Transocean, the company which leased the Deepwater Horizon rig to oil giant BP, has been fined $1.4 billion (£860 million) after admitting its role in the Gulf of Mexico disaster.
The deal with the US Department of Justice comes after BP agreed its own $4.5bn (£2.8bn) settlement with the US authorities in November, over criminal charges relating to the rig explosion in April 2010.
Swiss-based Transocean pleaded guilty to violating the Clean Water Act after its rig exploded, killing 11 workers, and spilling millions of barrels of oil. Transocean employed nine of the 11 workers killed in the disaster, according to Reuters.
Under the terms of the proposed deal, Transocean will pay its fine over five years, and the Department of Justice will not pursue further prosecution of the company.
Of the $1.4bn, $400 million was demanded of Transocean in criminal fines. Of this, $150 million will help protect the Gulf of Mexico, while another $150 million will fund spill prevention and response efforts there, the US DoJ said.
The US DoJ added that Transocean admitted its crew members were "negligent in failing fully to investigate clear indication that the Macondo well was not secure and that oil and gas were flowing into the well".
BP has long argued that it was not the only company responsible. In a statement, it said: "In settling, Transocean has acknowledged that it played a significant role and has responsibility for the accident. Transocean is finally starting, more than two-and-a-half years after the accident, to do its part for the Gulf Coast."
Assistant attorney general for the US Justice Department's Criminal Division Lanny Breuer said in a Press Association report: "Transocean's rig crew accepted the direction of BP well site leaders to proceed in the face of clear danger signs - at a tragic cost to many of them.
"Transocean's agreement to plead guilty to a federal crime, and to pay a total of $1.4 billion in criminal and civil penalties, appropriately reflects its role in the Deepwater Horizon disaster."
The Financial Times reported that shares in Transocean rose off the back of the news of its fines - Shareholders sent the shares climbing 10.6% in anticipation of a formal announcement, making it the FTSE Eurofirst 300's top performing stock.
“[A settlement] would be something that removes an overhang in that it gives them more options for free cash flow distribution,” Justin Sander, analyst at RBC Capital Markets, told the FT.
Halliburton, which was responsible for cementing the well, is the only company not to have settled over the disaster.
BP has been campaigning for the company to pay its share since the beginning of last year. It told PA on Friday: "Unfortunately, Halliburton continues to deny its significant role in the accident, including its failure to adequately cement and monitor the well."
The BP-contracted Deepwater Horizon was drilling the mile-deep well on April 20, 2010, when a surge of methane gas caused a blowout. The accident led to a months-long US deepwater ban and intense scrutiny of the offshore drilling industry, which is now booming worldwide despite lingering public concerns.