Page 384 - Pipeline Pigging Technology

Recent advances in piggable Y design

RECENT ADVANCES IN PIGGABLE WYE

DESIGN AND APPLICATIONS

INTRODUCTION

There are four subsea piggable wye junctions in the North Sea at present
(Fig. 1) and four more are on the way. The offshore oil and gas industry is quite
rightly cautious about having them, with concerns centring on whether they
can be reliably pigged. On the other hand, as operators concentrate on
developing the existing pipeline infrastructure, wyes show many advantages,
particularly in reducing the number of import risers on platforms from other
fields. These two main issues: the design of piggable wyes and their applica-
tions, are addressed in this paper. Ways of improving on present designs are
identified, and the potential for use of wyes in field development is discussed.
Regarding design, this paper reviews the designs that have been used to
date, the pigging tests which were carried out on them, and operators'
experiences of pigging them in practice. Based on recent work on the design
of wyes for two high-pressure gas pipelines, this paper goes on to suggest
ways of improving present designs to make them lighter and more easily
manufactured.
Typical field developments making use of wyes, tees and risers are
compared and contrasted to show where wyes are best employed. Putting in
a piggable wye is by no means a universal panacea, but there are instances
where it can eliminate additional risers by combining flows into a single riser,
or could change the field development concept from a collector platform to
a subsea junction at a safe distance from the platform.

NORTH SEA WYE JUNCTIONS

Table 1 shows the wyes presently planned and installed in the North Sea.
The first wye was installed by Occidental in 1978 on the 18-in gas pipeline
between Piper Alpha and MCP-01. Illustrated in Fig.2, it was made from a
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