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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.
Since the semi's major buoyancy members are located
below the surface, wave action on the vessel is diminished (fig.
2.6). Not only is heave greatly reduced, but the natural roll period
is much longer for the semi. In spite of these favorable motion
characteristics, a semi is subject to more limited loading conditions
since its deck is 40 to 70 ft above the water line during drilling.
Drillships can store tubulars and bulk materials low in the hull,
while the semi must stow them on deck. (Weight located high on a
vessel decreases stability; adding weight below increases stability.
When loaded, then, the high deck of the semi increases the
possibility of capsizing.)

Figure 2.5 - Semi anchored and drilling. (Courtesy Vetco)

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