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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,

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