Page 141 - Pipeline Pigging Technology

Pipeline Pigging Technology

Again, pull-through trials were performed and evaluated to decide which
was to be chosen for the Frigg lines.
Both the pigs performed extremely well in terms of sizing accuracy and
repeatability. In addition, they appear to be able to inspect near the girth weld
areas. However, large practical problems'were identified when running an
ultrasonic pig in a major gas line; that is, the pig needs to run in a liquid batch
to act as a coupling medium. The presence of any gas bubbles in the liquid
could cause loss of coupling, and therefore loss of inspection results.
This problem, in terms of disruption to the production and the logistics of
handling many hundreds of tonnes of liquid at either end of the line, at present
is still to be solved. For example, a slug of liquid 4km long (i.e. 2km either side
of the inspection vehicle) would typically be the amount of liquid required to
give some confidence for a 170-km inspection run. The British Gas pig was
subsequently chosen and run in the Frigg lines.

COMPARISON BETWEEN MAGNETICS AND
ULTRASONICS

Total Oil Marine pic believes, based upon test data, that in terms of pure
accuracy of defect depth, ultrasonics have a superior accuracy to magnetic
pigs. This is not unrealistic when one considers the physics involved in each
technique. However, magnetic pigs are more likely to pick up small, deep
corrosion pits which may be missed by the individual ultrasonic pulses.
Both 2nd-generation magnetic pigs and ultrasonic pigs are capable of
distinguishing between internal and external features; this is a major step
forward in attempting to identify the cause, and thereby possibly save a diving
campaign to investigate a feature.
The advantages and disadvantages of each type of pig are tabulated in Figs
5 and 6.
However, it appears that ultrasonic pigs are more suitable for running in
liquid lines, and we therefore have chosen the Pipetronix vehicle to run in the
12-in Alwyn -Ninian pipeline (15.4km long). Wax build-up on the wall of the
pipeline is a problem that must be carefully addressed before running an
ultrasonic pig; the wax prevents the ultrasonic pulses from reaching the pipe
wall.
Another important aspect which should be considered for offshore lines
is that more features occur internally, and in particular at the 6 o'clock
position inside the pipe. Damage or corrosion to the external pipe wall is rare.

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