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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 pipe
ram between it and the annular preventer. These components are
bolted or clamped together before being integrated into the BOP
stack framework. A mandrel is installed atop the bag-type
(annular) preventer for attaching the lower riser package (fig. 2.9).
The lower, middle, and upper support frames are flanged
and bolted to four large guide posts through which guidelines are
run. When the stack is landed on the ocean floor, these stack posts
fit over the posts on the PGS. Intermediate landing pads are
sometimes located on the posts at the level of the upper stack
frame to support the stack on the moonpool or spider beams while
the lower riser package is connected. Hydraulic connectors are
included, top and bottom, to connect the stack remotely to the
subsea wellhead housing and to attach the lower riser package to
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