That starts off with a "junk shot, " in which material such as rubber tires and golf balls is pumped into the shut-off valve, or blowout preventer, that sits atop the well.

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Other companies involved included Transocean, the owner of the rig and responsible for the safety valve known as the blowout preventer, and Halliburton, who provided cementing services.

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He said in such a situation, normal procedure would be for the contractor's crew to secure the well by closing off the huge valve sitting on the seabed known as a blowout preventer.

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Studies of the blowout preventer's two control pods suggest that a flattish battery and a dodgy valve meant that neither was in a fit state to close off the well automatically when they should have, which BP takes as evidence of poor maintenance by Transocean.

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