The process of removing cargo residues and gas from cargo tanks using water, chemicals, or crude oil washing prior to loading a new cargo.
Quick facts
Regulation
MARPOL Annex I
In practice
MARPOL Annex I controls the disposal of tank washings containing oil: water recovered during washing must be processed through the oily water separator or retained in the slop tank. Annex II governs residues from noxious liquid substance cargoes and requires pre-washing and ventilation to specified standards before washings may be discharged. The residue and washwater disposal records must be logged in the Cargo Record Book Part II. Port state authorities at many chemical and oil terminals require a tank cleanliness certificate from a recognised inspector before loading, and surveyors check tanks for odour, residual film, and rust condition.
Regulatory detail & full definition
Tank cleaning is the process of removing cargo residues, vapours, and contaminants from cargo tanks to a standard acceptable for the next cargo, for gas-freeing, or for entry by personnel. Crude tankers typically combine crude oil washing during discharge with a subsequent hot seawater wash, while product and chemical tankers may require multiple chemical cleaning stages to remove traces of previous cargo before loading an incompatible product. The Tanker Safety Guide (Chemicals) and ISGOTT provide practical guidance on cleaning sequences for specific cargo pairs.
MARPOL Annex I controls the disposal of tank washings containing oil: water recovered during washing must be processed through the oily water separator or retained in the slop tank. Annex II governs residues from noxious liquid substance cargoes and requires pre-washing and ventilation to specified standards before washings may be discharged. The residue and washwater disposal records must be logged in the Cargo Record Book Part II. Port state authorities at many chemical and oil terminals require a tank cleanliness certificate from a recognised inspector before loading, and surveyors check tanks for odour, residual film, and rust condition.