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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. 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
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