Page 392 - Pipeline Pigging Technology

Recent advances in piggable Y design

d) The web between the incoming branches is kept as long as possible
to maintain the separation between the bores. The crotch area,
where high stresses would otherwise develop, is machined back and
profiled locally.

Manufacture

Scoping calculations show that scaling up existing smaller-diameter de-
signs leads to problems with high weights and thick walls. Fig. 5 shows a graph
of predicted weight as a function of pipeline diameter for 2500psi pressure.
Concerns are that the thicker walls would lead to high costs in manufacture,
inspection and handling. The design illustrated in Fig.4 is, therefore, adopted,
with a smooth external profile and thinner walls suited to both forging and
casting manufacture and to ultrasonic inspection. This approach also shows
a considerable weight saving, as illustrated in Fig. 5.

FE analysis for operational loads

The behaviour of the wye under operational loads is determined using
finite-element modelling. Pressure containment, loads from the branch
pipework, and temperature differential stresses due to incoming streams at
different temperatures, are evaluated. Stress and fatigue levels are kept within
BS5500 allowables.
A full-PC version of ANSYS is used. Accounting for symmetry planes within
the wye, a quarter model is generated comprising typically 1200 8-noded
brick elements, as shown in Fig.6. A minimum of three elements are used
through the wall thickness. High stress gradients occur in the neighbourhood
of the wye crotch, and the mesh is further refined in this area to evaluate the
peak stresses.
The behaviour of the wye under pressure is to bend outwards at the
elongated sections where the bores are merging, as shown in Fig.6. The shape
of the cross section is arranged to resist the bending with thicker central walls.
This bending movement is also restrained at the crotch, which is conse-
quently the most highly stressed region. FE analysis determined that it is
necessary to cut back the area between the bores to relieve stress concentra-
tion. Under bending moments in the wye branches the stress intensifies in the
outside of the crotch, which was shown to need reinforcement and a smooth
profile. Stresses in the body of the wye were generally very low compared to
code limits, which points to the potential for further design optimization.

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