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(FSC) gate valves near each stack connection. These valves are
operated hydraulically. In the event of sudden pressure loss, a
spring cartridge in the valve operator automatically moves the
valve gate to the closed position. Choke-and-kill valves must be
rated for the same working pressure as the BOP stack. The upper
terminal fitting for the choke-and-kill lines on the BOP stack are
male stab subs that mate with the female connections on the lower
riser package.
Because of their relative inaccessibility, subsea BOPs
demand redundancy to an extent not required for surface stacks.
As a result, numerous combinations and configurations have been
tried with mixed success (fig. 2.9). The trend seems to be toward a
single-stack system (as opposed to a two- stack system where
minimal low-pressure equipment drills a shallow hole and lower
formations are drilled with a smaller diameter high-pressure stack).
However, the trend may be reversed if extremely high pressures
(greater than 10,000 psi) are encountered frequently in floating
drilling.
Of necessity, BOPs require highly efficient hydraulic
control systems capable of operating any of the functions on the
stack with precision and dispatch. For instance, high flow rates are
required for the large volumes needed to operate the ram and
annular preventers. These volumes may range from 5 gal for small
rams to 50 gal for a large annular preventer.
The NL (Koomey) system best illustrates redundant
hydraulic operation (fig. 2.10). The basis of this hydraulic control
system is a male control pod connected to the control receptacle
mounted on the upper frame of the BOP stack. This control pod
houses the SPM valves that direct the hydraulic fluid to operate the
various functions on the stack. The mating surface of the control
pod has outlets with individual seals that match inlets on the
conical sealing face of the female control receptacle. The female
receptacle has outlets that usually are connected by flexible high-
pressure hose to the various functions of the BOP stack.

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