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SCC Blamed for Recent Nova Rupture

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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