Page 399 - Pipeline Pigging Technology

Pipeline Pigging Technology

fitted in an existing pipeline, then the costs could go either way,
depending amongst other things on the pipeline lengths, the dura-
tion of the required shut down, and any penalty associated with
making the new line the same size as the existing.
Tie-in: Tying-in at a wye can be done without shutting down the
existing system. This has recently been demonstrated by the Gyda
tie-in. In comparison, construction work on a platform to tie-in an
entrant is likely to be more disruptive.
End of field life: If import risers are used and the original field is
depleted before the end of the pipeline life, it would need to be
maintained as a riser platform, or a subsea junction inserted. Using
a wye junction allows the original platform to be isolated and
decommissioned without affecting the rest of the pipeline users.
Emergency shut down: If import risers are used and there is an
emergency shutdown on the platform, the upstream fields will also
have to be shut down, whereas a wye junction would keep them
operating independently.
Shorter line: A wye junction can be placed to give the entrant the
shortest pipeline route. This is particularly so for a retro-fitted wye.

Wye junctions also have some drawbacks, and are by no means always the
best solution for tying-in an entrant. The main drawbacks are as follows:

Same size line: The wye junction's main use is to connect entrants of
the same size as the original pipeline. Whilst it is possible to connect
other sizes, these would not be piggable. There is typically a cost and
technical balance for an entrant between having, say, a smaller non-
piggable line to a tee, a larger piggable line to wye, or a longer
piggable line to a riser.
Subsea valves and protection covers: It would be feasible to have a wye
without valves. However, they are normally an operational require-
ment. For example, to tie-in an entrant without affecting the rest of
the system would normally need two valves on the branch of the
wye to give double-block-and-bleed isolation. For this reason, most
wyes to date have two isolation valves on each branch. If subsea
valves are used, it is necessary to have a protection cover.
Reverse pigging: Whilst not normally required in operation, it is
sometimes desirable to be able to pig in reverse during commission-
ing, for example in dewatering a line from the shore to the platform.
This would cause technical problems at a wye junction which is only
piggable in the convergent directions, and would require some form
of deflector plate for reverse pigging.
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