Page 137 - Pipeline Pigging Technology

Pipeline Pigging Technology

Corrosion coupons and probes are located at either end of an offshore
pipeline, and will not provide information in the areas of greatest interest, i.e.
downstream of a bend or at a low point in the gas line where liquid can
accumulate.
We therefore believed, since start-up, that we needed to monitor the
pipelines' internal condition as accurately as possible.

GEOMETRIC INSPECTION

Total Oil Marine pic has run a series of geometric pigs within the lines to
prove that the lines are free from dents or restrictions which may either give
cause for concern from the point of view of running a large inspection pig or
because it is known that dents, if associated with gouges, etc., can substan-
tially reduce the strength of the lines.
Geometric inspection is often used on major offshore lines prior to start-
up to confirm that the lines are free from harmful restrictions. This was also
performed on the Frigg Transportation System.
A T.D.Williamson geometric pig was run twice in each 32-in pipeline to
produce a "signature" for the line. It was run twice to attempt to identify
debris within the line which, in theory, should move from one run to the next.
Accuracy of the pig was about 1% of ID (internal pipe diameter).
For the 24-in Alivyn - Frigg pipeline, the signature was obtained in two
ways:

on the riser, by using a KIT (riser inspection tool) from H.R.Rosen;
in the pipeline, with the "out-of-roundness" pig developed by H.R.Rosen.

The order of accuracy of the vehicles were found to be 0.1mm, i.e. 0.01%
ID, for the RIT and 1.0mm, i.e. 0.1% ID, for the pipeline tool.
There is now no reason to systematically run geometric pigs to either
gather information about the line or to ensure the line is clear prior to running
an intelligent pig. The possibility of an unknown dent occurring since the last
survey can be checked by running a gauging pig. The first pig to be run has
a narrow body, such as a LBCC-2 or Vantage IV. This is followed by running
pigs with increasing gauging plate diameters. Finally, bi-dis are run, which we
have found to be the most efficient at removing both debris and liquid from
the line. A typical pigging programme is detailed in Fig.2; if the last pig and
gauging plate arrive undamaged, then the inspection pig can be run with
confidence.

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