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1 Introduction

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 tankers slop tanks with the decanted water being

discharged overboard into the ocean. Consequently, this operation resulted in inevitableoperational 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 carriers 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 enoughwater ballast to achieve the tankers required sailing ballast draught for the voyage.

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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.