BP shifted much of the blame for a rig blast that led to the United States' worst-ever oil spill onto its contractors Transocean and Halliburton.   

In a report released on Wednesday, BP defended decisions that US politicians have said represented cost-saving measures that contributed to the explosion on the rig in the Gulf of Mexico.   

These include its much-criticised single-casing well design, its choice to use fewer centralisers when cementing the well and its decision to replace heavy drilling mud, which was keeping the well under control, with lighter water.

The report, conducted by BP's head of safety, Mark Bly, highlighted eight key failures that, in combination, led to the explosion.

Most of these factors would normally be the responsibility of either Transocean, as rig operator, or Halliburton, which cemented the well.

However, BP denied it was shifting blame and accepted that one of its representatives, in conjunction with Transocean, had incorrectly interpreted a safety test that should have flagged up risks of a blowout.

Nonetheless, the report included some stinging criticism, directed at its contractors.
 'To put it simply, there was a bad cement job,' outgoing Chief Executive Tony Hayward said in a statement.

In line with previous comments from BP, driller Transocean was most in the firing line. 'Over a 40-minute period, the Transocean rig crew failed to recognise and act on the influx of hydrocarbons into the well,' BP said in a statement.   

Transocean and Halliburton were not immediately available for comment.

BP said a failure of the cement job allowed hydrocarbons to flow into the well and denied this was related to its insistence that Halliburton conduct the cement job with only six centralisers, rather than the 21 Halliburton recommended.   

'It would appear unlikely that the well design contributed to the incident, as the investigation found that the hydrocarbons flowed up the production casing through the bottom of the well,' Hayward said.

The choice of the number of centralisers used, as well as the decision to use only one drilling casing, is a part of the well design, a spokesman said.   

BP again highlighted the failure of the blowout preventer, an integral part of Transocean's rig, but manufactured by Cameron International, and said gas was vented onto the rig floor, rather than being diverted to the sea, as it should have.   

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 $8 billion so far responding to the spill but analysts expect the final bill to run to tens of billions. -Reuters