* BP shares up 2 percent (Adds detail, recasts)

By Tom Bergin

LONDON, Sept 8 (Reuters) - BP shifted much of the blame fora rig blast that led to the United States' worst-ever oil spillonto its contractors Transocean and Halliburton.

In a report released on Wednesday, BP defended decisionsthat U.S. politicians have said represented cost-saving measuresthat contributed to the explosion on the rig in the Gulf ofMexico.

These include its much-criticised single-casing well design,its choice to use fewer centralisers when cementing the well andits decision to replace heavy drilling mud, which was keepingthe 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 theexplosion.

Most of these factors would normally be the responsibilityof either Transocean, as rig operator, or Halliburton, whichcemented the well.

However, BP denied it was shifting blame and accepted thatone of its representatives, in conjunction with Transocean, hadincorrectly interpreted a safety test that should have flaggedup 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," outgoingChief Executive Tony Hayward said in a statement.

In line with previous comments from BP, driller Transoceanwas most in the firing line.

"Over a 40-minute period, the Transocean rig crew failed torecognise and act on the influx of hydrocarbons into the well,"BP said in a statement.

Transocean and Halliburton were not immediately availablefor comment.

COMPLEX CHAIN OF EVENTS

BP said a failure of the cement job allowed hydrocarbons toflow into the well and denied this was related to its insistencethat Halliburton conduct the cement job with only sixcentralisers, rather than the 21 Halliburton recommended.

"It would appear unlikely that the well design contributedto the incident, as the investigation found that thehydrocarbons flowed up the production casing through the bottomof the well," Hayward said.

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

BP again highlighted the failure of the blowout preventer,an integral part of Transocean's rig, but manufactured byCameron International, and said gas was vented onto the rigfloor, 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 surgeof crude that lasted until the well was capped on July 15, after4.9 million barrels of oil had leaked into the sea.

BP said last week it had spent $8 billion so far respondingto the spill but analysts expect the final bill to run to tensof billions.

(Reporting by Tom Bergin; Editing by Hans Peters, SitaramanShankar and Erica Billingham)