Pipe joints or couplings – Variable length – Telescopic with relative motion
Patent
1990-07-03
1991-12-03
Arola, Dave W.
Pipe joints or couplings
Variable length
Telescopic with relative motion
285900, 166355, F16L 2712
Patent
active
050694880
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for compensating the movements of a riser pipe running between a mobile offshore structure and a wellhead on the sea-bed, in which the riser pipe is connected fixedly to the offshore structure and at its upper end equipped with a sliding joint and a movement-compensated suspension assembly comprising at least one hydraulic cylinder for that section of the riser pipe which is below the sliding joint.
Such compensation for movement, employing a sliding joint, is common when oil and gas wells are being drilled from a mobile rig, for instance a semi-submersible rig or a drilling vessel. The sliding joint will here compensate for the varying distance between the well-head and the drilling rig which is caused by tides, the heaving movements of the rig because of waves, and the drift of the rig. During drilling, the pressure inside the riser pipe is comparatively low. However the pressure may increase if a shallow pocket of gas is encountered, and the sliding joint is therefore generally designed to withstand a pressure in the order of magnitude of 35 bar during a shorter period of time. However, in such blow-outs of shallow gas it has been found that the sliding joint will start to leak heavily after a short time, probably because the comparatively simple seals of the sliding joint have given small leaks which have rapidly become very much larger due to the flow of gas containing large amounts of highly abrasive impurities. Serious fires have started because of such leaks.
Another problem of previously known sliding joints is that only with difficulty can they be protected from being overloaded. If in an emergency the riser pipe is shut off at both ends while full of incompressible drilling mud, the riser pipe could most aptly be considered as an axially rigid pipe, and any heaving movements that the rig may make, would thus create a very much higher pressure in excess of what the sliding joint can tolerate.
Sliding joints based on the telescoping principle are subjected to axial forces which will seek to urge the various parts of the sliding joint apart and are commensurate with the pressure inside the riser pipe. These forces are absorbed by tension cables attached to the lower section of the sliding joint or the riser pipe directly below it, and these tension cables also serve the purpose of maintaining the tension in that part of the riser pipe which is below the sliding joint. It will be appreciated that if the pressure inside a riser pipe of an internal diameter of 540 mm load on the tension cables and the appurtenant system for movement compensation, necessitating over-dimensioning with the ensuing additional weight and costs.
Production platforms have traditionally been permanent ones, either of the jacket type or concrete gravitation platforms. These have, however, become too costly as the exploitation of oil has moved out to ever increasing depths, and instead mobile rigs such as e.g. tension rod platforms, have been employed. On marginal fields where a permanent platform would also prove too costly, drilling vessels have been used, and chain line anchored semi-submersible drilling rigs converted to production purposes.
Because the pressure inside a production riser pipe may easily be ten times higher than in a drilling riser pipe, this would have resulted in expansion forces in a possible normal sliding joint which would be difficult to handle without the costs becoming prohibitive. Production riser pipes for mobile production platforms where there has been a requirement for vertical accessability in the well, have therefore been constructed as an integrated unit suspended in tension systems and guides, capable of absorbing the necessary strokes and angular deviations. Such rigid riser pipes present the drawback that all operations must be based on moving systems (production trees, blow-out preventer valves etc.) with correspondingly complicated connections. Furthermore, a complicated, voluminous and expensive tension suspension system is required.
In order to
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Freyer Jan
Nergaard Arnfinn
Arola Dave W.
Smedvig IPR A/S
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