In the five months since ExxonMobil's Pegasus oil pipeline burst in Arkansas, two things have become clear. Flawed, 1940s-era welding techniques used when the Pegasus was built set the stage for the rupture, and an internal pipeline inspection failed to spot the problem just weeks before the spill.
The most critical question of all, however, has yet to be answered — What caused the pipe's long-dormant flaws — assumed to be J-shaped "hook cracks," in this case — to awaken and grow undetected until catastrophe struck?
"It is the extension of the hook cracks that is the key to this failure," said Patrick Pizzo, a professor emeritus in materials engineering at San Jose State University. "You have to get those cracks in motion in order to lead to a leak or a fracture."
Pizzo and several pipeline failure experts who reviewed the publicly available Pegasus reports say the pipe's cracks probably grew because of large swings in the pressure inside the pipe. So-called "pressure-cycle-induced fatigue" is one of the top four threats in pipelines that — like the Pegasus — were built from pre-1970 pipe that is predisposed to cracking and corrosion problems along lengthwise seams.
But there could be other factors, too, including problems associated with the type of product the Pegasus was carrying — an oil-like substance called bitumen that is mined in Canada and diluted to form diluted bitumen, or dilbit.
One theory is that dilbit made pressure swings inside the pipe larger and more frequent — and thus more harmful over time — because it's heavier and harder to push through pipe, and because its ingredients can vary more widely than conventional crude oils.
Another theory holds that an excessive amount of hydrogen accelerated or triggered the crack growth. In that scenario, the destructive hydrogen could have come from an overcharged corrosion protection system or from the sulfur-heavy dilbit being carried by the Pegasus.
A metallurgical report on the ruptured segment of the Pegasus concluded that substandard manufacturing methods left tiny cracks near the pipe's seam. Those cracks grew until the steel pipe split, sending crude oil gushing from a 22-foot-long gash. Hook cracks are typically formed at the steel mill. But the metallurgical report said that in this case the hook cracks were probably formed later, after micro-cracks from the manufacturing process grew and merged during service. The report didn't say what caused the cracks to grow.
Answering that question is vital, because 30 percent of the nation's 180,000 miles of onshore hazardous liquid pipelines could have manufacturing flaws similar to those on the Pegasus. Operators of those pipelines need to know if they should conduct new inspections, rethink assumptions about crack growth, or adjust the way they operate their lines.
If dilbit turns out to have been a factor in the Pegasus spill, it would further inflame the debate over U.S. imports of Canadian bitumen. That, in turn, would put the spotlight on other flawed pipelines that carry dilbit and would also provide fodder for opponents of the proposed Keystone XL pipeline, which would carry dilbit from Canada to oil refineries on the Gulf Coast.
The federal Pipeline and Hazardous Materials Safety Administration (PHMSA) is investigating the Pegasus failure but has not discussed its work. Exxon spokesman Aaron Stryk said the company would not comment on the matter until the probe is complete.
PHMSA investigators are analyzing the toughness of the steel, residual stress, mechanical properties, chemical analysis of deposits and resistance to environmentally assisted cracking, among other things, according to an Exxon presentation to Arkansas officials. Pressure cycling and hydrogen cracking are types of environmentally assisted cracking.
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