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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 as
much as they do when it floats on the surface. A semisubmersible rig therefore offers a more stable
drilling platform than a drill ship that drills while floating on the water's surface.
Large cylindrical or square columns extend upward from the pontoons. The main deck rests
on top of the columns. The main deck of a semi is big. Semis (short for semisubmersibles) often use
anchors to keep them on the drilling station. Workers release several large anchors from the deck of
the rig. An anchor-handling boat crew sets the anchors on the seafloor.
Besides being good rough-water rigs, semis are also capable of drilling in water thousands
of feet (metres) deep. While many semis work in water depths ranging from 1,000 to 3,500 feet
(300 to 1,000 metres), the latest are capable of drilling in water depths of 8,000 feet (2,500 metres).
Semis can drill holes up to 30,000 feet (10,000 metres) deep. Indeed, semisubmersibles are among
the largest floating structures ever made. The biggest ones soar to over 100 feet (30 metres) tall and
their main decks can be almost as big as a football field-that's 3,000 square yards (2,500 square
metres).
Drill Ships
A drill ship is also a floater. Drill ships are very mobile because they are self-propelled and
have a streamlined hull, much like a regular ocean-going ship. A company may therefore choose a
drill ship to make hole in remote waters, far from land. A drill ship is a good choice for drilling
remote locations. For one thing, it can move at reasonable speeds under its own power. Secondly,
its ship-shaped hull can carry a large amount of the equipment and material required for drilling.
Frequent resupplying from ashore base is therefore not necessary.
While many drill ships operate in water depths ranging from 1,000 to 3,000 feet (300 to
1,000 metres), the latest can drill in water depths approaching 10,000 feet (3,000 metres), or nearly
2 miles (3,2 kilometres). They can drill holes over 30,000 feet (10,000 metres) deep. These big drill
ships are more than 800 feet (250 metres) long, which is almost as long as three football fields laid
end to end. They measure some 100 feet (30 metres) wide, or a little wider than a football field.
Their hulls tower more than 60 feet (18 metres) high, about that of a six-story building.
Anchors keep some drill ships on station while drilling, but those drilling in deep water
require dynamic positioning. Dynamically positioned drill ships use computer-controlled thrusters
and sophisticated electronic sensors. Thrusters are power units with propellers that the builder
mounts fore and aft on the drill ship's hull below the waterline. Once the dynamic positioning

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