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systems are now used. Diverters are low-pressure annular blowout preventers used

to direct the flow of fluids away from the rig floor. After drilling, the riser is pulled

because it is too small to accept the 20-in. casing connectors. Then the wellhead

and casing are run and cemented with returns to the sea floor. While the cement

sets, the riser is again run with the BOP stack. With the surface casing sealing off

the relatively low-pressure formations, the higher pressure formations (usually

3,000 ft or more below the mud line) can be controlled.

Use of an 183/4-in. wellhead and BOP stack permits the 17'/2-in. hole for

the 13%-in. casing to be drilled without underreaming, eliminating considerable

drilling time. After the casing has been cemented and sealed, holes for the

remaining casing strings are drilled. Each casing is run, cemented, and sealed. The

depth for each casing string is based on the fracture gradient at the shoe of the

surface casing, estimated formation pressure at setting depth for the next casing,
and the estimated pressure gradient of the fluid in the open hole.

Blowout Preventer Staak

The blowout preventers maintain control over potential high-pressure

conditions that may exist in the formation being drilled. The BOP can shut in the

well under pressure while formation material, fluids, or gases that have invaded the

well bore are circulated out. Standard sizes and pressure ratings for subsea BOPs

are 16%-in., 5,000 or 10,000 psi; 183/4-in., 10,000 or 15,000 psi; and 21 Win.,

10,000 psi for a single stack system. Subsea wellhead systems, the BOPs, and the

wellhead itself generally have the same nominal ID and pressure rating. The BOP

components are integrated into a structural steel framework with four stack posts.

This assembly, then, is referred to as a BOP stack, BOP, or the stack.

A typical BOP stack consists of a wellhead connector housed in the lower

framework; three pipe rams, a blind shear ram, and the annular preventer are

encased in the middle framework. Of the four ram preventers, the blind shear

usually is placed just below the annular preventer or is positioned so there is one

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