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from the bottom of the 30-in. diameter open-ended shoe joint. A
stabilizer is positioned above the jet sub to centralize it inside the
30- in. casing. The plugs in the running tool are removed, allowing
the jetted returns to rise inside the casing and spill out of these
ports onto the ocean floor. Sea water is used as the jetting fluid.
The 30-in. casing string with permanent guide structure and
guidelines are run until the shoe joint reaches the mud line. As the
formation is washed by the action of the jetting nozzle, the casing
is lowered slowly into the resulting cavity. This procedure is
continued until the PGS is a few feet above the mud line. With the
assembly in its final position, the drill string is rotated to the right
to release the housing running tool. The jetting assembly, with the
housing running tool, is then retrieved. Formation friction on the
30-in. casing is sufficient to anchor and hold the casing in place.
The 30-in. casing is considered installed at this point. Recently,
30-in. casing strings have been drilled or jetted into position using
dynadrills or turbodrills (downhole drill motors) within the casing
string instead of the more conventional jetting tool described
above.
The 30-in. casing, usually set to a depth of 80 to 300 ft,
provides only structural support and will not withstand pressure.
The depth of the 30-in. casing is determined by the ability of the
soil to support the wellhead and other equipment; vertical loading
and overturning moment are the criteria. If riser is used when
drilling hole for the 20-in. surface casing, the ability of the
formation to withstand the hydrostatic pressure of the mud column
in the riser must also be taken into account.
For the 20-in. (or surface) casing, a pilot hole is drilled and
then opened to 26 in. This casing normally is set to about 1,000 ft
below the mud line. In the past, holes for the surface casing were
drilled with mud and cutting returns to the sea bed. However,
shallow gas sands have caused blowouts, so risers with diverter
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.

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