Investigators believe a recent line break on the NOVA GasTransmission Ltd. (NGTL) system was caused by stress corrosioncracking (SCC). The line break occurred Feb. 7 about 18 miles northof Highway 11 and 45 miles southwest of Drayton Valley. There wereno injuries. The pipeline involved in the line break wasconstructed in 1980. “We will take the data and evidence wegathered at the site and prepare a more detailed analysis toconfirm our preliminary assessment,” said John Walker, vicepresident plant maintenance.
SCC forms on the exterior surface of underground pipelines as agroup of hairline cracks. It is the result of the interactionbetween pipeline stress, corrosion and certain environmental soilconditions. Since the mid-1980s, NGTL has been monitoring itspipeline for SCC, and found few instances of it.
Repair crews worked around the clock to remove the damaged42-inch diameter pipe and install approximately 328 feet of newpipe to replace it. This pipe section will be tested and placedback into operation around the end of February.
An Aug. 8 line break on NOVA also is believed to be the resultof a combination of external corrosion and stress corrosioncracking. In July 1997, Canada’s Transportation Safety Board saidfurther safety precautions were needed in the wake of a rupture andfire involving two parallel lines of TransCanada Pipe Lines (seeNGI July 28, 1997). That rupture also was caused by SCC. The safetyboard, an advisory-only agency, urged the National Energy Board(NEB) of Canada and the industry to “harden” emergency shutdownsystems and to review the design of systems at compressor stations.
Joe Fisher, Houston
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