Page 397 - Pipeline Pigging Technology

Pipeline Pigging Technology

line, would be installed next to it. The pipeline spool would be removed and
replaced by dogleg spoolpieces to tie in the wye. The pipeline system would
then be leak tested and p re-corn missioned.
The two valves on each branch allow either branch to be isolated whilst
the rest of the pipeline system is operational. This function could be used, for
instance, during a pipeline repair, for tying-in another pipeline,
decommissioning a branch line, or pressure testing an ESD valve. It is always
worth considering, however, whether all the valves are strictly justifiable.
At a later date the entrant pipeline would be installed and connected to the
spare branch. In the case of a gas line, it would normally be dewatered to a
pre-commissioning valve, a spoolpiece would be connected across to the
wye, tested and blown down, and the entrant pipeline dried prior to
commissioning. An entrant to an oil system could avoid the extra pre-
commissioning valve by testing against the wye valves and dewatering back
to the platform. Again, there are many variations on this depending on the
relative timing of the main pipe, wye and entrant pipe installations.

WYE vs RISER CONNECTION

The main alternative to a wye junction is to connect the second pipeline
via a riser. Fig.8 compares the field configurations resulting from wye and riser
tie-ins. Several advantages and a few disadvantages arise from having the wye
as opposed to the riser as discussed below. First the advantages:

Safety: as can be seen from Fig.8, the wye junction eliminates the need
for an additional import riser on the platform, and is thus a safer
solution from the viewpoint of the platform, particularly for gas
pipelines.
Field layout The wye junction can be sited away from the platform
avoiding seabed congestion around the platform. This leaves the
field free to be developed using satellite wells and flowlines, for
example, without being crowded by incoming pipelines from other
fields. It also allows the field layout to be planned with greater
certainty, keeping pipelines and flowlines in corridors with safe
anchoring areas between, avoiding spoolpieces under boat-loading
areas, etc.
Cost. The wye will normally show cost advantages over a riser, particu-
larly if the riser has to be retro-fitted, or a cantilever extension has
to be added for the pig receiver. If, however, the wye has to be retro-

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