Page 76 - 4714

can be cylindrical columns, somewhat like pillars, or they can be
open-truss structures which resemble a mast or a derrick.
Whether it has columnar or open-truss legs, when a jackup's
barge hull is positioned on the drilling site, the crew jacks down the
legs until they contact the seafloor. They then raise, or jack up, the
hull above the height of the highest anticipated waves. The drilling
equipment is on top of the hull. The largest jackups can drill in water
depths up to about 400 feet (about 120 metres), and are capable of
drilling holes up to 30,000 feet (10,000 metres), or close to 5,05
miles,deep.
Floating Units
Floating offshore drilling rigs include semisubmersibles and
drill ships. Semisubmersibles, because of their design, are more stable
than drill ships. On the other hand, drill ships can carry more drilling
equipment and supplies, which often make them the choice in remote
waters.
Semisubmersibles
Most semisubmersible rigs have two or more pontoons on
which the rig floats. A pontoon is a long, relatively narrow, and
hollow steel float with a rectangular or round cross section. When a
semisubmersible is moved, the pontoons contain mostly air so that the
rig floats on the water's surface. In most cases, towboats then tie onto
the rig and move it to the drill site. However, some semisubmersible
rigs are self-propelled−they have built-in power units that drive the rig
from one site to another.
Semisubmersibles get their name from the fact that in the
drilling mode the rig is not submerged to the point where its pontoons
contact the sea bottom. Instead, rig personnel carefully flood the
pontoons to make them submerge only a few feet (metres) below the
water's surface. Thus, the rig is "semisubmerged" (If the pontoons
contacted the sea bottom, the rig would be "submerged.") With its
pontoons submerged below the waterline, waves do not affect the rig

75