Page 229 - Pipeline Pigging Technology

Pipeline Pigging Technology

works had a significant impact on the way the valve assembly was installed.
These indicated that all flanged joints should be leak tested at 1.1 MAOP,
whereas a minimum number of new welds could be inspected by 100% NDT.
This meant that in order to avoid pressure testing the whole line, the flanged
valve had to be pre-tested with flanged pup pieces already in place, rather
than welding-in the two flanges offshore and then bolting in the new valve.
In practice, the differential pressure across the pig train in the offshore
phase was slightly less than that anticipated from the trials; this may have been
due to condensate present in the line. The pressure required to 'flip' the entire
train to return it back to the platform on completion of the operation was
lO.Sbarg. Combined with the static head of diesel available, this meant that
the pig train would have held back a DP of up to ISbarg.

SUBSEA VALVES

Following the success of the high-DP pig train for pipeline isolation for
topsides' valve installation, its application for subsea valve installation was
studied. The application for subsea works introduced several new factors into
the pig train design concept.
Firstly, because the construction work would be carried out subsea, it was
necessary to launch the pig train with water to provide the necessary working
environment for the divers. This would be advantageous for control and
positioning of the pig train, as water is largely incompressible and easy to
meter. It would, however, mean that some method of recommissioning the
pipeline would be required.
The design premise for the pig train was also altered by the construction
work being subsea. It was always intended that the pipeline would be vented
down to static head pressure subsea, i.e. approximately 13bar. With the pig
train in position and the pipeline cut, the pig train would be in dynamic
balance, with 13bar gas pressure on one side and 13bar static head on the
other.
The differential pressure capability of the pig train would only come into
play in an emergency situation. Initially, this was taken to be inadvertent
pressurization from the far end with gas moving the pigs towards the divers.
However, this was found to be highly unlikely as, in this case, gas injection was
not possible. Further examination of the system gave a worst-case scenario of
a topsides' leak or rupture at the far end leading to pipeline depressurization.
The full static head would then be acting across the pig train, and the divers
could potentially be sucked into the pipeline if the pig train moved. It was

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