a floating offshore drilling structure that has hulls submerged in the water but not resting on the seafloor. Living quarters, storage space, and so forth are assembled on the deck. Semisubmersible rigs are either self-propelled or towed to a drilling site and either anchored or dynamically positioned over the site or both. Semisubmersibles are more stable than drill ships and are used extensively to drill wildcat wells in rough waters such as the north sea. (see dynamic positioning.)
