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the Sperry Sun crew could only measure returns sent to the mud pits and could
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not measure flow volumes sent overboard.

F. The Blowout

On April 20, 2010, at around 9:40 p.m., powerful pressures from the well
caused mud to flow up from the well. Mud spilled on the rig floor as the well
began to blowout. The crew responded to the situation by diverting the flow to
the mud gas separator, part of the diverter system to which the crew could direct
fluids coming up from the well. At this time, crew members likely realized that
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they had lost control of the well and attempted to regain control of the well by
activating the BOP stack’s upper annular preventer and the upper variable bore
ram.
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The mud‐gas separator, to which the crew had diverted flow from the
well, was quickly overwhelmed and failed, causing a gas plume to fill the rig
floor. The gas quickly ignited, causing the first explosion on the rig at 9:49 p.m.
Approximately ten seconds later, a second larger explosion occurred and the fire
onboard the rig spread rapidly. Shortly after the second explosion, the rig lost
power and experienced a total blackout.

At approximately 9:56 p.m., the rig’s subsea engineer attempted to
activate the BOP stack’s emergency disconnect system from the BOP panel on the
rig’s bridge. The emergency disconnect system is designed to activate the BOP
stack’s blind shear ram and disconnect the rig’s lower marine riser package
(“LMRP”) from the well. The BOP panel apparently indicated that the
emergency disconnect system was activated, but the rig remained connected to
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the well and hydrocarbons continued to flow uncontrolled from the well.
About four minutes after the attempt to activate the emergency disconnect
system, personnel on the Deepwater Horizon’s bridge manually sounded the
general alarm and made a muster call for personnel to gather at designated

45 Testimony of Joseph Keith, Joint Investigation Hearing, December 7, 2010, at 135.
46 The mud gas separator, and the rig crew’s decision to use it to handle the influx of
hydrocarbons from the well, is discussed in detail in Section V‐D of the findings and conclusions.
47 DNV, Forensic Examination of Deepwater Horizon Blowout Preventer, March 20, 2001, (DNV
Report) at 4. As discussed in later in this Report, the upper annular preventer and the upper
variable bore ram are two BOP stack components that are used by rig crews in well control
events. Neither component, however, is designed to shear the drill pipe and completely seal the
well – the blind shear ram on the BOP stack is designed to perform these functions.
48
Testimony of Chris Pleasant, Joint Investigation Hearing, May 28, 2010, at 123.

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