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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.)

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