Study points to pipe wall failure
Cracks in Enbridge pipeline likely existed from the beginning
Walter Strong
Northern News Services
Published Monday, December 8, 2014
PEHDZEH KI/WRIGLEY
The Transportation and Safety Board of Canada has completed its investigation into the cause of a leak identified early in 2013 along the Enbridge Norman Wells to Zama pipeline approximately 55 km south of Wrigley.
Work at the Enbridge spill site located roughly 45 km south of Wrigley on the Norman Wells pipeline on Sept. 6, 2011. The National Energy Board is still investigating that leak, but the Transportation Safety Board has completed its study of a 2013 leak. - photo courtesy of Enbridge Pipelines Inc. |
The board, which published its report on Dec. 4, concluded the leak was due to "internal stress corrosion" that likely began before the pipeline was put into service in 1985.
Pre-service pressure testing, and likely the use of methanol as a drying agent at the time, produced stress fractures near welds that propagated through up to 98 per cent of the 6.9 mm pipewall thickness.
Normal pipeline operations then forced the stress fracture to expand through the remaining two per cent of pipewall thickness.
This was a small fracture, up to 3.4 mm long. No free-standing oil was located near the cracks, although 54 cubic metres of oil-contaminated soil were removed from the site during clean-up.
Similar cracks were found at three other locations following 60 site excavations during the winter of 2012-13. The dig sites were determined after Enbridge passed an ultrasonic crack detection tool through the pipe.
Contaminated soil was found at each of these three further locations, although no standing oil was present on inspection. In one case, a section of pipe was removed for analysis and replaced. The other two were sleeved and earmarked for further evaluation.
A larger leak in 2011 about 45 km south of Wrigley released between 700 and 1,500 barrels of crude contaminating 8,980 tonnes of soil, or 477 truckloads. The contaminated soil was removed and hauled to a landfill in northern B.C.
That leak was blamed on a similar through-wall stress fracture, as was an earlier leak in 1992.
At least 33 monitoring wells were dug at the site, including 10 deep wells, for a minimum two-year environmental monitoring period.
Pehdzeh Ki First Nation Band Manager Mark Macneill of Wrigley said that monitoring period is up and contractors are preparing to decommission the test wells.
"They've been sampling water on a regular basis," Macneill said. "Everything has come up fine."
The 869 km pipe between Norman Wells and Zama was constructed during winter months in three segments: Norman Wells to Wrigley (336.4 km); Wrigley to Mackenzie (249.8 km); and Mackenzie to Zama (283.4 km).
Each of the six identified leaks are confined to the Wrigley-Mackenzie portion of the pipeline. The TSB ruled out contractor error for the failure.
"The causal stress corrosion cracking is in no way related to contractor error," said Daphne Snelgrove, engineer and TSB senior pipeline investigator.
Snelgrove said it was not entirely clear why the leaks have been confined to the Wrigley-Mackenzie portion of the pipeline.
"The methanol may have been in that segment (of the pipeline) longer than in the other two segments," she said. "The records are still being looked into on that."
The Enbridge Norman Wells pipeline continues to operate with NEB approval at reduced pressure - approximately half maximum operating pressure. The TSB report stated that Enbridge had demonstrated to the NEB that this is safe for the remaining intended life of the pipeline.
"There are no plans to replace portions, nor the entirety of the pipeline," stated Enbridge spokesperson Graham White in an email to News/North.
"It will continue to be monitored and maintained for safety as part of our integrity management program."
