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anticipated loading conditions, spaced to support topsides
modules, and tuned for a natural period of at least 20 seconds.
These columns are supported by two parallel pontoons or a ring
pontoon. Pontoons are sized to provide adequate buoyancy to
support all weights and vertical loads, and proportioned to
maximize heave damping.

Figure 2.4 - Semi-submersible platform Sonat

Semisubmersibles have many deck configurations. The
basic shapes are triangular, rectangular, and pentagonal. The first
triangular semi, Ocean Driller, appeared in 1963, followed by
Bluewaterll, the first rectangular semi, in 1964. These vessels,
supported by legs (usually three to ten) from deck structure to
submersible hulls, generally drill in water depths from 200 to
2,000 ft. In 1970, semis were equipped with thrusters to facilitate
towing and positioning over the well (fig. 2.5). Most semis built
after 1973 have some form of self-propulsion.
The first DP semi, the Sedco 709, was commissioned in
1978. Dynamically positioned semis still are not common,
generally due to the power requirements for positioning the
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