At issue were the choice for the design of the well, preparations for and tests of the cement job, and assurances that the well was properly sealed on the top.

Among other things, BP apparently rejected advice of a sub-contractor, Halliburton, in preparing for a cementing job to close up the well.

BP rejected Halliburton's recommendation to use 21 centralisers to make sure the casing ran down the centre of the well bore. Instead, BP used six.

In an e-mail on 16 April, a BP official involved in the decisions explained: "It will take 10 hours to install them. I do not like this."

Later on the same day, another official recognised the risks of proceeding with insufficient centralisers but added: "Who cares, it's done, end of story, will probably be fine."