At 8:20 p.m. on the day of the explosion the pressure data suggest there was a constant flow ofsea water being pumped into the drill pipe that was displacing the heavier mud system which was the primary pressure control for the well.
On May 26th BP, the company responsible for the well that Deepwater Horizon had drilled, started to pump heavy drilling mud into the well's blowout preventer, a set of valves on the sea floor.