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With an electrohydraulic control system, electrical power, fed through a

multicore conductor, actuates solenoid-operated valves in the control pod. Then,

only a single hydraulic hose is required to deliver the hydraulic fluid to the

accumulators. Some systems have incorporated the accumulator fluid supply line

into the integral riser system.

Pure electric control or multiplex systems are also being manufactured

where electric signals and electrical power are sent to the BOP stack through a

single, multicore conductor connected to each control pod. Acoustic control

systems generally are not being considered as a prime operating system but are

being employed as a backup control system for subsea BOPs.

If and when a higher-then-expected well bore pressure is encountered while

drilling, the annular and ram preventers can be closed to seal around the drill pipe.

The pressure buildup is controlled by circulating heavier drilling mud through the
drill pipe and back through the choke or kill lines. Once the mud column pressure

exceeds the formation pressure, the preventers can be opened and drilling may
continue.

In the event sea conditions make it imperative to disconnect from the BOP
stack, an emergency drill pipe hangoff tool can be landed in the subsea wellhead

housing. The well can then be sealed by closing the bottom pipe ram around the

drillstem of the emergency drill pipe hangoff tool. The running tool is released

from the hangoff tool, and the remaining drill pipe may be retrieved. At this point,

the blind ram is closed above the hangoff tool and the hydraulic connector in the

lower riser package is disconnected. The drilling vessel is then in a position to ride

out the emergency, leaving the well in a secure and shutin condition. During the

reconnection and reentry procedure, it is important to circulate through the choke-

and-kill lines so any pressure built up under the closed rams is bled off before the

drill pipe is lowered and reconnected.

Only under extreme emergency conditions are the blind shear rams closed

on a drillstring. In such case, one of the pipe rams below the shear rams preferably

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