The rupture in the ExxonMobil pipeline that sent a river of oil through a suburban neighborhood in Mayflower, Ark. is now known to be 22 feet long and 2 inches wide. That's almost four times longer than the six-foot pipeline tear that sent more than one million gallons of Canadian dilbit into Michigan's Kalamazoo River in 2010, the worst accident of its kind in U.S. history.

The size and speed of the release through a long opening, thin as a mail slot, shines a spotlight on just how quickly oil pipeline accidents can turn into catastrophes. Between 200,000 and 420,000 gallons of heavy oil spewed out of the 65-year-old pipeline without warning on March 29, Good Friday afternoon, forcing the evacuation of 22 suburban homes.

Few Americans realize how much pressure is needed to operate a pipeline like the Pegasus, which moves more than 90,000 barrels a day of crude across four states, from Illinois to Texas. That's almost four million gallons of heavy oil being pushed over an 850-mile distance in a single day.

When a rupture occurs, so much force is released that large amounts of oil can pour through the breach in minutes.

"People just don't gather how high these things can go," said Richard Kuprewicz, president of the pipeline consulting firm Accufacts Inc. "For the average person, they're just exotic pressures." But if pipeline operators drop their guard, he said, pipelines "can be highly destructive."

At the time of the rupture, the pipeline was operating at 708 psig (pound-force per square inch gauge), about 14 percent below its maximum operating pressure of 820 psig. That's more than twice the pressure of a fire hose, which can spray water 30 floors into the air. But a fire hose is a few inches in diameter, and the Pegasus is 20 inches wide.

Kuprewicz said 708 psig is considered "moderate" for oil transmission lines. Some pipelines can run at slightly above 1400 psig, he said, "which is perfectly legal and technically sound if the pipe is of high integrity."



However, the fact that the Pegasus ruptured while running below maximum pressure "is not good," Kuprewicz said, because it means something was wrong with the pipeline's integrity management. Pipelines are supposed to be safe even if they operate at slightly above the maximum operating pressure, he said, so the Pegasus line "failed at a negative safety margin."

Exxon says it shut down the Arkansas pipeline within 16 minutes after a pressure drop in the line was detected, although questions remain about how the company detected the dip, and when.

According to preliminary findings from the U.S. Department of Transportation, Exxon closed two valves that were 18 miles apart in order to isolate the section of pipe that had ruptured. If the line was full when the break occurred, that 18-mile section of would have contained more than 1.5 million gallons of oil.

Exxon said it began work on Sunday to excavate the damaged, underground pipe. It will be hauled away to a laboratory where scientists will try to determine the precise cause of the rupture.

A Corrective Action Order from the Department of Transportation said Exxon would use "stopple valves" to aid in the excavation. The process involves cutting into the pipeline and inserting a disk a few feet from either end of the rupture site to isolate the failed segment, Kuprewicz said. Because the pipeline is 20 inches in diameter, he added, the equipment needed is fairly large and heavy, so the process could take some time.

Kuprewicz described the tear as a small "fish mouth" opening that's wider in the middle and narrow at the ends.

A fish mouth opening is a "classic fingerprint" of a "typical liquid pipeline rupture," he said. It rules out the possibility that the rupture was caused by third-party damage such as construction activity.

"A metal lab analysis is now required to detail where in the manufactured weld seam the failure originated, and why," he said in an email. Fish mouth ruptures are "usually associated with crack-like features" that develop in pipelines for a variety of reasons.

Kuprewicz said the size of the tear wasn't unusual.

"I've seen longer, wider failures along or near certain manufacturing seams," he said. "The opening is a fuction of fracture mechanics and depends on a lot of factors."

Cracks and crack-like defects were responsible for the 2010 pipeline accident in Michigan, on a 30-inch pipeline owned by Enbridge Inc. That tear was about six and a half feet long and five inches wide at its largest point. In its investigation of that spill, the National Transportation Safety Board determined that Enbridge—the company that owned the pipeline—had failed to address defects found years before the accident.

The Arkansas spill has drawn new attention to the Keystone XL project, the much-debated 36-inch pipeline that will carry diluted bitumen from Alberta, Canada to Steele City, Neb. if it is approved by the Obama administration, the Keystone's northern segment would cross the Ogallala aquifer, a crucial source of water for the Plains states. It would carry up to 830,000 barrels of oil a day, nearly 10 times the capacity of the Pegasus line, and operate at a maximum pressure of 1,308 psig.

Pipelines that carry dilbit typically run at higher pressures than those transporting conventional crude. Environmental groups say that's one of several reasons why dilbit corrodes pipelines more easily than conventional crude oil, although the oil industry says dilbit poses no more risk than other oils. A National Academy of Sciences team is studying the issue, and its report will be released this summer.

InsideClimate News reporter Maria Gallucci contributed to this report.

