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semi-submersible
semi-submersible adj. and n. 
/ˌsɛmisəbˈmərsəb(ə)l/ (applied to) an offshore drilling platform or barge equipped with submerged hollow pontoons that can be flooded with water when the vessel is anchored on site in order to give it stability against waves and wind.
Brit. /ˌsɛmɪsəbˈməːsᵻbl/
, /ˌsɛmɪsəbˈməːsᵻbl/U.S. /ˌsɛˌmaɪsəbˈmərsəb(ə)l/
, /ˌsɛˌmaɪsəbˈmərsəb(ə)l//ˌsɛmisəbˈmərsəb(ə)l/ΚΠ
1962 World Oil Sept. 96/3 The rig is an all weather, semi-submersible which is submerged to the 40 foot level in normal drilling operations.
1963 World Petroleum Aug. 47/1 The semi-submersible vessels give a desired balance between cost, safety and performance.
1975 North Sea Background Notes (Brit. Petroleum Co.) 11 Semi-submersibles can be used for drilling when resting on the sea bottom, but they are generally employed in the floating position.
1980 Christian Sci. Monitor (Mid-western ed.) 4 Dec. 11/1 A third delineation well..was spudded on Nov. 14 five miles west of P-15 by the newly arrived semi-submersible drilling rig.
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