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.
An investigation by the Pipeline and Hazardous Materials Safety Administration has thus far resulted in a preliminary $2.66 million fine for Exxon's "probable" violations of pipeline regulations, but details of the agency's findings have not been released. Exxon has appealed the findings.
Knowing and publicizing the failure cause and all the other relevant information is important because Exxon may seek to restart the damaged northern section of the Pegasus. Without knowing exactly what happened, the company cannot assure regulators and the public that it can prevent a recurrence. That's critical, because the 647-mile northern leg of the Pegasus is demonstrably vulnerable to ruptures, and it runs through parts of four states and 18 sources of drinking water in Arkansas alone.
In addition, more than a quarter of the nation's hazardous liquid pipelines—or more than 50,000 miles—have aging seams made via similarly problematic methods, according to PHMSA data. What happened on the Pegasus may yield important lessons for making those other pipelines safer.
"Before you start the pipeline up in any form or manner, you better have a handle on why this thing failed below [maximum operating pressure]," said Richard Kuprewicz, a pipeline safety expert who is advising an Arkansas water utility. "If you try to start this line up without having enough people look over your reasons for why this pipe failed, and buy into it, you could find yourself with another rupture."
The other big questions about the spill left unanswered are:
If there were dangerous manufacturing flaws in the 1947-1948 Pegasus pipe, what caused them to become active and grow into a crack large enough to rupture the pipeline all these years later?
Did the contents of the pipe—highly viscous diluted bitumen from Canada's tar sands—play a role in the failure, either by exacerbating swings in the pipeline's internal pressure or by introducing excess hydrogen that hastened crack growth?
Did the size and frequency of pressure changes inside the Pegasus lead to "pressure-cycle-induced fatigue" that weakened the pipe or worsened existing cracks?
What role did the combination of major operational changes on the Pegasus—restarting it after years of inactivity, reversing its direction, boosting its capacity by 50 percent, and changing the product in the pipe to thicker, higher sulfur dilbit—play in the pipe's failure?
Did Exxon's corrosion-protection system malfunction and allow errant hydrogen to widen existing cracks?
Why did Exxon's 2010 and 2013 Pegasus tests fail to flag any problems in the place where it burst?
How can Exxon prove to regulators and the public that the Pegasus can safely carry more than 90,000 barrels per day of Canadian dilbit?
Exxon likely knows these answers by now, according to experts, and the company is expected to provide them before April 7, the date Exxon must give federal regulators a remedial work plan describing how it intends to fix the Pegasus and prevent future ruptures. To do that, Exxon must also explain what happened and provide evidence to back up the company's conclusions about what caused the Pegasus to fail.
Earlier this week, Exxon spokesman Aaron Stryk said the company "will share the results of our investigation prior to submission of the remedial work plan."
John Tynan, former watershed protection manager for Central Arkansas Water and now its director of public affairs, hopes to hear about Exxon's conclusions on Friday. That's when he and others from the water utility will meet with Exxon officials in Houston to discuss the Pegasus pipeline.
"It's actually taken them a long time to figure out what happened," Tynan said. "We're eager to see what information Exxon is in a position to share, both about the causal factors related to the Mayflower ruptures and also how they plan to proceed forward."
The pipeline, which stretches from Patoka, Ill. to Nederland, Texas, burst open about eight miles from where it enters the Lake Maumelle Watershed. The watershed feeds the Central Arkansas Water system that supplies water to 400,000 people in and around Little Rock.
With more than 13 miles of the Pegasus pipeline running through that watershed, the water utility has been pushing hard for safety upgrades and detailed information about the pipeline's condition and last year's failure.
Central Arkansas Water has asked Exxon to move the Pegasus out of the watershed, and filed a notice of intent to sue Exxon under the Pipeline Safety Act. The filing alleges that Exxon's pipeline safety plan was inadequate, that the pipe should have more shut-off valves in the watershed, and that it failed to reassess the pipeline for cracks within a five-year deadline, despite having found pipe defects within the Maumelle watershed in 2006.
Some of CAW's assertions were included in PHMSA's November Notice of Probable Violation and Proposed Compliance Order.
Once Exxon reveals its conclusions about the cause, attention will shift to spill prevention. Other parts of the Pegasus pipe are likely to contain manufacturing defects that could progress to rupture without careful maintenance and oversight.
"We want to have definitive proof that Exxon's integrity management effort can reliably and accurately identify all of the threats that may lead to cracks resulting in rupture, and that appropriate safeguards are in place to prevent that from happening in the Maumelle watershed," Tynan said.
Exxon has already sought permission to restart the southern leg of the Pegasus pipeline, a 211-mile Texas section that connects Corsicana to Nederland. The restart plan Exxon submitted to PHMSA for that segment, which has not been made public, is assumed to include the testing plans outlined in a fact sheet on the southern section provided to the water utility in February. In that, Exxon said it would test the line after it is operational using two kinds of internal inspection tools, or "smart pigs," to verify the safety of the pipeline.
Exxon said it plans to operate the southern section at reduced pressure through 2014. Stryk said Exxon will share the results of the post-startup tests with PHMSA to determine whether the Texas segment may return to its normal operating pressure.
"I can't take a position on whether that's appropriate for the southern section," Tynan said. But if Exxon presents a similar restart plan for the northern segment, "we do not believe that would be sufficient to protect the watershed from a possible rupture."
In a letter to PHMSA, Exxon targeted March 28 for reopening that part of the pipeline, but as of Thursday evening, PHMSA had not yet approved Exxon's restart plan.
March 28 - Central Arkansas Water meets with Exxon in Houston. The meeting could yield news about the secondary causes of the Mayflower spill and Exxon's plans for the pipeline.
March 28 - The deadline Exxon mentioned in its request to restart the southern leg of the Pegasus
March 29 - The one-year anniversary of the Mayflower spill. Rita Beving, a Texas organizer for Public Citizen, said a vigil is planned for Saturday evening in Dallas in support "for those Mayflower residents who have suffered, and with the concern for those who may be suffering in the near future from this same pipeline."
April 7 - The latest deadline for Exxon to submit a remediation plan to PHMSA explaining what they will do to make the pipeline safe.
June 11 - Hearing on Exxon's appeal of PHMSA's $2.6 million fine. The hearing will be held at PHMSA's Southwest Region office in Houston, Texas.