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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The joints between the half-pipes have rubber seals to stop the contents leaking, and each half-pipe has a vertical pipe connected to it, to introduce or dispose of the fluid concerned.

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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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