Page 159 - Pipeline Pigging Technology

Pipeline Pigging Technology

been used in pipelines where excessive pig wear has occurred. The
possibility also exists of modifying the shape of these discs, and of
prolonging the "sleeping" time.

Cups: Traditionally, pigs were equipped with sealing units shaped as
cups; the use of discs is a relatively-modern technique. Cups are
claimed to last longer, although discs, however, are known to
perform better. A combination of discs and cups will be further
evaluated.

Cup shape: Traditionally, a spherical cup shape has been used. Today,
conical and parabolic cups are also available on the market. This will
be further evaluated if cups are to be used.

Increase the oversize of the sealing discs: This will provide more
material to wear down before sealing is lost. However, average wear
may be faster. This will also be further investigated and tested.

Disc bending moment". An optimization study on disc bending moment
will be performed to evaluate the distance from the pig "body" to the
tip of the disc and the disc thickness and stiffness in order to obtain
optimum parameters for the Sleipner to Zeebrugge pipeline.

Forced rotation of the pig: From the wear characteristic of mechanical
pigs, it is evident that pig rotation is limited. By forcing the pig to
rotate, for instance by an offset wheel, the effective length of each
pig run may be improved.

Prior to selecting the pigs to be used in Zeepipe, all of the above aspects
will be evaluated. Currently, the most promising concept is regarded to be the
use of wheels, possibly in combination with further general improvements of
the pig. When the pig design has been concluded, different opportunities for
testing will be employed.
Apart from the more standard tests performed in the workshop and in test
loops, these pigs, together with standard off-the-shelf pigs, will be subjected
to full-scale tests in existing gas transmission systems.
The most important and relevant test, however, will be during the
intermediate testing of the Zeepipe pipelines after the lay seasons 1991 and
1992, and two purpose-designed pigs are planned to be included in the
intermediate testing pig train. The timing of these operations will allow
further modifications to be implemented and a retest carried out, if required,

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