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responding to a well control event, but the Panel found no evidence that the rig
crew did so at the time.

D. The Use of the Mud Gas Separator

A mud gas separator is used on a drilling rig to capture and separate gas
from the drilling fluid returned from the well. After separation, the mud flows
down into the pits and the gas is vented up in the derrick. A mud gas separator
is typically used to divert small volumes of hydrocarbons during a kick. If a
large flow is sent to the mud gas separator, there is a risk that the vessel will fail
and create the possibility of gas ignition. If there is potential for a large flow, the
safer option is to divert the flow overboard using one of two diverter lines. The
diverter line is typically used during a well control operation when: (a) the gas
flow rate is too high for the mud gas separator; (b) hydrates are forming in the
gas vent line from the mud gas separator; (c) the gas is found to contain
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hydrogen sulfide; or (d) the mud system is overloaded.

BP’s well control manual states that the mud gas separator should be lined
up at all times when a kick is being displaced, but there is a limit to the volume
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of gas that each mud gas separator can safely handle. Transocean’s well
control handbook indicates that if gas has migrated or has been circulated above
the BOP stack before the well is shut in, the choke manifold and mud gas
separator may no longer be available to control the flow rates when the gas in the
riser reaches the surface. Both companies recommend using the diverter lines
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when flow rates are too high for the mud gas separator.

Special precautions and procedures are necessary to avoid the effects of
the rapid expansion of gas in the riser, particularly in deepwater operations.
According to the Transocean well control manual, there is approximately four
times the mass of gas in a 15 barrel influx in 6,000 feet of water as there is in the
same influx in 1,500 feet of water. Early recognition of the warning signals and
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rapid shut‐in are the key to effective well control. By taking well control
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actions quickly, the crew can minimize the amount of formation fluid that enters
the wellbore. According to the Transocean well control manual, the rig crew can

271 Hydrogen sulfide is a chemical compound that is highly flammable.
272 BP‐HZN‐MBI00000060‐61 (pages 1‐4‐4 and 1‐4‐5).
273 BP‐HZN‐MBI00131628‐30 (pages 21 and 23).
274 Id. (Section 8, Subsection 4, page 22 of 28).
275 Id. (Section 3, Subsection 2, page 1 of 2). This is due to gas expanding as it moves towards the
surface and as compressive pressure decreases.

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