The United States has hit drilling rig operator Transocean Deepwater with $US1.4 billion ($A1.3 billion) in criminal and civil fines for its role in the 2010 Deepwater Horizon oil leak disaster in the Gulf of Mexico.
Transocean agreed to plead guilty to violating the Clean Water Act in the disaster, in which a blowout on its drilling rig killed 11 and sent some 4.9 million barrels of oil into the Gulf, the Justice Department announced on Thursday.
The blowout took place on April 20, 2010 as Transocean's Deepwater Horizon vessel was drilling a well for oil giant BP.
In pleading guilty, the department said, Transocean had admitted its rig crew was "negligent in failing fully to investigate clear indications that the Macondo well was not secure and that oil and gas were flowing into the well".
The firm was ordered to pay $US400 million ($A382.76 million) to resolve criminal charges and another $US1 billion in civil penalties, partly to fund spill prevention and environmental restoration in the five states hit by the three-month-long spill.
"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," said Assistant Attorney General Lanny Breuer.
"Transocean's agreement to plead guilty to a federal crime, and to pay a total of $US1.4 billion in criminal and civil penalties, appropriately reflects its role in the Deepwater Horizon disaster."