BP will try to whittle away at $13.7 billion (£9.03bn) in potential fines under the Clean Water Act as the penalty phase starts on Tuesday (January 20) in its trial over the 2010 Gulf of Mexico oil spill.

Several billion dollars in potential fines were avoided on Thursday (January 15) when US District Court Judge Carl Barbier put the size of the spill at 3.19 million barrels. That was well below the government's estimate of 4.09 million barrels, which could have led to $17.6 billion in fines for the worst offshore disaster in US history.

The Clean Water Act penalties would come on top of more than $42 billion the oil major has set aside for cleanup, compensation and fines. About 810,000 barrels were collected during cleanup.

Clean Water Act rules say when assigning penalties the court must take into account BP's ability to pay, history of past violations, and steps it took to clean up the spill.

The US government named as the defendant in the case BP's exploration and production unit, known as BPXP, which could conceivably limit the size of the fine.

Environmental groups say it could take decades for the Gulf ecosystem to recover. They disagree with BP's assertions that the Gulf has recovered better than expected.

Under a "gross negligence" ruling Barbier issued in September, BP could be fined a statutory limit of $4,300 for each barrel spilled, though he has authority to set lower penalties.

BP has also filed motions saying the maximum fine per barrel is in fact just $3,000 because Congress never passed laws to adjust it for inflation.

A simple "negligence" ruling, which BP sought, caps the maximum fine at $1,100 per barrel.