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Workers collect oil in bins during the cleanup of the Rainbow pipeline break northeast of Peace River, Alta.Ian Jackson/The Canadian Press

Plains Midstream Canada says it has found no reason to change its pipeline repair practices after a maintenance job gone wrong led to a major spill in northern Alberta.

The rupture to the Rainbow pipeline late last month, which leaked 28,000 barrels not far from the small Cree community of Little Buffalo, is the worst oil spill in the province in decades. The company has blamed a routine maintenance excavation last year that did not properly compact soil underneath the pipe, allowing the ground to settle and the pipe to flex and, eventually, crack.

The repair work was done by third-party contractors and overseen by integrity engineers for Plains Midstream which, like other pipeline companies, did not require a provincial signoff to complete its work.

But Stephen Bart, vice-president of operations for Plains Midstream, said the company believes its procedures don't need to be changed - despite not knowing what went wrong in what the company is calling "a really isolated incident."

"We're doing an investigation to find out why we didn't achieve the compaction we would have hope to," Mr. Bart said in an interview Tuesday.

"How that came to be, we're not sure. But we can say the procedures we use are appropriate for this work. For whatever reason, the compaction wasn't achieved. But we'll be certain to achieve it on all of our work going forward."

The problem with the soil compaction was exacerbated by the fact that the excavated area of pipe was outfitted with a decades-old repair fitting, essentially a metal sleeve welded onto the pipe to fix problems with corrosion.

That kind of fitting "makes the pipe stiffer at this location," Mr. Bart said. "So when the pipe settled, the stress was concentrated at the edge of the thicker to thinner part."

Today, such repairs are done with an epoxy-coated sleeve that is more flexible and not welded to the pipe.

Plains Midstream, the Canadian subsidiary of Plains All American Pipeline L.P. , has checked for other excavations done near such sleeves, and found none, said Mr. Bart, who is speaking to media as part of a corporate effort to boost communications and transparency about the spill.

The company has now cleaned up 36 per cent of the spilled oil and, with the pipe repaired, has requested to restart operations. But it has to receive approval from Alberta's energy regulator, the Energy Resources Conservation Board.

Meanwhile crews are working 18-hour days in a cleanup operation expected to take two to six months.