Page 139 - Pipeline Pigging Technology

Pipeline Pigging Technology

A summary of the different methods of checking internal geometry of
pipelines is given in Fig.3.

INTELLIGENT PIGGING

Soon after start-up in 1979-80, the market of inspection pigs was investi-
gated and tests made with the reputable pigs of the day, or Ist-generation
magnetic pigs. These were "metal-loss pigs" working on the principle of
magnetic-flux leakage detection. Total Oil Marine pic constructed a test line
for pull-through tests; the line included a valve, barred-tee, etc., together with
artificial defects in the line to evaluate the pigs' detection and sizing capacities
as well as their reliability. An additional test line with a 3D bend, similar to the
one installed offshore, was used, through which the pigs were pushed by
water, to confirm their capabilities of passing a 3D bend.
The Linalog pig was chosen to be run in the Frigg lines. The first survey
commenced in 1981, and a total of six runs were made, one in each half line
and two further re-runs or second inspections.
During the first four runs, very little was found which required further
investigation. However, minor features were reported, and these were
checked following the second run. The following was concluded:

some indications found by the first run disappeared from the second
run;
the detection accuracy was not good enough to conclude any trend.

Even with careful cleaning of the lines, such a long line (over 170km) can
still have small items of debris. These produce spurious indications which
cannot be distinguished from real defects or areas of metal loss.
The grading method used by Ist-generation vehicles was not sufficiently
accurate to determine trends unless the trends were so marked that questions
concerning the pipeline integrity would have to be asked. This was not the
case for the Frigg pipelines. We are looking for small features which could
lead to identifying trends in the pipelines' condition.
The Linalog defect grading system is given in Fig.4, but we consider it to
be too wide a spread for the type of defects expected in offshore lines.
Therefore in 1987, Total Oil Marine pic investigated the new pigs available on
the market, namely the British Gas 2nd-generation magnetic pig and the
Pipetronix ultrasonic pig.

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