The process of removing hydrocarbon vapours, liquids or residues. Tank cleaning may be required for one or more of the following reasons:
1. To carry clean ballast.
2. To gas free tanks for internal inspections, repairs or prior to entering dry dock.
3. To remove sediments from tank top plating. This may be required if the vessel is engaged in the repetitive carriage of fuel oil or similar sediment settling cargoes. Although washing may not be necessary in between consecutive voyages, assuming the cargoes are compatible, many Ship Owners have found it prudent to water wash a small group of tanks on a rotation basis between voyages, thus preventing any large accumulation of sediments.
4. To load a different and not compatible grade of cargo.
Washing in between carrying different grades of cargo is the most common reason for tank cleaning. In most cargo sequences on product tankers, the cleaning may consist of no more than a simple hot or cold seawater wash. A simple water wash will disperse many types of chemicals and has been found effective between clean petroleum products such as gasoil and kerosene. However, it should be noted that there is a number of grade sequences, particularly in the petroleum product trade, where no washing at all needs to be be carried out. Thus the decision for necessary tank cleaning required in such trades is often made only when knowledge of the next grade to be loaded is obtained.