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heave damping.

Figure 6.11 - 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. 6.12). 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 massive structures. However, the advanced- technology DP system

offered in one new ice-class semi design claims station- keeping with enough

precision to achieve three times the downhole drill rate of other hostile-

environment semis.

In addition to station-keeping, other factors considered when selecting a

drilling vessel are motion-compensation ability, load and stowage capacities,

location of the rotary table relative to the vessel's center of gravity, mobility, and

the experience and skill of the personnel. The primary advantage of the semi over a

drillship in rough seas is its stability—its resistance to capsizing.

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