Page 401 - Pipeline Pigging Technology

Pipeline Pigging Technology

Flow limitations: To ensure the passage of pigs through the wye, there
has to be adequate flow in the main line and no reverse flow in the
branch. For a pipeline system which needs periodically to be coated
by a slug of corrosion inhibitor held between two batching pigs,
there may be limitations on the flow conditions at the wye to avoid
loss of inhibitor up the second branch.

WYE vs TEE

Tees normally have the advantage of being relatively small and light such
that they can be laid with the pipeline and need only a small protection cover.
Their main application is for tying-in smaller-diameter pipelines. They are not
readily piggable and would require specialist techniques such as gel or foam
slugs, or a subsea pig trap.
Fig.9 illustrates a subsea pig trap for a gas pipeline. The deployment,
operation and retrieval of this device would be a costly exercise unsuited to
routine pigging. It could, however, be justified for intelligence pigging.
Overall, the applications of wyes and tees are quite distinct, in that wyes
are suited to a same-sized piggable entrant, and the tee to smaller, rarely-
pigged entrants.

CONCLUSIONS

a) The technology for designing and manufacturing piggable wyes is now
maturing. This paper details the features to ensure that the junction is reliably
piggable, operates within allowable stress levels, and can be manufactured.
b) A successful operational track record for wye junctions has been built
up in the North Sea, and they are now being used in increasing numbers.
c) Wyes provide an alternative to import risers for the connection of other
fields to a pipeline system, and in many cases will show cost and safety
advantages both in installation and operation.

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