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against corrosion from subsurface mineral waters and electrolytic
action from the outside. Cementing also helps strengthen casing
resistance to burst pressures.
In the steel mill, each piece of casing is drifted, an internal
measurement that indicates the guaranteed minimum inside
diameter of the joint. Drift diameter is less than the calculated
inside diameter and is an important consideration when using
downhole tools. Line pipe usually is not drifted unless this
operation is required and negotiated with the pipe manufacturer.
The maximum internal pressure for a particular casing
string is given as the minimum internal yield pressure in pounds
per square inch (psi). If this pressure is exceeded, the pipe will not
retain its original form and rated strength. The well pressure
should never exceed the minimum internal yield of the innermost
casing string. Collapse resistance and internal yield pressure are
critical factors when high pressures migrate into the annulus
between two casing strings. If this occurs, either the inner casing
string may collapse or the outer string may burst if the
corresponding pressure ratings are exceeded. The two most
common occurrences causing this type of problem are when high-
pressure fluids or gases channel a poor cement job or when high
pressure bypasses the casing hanger packoff seal. When casing is
used to support a load in tension, the indicated minimum body
yield strength represents the maximum allowable load.
Line pipe normally is used for 30-in. and 20-in. casing
strings and marine riser rather than high-strength casing. Usually
each casing string will be supported at the mud line. One exception
is the practice of hanging the 7-in. casing on a liner hanger inside
the 9%-in. casing. This reduces the amount of 7-in. casing
required.
As an alternative to drilling the 36-in. diameter conductor
casing, casing may be jetted into place. This procedure establishes
the 30-in. casing, permanent guide structure, and the guidelines in
one operation and does not necessarily require the use of the
temporary guide base. Soft-bottom conditions are prerequisites to
using the jetting method.
The 30-in. housing and 30-in. open-ended conductor is
made up, run, landed, and locked in the PGS, which sits on support
beams as discussed previously. A special jetting tool or bit is made
up on a drill pipe stinger that is spaced out inside the 30-in.
diameter conductor to position the jetting tool about 10 to 18 in.

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