Since at least 2006, ExxonMobil has known that its 1940s-era Pegasus pipeline had many manufacturing defects like the faulty welds that recently sent crude oil spewing into an Arkansas neighborhood. The company also knew that the seams of the pipe have been identified by the industry as having another dangerous flaw: They are especially brittle, and therefore more prone to cracking.
"Having a crack or flaw in a pipeline is a whammy," said Patrick Pizzo, a professor emeritus in materials engineering at San Jose State University. "But having a crack embedded in brittle material, such as the heat-affected zones of the pipeline seams—that's a double whammy."
Despite those inherent risks, Exxon added new stresses to the Pegasus by fundamentally changing how it used the line. It began carrying a heavier type of oil, reversed the direction of the flow and increased the amount of oil that surged through it.
Old pipelines that are well maintained and carefully monitored can withstand such changes, experts say. But doing so is significantly harder for lines that—like the Pegasus—were built from pre-1970 pipe that is predisposed to cracking and seam corrosion problems.
In early 2006, before the 858-mile Pegasus was reconfigured, Exxon tested its strength by conducting a hydrostatic pressure test. Hydrotests are expensive and disruptive but they are also considered the most reliable way to detect and eliminate perilous cracks on problematic pipelines. Operators must suspend business and empty the pipeline, then pump water through it at higher-than-normal pressure to force vulnerable parts of the pipe to fail—a process that’s repeated until the pipe and its repairs hold the water without leaks or splits.
After a series of ruptures and repairs, the Pegasus cleared the hydrotest. Soon after, Exxon started using it to ship a Canadian heavy crude oil-like substance known as diluted bitumen, or dilbit, from Patoka, Ill., to refineries on the Texas Gulf Coast. Two years later, Exxon expanded the pipeline's capacity by 50 percent to 90,000 barrels per day.
Exxon declined to say whether the expansion also boosted the line's operating pressure, and if so, by how much.
"All I can say at this time is that prior to the incident, the pipeline was operating under normal pressure and flow conditions," said Exxon spokesman Aaron Stryk.
Many key details about the March 29 spill in Mayflower, Ark. have yet to be made public, and the pipeline failure is still under investigation by the federal Pipeline and Hazardous Materials Safety Administration (PHMSA). The agency recently released the metallurgical report on the broken section of pipe as well as the results of the 2006 hydrotest—information PHMSA had previously declined to make public. Other information, including data and analysis from inspections in 2010 and 2013, has not been circulated outside a small group of politicians and agencies.
What has emerged so far suggests that, at a minimum, Exxon took calculated risks given the known condition of the 20-inch pipeline and either used a flawed integrity management plan, or had a good plan and didn’t adhere to it.
"I'm looking at this from a distance ... but they were not very careful," said Mohammad Najafi, a pipeline construction expert and professor at University of Texas at Arlington, who has reviewed the metallurgical report and hydrostatic test results. "They should have been more concerned about the pipe."
Stryk, the Exxon spokesman, said the company "established a system for determining seam-failure susceptibility and for initial and subsequent seam integrity assessments." That system, he noted, "has been reviewed by PHMSA, is modeled after [the agency's] recommendations and guidelines for seam assessments, and is consistent will all applicable pipeline regulations."
PHMSA last reviewed the Pegasus integrity management plan during a special inspection in March and April 2011, Stryk said.
The section of the Pegasus that burst open in March was operating at a pressure of 708 pounds per square inch, well below its maximum operating pressure of 820 psi, according to PHMSA, which regulates most of the nation's liquid fuel pipelines.
The line split apart, opening a 22-foot gash and sending 5,000 barrels—or 210,000 gallons—of dilbit into a nearby neighborhood and lake. The noxious fumes and sticky flood forced 22 families to flee their homes.
Testing the Pipe