By ALISON SIDER
An independent report found that a rupture in an ExxonMobil pipeline that spilled thousands of barrels of oil in Mayflower, Arkansas earlier this year was caused by defects dating back to when the pipe was built in the 1940s, the company said.
An independent metallurgical laboratory looked at the section of the pipe that ripped open in March, spilling an estimated 5,000 bbl of oil into a residential neighborhood in the small town about 25 miles from Little Rock.
ExxonMobil said Thursday it is reviewing the results of that assessment, which were provided to the company and the Pipeline and Hazardous Materials Safety Administration (PHMSA) on Wednesday.
The Texas oil giant said that, according to the report, hook-shaped cracks along the pipe's seams were the root cause of the line's failure, not corrosion. The cracks are related to an outdated welding process that is no longer performed on new pipes, but that still affects thousands of miles of pipelines in use across the US.
The lab also found that the pipeline's limited flexibility contributed to the incident, ExxonMobil said. The report has not been made public, and the PHMSA is still reviewing it, a spokesman for the agency said.
The section of the pipe that ruptured is more than 60 years old, but it passed a high-pressure test in 2006 and an internal inspection in 2010 that sought to measure metal loss and other anomalies, ExxonMobil said.
A spokesman for ExxonMobil said the full results of a more detailed inspection the company conducted earlier this year are not yet available. That inspection, called a transverse flux, uses a magnetic field to find corrosion along seams. It can also detect cracks and other defects, although not as reliably as it can detect corrosion, according to the PHMSA.
After the results come out, the company will review whether it needs to make changes to its pipeline integrity management program.
The type of cracks found to have caused the rupture in Mayflower are associated with some types of an early welding process called electric resistance welding, according to the PHMSA. That process hasn't been used on new pipelines since 1970, but approximately 25% of the 182,500 miles of liquid fuel pipelines across the US were welded that way, according to the most recent federal data.
Other pipeline ruptures have been linked to electrically welded pipe over the years, including a 2007 break in a liquid propane pipe near Carmichael, Mississippi, which caused a fire that killed two people.
In 2011, the PHMSA commissioned a study of how to detect problems in these pipes and prevent them from failing while in use.
Arkansas and the US government have sued ExxonMobil for allegedly violating pollution laws. The Arkansas attorney general has not yet seen the testing results, a spokesman for the office said.
Dow Jones Newswires