Oct 17 (Reuters) - Tesoro Logistics LP detectedanomalies during an inspection of its 20-year-old North Dakotapipeline just days before the line ruptured and spilled 20,600barrels of oil onto farmland, the company said on Thursday.
The six-inch pipeline was carrying Bakken oil to theStampede rail facility outside Columbus, North Dakota, when itruptured. A farmer harvesting wheat discovered oil spouting fromthe line on Sept. 29.
A robot, known as a "smart pig," detected anomalies duringwhat Tesoro called routine internal inspections of the pipelineSept. 10 and 11.
"We were awaiting results of the analysis of that inspectionwhen the leak was reported," Tesoro spokeswoman Tina Barbeesaid.
North Dakota regulators said corrosion on the pipeline mayhave caused the spill.
The U.S. Department of Transportation's Pipeline andHazardous Materials Safety Administration was on siteinvestigating the spill despite the government shutdown over thelast few weeks.
The pipeline remains shut down while the federal regulatorlooks into how Tesoro responded to the spill, its control-roomprocesses and records and whether non-compliance contributed tothe pipeline's failure, said Jeannie Shiffer, PHMSA's directorfor Governmental, International, and Public Affairs.
The ruptured pipeline runs 35 miles from Tioga to BlackSlough in North Dakota. It was built by BP Plc in 1993.
It is a part of Tesoro's "High Plains" pipeline system inNorth Dakota and Montana that gathers oil from the Bakken shaleand delivers it to another pipeline and to Tesoro's 68,000barrels-per-day Mandan refinery.
Texas-based Tesoro bought the pipeline and the refinery fromBP in 2001.
The spill is the largest on U.S. soil since an Exxon Mobil pipeline spilled 5,000 to 7,000 barrels of heavyCanadian crude in Mayflower, Arkansas, last March.
It is the largest oil leak in North Dakota since hydraulicfracturing unlocked massive oil reserves in the Bakken shale. InAugust North Dakota produced more than 911,000 barrels of oilper day.
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