Page 41 - 6200

Submersibles and jackups contact the seafloor when drilling. The lower part of a
submersible's structure rests on the seafloor. In the case of jackups, only the legs contact the
seafloor.
Submersibles
A submersible MODU floats on the water's surface when moved from one drilling site to
another. When it reaches the site, crew members flood compartments that submerge the lower part
of the rig to the seafloor. With the base of the rig in contact with the ocean bottom, wind, waves,
and currents have little effect on it.
Posted-Barge Submersibles
The first MODU was a submersible. It drilled its initial well in 1949 off the Gulf Coast of
Louisiana in 18 feet (5.5 metres) of water. It was a posted-barge submersible−a barge hull and steel
posts (columns) supported a deck and drilling equipment. It proved that mobile rigs could drill
offshore. Posted barges are now virtually obsolete, however, because newer and better designs have
replaced them.
Bottle -Type Submersibles
About 1954, drilling moved into water depths beyond the posted barge's capabilities, which
was about 30 feet (9 metres).
So, naval architects designed bottle-type submersibles. A bottle-type rig has four tall steel
cylinders (bottles) at each corner of the structure. The main deck lies across several steel supports and
the bottles. The rig and other equipment are placed on the main deck. When flooded, the bottles cause
the rig to submerge to the seafloor.
In their heyday in the early 1960s, the biggest bottle-type submersibles drilled in 150-foot
(45-metre) water depths. Today, jackups have largely replaced them; jackups are less expensive to
build than bottle-types and can drill in deeper water. Rather than completely scrap their bottle types,
however, rig owners modified some of them to drill as semisubmersibles, which are still in use.
(Semisubmersibles are covered shortly.)
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 can be cylindrical
columns, somewhat like pillars, or they can be open-truss structures which resemble a mast or a

17