Page 390 - Pipeline Pigging Technology

Recent advances in piggable Y design

Further research work has been carried out by Seanor Engineering AS for
BP Norway as part of the BP diverless subsea production system (DISPS)
project. Seanor developed compact 12-in convergent and divergent wyes for
use in pigging flowlines from a platform to a template, around a crossover
loop and back to the platform. These were successfully tested in the vertical
on water, air and water/air mixtures. A preference for long-bodied (1.5D)
cupped pigs was established. These DISPS designs have not yet been used in
operation, but they form the ground work for future developments using
active-diverter wyes and compact-converger wyes.

ADVANCES IN DESIGN APPROACH

The following paragraphs describe an enhanced approach recently adopted
to produce economical designs for two large-diameter high-pressure wye
pieces. The main areas addressed are piggability, pressure containment, and
manufacture. Fig.4 illustrates the main features of the design.

Piggability

Piggability is a function of the profile of the internal bore. As detailed
above, a great deal of research and development work has been carried out
in this field, as a result of which the following features are incorporated:

a) The angle between the branches is set at 30°. Sharper angles increase
the length over which the bores merge, which would increase the
probability of a pig coming to rest in the wye with the flow by-
passing around it. Larger angles mean that the pigs have to turn more
sharply into the outlet, with correspondingly larger impact forces
and accelerations. Model tests indicate that 30° is the optimum
angle.
b) The bore in the section where the branches merge is enlarged to 105-
110% of the pipeline internal diameter. This is large enough to allow
the pigs to contact surfaces and expand out to their unrestrained
diameter, hence reducing the friction on the pig as it passes through
the wye.
c) The region just before the exit bore is smoothly profiled with
minimum radii of 5 diameters in the longitudinal planes. The reduc-
tion in bore is made gradually, over a distance of about one diameter.

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