Page 199 - Pipeline Pigging Technology

Pipeline Piggfng Technology

ALYESKA'S EXPERIENCE

During the past three years, Alyeska Pipeline has had an opportunity to use
magnetic-flux and ultrasonic corrosion pigs to monitor the condition of the
transAlaska pipeline. The company has had the resulting opportunity to
compare the capabilities of the two inspection technologies using two
specific pigs: the IPEL magnetic-flux pig and the NKK ultrasonic pig.
The environment for operating pigs in the Alyeska pipeline is challenging.
Current throughput in the 4&in diameter pipe is 1 .85million brl/d, producing
an average pig speed of 6.53mph or 9.57fps. Oil temperature varies from
125'F to 100°F. The pipe wall is 0.462- and 0.562-in thick, in grades of X&,
X-65 and X-70.
Alyeska contracted with IPEL in 1987 to run its magnetic-flux pig after a
thorough review of the pig capabilities and physical characteristics. The pig
was run in the summer of 1987 and the fall of 1988. The 1987 run produced
a final report of 12 potential corrosion anomalies. Field excavation of each of
these anomaly locations did not find any pipeline corrosion. A second run was
made in 1988 with minimal hardware changes to the pig. The results of a
subsequent grading analysis produced 241 possible corrosion anomalies.
Field investigation in 1989 and 1990 verified corrosion in 122 of the 189
locations investigated. Because of the relatively-high success ratio in identify-
ing metal loss, PEL was asked to do a second grading of the data based on the
results of the verifying field data. The results of the regrading produced an
additional 178 possible corrosion locations. The total reportable corrosion
anomalies from the 1988 pig run is 419. As of December, 1990, Alyeska has
field-inspected 31 2 of these anomalies with the following results:
Ultrasonic corrosion pig

Alyeska has been working with the NKK Corporation since 1984 discuss-
ing the possibility of developing and testing a 48in diameter corrosion pig
using ultrasonic transducers. After years of development by NKK and several
test runs in the TransAlaska pipeline, the NKK pig ran its maiden run in June,
1989. This run reported 419 possible corrosion anomalies. Field investigation
of 280 locations of the 413 possible sites found 194 corrosion anomalies, a
successful call rate of 6%. It must be noted that this fmt report by NKK was
based upon the grading criterion that three adjacent circumferential trans-
ducers must collect data indicating metal loss before corrosion can be
reported. Alyeska believes that this criterion may be improved, even though

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