Northwest Pipeline said longitudinal stress-corrosion cracking caused a pipeline rupture on its 26-inch diameter line near Lake Tapps, WA, on May 1. A report by a metallurgical firm from Texas determined the cause. It is the first time that stress-corrosion cracking, the Canadian pipeline industry’s nemesis, has caused a failure on the Northwest system, the company said. Landslides are usually the cause of pipeline ruptures on Northwest.

There were no injuries caused by the rupture, which also did not produce a fire. Valves were closed within an hour and a half and gas was rerouted, a Northwest spokeswoman said. There was no loss of service. Back up service was completed in 24 hours.

“Over the past five years, significant portions of the 26-inch pipeline between Sumas and Washougal have been uncovered and examined nearly 300 times and no critical flaws, like those that caused the May 1 incident, were discovered,” the company told customers in a notice on its bulletin board. “The circumstances leading up to the recent line break appear to be local in nature.”

The spokeswoman said the line had been pigged as recently as 1996 and had been hydrostatically tested in 1992. No flaws in the system were found.

To ensure the reliability of the 26-inch line, Northwest has installed a new section of pipe where the break occurred, initiated Flame Ionization Testing between Washougal and Sumas, initiated a Close Interval Survey from Sumner to Snohomish to validate the cathodic protection on the line and met with the Department of Transportation’s Office of Pipeline Safety (OPS) to outline an integrity assurance plan that includes a combination of internal inspection tools, the company said.

“Williams Northwest Pipeline has been safely and reliably delivering natural gas to customers in the Pacific Northwest for more than 45 years,” the company said. “We are undertaking the necessary steps to ensure the integrity of our pipeline and working with OPS to finalize a plan for the continued safe operation of our pipeline system in western Washington.”

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