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Arctic Submersibles
A special type of submersible rig is an arctic submersible. In
the arctic, where petroleum deposits lie under shallow oceans such as
the Beaufort Sea, oil companies knew that jackups and conventional
barge rigs would not be suitable. During the arctic winter, massive
chunks of ice form and then move with currents on the water's surface.
Called "floes," these moving ice blocks exert tremendous force on any
object they contact. The force is great enough to destroy the legs of a
jackup or the hull of a conventional ship or a barge.
Arctic submersibles therefore have a reinforced hull, a caisson.
One type of caisson has a reinforced concrete base on which the
drilling rig is installed. When the sea is ice-free in the brief arctic
summer, boats tow the submersible to the drilling site. There, workers
submerge the caisson to the sea bottom and start drilling. Shortly,
when ice floes form and begin to move, the arctic submersible's strong
caisson hull deflects the floes, enabling operations to continue.
Inland Barge Rigs
A fourth submersible is an inland barge rig. It has a barge hull
– a flat-bottomed, flat-sided, rectangular steel box. The rig builder
places a drilling rig and other equipment on the barge deck. Inland
barge rigs normally drill in marshes, bays, swamps, or other shallow
inland waters. By definition, barges are not self-propelled; they have
no built-in power to move them from one site to another. Therefore,
boats tow them to the drilling location. When being moved, the barge
floats on the water's surface; then, when positioned at the drilling site,
the barge is flooded so that it rests on the bottom ooze. Since they
often drill in swampy shallow waters, drilling people often call inland
barges "swamp barges."
Jackups
A jackup rig is a widely used mobile offshore drilling unit. It
floats on a barge hull when towed to the drilling location. Most
modern jackups have three legs with a triangular-shaped barge hull;
others have four or more legs with rectangular hulls. A jackup's legs

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