MARPOL was devised by the International Maritime Organisation (IMO) (a branch of the United Nations); it was the result of a general feeling that existing oil pollution prevention regulations were inadequate because of the continuing increase in the size of oil carriers and of the amount of oil and liquid chemicals being transported by sea. The MARPOL convention was first given effect in the United Kingdom by the introduction into English law by Section 20 of the Merchant Shipping Act 1979. As a result of the adoption of the Convention by 15 States (which was the minimum requirement and was achieved by October 1983) implementation was brought about in the United Kingdom by the Merchant Shipping Prevention of Oil Pollution Order 1983. MARPOL 1973 introduces the requirement for Crude Oil Washing (COW). This involves the use of high pressure jets of crude oil as a means of tank cleaning. This is of beneficial effect in that it greatly reduces the quantity of oil remaining on board after discharge and thus reduces the risk of pollution. Newly built crude oil carriers exceeding 20,000 DWT are required to be fitted with both segregated ballast tanks (SBT) and COW. Existing crude oil tanker carriers of 40,000 DWT or over must be fitted with either SBT or COW. MARPOL 1973 calls for Segregated Ballast Tanks SBTs to be fitted to new tankers of 80,000 DWT or over. This enables the Master, after cargo discharge, to take on seawater to compensate for the weight of the cargo discharged and thus provide stability, strength and manoeuvrability. The SBT idea was aimed towards the purpose of avoiding the discharge into the sea of dirty ballast water which is such a commonplace result of the carrying of ballast water cargo in tanks and the inevitable consequential mixing of oil and water. Another feature of MARPOL 1973 was the need of any vessel of 400 GRT or more to fit oil-from-water separating equipment or filter system. Also any ship exceeding 10,000 GRT should be fitted with an oil discharge monitoring and control system.