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Since the semi's major buoyancy members are located below the surface,

wave action on the vessel is diminished (fig. 6.13). 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 6.12 - Semi anchored and drilling. (Courtesy Vetco)

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