By Tom Bergin and Ayesha Rascoe

LONDON/WASHINGTON (Reuters) - A BP Plcinvestigation of the Gulf of Mexico disaster played down thecompany's role in the world's worst offshore oil spill, seekingto share the blame with its contractors.

The 193-page internal report made public Wednesday drewfire from U.S. lawmakers and one of the contractors,Transocean, called it a "self-serving" attempt by the Britishenergy giant to escape responsibility for the "fatally flawed"design of its deepsea Macondo well.

The report threatened to reignite public anger over themassive spill, which caused an environmental catastrophe alongthe U.S. Gulf Coast, devastated tourism and fishing in the areaand damaged President Barack Obama's popularity.

BP investigators were unable to identify any single actionor inaction that caused the Deepwater Horizon rig to blow up onApril 20, killing 11 workers and rupturing the well a milebelow the surface.

"Rather, a complex and interlinked series of mechanicalfailures, human judgments, engineering design, operationalimplementation and team interfaces came together to allow theinitiation and escalation of the accident," the report said.

"Multiple companies, work teams and circumstances wereinvolved over time."

Investors had been eagerly awaiting the report to find outwhether BP would be able to share the potential costs of thespill -- estimated by some analysts to exceed $50 billion.

BP shares traded in New York were up 3.2 percent and itsLondon-traded shares closed 1.3 percent higher. Shares ofTransocean rose 2.0 percent in New York and those of oilservices company Halliburton, the other major contractor, wereup 1.5 percent.

EIGHT KEY FAILURES

The rig explosion unleashed a surge of crude that lasteduntil the well was capped three months later on July 15, after4.9 million barrels of oil had leaked into the sea.

The report, overseen by BP's head of safety, Mark Bly, alsosingled out Halliburton.

Halliburton said the BP report contained "substantialomissions and inaccuracies" and stressed it was fullyindemnified for any allegations in the document.

Transocean also criticized BP's findings, with spokesmanLou Colasuonno saying the well's blowout preventer was"inspected, tested and went through a rigorous maintenanceschedule prior to being placed on the Macondo well and was thentested weekly, right up until 72 hours prior to the blast."

"Any statement to the contrary is false," he said.

BP defended some of its own decisions, which have beencharacterized by U.S. politicians as cost-saving measures thatcontributed to the disaster.

These included:

-- BP's much-criticized single-casing well design;

-- The use of fewer centralizers (devices used to ensurethe cement casing is applied evenly around the well) thanrecommended and;

-- Replacing heavy drilling mud, which was keeping the wellunder control, with lighter water.

The report highlighted eight key failures that, incombination, led to the blowout of the Macondo well and thesubsequent explosion.

"It would appear unlikely that the well design contributedto the incident," BP's outgoing Chief Executive Tony Haywardsaid.

"HAPPY TO SLICE UP BLAME"

BP, which has seen almost $70 billion wiped off its marketvalue since April 20, has been trying to rehabilitate itstarnished public image, spending millions of dollars onpositive television and newspaper advertising.

Hayward has faced withering criticism from U.S. lawmakersfor initially playing down the scale of the disaster and thepotential impact on the environment.

"This report is not BP's mea culpa," said Democraticcongressman Edward Markey, an outspoken critic of BP's handlingof the disaster. "Of their own eight key findings, they onlyexplicitly take responsibility for half of one. BP is happy toslice up blame, as long as they get the smallest piece."

Energy industry analysts were also not convinced by the BPfindings, noting its investigators did not have access to allthe relevant employees among the contractors.

"Make no mistake, our view remains that this is BP's well,and BP is in charge of design and execution," Houston energyinvestment boutique Tudor Pickering Holt & Co said in a note toinvestors.

BP denied it was shifting blame but its report includedstinging criticism of Transocean and Halliburton.

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

But Transocean said BP was seeking to conceal the criticalfactor that led to the rig explosion -- the well design.

"In both its design and construction, BP made a series ofcost-saving decisions that increased risk," it said in astatement.

In pointing the finger at Halliburton and Transocean, BPsaid:

-- Halliburton had used an "unstable" cement mixture thatallowed hydrocarbons to leak into the well;

-- There was "no indication" that Transocean had tested theautomatic shut-off function on the blowout preventer before itwas used on the Deepwater Horizon rig.

-- The rig crew diverted the flow of drilling mud andhydrocarbons into the wrong system after the blowout occurred.

This meant gas vented onto the rig floor, rather than beingdiverted toward the sea, where it would have been much lesslikely to cause a blast. (Additional reporting by Matt Daily in New York and KristenHays in Houston; Writing by Ross Colvin; Editing by JohnO'Callaghan)