Page 477 - Pipeline Pigging Technology

Pipeline Pigging Technology

It is conceivable that research and development such as this could lead to
the safe operation of multi-phase pipelines with significantly smaller liquids'
handling facilities (e.g. slug catchers) and the virtual elimination of slugging
- a pipeliner's "dream come true"....

PIGS FOR DIFFERING DIAMETERS

"Double-diameter" pigs are capable of traversing pipelines which have
been built with more than one nominal diameter in between pig traps.
To reduce shipping costs, it was not unusual to design a pipeline with two
diameters, so that the smaller pipe could be transported inside the larger one.
This resulted in initial savings, but maintenance costs are generally higher and
pigging is, at best, a compromise.
Most pig manufacturers have "double-diameter" pigs in their range, typical
of which is the WCK-3DD of T.D.Williamson (Fig.5). One company in
particular, S.U.N.Engineering, has done considerable work on the develop-
ment of double-diameter pigs, and it now has a range which is capable of
running in lines which have three, and in some cases even four, nominal pipe
sizes between the large and the small diameter. A typical S.U.N. pig is shown
in Fig.6.
Very few new pipelines are now laid with more than one diameter
between traps. However, the increasing need to tie-in marginal fields to
existing export pipelines is highlighting the importance of developing pigs
which are capable of extreme double-diameter performance. Precisely how
"extreme" will need to be studied carefully, but lOin or 12in into 30in (i.e. 9
or 10 pipe sizes) may well be typical.
If only liquids' removal was required, a sphere or a foam pig run through
a tee may well suffice, but this would not be adequate for effective solids'
removal. Most importantly, intelligent pigging would be impossible.
Effective cleaning and intelligent pigging will require an arrangement
similar to that shown in Fig.7. This shows a wye installed in the export line,
with a reducer upstream on one leg to enable the smaller-diameter marginal
field line to be tied-in.
It is unlikely that an intelligent pig can be designed to have extreme
double-diameter capability, so the challenge will be to design a conventional
pig which can both clean the small-diameter line and/or tow an intelligent pig
behind it.
Clearly, this will require some radical design and some extensive trials - all
of which must be funded. But the rewards to the operators of marginal fields

458