Intertanko - Introduction to COW

Regulations laid down in the 1978 Protocol to the 1973 Marine Pollution Convention (MARPOL 73/78) require the cargo tanks of crude oil tankers to be cleaned using a procedure called crude oil washing (COW). With the COW procedure the crude oil cargo itself is used as the cleaning medium. During the 1960s it was discovered that crude oil, when applied to the cargo still remaining on tank floors and clinging to the tank structures, using tank cleaning machines, effectively dissolves and dilutes these residues and mixes it in with the rest of the cargo which is being discharged ashore by the cargo pumps.

Prior to the advent of COW, cargo tanks were washed with sea water on their ballast voyage to the next loading port. The mixture of oil and cleaning water resulting from this type of cleaning operation could settle out in the tanker’s slop tanks with the decanted water being discharged overboard into the ocean. Consequently, this operation resulted in inevitable operational discharges of oil-water mixture into the sea.

However, the use of crude oil to COW the tanks means that the solvent action of the crude oil makes the process far more environmentally friendly than when water is used. Additionally, after undertaking COW, the volume of cargo residues left in the tanks is greatly reduced removing the subsequent risk of operational discharges at sea.

Modern tankers are designed with segregated ballast tanks (SBT), and there are only a few stipulated occasions on which seawater comes into contact with the oil cargo system during the course of normal tanker operations. The requirement for new crude oil tankers to be built with double hulls, introduced in the 1990s, has further improved the efficiency of COW operations because more of the structural support members are placed outside the cargo tank and on these types of ships, the amount of crude oil residues left in the cargo tank following discharge is much reduced. Overall, the COW procedure and ship design changes have greatly reduced the need for operational discharges from tankers.

Regulation 13B of Annex I of MARPOL 73/78 requires that the COW installation and arrangements onboard a tanker should comply with the provisions of the “Specifications for the Design, Operation and Control of Crude Oil Washing Systems” adopted by the International Maritime Organization (IMO) in 1978. The COW regime requires that before departure on a ballast voyage, after the complete discharge of cargo, sufficient tanks shall have been crude oil washed to preclude the ballasting of a cargo tank without it having been crude oil washed. On SBT ships approximately 25 per cent of the crude oil carrier’s cargo tanks need to be washed, in the prescribed manner, on every voyage for sludge control purposes provided that no tank need be crude oil washed for sludge control purposes more than once in every four months. For tankers with insufficient SBT capacity, the number of tanks to be crude oil washed has to be increased above this minimum level in order to render sufficient cargo tanks “clean” enough (as defined by the regulations) to take onboard enough water ballast to achieve the tanker’s required sailing ballast draught for the voyage.

In addition to the regulatory controls governing the use of COW, commercial or charter party requirements may require the tanker operator to carry out a greater or lesser degree of COW than the specified minimum in order to maximise the discharge of the crude oil cargo. Notwithstanding these commercial pressures for the extent of COW to be undertaken, at no time should a tanker undertake less than the minimum levels specified in paragraph 6 of Section 1 of the mandatory onboard COW Manual.

Although the MARPOL 73/78 COW regime has proved to be eminently successful in minimising tanker operational discharges and improving cargo outturns during the last two decades of the 20th century, the tanker industry has also been learning more about the behaviour of crude oil cargoes over the period. A number of research projects1 have led to a better understanding of the COW process and how it could be further improved. As a result of this work and at the initiative and suggestion of INTERTANKO, in 1999 the IMO adopted amended COW requirements that are laid down in the revised “Specifications for the Design, Operation and Control of Crude Oil Washing Systems”. These revised Specifications can be found in the 2000 Edition of the “Crude Oil Washing Systems” publication, issued by the IMO.

From an operational perspective the changes provide a more realistic and accurate way of determining the suitability of a crude oil for use in crude oil washing.