The charges were in connection with oil spills and corrosion discovered in Prudhoe Bay oil field transit pipelines in 2006.

Cohen said BP acknowledged deficient maintenance on field transit lines in the plea agreement. A low flow of oil in the pipeline resulted in a buildup of sludge that created an environment for bacteria-induced corrosion.

The sludge should have been cleared out by maintenance pigging, but BP had not run maintenance pigs or a “smart” pig to detect internal corrosion for eight years.

“We submit that this is proof of negligence,” Cohen said.

BP's attorney, Jeff Feldman, said the company has had a rigorous corrosion-protection program across most parts of Prudhoe Bay. “However, the company underestimated the corrosion potential in the transit lines,” Feldman told the court.

The parties agreed that the plea agreement does not cover potential civil proceedings by the state and federal governments over a crude oil spill in March 2006 that led to the release of around 200,000 gallons of oil.

BP spokesman Ronnie Chappell said discussions regarding the civil proceedings are underway with both federal and state governments.

In the criminal case, BP as charged with negligence of maintenance in the transit pipelines, allowing the sludge to accumulate which resulting in corrosion and an almond-sized hole that led to the March 2006 spill.

The charges covered a spill in Prudhoe Bay's western operating area discovered on March 5, 2006. The plea agreement and fine covers violations in March in addition to another Prudhoe corrosion-related spill in the eastern operating area of the field that occured August 2006.

Cohen said the leak detection system in the pipeline did not detect the leak. It was discovered by a BP employee doing a field inspection after oil had been leaking for about five days, Cohen told the court.

BP will be on probation for three years as a part of the plea agreement but can have its probation reduced to one year if certain performance benchmarks are achieved, Cohen said.

Part of the agreement with the government is that BP replace the aged transit pipelines and install improved leak detection systems, which the company is now doing.

Federal judge Ralph Beistline commended BP for cooperating with the federal investigation but also admonished the company, saying BP should “put a higher priority on maintenance and protecting the environment and a little less priority on profits.”

Some $12 million of the $20 million fine is being paid to the federal treasury. Another $4 million will go to the state of Alaska and $4 million is being paid to the U.S. Fish and Wildlife Foundation to help fund environmental research on the North Slope, Cohen told the court.

BP initiated a $260 million Prudhoe Bay pipeline replacement project after the transit line corrosion was detected in 2006. Replacement and upgrades of the field pipelines is half complete, BP spokesman Steve Rinehart said.