One thing that has been bothering me, the decision to remove the heavy mud from the riser prematurely. No one has answered that one yet. That was a human decision way out of line with historical operating procedure. If you install a blowout preventer with dead batteries then take away your first line of defense(heavy mud),I would say you can expect a rig explosion.
‘Mud weight is the first round of defense’
If the final cement plug wasn’t in place yet, removing the mud would be at odds with “good oil-field practice” outlined in 2003 by the federal Minerals Management Service. The MMS report, prepared by WEST Engineering Services, warns against single-point failures — counting on one mode of protection — by saying that “mud weight is the first round of defense against a kick, followed up by” the blowout preventer. Removing the mud left the blowout preventer as the only failsafe.
“To displace mud above the position of the upper plug with water before setting the upper plug means that you are relying on one barrier for the duration; this is not good,” said a deepwater drilling expert who did not want to be identified because he does business with BP. The expert is not involved in the Deepwater Horizon project.
While the mud could have given workers more time to react to the blowout, the accident itself and the oil spill originated in the failure of the cement and the failure of the blowout protector. Read more..