BISMARCK, N.D. (AP) — Tesoro Corp. on Friday restarted a North Dakota crude oil pipeline that ruptured and spilled more than 20,000 barrels of oil in a North Dakota wheat field.

The San Antonio-based company shut down the 35-mile long underground pipeline on Sept. 29, after a farmer discovered oil seeping from the ground in his wheat field near Tioga, in the northwest corner of the state.

Tesoro said it had received and accepted a safety order from the federal Pipeline and Hazardous Materials Safety Administration that sets conditions to restart the pipeline.

Oil began coursing through the pipeline Friday morning, Tesoro spokeswoman Elizabeth Watters said, minutes after the company issued a statement that it accepted the federal agency's safety plan, which requires such things as frequent aerial and ground inspections and additional leak detection equipment.

The federal agency, which oversees pipelines throughout the country, did not immediately return telephone calls from The Associated Press on Friday.

Federal regulators have said a preliminary investigation shows a possible lightning strike might have caused the rupture in the 6-inch-diameter steel pipeline that runs from Tioga to a rail facility outside of Columbus, near the Canadian border.

Tesoro said in a statement that the damaged portion of the pipeline was removed and sent to an independent laboratory for analysis. Tesoro has said the hole in the 20-year-old pipeline was a quarter-inch in diameter.

"While final results are pending, the preliminary report indicates that the likely cause of the small diameter hole in the pipeline was from electrical discharge," the company's statement issued Friday said. "The source of the electrical current remains under investigation. According to the initial lab report, there were no signs of corrosion or other defects at the failure location."

The company said the entire will have additional monitoring "to detect patterns that might indicate a leak."

The North Dakota Health Department was told about the spill on Sept. 29. 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. Regulators said the spill caused no damage to water or wildlife.

Tesoro estimated it would cost $4 million to clean up the spill. The company said it has cleaned up about 4,500 barrels of oil so far.

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