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operations are normally rated at 2,000, 5,000, or 10,000 psi working pressure.

The hydraulic connectors used between the wellhead and the BOP and

between the BOP and the riser are controlled from the surface and are of two basic

types: mandrel and collet. Mandrel-type connectors utilize hydraulically actuated

cams to drive locking dogs into the grooves machined into the wellhead. The collet

connector uses a series of collet fingers to form a funnel configuration to guide the

connector over the well hub. Both types of connectors use AX or VX rings for

sealing.

Usually, two choke-and-kill lines are run down to the BOP stack, either

integrally with the riser or as independent lines. Choke-and-kill valves provide

subsea shutoff of the high-pressure lines and are part of the stack, controlled by the

BOP control system. Most BOP stacks have connections below all rams to allow

different choke-and-kill hookups, as many operators have specific requirements
with regard to the placement of choke-and-kill entry lines within the stack.

Each choke-and-kill line usually carries two fail-safe close (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. 6.18). 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.
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