What caused Exxon’s Pegasus pipeline to split apart while the line was running well below maximum pressure? It's still anyone's guess.

By Elizabeth Douglass

It's been a year since a broken oil pipeline sent an estimated 210,000 gallons of Canadian dilbit into an Arkansas neighborhood, but there's still a long list of unknowns about the spill.

The most critical mystery yet to be resolved for the public: What caused ExxonMobil's Pegasus pipeline to break apart March 29 while the line was running well below its maximum approved pressure?

All the public knows now is that a metallurgical report concluded that substandard pipe-making methods left tiny cracks near the lengthwise seams on the 1940s-era northern Pegasus pipe. Those micro-cracks grew and merged during service to become dangerous "hook cracks," and then something—or a combination of things—caused at least one hook crack to open up a 22-foot gash in Mayflower, Ark.   

The report didn't determine what caused long-dormant manufacturing defects to awaken and expand, and didn't say whether the way the Pegasus was being operated, or the properties of the dilbit had a role in promoting crack growth on the pipe.

The metallurgical report was completed in July. There's been no news about the cause or the pipeline's condition since. Exxon has only said that it was conducting additional tests on the line to ferret out all the factors that contributed to the failure.


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