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Question: When connecting a sub-sea high pressure line with 11" 10M flanges, connections made-up on the surface leaked on test after they were lowered to the sea floor. Our special efforts to assure no leaks after surface make-up included adding a test port to one flange in each connection to test the BX gasket seal after tightening all the bolts. Simultaneous hydraulic tensioning assured the API recommended tension in all the bolts. The flanges had standard BX ring grooves cut into the base metal of the flanges; we used BX gaskets made of 316 stainless steel. The ring grooves and gaskets were wiped completely clean before assembly. Surface testing of each connection revealed no leaks after make-up. Once lowered to the sea floor, sections of the line received a hydrostatic test and a slow bleed off of test pressure revealed that some of the connections now leaked. Recovery of the leaking sections and disassembly of the flanged connections revealed no apparent problem. Re-examining the ring groove surface finish and gasket fit revealed no apparent cause. Can you recognize from our description anything we may have overlooked, or anything we may do to remediate this problem? Answer: If you make-up a line with connections of this size and pressure on the surface, and attempt to lower the line section by section to the sea floor, the "equally tensioned" bolts will experience increased strain on the lower side of the flanges due to bending in the line. This increased strain will allow some separation of the gasket seal area allowing the gasket to shift slightly in the groove and destroy the intimate contact necessary to maintain a seal. Because this strain may prove unavoidable, it may help to apply a thin layer of lubricating grease uniformly all around and all over the ring gaskets before installing them in the connections. This lubrication may allow the slight movement of the ring gasket in the ring groove, due to the strain applied to the bolts during lowering the line, to correct itself as the line settles on the sea bottom and the test pressure acts on the gaskets to re-center them in the seal groove. Caution: We can't be certain that the description of this problem, or the answer suggested, explains the full extent of the original problem or the definitive solution. However, the operator submitting the question reports that no further leaks occurred after implementing the suggested lubrication of the ring gaskets on the approximately 200 connections making up the mentioned line. Later added note: Loss of seal may allow sea water to enter a ring groove under a ring gasket. This water under a ring gasket may prevent the gasket from regaining the intimate contact necessary for sealing. The use of SBX Ring Gaskets, now manufactured specifically for sub-sea connections, should provide further assurance of a reliable seal. API 17D specifies increased bolt tension for sub sea flange connection of API 6BX, API 17D SS and API 17D SV flanges. Click here to learn more about API Spec 17D SS and SV flanges.
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