London - BP attempted to place much of the blame for the rig blast that led to the United States' worst-ever oil spill on its contractors, Transocean and Halliburton.
In a report released on Wednesday, BP also defended some of its own decisions, which have been characterised by U.S. politicians as cost-saving measures that contributed to the Gulf of Mexico disaster.
These include BP's much-criticised single-casing well design, the use of fewer centralisers when cementing the well than recommended, and replacing heavy drilling mud, which was keeping the well under control, with lighter water.
Centralisers are devices used to ensure that the cement casing is applied evenly around the well.
The report, conducted by BP's head of safety, Mark Bly, highlighted eight key failures that, in combination, led to the blowout of the Macondo well and to the subsequent explosion.
Most of these factors would normally be the primary responsibility of either Transocean, as rig operator, or Halliburton, which cemented the well.
"Self serving"
Transocean slammed the report.
"This is a self-serving report that attempts to conceal the critical factor that set the stage for the Macondo incident: BP's fatally flawed well design. In both its design and construction, BP made a series of cost-saving decisions that increased risk," it said in a statement.
BP Chief Executive Tony Hayward said "It would appear unlikely that the well design contributed to the incident."
BP denied it was shifting blame, but its report included some stinging criticism of the two contractors.
"Over a 40-minute period, the Transocean rig crew failed to recognise and act on the influx of hydrocarbons into the well," BP said.
A spokesman declined comment on whether BP staff were also supposed to be monitoring the pressure in the well.
Companies such as Royal Dutch Shell monitor all their wells in real time from central locations. so that increases in pressure as happened in BP's well would be spotted before a blowout occurred.
Halliburton officials were not immediately available for comment.
Complex chain of events
BP also denied its insistence that Halliburton conduct the cement job with only six centralisers, rather than the 21 Halliburton recommended, was a factor in the blast.
BP said it was the "unstable" cement mixture Halliburton used that allowed hydrocarbons to leak into the well.
BP again highlighted the failure of the blowout preventer (BOP), an integral part of Transocean's rig, but manufactured by Cameron International.
"The investigation team found indications of potential weaknesses in the testing regime and maintenance management system for the BOP," the report said.
The British company also criticised the rig crew for diverting the flow of drilling mud and hydrocarbons into the wrong system after the blowout occurred.
This meant gas vented onto the rig floor, rather than being diverted toward the sea, where it would have been much less likely to cause a blast.
The oil company, which has seen almost $70bn wiped off its market value due to the disaster, did accept that one of its representatives, in conjunction with Transocean, had incorrectly interpreted a safety test that should have indicated hydrocarbons were leaking into the well.
Investors were eagerly awaiting the report for indications as to whether BP would be able to share the potential costs of the spill -- estimated by some analysts to exceed $50bn -- with the contractors or its partners.
BP shares traded up 1.5% at 16:03, in line with the STOXX Europe 600 Oil and Gas index.
Anadarko Petroleum, which owns 25 percent of the well, has refused to pay its share of cleanup costs, accusing BP of gross negligence.
If gross negligence was proven BP would be unable to recoup costs from Anadarko, or Japan's Mitsui, which owns 10 percent of the well.
It would also open BP up to federal fines of potentially in excess of $20bn.
Transocean criticises omissions
Echoing criticism by lawmakers, Transocean highlighted how BP had decided against conducting a test, called a cement bond log, which might have shown whether the cement job had integrity.
The BP report does not mention such a test.
Transocean also repeated criticism that BP ordered that the mud in the well be circulated for a shorter period than is considered best industry practice.
The Deepwater Horizon drilling rig exploded on April 20, killing 11 workers. It sunk two days later, unleashing a surge of crude that lasted until the well was capped on July 15, after 4.9 million barrels of oil had leaked into the sea.
BP said last week it had spent $8bn so far responding to the spill.
It was unclear whether BP's well site leader, Donald Vidrine, contributed to the report, although BP cites "interviews" as the source of information about discussions between Vidrine and Transocean staff shortly before the blast.
Vidrine has failed to testify to federal investigators at least twice, citing ill health, and any contribution by him to BP's report could prompt criticism.