Page 239 - Pipeline Pigging Technology

Pipeline Pigging Technology

Custom designs - Because an operator sometimes faces unique pigging
situations, he has the occasional requirement for a pig that is not available "off
the shelf'. Due to the method of moulding foam and applying external
coverings, it is relatively simple to design and build a polly pig for a particular
pipeline problem.
Less risk of a "stuck" pig - with the flexibility offered by the polly pig, there
is less risk that it will get stuck at a dent, partially-closed valve, or some other
unknown obstruction. A foam pig can easily deform to accommodate diam-
eter reductions, and in the event that it does get lodged in the line, it will have
a tendency to break apart if sufficient differential pressure is applied.

Cleaning ability- an efficient cleaning pig serves two functions while inside
a pipeline. First is the scraping or wiping of the pipe surface; the second is to
assist in moving the deposits out of the pipeline. There is more surface-
bearing area on a foam pig than on any other standard-sized, conventional
design. For instance, in a 24-in pipeline, a polly pig has three times more
surface in contact with the pipe wall than a four-cup mandrel pig, and seven
times that of a sphere. The foam pig has a jetting-type by-pass between the
surface-bearing area and the pipe wall to assist in suspending deposits such
as scale or wax ahead of the pig. This reduces the risk of solids piling up in
front of the pig and possibly causing the pig to get stuck.
Removal of solids from a pipeline always involves a certain level of risk. If
the solids pile up ahead of the pig, they can form a plug and possibly cause the
pig to stop moving. One concept, or method, available to the operator faced
with cleaning a severely-fouled pipeline, is the "progressive pigging" proce-
dure utilizing foam pigs. If a pipeline has accumulated a large volume of
deposits such as paraffin or scale, it can be difficult, and sometimes disastrous,
when an attempt is made to remove too much of the material during any given
pig run. Using polly pigs, an operator can take advantage of the density ranges,
various designs and diameter sizes to safely remove the solids in stages. Soft,
undersized pigs are initially run through the line to remove any loose, or soft,
deposits, followed by progressively larger, firmer, and more aggressive pigs.
The natural by-pass between the cleaner and the pipe wall helps to keep the
solids in suspension ahead of the pig. This procedure gives the operator more
control over what is taking place inside the pipeline, and reduces the risk of
bridging the flow. Since it is difficult to accurately predict the build-up
throughout the piping system, the flexibility of the foam pig allows for a
degree of error if the deposit is heavier than predicted.

220