BP was forced to shut down the eastern half of the 400,000 barrels-per-day Prudhoe Bay field in August after a government ordered internal inspection of the segment of pipeline where the sediment had been building up revealed severe corrosion in over a dozen places and several small leaks.

BP's own corrosion detection program, which largely relied on external tools to monitor the state of the pipeline, had not revealed any problems with the line.

The shutdown came less than six months after a corroded segment of the western oil transit line ruptured, spilling at least 200,000 gallons of crude oil onto the Arctic tundra, prompting a federal criminal investigation and a US Congressional probe into BP's Alaska operations.

BP's reputation in the United States has been battered by a series of accidents and scandals since March 2005 when a deadly blast at a Texas refinery killed fifteen workers and injured scores more.

PIGGING IMPRACTICAL

US Congressional investigators and officials with the Department of Transportation's pipeline oversight office are focusing their investigations into the corrosion problems at Prudhoe Bay and the lack of pigging of both oil transit pipelines.

Investigators believe that the accumulated sediment is believed to be a major factor behind the corrosion in both the eastern and western pipelines.

BP officials have acknowledged that sediment build-up can create the potential for microbiologically induced corrosion and that in that case sediment should be removed with a pig. The company has not yet determined whether sediment in the pipelines played any role in the corrosion at Prudhoe Bay.

BP said in an August 2002 letter to the Alaska Department of Environmental Conservation (ADEC) that cleaning the section of pipeline on the eastern side of the oil field would be "impractical" due to the slow speed at which oil was running through the pipeline segment.

ADEC had ordered BP to pig another section of the eastern oil transit line, as well as the entire western oil transit line if necessary as part of a July 2002 Compliance Order to improve pipeline leak detection systems.

BP argued in the August letter that it should be released from the requirement to pig the eastern and western pipelines as no significant buildup of sediment had been detected.

ADEC agreed with BP in August and later again in December 2002 that pigging was not necessary to comply with the leak detection order.

"In 2002 all of the discussion was focused on the impact that sediment had on the leak detection equipment. The cause and effect element of sediment in those lines and corrosion was generally not being discussed," said BP Alaska spokesman Daren Beaudo.

In the summer of 2006, BP told federal regulators that the sediment in the section of the eastern transit line that leaked was an inch deep.

BP had not pigged the western pipeline since 1998, while the eastern line had not been pigged since 1990 partly because there were no facilities to handle the sediment that would be produced by pigging.

Following the March spill, federal regulators ordered BP to build facilities to handle the produced solids from any pigging operations and ordered the company to institute a regular pigging program.