Page 154 - Pipeline Pigging Technology

The Zeepipe challenge

Fig.3. Pre-commissioning/comniissioning pig.

If the discs for some reason are exposed to strong forces or vibration, tear
may occur and in extreme cases the steel flanges on the pigs may come into
direct contact with the pipe wall. The main concern related to wear is loss of
sealing capability. If by-pass occurs, the driving force will be reduced, causing
the pig velocity to slow down compared to the fluid velocity. However, even
large by-passing should not prevent the pig from travelling at a reduced
velocity. As an example, purpose-made pigs are reported to be fabricated
with up to 25% by-pass ports.
Experience from other pipelines confirms that even pigs having metal
contact with the pipe wall can pass through a pipeline without major
difficulties. A worn cleaning pig will therefore be propelled through the
pipeline, i.e. it will not get stuck, as long as the pipeline is free from
obstructions.
The main concern is therefore related to loss of sealing and cleaning effect,
i.e. loss of working capability.
The sealing effect is most critical during the dewatering operation. This is
because the amount of water left in the pipeline will depend on pig wear. In
extreme cases, excessive amounts of gas may by-pass the dewatering train
and accelerate the deterioration of the train, i.e. gas in the train will reduce
the dewatering efficiency.

Inspection pigs

Recent advances in intelligent pigging technology have made it possible
to inspect an 810-km pipeline without intermediate pigging stations. There
are several examples of pigs having accumulated more than 1000km of
pigging distance in gas systems without change of discs.

135