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Image courtesy of Pacific Southwest Region USFWS/Flickr.

The Pipeline and Hazardous Materials Safety Administration (PHMSA) said Wednesday that it has pinpointed external pipeline corrosion as the potential cause behind a 2015 oil spill in Santa Barbara, California.

In the report, the agency said preliminary findings indicate that external corrosion was the “root cause” behind the Line 901 pipeline failure that polluted a four mile stretch of Refugio State Beach and created oil slicks along the coastline.

Line 901, operated by Plains All American, has been shutdown since the spill along with the nearby Line 903.

The report found that the leak began after an unintentional pump shutdown occurred while a Plains technician was disconnecting and removing a motor from a non-operational pump at the Sisquoc station on Line 903.

When the pump on Line 903 stopped operating the pressure in Line 901 increased.

The PHMSA said Line 901 failed between 10:55 and 10:56 am Pacific Daylight Time while operating at a pressure of 750 psig , or 56 percent of its maximum operating pressure.

The failed pipe segment was analyzed by third-party metallurgical experts, Det Norske Veritas – GL (DNV-GL), who found that the “leak occurred at an area of external corrosion that ultimately failed in ductile overload under the imposed operating pressure.”

“The morphology of the external corrosion observed on the pipe section is consistent with corrosion under insulation facilitated by wet-dry cycling,” the report added.

Pipe External Surface at the Line 901 Failure Site. Image courtesy of the PHMSA.
Pipe external surface at the Line 901 failure site. Image courtesy of the PHMSA.

The agency also found that “Plains’ existing corrosion control system is not preventing external corrosion of the pipe under insulation.”

Plains initially estimated that the ruptured line released about 2,400 barrels.

However, after the company completed a PHMSA-mandated purge, Plains indicated that as many as 3,400 barrels may have been released.

The PHMSA said that Plains has verbally reported a spill estimate of 2,960 barrels but added that the company has not provided technical documentation or a final accounting report to the agency.

Plains has reportedly recovered 997 barrels of crude oil, the agency said.

The PHMSA expects to release its final accident investigation report in late Spring 2016.