BP warned well cap was 'unstable'

Investigators in the US say BP had been warned that cement used to seal the Deepwater Horizon well was unstable before it blew out, causing a massive oil leak in the Gulf of Mexico.

The National Oil Spill Commission has been looking into the causes of the explosion that killed 11 workers and led to the largest offshore oil spill in US history.

A White House Panel said Halliburton, which was BP's cementing contractor, used flawed cement when securing the bottom of the well. This contrasts with the company's statements claiming its tests showed the mixture was stable.

The US company conducted four tests between February and April on the cement slurry used for the Macondo well, with three concluding that the mix was unstable.

Halliburton shared the results of one of the tests with BP, which designed the well, in an e-mail on 8 March, according to Fred H Bartlit, the chief investigator for the National Commission on the BP Deepwater Horizon Oil Spill, a US presidential panel examining the spill.

In a letter to the commissioners, Mr Bartlit said that in passing the test result to BP "there is no indication that Halliburton highlighted to BP the significance of the foam stability data or that BP personnel raised any questions about it".

The letter is the first finding of one of several official investigations into the 20 April explosion that killed 11 men and caused the worst oil spill in US history.

Shares in Halliburton, which has previously said tests it conducted on the cement showed it was stable, plunged in New York, while the cost of insuring its debt jumped. Mr Bartlit's letter was released after the stock market closed in London. BP's US shares were flat.

Though oil has not flowed from the well since the middle of July, BP, Halliburton and Transocean, the owner of the Deepwater Horizon rig, face multiple investigations in the US into the explosion. If any of these inquiries uncover evidence of gross negligence, the financial penalties could run into billions of dollars.

Without identifying the cement work as the central cause of the spill, Mr Barlit says that "we have known for some time that the cement used to secure the production casing and isolate the hydrocarbon zone at the bottom of the Macondo well must have failed in some way".

BP's own internal inquiry into the accident found that after a "bad cement job" by Halliburton, the valve at the bottom of the well failed to prevent oil bursting out of the reservoir. BP pointed the finger towards Halliburton, but it did not assess how BP staff had monitored the cement job.

After BP's inquiry, Halliburton strongly denied any culpability.

Thomas Roth, Halliburton's vice president in charge of cementing, blamed BP's structural design rather than his company's work, was to blame for disaster.He said the entire well was "flawed" accusing BP of failing to take enough safety precautions.

The letter from Mr Bartlit explains that the commission instructed Chevron to run tests on a batch of cement slurry provided by Halliburton and similar to that used in the Macondo well.

When Chevron's tests found the mix to be unstable, investigators asked that Halliburton hand over all the data on the tests it had run on the cement.

"Halliburton and BP both had results in March showing that a very similar foam slurry design to the one actually pumped at the Macondo well would be unstable, but neither acted upon that data."

Halliburton said it would be releasing a response later, while BP could not be reached for comment.

However, in earlier evidence before the joint Coast Guard-Bureau of Ocean Energy Management investigative panel, Halliburton engineer Jesse Gagliano, when asked if he would pour the same cement again, said he would. "I am comfortable with the slurry design," he said.