1 will review the industry’s state-of-the-art

capabilities, and also the oil industry’s envisaged

installation needs, over the next 10 years. The findings of

each will be used to populate a capabilities and

requirements database.

The state-of-the-art capability data will be gathered

from the installation contractor participants, and the

installation contracting sector as a whole. Data will also

be gathered from the specialist supply sector (for instance

of specific deepwater deployment system components, or

of lift line dynamic modelling software).

The installation requirements will be gathered from

the oil company participants, with the specific aim of

reflecting the geographical spread and metocean-severity

spread of their deepwater prospects. The data will also

seek to reflect a full range of development solutions and

hardware components involved. This has implications for

the positioning accuracy, number of lifts, component

criticality, connectability requirements, life-of-field

intervention requirements, and configuration of each

component.

Phase 1 will then combine and compare these twoaspects – capabilities and requirements - and identify

technical uncertainties and capability gaps. These will be

reviewed, ranked and prioritized during the course of a

specially facilitated “mid-flight” project workshop. This

workshop will result in a key deliverable of Phase 1,

where specific technical uncertainties are targeted, and

plans developed to address them.

Phase 2 will address these targeted technical

challenges in detail. Tasks performed during Phase 2 are

likely to be of three distinct types:

•

modelling tasks, which aim to develop, validate andapply 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);





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• procedural studies, with the objective of assessing

how subsea operations should be carried out,

whether they are feasible and economic, identifying

key areas of difficulty, and finding practical

solutions.

The deliverables of Phase 2 will vary, dependingon the nature of the task. However, likely deliverables

are:

• 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 allowtechnical developments to reach prototype stage,

and recommendations about actions and priorities

for further work.

CONCLUSIONS

The DISH project will benefit the offshore industry

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.

ACKNOWLEDGEMENTS

The authors wish to thank all those who have

supported the launch of the DISH project. Particular

thanks are due to Stewart Willis of Stolt Offshore, and Ian

Edwards of Halliburton who provided much of the

background information on existing deepwater lifting andlowering limitations given in this paper.

REFERENCES

[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

Limited, and Offshore Technology Management Limited,

November 2000.





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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].

deepwater installation of subsea

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