Crude oil washing (COW) is a system whereby oil tanks on a tanker are cleaned out between voyages not with water, but with crude oil - the cargo itself. The solvent action of the crude oil makes the cleaning process far more effective than when water is used. (There is usually a final water rinse but the amount of water involved is very low.) The system helps prevent pollution of the seas from operational measures.
Crude oil washing was made mandatory for new tankers by the 1978 Protocol to the MARPOL Convention. Regulation 33 of MARPOL Annex I requires every new crude oil tanker of 20,000 tons deadweight and above to be fitted with a cargo tank cleaning system using crude oil washing.
Regulation 33 states that COW installation and arrangements should comply with at least all of the provisions of the Specifications for the Design, Operation and Control of Crude Oil Washing Systems adopted by IMO in 1978, as may be revised.
In 1999, IMO adopted revised specifications for COW by resolution A. 897(21) Amendments to the revised Specifications for the Design, Operation and Control of Crude Oil Washing Systems (Resolution A.446(XI), as amended by resolution A.497(XII).