Page 235 - Pipeline Pigging Technology

Pipeline Pigging Technology

surface-bearing area (that part of the pig that touches the pipe wall) to add
wear resistance and sealing ability. Abrasive surfaces such as steel wire
brushes can be added to increase the cleaning and scraping ability.

HISTORY

It is not certain when the first flexible foam pig was put into a pipeline, or
who came up with the idea. The first recognized foam pig was patented
(Wheaton) in 1954 for use in the diary industry. A low-density foam cylinder
(resembling furniture cushion material) was inserted, under a vacuum, into
the milking system, displacing the liquid and making the cleansing process
more efficient. On one end of the cylinder a thin layer of rubber gasket
material was applied to act as a seal against the vacuum. The coated cylinder,
or "swab" as it became known, was also used in pressurized pipe applications.
Although it worked well for light cleaning and drying in short length lines, it
had a tendency to break apart, and thus had a limited use.
In I960, a major oil company required a flexible pig that would remove a
build-up of anaerobic bacteria in a water-injection system constructed from
transite pipe. The short-radius 90° bends contained in the system would not
allow successful passage of a sphere or mandrel pig, and the low-density swab
would not clean the deposits sufficiently. The oil company enlisted the help
of a firm that was involved in manufacturing packaging materials and other
products from a new polyether, open-cell foam system. The material was
nearly as flexible as the soft foam used in swabs, but had a greater tear strength
and firmness. The higher-density foam was moulded in the shape of a bullet.
The nose of the pig was parabolic, to help negotiate the bends, and the base
was concave similar to the back side of a cup, to assist in sealing. Called the
'Polly Pig', it negotiated the system and removed the deposits from the pipe
wall without losing a seal or plugging in the tight bends.
The next stage in the evolution of the polly pig was the addition of an
external coating. The foam systems available in the 1960s were not very
durable and had a tendency to wear quickly and break apart under the
stressful conditions found in cross-country pipelines. To strengthen the foam,
a flexible, polyurethane elastomeric coating was applied to the exterior of the
foam body. The base was coated to minimize by-pass through the pig body,
and the nose was coated to resist wear when the pig negotiated bends in the
pipeline. The surface bearing area of the foam body was covered with a spiral
pattern of the coating to give the pig greater wear resistance and wipe the
pipe more efficiently. In an effort to increase its sealing ability, another series

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