BP has agreed to pay a record $50.6m (£32.5m) fine for failing to correct safety hazards at its Texas City plant.
An explosion at the refinery in 2005 killed 15 people.
The US Occupational Safety and Health Administration (OSHA) said the penalty "rightly reflects BP's disregard for workplace safety". BP said it had worked to enhance safety at the plant.
OSHA is still working to collect another $30m from the company for other penalties that BP is contesting.
The fine is the largest handed out by OSHA, which is part of the Labour Department.
"This agreement achieves our goal of protecting workers at the refinery and ensuring that critical safety upgrades are made as quickly as possible," said Secretary of Labour Hilda Solis.
BP also agreed to spend $500m on fixing safety problems at the refinery.
"We respect OSHA's concerns and have addressed them in this agreement," said Iain Conn, BP's global head of refining.
The agreement is the latest financial penalty for BP following the Texas explosion.
The company paid a $21.3m fine to OSHA in 2005 and entered into a four-year agreement to repair hazards at the refinery.
In 2007 the oil giant agreed to pay $373m to settle a number of both criminal and civil charges.
In October last year, following an inspection of safety practices at the Texas plant, OSHA issued BP with an $87.4m fine, made up of two parts.
The first part cited 270 violations that BP had failed to correct after the explosion, totalling $56.7m, later corrected to $50.6m, which BP has now agreed to pay.
The remaining part of the fine, of more than $30m, is for 439 "new wilful violations". BP continues to contest it.