As early as five hours before the explosion drillers noticed unexpected loss of fluid in the riser pipe, indicating leaks in a part of the blowout preventer.

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Inside the riser pipe is a space about half a meter in diameter, large enough to hold the spinning drill pipe and bit and to allow the return flow of drilling fluid.

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One ROV got a saw lodged in the side of the leaking riser pipe in early June, for example, causing BP to rethink its strategy for trying to stop the gushing leak.

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Within about a one square mile area around that wellhead and the riser pipe and everything else, at any particular time you could have between 14 and 20 ROVs operating down there.

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Locating a ship resting on the floor of the ocean for many decades is somewhat different than finding a submersible to get to that depth and plugging many leaks in a riser pipe and in a blowout preventer.

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Allen said that in preparation for the storm, crews had pulled the drill string out, put a plug known as a "packer" in the casing pipe and filled the riser with seawater so the pipe would be stable.

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The pipe would have to be a portion of the riser, which would have to still be attached to the BOP, but crimped and bent over.

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