Page 230 - Pipeline Pigging Technology

Pipeline isolation - available options

therefore decided that the pig train should be designed to hold the full static
head pressure (13barg) plus a factor of safety. Due to the cumulative nature
of the DP across the pigs, the factor of safety required can be relatively low,
because in losing one pig, for example due to damage, we only lose a small
percentage of the entire system's capability. The design requirement for the
pig train was therefore set at 15barg.
The use of nitrogen within the pig train also required careful considera-
tion. Whilst slugs of nitrogen were desirable to minimize diffusion of gas along
the train, their use would create other problems. When launching the pig
train for topsides' isolation into a pipeline at zero pressure, it had been
possible to vent off the residual nitrogen pressure after launching the first two
pigs. Launching the second part of the train had only compressed this to
approximately O.lbarg.
In the subsea case, this would not be possible when launching against a
pressure of 13barg. The nitrogen slugs would therefore act as springs with the
potential of pushing the pig train back towards the worksite after reducing
the launch pressure to static head pressure.
Examining the pressure profiles across the pig train, and the positioning of
the nitrogen slugs, became an important part of developing the pig train.
With a te-in difference in wall thickness between thick- and thin-wall, the
DP capability of the pigs was relatively low. A comprehensive testing
programme was undertaken to evaluate the effect of wear on the pigs and
long-term liquid retention capability, as well as disc material compatibility
tests with the various fluids with which the pigs would be in contact (bearing
in mind contact could last up to 60 days).
The pig train was designed with three pigs at the front, separated by slugs
of nitrogen. Again, the main purpose was to minimize the diffusion of gas
towards the worksite. These were then followed by four slugs of
recommissioning fluid trapped between high-differential pigs; a further eight
high-differential pigs separated by slugs of inhibited water would complete
the train. A standard bi-di would be added at the rear of the train to remove
the hyperbaric spheres on the way out. The lengths of all the liquid slugs were
sized to give the necessary spacing when receiving the train, to ensure that
none of the train left in the line would be in the other ball valves.

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