Crude oil washing (COW) is a system whereby oil tanks on a tanker are cleaned out between voyages not with w ater, 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. COW is mandatory on new tankers under the International Convention for the Prevention of Pollution by Ships (MARPOL). 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 seaw ater. 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 cons iderable amount of oil getting into the sea. The ballast water, which was pumped overboard to make way for a fresh cargo ofoil, 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 la nd. 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. During the course of the return voyage to the loading terminal this mixture separates. Oil, being lighter than water, gradually floats to the surface leaving the denser water at the bottom. This water is then pumped into the sea, leaving only crude oil in the tank. At the loading terminal fresh crude oil is then loaded on top of it. The process had advantages for the owner of the oil, since the oil normally lost during tank cleaning can be saved (as much as 800 tons of oil on a large tanker), but the main beneficiary was the environment. Some experts believe that without load on top the amount of oil being dumped into the sea as a result of tank cleaning could have reached more than 8 million tons a year. 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.
