Page 143 - Pipeline Pigging Technology

Pipeline Pigging Technology

Fig.6 (top). Advantages and disadvantages of ultrasonic pigs.
Fig.7 (bottom) Typical double joint prior to shipment offshore.

Therefore, ultrasonic pigs could be more suitable offshore, as any loss of
coupling is likely to be due to gas bubbles at the 12 o'clock position.
We see this is one of the advantages of ultrasonics over magnetics for
offshore lines. We are looking for corrosion-type problems, and therefore the
accuracy of survey from one year to another is important.
However, given the above, we consider at present the practical and
logistical problems of running an ultrasonic pig in a major gas line are
unresolved.
The second-generation magnetic pig appears not to be as accurate when
defining defects, depths, etc., although it is stressed that this is a high-quality
vehicle which can certainly reliably detect metal loss features at depths well
below where failure of the line could occur.

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