Page 200 - Pipeline Pigging Technology

Corrosion inspection of the Trans-Alaska pipeline

the technique can measure pits as small as 1.75in in diameter and as shallow
as 10% of the pipe wall.
Alyeska has asked NKK to institute grading a sample of the pig data based
on the criterion of a single or two adjacent transducers. That is, corrosion will
be reported when one or two transducers collect data which reflects metal
loss greater than 10% of the pipe wall. This will provide measurement of pits
as small as 0.5in in diameter.
Single- or double-transducer grading is a feasible objective, but in the early
production stage of the NKK pig development this is not practical because:
1. Single- or double-transducers do not "read" the same location on the
pipe wall for each pig run.
2. NKK computer-assisted grading is a very labour-intensive process.
3. The computer-assisted/manual grading process increases the poten-
tial for analysis errors.
4. The increased pipe-wall coverage capability of the single transducer
is second choice to additional pig runs.
5. The Alyeska pipeline's 800-mile length is a staggering inspection
assignment without a fully-computerized analysis process.

Alyeska is continuing to investigate the results of the reported corrosion
anomalies from the IPEL and the NKK pigs to meet its corporate commitment
of no oil leaks. Alyeska has scheduled the 1991 pig run by NKK for August.

Magnetic flux vs. ultrasonic technology

Alyeska's pig inspection programme provides a unique opportunity to
compare the results of a sophisticated magnetic-flux pig and the high-tech
ultrasonic corrosion pig. The differences between the two technologies are
well known. The magnetic-flux technology uses sensors to determine the
change in the flux field due to corrosion anomalies. The ultrasonic technology
uses transducers to send high-speed sonic waves to the inner and outer pipe
wall, and measures the time difference between the time of the pulses to
calculate the wall thickness. The obvious difference between the two is that
the magnetic flux is a detection and interpretation method, whereas the
ultrasonic method is a measurement method.
The following data is based on the 1988 run of IPEL and the 1989 and 1990
NKK pig runs. We believe that this data supports the assumption that
ultrasonic pigs may be more accurate due to their measurement capability.
Considerations in the decision of selection of which pig technology to use
in a pipeline system are as follows:

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