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Everything you always wanted to know about flexible pipe* (*but were afraid to ask)
Transcript of Everything you always wanted to know about flexible pipe* (*but were afraid to ask)
What is it?
Smooth Bore Liner
Other 'umbilical' lines
Who makes it?
NKT Flexibles - Denmark
Coflexip - France
Short Lengths (25 Tonnes)
Long Lengths (to 2500 Tonne)
(to 250 Tonne)
Everything you always wanted to know about flexible pipe*
(*but were afraid to ask)
Not many people make flexible pipe.
In fact, only three firms worldwide, each
of which grew out of the sub-sea cable
NKT Flexibles are based in Denmark. They have recently been acquired by NOV (and are not called NKT any more).
Wellstream were originally a US firm that opened a satellite facility in Newcastle, which is now their global HQ. They were recently acquired by GE (but are still called Wellstream).
Coflexip were acquired some time ago by Technip, and everyone now calls them Technip.
They like to claim that they were the inventors of flexible pipe, and get quite cross if you mention Operation Pluto.
The key components of a
flexible pipe are:
Sometimes, in order to reduce the turbulence within the flow, a 'smooth bore liner' is added as the first layer of the pipe.
Such pipes are only used in specialist applications, such as dry gas export lines, where 'singing risers' and low flow rates have caused problems
The most common first layer, however, is formed using a self-interlocking stainless steel tape.
Note, this layer is directly exposed to the well fluids, and is normally a corrosion resistant alloy.
Next is the pressure sheath, which keeps the well fluids in the pipe. Much as the rubber bladder in a football contains the air without possessing any intrinsic structural strength, it must be supported by other layers of the pipe.
It is a continuously extruded layer of thermoplastic, and must be formed in a single operation. No repairs are allowed.
The internal pressure is contained by the pressure vault or pressure armour. This is wound around the pipe much like the carcass layer, but is considerably stronger.
The cross section is constructed to allow a limited degree of curvature. Too much curvature and the interlocking will disengage - a failure mode that cannot be repaired.
Tensile armour, as the name suggests, supports the tensile loads within the pipe. These can be considerable, particularly in deep waters.
They are wound in opposite handed pairs around the pipe to ensure torsional balance.
Normally fabricated with high tensile steel wires, there are moves towards carbon fibre rods to improve the strenght-to-weight ratio.
Tensile armour, however, cannot support the compressive loads that may occur. The stability of the tensile armour is improved by adding layers of kevlar tape.
Layers of insulation may then be added to improve flow assurance.
Indeed, some designs add a range of additional hydraulic lines, electrical cables, fibre-optic lines and heating elements
The final part of the puzzle are the end terminations - you need at least two.
Each of the layers performs its own particular function and must be terminated in a particular way. This is also the point where solid and rigid pipe codes collide, so there are two sets of requirements to meet.
It can take five years to train someone to make a termination correctly
The classic application for flexible pipe is to link a mobile FPSO on the surface with static sea bed assets.
For such dynamic applications, there is really no other option than flexible pipe. There is, however, no reason why it cannot be used for static applications too.
The flexibility of the pipe allows the FPSO to move relative to the riser base. The extra pipe has to be kept somewhere, so shallow water installations can get complicated.
In shallow water, there is nowhere for the extra pipe to go. Complicated 'double hump' installations are possible, but the riser will necessarily impact on the sea bed and may need additional protection.
Flexible pipe may also be installed
'flexibly'. Large, single purpose
vessels are available (below), but
may not be required for simple
Short lengths - flow lines between well heads and manifolds for example - can be delivered on 'pallets'.
Although more expensive, the use of flexible pipe provides greater dimensional tolerance within subsea developments.
Very long lengths (up to 20km for 6-inch pipe) can be dispensed from a 'carousel' within the hold of a purpose built ships.
Such vessels are expensive and unlikely to be anywhere near by - adding to the costs of deployment.
Most flexible pipe is delivered on reels (of reasonably standard dimensions) up to 10m in diameter. Such lengths (say 2km of 6-inch pipe) are sufficient for the majority of applications.
They can be handled with readily available equipment that can be attached to any support vessel.
Pipes are dispensed in a controlled manner using caterpillar tensioners, such as the 4-track variety shown.
They must also be 'overboarded in a controlled manner such that the minimum bend radius is not exceeded.
At one end of the scale, a simple chute will do...
More complex (and expensive) systems are available that allow a range of ancillaries to be attached on the way.
The whole process is story boarded and modeled in exquisite detail to ensure that the pipe and fittings are not damaged during installation.
There are many local implementations of flexible pipe, for example:
Why is flexible pipe particularly suited to Australia?
It is much faster to lay (up to 1000m per hour). This can make installation during cyclone season a simpler thing to schedule.
It uses the same installation kit as the umbilicals and cables that will also be part of the development.
All the testing and fabrication is carried out at the factory - far simpler to deploy in remote areas.
The pipes can be - and often are - recovered and redeployed, spreading the costs over multiple deployments