BP plc (NYSE:BP) (LON:BP) says it will launch a vigorous defense next week when it goes to court with the U.S. government in connection with the Deepwater Horizon oil spill. NASDAQ reports that the oil giant will defend against one of the fines imposed on it by federal authorities. In November, a $4.5 billion settlement offer was made by BP. That settlement wouldn’t apply to the fine the company faces under the Clean Water Act, which could be as much as $21 billion or as little as $13 billion, depending on the final estimation of the number of barrels which spilled in the 2010 spill in the Gulf of Mexico.

BP

Federal officials estimated that the spill was approximately 4.9 million barrels of oil. However BP plc (NYSE:BP) (LON:BP) believes that estimate is overstated by 20 percent or more. According to the company, the government’s estimate also includes 810,000 barrels that BP was able to capture from the leaking well, and it says those barrels never made it into the environment.

In the calculation of the fine for violation of the Clean Water Act, BP plc (NYSE:BP) (LON:BP) believes that the highest amount that should be estimated is 3.1 million barrels. The company also said that the U.S. Department of Justice agreed with it on that estimation.

BP plc (NYSE:BP) (LON:BP) also said it expects its fine to be lower than the Clean Water Act’s statutory maximum of $1,100 per barrel of spilled oil or $4,300 in the case that BP was proven to be grossly negligent in its actions before the spill. The company does not believe it was grossly negligent.

However the DOJ has accused BP of being grossly negligent in connection with spill.