The success was realized by Furmanite working with Subsea 7, whose ROV systems were used to install the bespoke Furmanite-designed clamp and to mix and inject the specially-formulated sealing compound — quite a challenge to achieve by remotely operated vehicle.
The 10-inch leaking flange joint was situated on the flowline termination assembly at the base of the Foinaven water injection riser, which feeds injected water to different parts of the reservoir. Seawater ingress at the joint had caused corrosion, which was threatening the output capacity of the field.
Furmanite's clamp solution was designed to cover the flange connection. "Space was limited and the clamp had to be operable by ROV, with no opportunity to use heavy lifting gear, so while designed to cover the flange connection the clamp was also kept to minimum dimensions (some 666mm wide, 680mm high and 140mm deep, weighing around 200kg).
It was also engineered to be compatible with Subsea 7's bespoke ROV deployment system," Furmanite project engineer Mike Bowerbank explained.
Installation involved lowering the clamp over the pipe, with the lower half suspended by one clamp bolt; the clamp rotated until both halves were in-plane with each other, normal to the pipe axis; and (with both bolts now in place) the lower half drawn up on the bolts, bringing the clamp halves together.
With the two halves some 100mm apart the clamp was moved along the pipe and positioned over the flanges, at which point the bolts were initially tightened to seat and compress the seals, and then fully tensioned to the necessary pre-load to ensure that internal pressure forces would not part the clamp halves.
Critical to the success was the sealing resin, which was injected once the clamp was in place to seal the gap and bolt clearances. The challenge was to get the resin mix absolutely right to match the leak profile – in other words sufficiently fluid to allow injection and ensure it reached the leak point, while sufficiently viscous to ensure it bridged the 60mm hole without extruding into the pipeline.
In keeping the clamp as small as possible (covering the flanged connection rather than fully enclosing it), the bolt clearances also had to be considered, as these in effect provided 32 potential leak points that, with full pressure restored, the water would seek to find once the original leak path was sealed. It was therefore crucial that the resin must reach and seal these too.
A further challenge was to ensure that the resin could be mixed and injected remotely. "Traditionally for subsea applications the sealing resin would be mixed on the surface, and divers used to install and inject the clamp," Bowerbank explains.
"Clearly in this instance, with the depths involved, the whole process had to be undertaken by ROV. If the resin were sent down ready-mixed it would have cured before it could be injected, so a special remotely-operated injection and mixing system had to be developed. This involved injecting from separate storage cylinders through a mixer unit to ensure a good homogeneous mix, and then into the clamp."
Bowerbank adds that extensive testing was undertaken to ensure that this crucial element in the repair design would work as required. Two clamps were manufactured so that one could be used for testing purposes, and two test spools were manufactured (at Furmanite's facility in Carlisle), with the appropriate type of 10-inch flanges and a representative defect in the gasket.
The testing was undertaken at both Furmanite's and Subsea 7's premises. A test rig was built to the Subsea 7 skid injection unit specifications, and the two-part compound injected into the flange via the clamp, with the injection rig hoses, clamp and test spool flooded with water to simulate seabed conditions. The seal was then pressure-tested, following the cure period, by pressurizing the test spool.
The clamp was successfully installed and injected, sealing the leaking joint, in just 24 hours (including injection and cure), ensuring that water injection pressure could be maintained.
Commenting on the project, BP senior subsea engineer Sandy Meldrum said: "We were delighted with the solution design and implementation, and the co-operation between Furmanite and Subsea 7. The leak was presenting a problem to our production performance – a situation we had to resolve, preferably without having to shut down. We've long worked with Furmanite and know their capability to engineer bespoke solutions where required."
Bowerbank adds: "This is another example of our ability to develop solutions to meet customers' needs, helping them keep their assets operating and earning at maximum capacity."
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