Page 227 - Pipeline Pigging Technology

Pipeline Pigging Technology

Fig.l. Primary dynamic seal - intended pig train position.
thicknesses. For example, a high-DP pig developed for 24-in pipe will not give
similar results at 36in, because the area of contact on the pipe wall changes,
the relative distance between the disc support flange and the pipe wall is
different, and hence the deformation of the disc is altered.
Differing wall thicknesses have an even more marked effect on DP
capability as one might imagine. DP's obtainable in pipe of constant bore are
more than halved in the pipe configurations where we have a ^-in difference
in wall thicknesses due to the damage caused by heavier-wall pipe.
If reproducible results are required in the field, then tests will be required
to establish the particular figures for a given set of pipeline parameters.
On this initial topsides' isolation, the pig train was designed using the
following parameters:

i) the front part of the train would aim to provide the main interface to
prevent migration of gas towards the worksite;

ii) the second part of the train would provide the differential holding
capability which would provide a large factor of safety in the event
of inadvertent pressurization or pipeline rupture. This would be
achieved by two means; firstly by using high-DP pigs, and secondly
by using slugs of liquid between the pigs to create a static head
should the pig train start to move up the riser.

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