The blowout preventer's control pod, which was also originally connected to the lost rig, has been taken up to the Q4000, where it has been tested and attached to new cables.

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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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At these depths, everything must be done with remote-control robots that install the blowout preventers, submersible pumps and other devices to manage the flow of oil and water on the ocean floor.

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Mr Wells says that once the control pod has been reinstalled on the blowout preventer and the manifold is attached to the Q4000, the top kill could start.

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