A tank cleaning method using crude oil cargo as the washing medium to dissolve and remove waxy residue from tank surfaces.
Quick facts
Regulation
MARPOL Annex I
In practice
COW may only be carried out simultaneously with cargo discharge, when the inert gas system is operating and maintaining the required oxygen-deficient atmosphere. The tanker officer responsible maintains a detailed washing programme, documenting which tanks are being washed, the duration, and the washing medium temperature and pressure. Fixed washing machines must have their calibration and drive systems verified before each operation, and any malfunction must be recorded and reported. Port state control officers inspect COW records and the COW Manual as part of tanker-specific MARPOL compliance checks.
Regulatory detail & full definition
Crude oil washing is a tank cleaning technique in which crude oil cargo, warmed and pressurised, is sprayed through fixed or portable washing machines onto the internal surfaces of cargo tanks to dissolve and dislodge waxy residue that would otherwise remain as clingage after discharge. It reduces water-borne waste, increases cargo outturn, and shortens port time compared with hot seawater washing. MARPOL Annex I Regulation 33 and the COW Manual — which must be approved by the flag administration — prescribe the procedures, washing machine standards, and record-keeping requirements.
COW may only be carried out simultaneously with cargo discharge, when the inert gas system is operating and maintaining the required oxygen-deficient atmosphere. The tanker officer responsible maintains a detailed washing programme, documenting which tanks are being washed, the duration, and the washing medium temperature and pressure. Fixed washing machines must have their calibration and drive systems verified before each operation, and any malfunction must be recorded and reported. Port state control officers inspect COW records and the COW Manual as part of tanker-specific MARPOL compliance checks.