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19
To:
Subject:
Leaking flange connections in sub-sea line
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 for API 17D specified torque values for Fluoropolymer coated bolts and nuts
connecting flanges in sub sea applications.
Click here to learn more about
API Spec 17D SS and SV flanges.
Click here for Q & A 21 Ring Gasket Evaluation
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