As the briefing heads towards the end, questioning comes back to the “negative pressure test”, used to determine whether the cement job had been successful and the well had been sealed properly so no oil and gas could escape.
On the afternoon and evening of April 20, the rig crew had two attempts at conducting a negative pressure test, intended to check what would happen if the heavy drilling fluid were flushed out of the riser and replaced with lighter sea water, as part of the standard procedure for disconnecting the rig and moving away.
The test came up with results that were potentially worrying: there was a “pressure abnormality” on the drill pipe that could have been caused by the well not being properly sealed off from the oil reservoir. According to the inquiry’s version of events, a Transocean engineer suggested that the high pressure reading could have been misleading, and BP’s well site leader accepted that explanation.
A further indication from the test also misleadingly suggested that the seal was holding, possibly because the pipe used had become clogged with viscous “spacer” fluid, used to separate the mud from the water while the mud was being flushed out.
Between them, the BP and Transocean engineers decided that the worrying result could be ignored and the reassuring result could be accepted and carried on with the procedure to disconnect the rig. When the heavy mud was replaced with sea water, one of the key barriers preventing oil and gas from rising up the pipe to the rig was removed, with fatal consequences.