SCC Blamed for Recent Nova Rupture
Investigators believe a recent line break on the NOVA Gas
Transmission Ltd. (NGTL) system was caused by stress corrosion
cracking (SCC). The line break occurred Feb. 7 about 18 miles north
of Highway 11 and 45 miles southwest of Drayton Valley. There were
no injuries. The pipeline involved in the line break was
constructed in 1980. "We will take the data and evidence we
gathered at the site and prepare a more detailed analysis to
confirm our preliminary assessment," said John Walker, vice
president plant maintenance.
SCC forms on the exterior surface of underground pipelines as a
group of hairline cracks. It is the result of the interaction
between pipeline stress, corrosion and certain environmental soil
conditions. Since the mid-1980s, NGTL has been monitoring its
pipeline for SCC, and found few instances of it.
Repair crews worked around the clock to remove the damaged
42-inch diameter pipe and install approximately 328 feet of new
pipe to replace it. This pipe section will be tested and placed
back into operation around the end of February.
An Aug. 8 line break on NOVA also is believed to be the result
of a combination of external corrosion and stress corrosion
cracking. In July 1997, Canada's Transportation Safety Board said
further safety precautions were needed in the wake of a rupture and
fire involving two parallel lines of TransCanada Pipe Lines (see
NGI July 28, 1997). That rupture also was caused by SCC. The safety
board, an advisory-only agency, urged the National Energy Board
(NEB) of Canada and the industry to "harden" emergency shutdown
systems and to review the design of systems at compressor stations.
Joe Fisher, Houston
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