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7/27/2019 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
1
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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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
2
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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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
3
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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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
4
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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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
5
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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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
6
• 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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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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Figure 1 - Field Depth Trend - from [1].
Figure 2 - Subsea installation weight trends - from [1].
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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
8
Figure 3 - Effect of current on lifting line and load position - from [2].
Figure 4 - The Bluewater Powerpod.
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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
9
Figure 5 - The Curly Wurly System.
Figure 6 - Deepwater installation issues - from [2].