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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