BP focused too much on the little details of personal worker safety instead of the big systemic hazards that led to the 2010 Gulf of Mexico oil spill and was not as strict on overall safety when drilling rigs involved other companies that they hired, a US government safety panel has concluded.

Eleven workers were killed in the April 2010 explosion on the Deepwater Horizon rig and about 757m litres of oil flowed into the Gulf from the blown-out Macondo well. The company had the lease on the well, but the drilling rig was owned and operated by another company and BP has blamed drilling contractor Transocean.

That contractor-owner split made a difference in major accident prevention, the US chemical safety board concluded in a presentation to be made at a hearing in Houston on Tuesday.

"BP applied lesser process safety standards" to rigs contracted out than to its own facilities, the safety board managing director, Daniel Horowitz, told the Associated Press. "In reality, both Transocean and BP dropped the ball on major accident hazards in this case."

The oil company "did not conduct an effective comprehensive hazard evaluation of the major accident risks for the activities of the Deepwater Horizon rig or for the Macondo well" because BP's large risk evaluation programme "looked only at BP assets, not drilling rigs that it contracted" to other firms for operation, investigators said in the 50-page presentation.

A BP spokesman said the company "stepped up" and developed more rigorous safety indicators following the accident.

The safety board said when BP looked at offshore projects it "focused on financial risks, not process safety risks". And after the Deepwater Horizon explosion, the company's own accident investigation report "recommended requiring hazard reviews of BP-owned and contracted rigs," the safety board's presentation says.

"That's very disturbing because the Gulf of Mexico belongs to the American people," said former senator Bob Graham of Florida, who co-chaired another government oil spill investigation. The chemical safety board's findings pointed to BP having differing standards for what it owned and operated and what it did not.

"If that's true, it's reprehensible," said Graham.

The board's presentation says there is a difference between worker safety and making sure the entire rig and well are safe, and that was where owner BP and rig operator Transocean were "inadequate". And that same lack of focus on the bigger picture of safety bore an "eerie resemblance" to what the safety board found in its investigation into the 2005 explosion at BP's Texas City refinery that killed 15 people, safety board investigator Cheryl MacKenzie said in a news release.

The federal oil spill commission report, co-chaired by Graham, and a National Academy of Engineering investigation found similar problems.

Reducing lost time for workers and making sure they wear the right kind of boot is important, "but that really doesn't have much to do with system safety," said Donald Winter, who chaired the National Academy of Engineering investigation. "It is fundamentally different."