Page 226 - Pipeline Pigging Technology

Pipeline isolation - available options

hydrate formation and minimize corrosion. This is both costly and time-
consuming. It is therefore only really feasible to flood and commission short
pipelines of small diameter.
Nitrogen purging the pipelines can also be very expensive on larger sizes
of line. Due to vaporisation of condensate, etc., even this doesn't guarantee
to make the line perfectly safe. A local isolation is usually required, again in the
form of a sphere or stopper, to prevent vaporised liquids coming into the
worksite area.
The alternative to this, particularly on longer trunklines, is to carry out a
local isolation. Several techniques have been examined for carrying out this
type of isolation, including: tethered inflatable stoppers and bags inflated by
an umbilical, remote-controlled stopper pigs, and high-differential high-
sealant pig trains.
McKenna and Sullivan has had particular experience with the high-
differential pig train, which has been used successfully on several operations.
The concept of the high-differential pig train was specifically developed to
meet the needs of operators requiring this localized isolation. Due to the short
time period available on the first project where this was used, the pig train was
decided upon because insufficient time was available for development and
manufacture of other systems.
The pig-train concept was seen as utilizing proven basic technology in the
form of bi-directional pigs and with an in-built factor of safety due to the
number of pigs being used. Trials were carried out to develop two types of
pigs: (a) a high-sealant pig to provide the main gas interface, and (b) a high-
differential pig to provide a factor of safety in the event of either inadvertent
pressurization of the line or rupture of the line which could cause it to fill with
water and pressurize.
A test loop was built to simulate conditions in the pipeline. This consisted
of a section of light-wall pipe, a section of heavy-wall pipe and a 90°bend.
Various disc configurations were tested on a standard bi-directional pig body.
Different oversized discs were used in varying configurations to try to achieve
the best combination of either sealing characteristics or high-differential
characteristics without damaging the discs or the pig body. Many combina-
tions were initially tested, from the original bi-di configuration up to the point
where the force across the pig was so great that the discs tore under the stress.
Eventually an optimum disc configuration was found, where no damage
occurred to the pig and the maximum differential pressure (DP)/sealing
capability was achieved.
Subsequent testing of pigs on other pipework systems has led to further
development of this initial concept. Unfortunately, from the operator's point
of view, it has become clear that the suitability of a particular pig for providing
high DP is unique to the size of pipe involved and the difference in wall

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