Page 144 - Pipeline Pigging Technology

10 years of intelligent pigging

1988 INSPECTION OF LINE 1 SOUTH

The British Gas inspection vehicle was run in the Frigg line 1 from MCP01
to St.Fergus during September, 1988. No disruption occurred to normal
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production, with a flowrate of 8 x 10 SCM/day and a speed of 2m/s. The 175-
km long pipeline was inspected in one pass.

Results

Four external features above the British Gas reporting threshold (see Fig.4)
were reported on the line. In addition, British Gas was requested to investi-
gate the next seven severe features. All 11 features were found to have a
common link, namely that they were within approximately 400mm of a
circumferential girth weld and external to the pipe wall. This indicated that
perhaps some kind of handling damage occurred during pipeline fabrication
and construction. Further investigations were made into the pipe history
archives to identify any other common cause or links. If this could be
established, it could be unnecessary to undertake any diving work for further
investigations.
Two major problems exist with diving work for investigating a feature -
these are:

the possibility of further damaging the line cannot be ignored; and
the cost is probably 100 times more expensive than investigation of an
onshore line, typically£0.5 million to investigate one or two features
offshore in the Northern North Sea.

Another common link between all the 11 features was their shape and size.
All were relatively local features with typically an axial length of 20-30mm, a
circumferential length of 30-70mm with the depth varying up to a maximum
of 48% of wall thickness.

INVESTIGATIONS

Detailed study of the pipeline history archives resulted in a common
fabrication aspect for all the 11 features. The pipeline was originally fabri-
cated in 12-m lengths and then joined or double-jointed to make 24-m lengths

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