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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. 6.14). 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
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