Page 395 - Pipeline Pigging Technology
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Pipeline Pigging Technology
APPLICATIONS
The principal use for a wye is to connect two pipelines of the same
diameter such that both can be pigged. Example applications are:
a) connecting an entrant into a pipeline at its closest point so as to
minimize the total pipeline length;
b) inserting a wye at the base of a riser to tie-in a second entrant to the
one riser, thus retaining the same number of risers and avoiding the
expense of retro-fitting ones;
c) combining a wye and subsea isolation valve installation;
d) stacking wyes in series, always retaining a piggable inlet to the
pipeline system for future entrants.
The alternatives to wyes are risers and tees. These are compared in the
following sections. Table 2 sets out the broad areas of application for each.
First of all, however, a characteristic arrangement for a wye junction (Fig.7)
is considered. This would be adapted to suit a particular job, but serves to
illustrate a few points as follows.
The offset layout shown in Fig.7 is mainly a function of the installation
method. Typically, the main pipeline would be installed with a flanged spool.
The wye, valves and protection frame, which would be too big to be laid in
Junction type Entrant line Pigging requirement
size
tee smaller infrequent
riser smaller routine
wye same infrequent or routine
none: lay another larger infrequent or routine
trunkline
Table 2. Main applications for riser, wye and tee junctions.
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