BISMARCK, N.D. (AP) - A lightning strike may have caused a pipeline rupture that spilled more than 20,000 barrels of oil in a North Dakota wheat field, federal regulators said in a report issued Thursday.
The U.S. Department of Transportation's Pipeline and Hazardous Materials Safety Administration said a preliminary investigation of the Tesoro Corp. pipeline break "points to a strong electrical discharge as the cause of the failure."
Damon Hill, a spokesman for the agency, said a final determination as to the exact cause has not been made. The agency based its initial findings on mechanical and metallurgical analysis of a section of the 6-inch diameter steel pipeline that runs underground about 35 miles from Tioga to a rail facility outside of Columbus, near the Canadian border, he said.
"It's possible it may have been a lighting strike," Hill said. "It's possible it could have struck the pipeline or another conduit near the pipeline, such as a fence."
Tesoro has said the hole in the 20-year-old pipeline was a quarter-inch in diameter. North Dakota regulators previously had said the hole may have been caused by corrosion.
"The cause of the electrical discharge is under investigation," Tesoro spokeswoman Elizabeth Watters said Thursday.
The break in the pipeline happened in a remote area, and officials said no water was contaminated or wildlife hurt.
The preliminary results of the break were published in a safety order issued Thursday that sets conditions for Tesoro to restart the pipeline. The order requires the company to perform frequent aerial and ground inspections and install additional leak detection equipment.
The North Dakota Health Department was told about the spill on Sept. 29, after a farmer whose combine's tires were coated in crude discovered oil gurgling from the ground in his field in the northwest corner of the state. Although the state initially thought just 750 barrels of oil was involved, it turned out to be one of the largest spills in North Dakota history _ an estimated 20,600 barrels covering 7.3 acres of land, or about the size of seven football fields. One barrel of oil is 42 gallons, and the amount that spilled near Tioga would fill more than 30 rail tanker cars.
Tesoro estimated it would cost $4 million to clean up the spill.
The Pipeline and Hazardous Materials Safety Administration said sections of the old pipe were sent to a lab for analysis. The agency said new pipe has been installed and tested, and pressure and flow detection systems have been installed.
In addition to the corrective measures included in the safety order, the agency has required Tesoro to submit a restart plan for the entire pipeline segment.
Hill said his agency could allow Tesoro to restart the pipeline before the final investigation is done, if Tesoro meets corrective measures.
Watters said the company does not have a timeline to restart the pipeline.
"We're continuing to work on getting restart approval," she said.
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