Tankers carry their cargo in a number of tanks or compartments within the hull of the ship. Before the introduction of segregated ballast tanks, tanks were cleaned after the oil was discharged and about one third of them filled with seawater so that the ship's propeller is properly immersed and it has correct handling and sea-keeping characteristics. This process is known as ballasting.

In the early days of oil tanker operations it was a common practice to clean tanks by means of jets spraying seawater. The jets washed the oil residues from the tank surfaces, resulting in a mixture of oil and water which collected at the bottom of the tank and was then pumped overboard. This naturally led to a considerable amount of oil getting into the sea. The ballast water, which was pumped overboard to make way for a fresh cargo of oil, was also contaminated.

In the 1950s, there were no alternative ways of cleaning tanks. The OILPOL Convention, adopted in 1954, tried to alleviate the pollution from this process by prohibiting the discharge of oil or oily mixtures within 50 miles of land. This limit was extended to 100 miles in certain areas which were regarded as being particularly endangered.

In the late 1960s, concern about the waste of oil and pollution caused by this process led the industry to look for an alternative. The result was to become known as "load on top".

Under load on top, tanks were cleaned as previously using high-pressure hot-water cleaning machines. However, instead of pumping the resulting oily mixtures overboard, they were pumped into a special slop tank.

Development of Crude Oil Washing

The introduction of load on top was a great contribution to the fight against marine pollution but it did not completely eliminate pollution resulting from tank cleaning operations. Although the amount and rate of discharge is carefully regulated the process still resulted in some pollution occurring. In the late 1970s an improvement was introduced. Instead of using water, the tank cleaning machines used crude oil - in other words, the cargo itself.

When sprayed onto the sediments clinging to the tank walls, the oil simply dissolved them, turning them back into usable oil that could be pumped off with the rest of the cargo. There was no need for slop tanks to be used since the process left virtually no slops. The process became known as Crude Oil Washing (COW).

Crude oil washing meant that the mixture of oil and water which led to so much operational pollution in the past was virtually ended. At the same time, the owner is able to discharge far more of this cargo than before, since less of it is left clinging to the tank walls and bottoms.

Crude oil washing regulations

Crude oil washing was made mandatory for new tankers by the 1978 Protocol to the MARPOL Convention.

MARPOL Annex I Regulation 13 (6) 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 13B 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).