The OPA90 and equivalent IMO requirements must be satisfied in the structural design of ships carrying oil cargoes. The primary requirements are to arrange a double bottom to be of a required minimum height, and double sides of a required minimum width. To reduce the outflow of pollutant cargoes in a ship collision or grounding accident, the OPA90 and IMO both require that the minimum width w of each wing ballast tank or void space is not to be less than 0.5+DWT/20 000 (m) or w = 2.0 (m), whichever is the lesser, where DWT is the deadweight of ship in tonnes. In no case is w to be less than 1.0 (m). The OPA and IMO also require that the minimum vertical height h of each double-bottom ballast tank or void space is not to be less than 2.0 m or B/15 (B = ship's beam), whichever is the lesser, but in no case is the height to be less than 1.0 m.