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The first and second casing strings are cemented with
cement returns to the ocean floor.
The last joint of casing is made up to a casing hanger so it
may be suspended permanently prior to cementing.
Flowby ports provide for mud returns. After a casing string
is landed, cement slurry is pumped down the casing and casing
shoe and up through the casing/hole annulus. Cementing plugs are
released remotely. Casing seals must be remotely run and set.
Special tools are required for testing. Wear bushings must be used
to protect the wellhead.
Pipe diameters may vary, depending on conditions and
anticipated depth, but typically follow the standardized sizes,
grades, and pressures.
The hole sizes and casing programs used in offshore
exploration drilling are essentially the same whether a jackup
drilling vessel (using a mud line suspension system) or a floating
drilling vessel (using a subsea wellhead system) is drilling the
holes. The setting depths and grades of casing are in no way
standardized. The outline is only a generalization of what is
practiced most frequently.
When drilling a hole for a particular casing string, the size
of the hole obviously must exceed the size of the casing. Other
factors that influence the hole size are shale formations that may
expand into the hole and reduce the original hole size, joint ODs
that are usually larger than the OD of the casing unless flush joint
casing is used, and sufficient casing-to-hole annulus required for
adequate circulation and cementing.
In some areas, regulatory agencies dictate the number and
size of casing strings in a particular drilling application. However,
abnormal formation pressures also may necessitate use of up to six
casing strings. Corresponding equipment modifications have led to
the development of many specialized wellhead systems.
One such system involving a four-hanger housing is a
Vetco 183A-in., 10,000-psi WP SG-5 single stack to accommodate
an 183/4-in. x 13%-in. x 95/s-in. x 7-in. casing program (fig. 2.7).
The 18%- in. wellhead housing will support the 20-in. surface
casing and provide landing and sealing areas for the successive
casing hangers and packoff assemblies. The housing also provides
the structural support on which the BOP stack (and, subsequently,
the completion tree) will be landed, connected, and sealed. The
function of the casing hangers is to hang off and support the load

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