What is crude oil washing (COW) on a tanker?

Crude oil washing turns the tanker's own cargo into its tank-cleaning fluid, which sounds almost too simple to work but is genuinely effective because crude oil is a natural solvent for the waxy, asphaltenic clingage that sticks to steel tank surfaces during a voyage.

While a cargo tank is being discharged, some of the crude already flowing through the cargo lines is diverted to fixed tank-cleaning machines, deck-mounted units that rotate through programmed cycles and direct high-pressure jets of crude across the internal surfaces of the tank, top, sides and bottom, working through the tank systematically as the liquid level falls so surfaces are washed as they are exposed. The oil striking the walls dissolves and lifts the clingage, and because washing takes place while the tank is being pumped out, the dissolved residue simply drains down and leaves with the rest of the cargo rather than needing separate handling, which is the core efficiency gain: cargo that would otherwise remain stuck to the tank and eventually become slop is instead delivered to the receiver.

Before crude oil washing existed as standard practice, tanks were washed with seawater after discharge, which stripped the clingage into an oily water mixture that then had to be separated, with the oily fraction retained as slops and the water discharged only after meeting oil content limits, a process that generated large volumes of oily waste and created real pollution risk when it went wrong. COW cut both the volume of residue left in the tanks and the amount of oily water generated, which is why MARPOL Annex I makes it mandatory on new crude oil tankers of 20,000 dwt and above, alongside segregated ballast tank requirements, part of the wider tightening of tanker pollution rules.

Because the operation involves spraying and atomising crude oil inside an enclosed tank, explosion risk is the overriding safety concern, so COW is only carried out with the tank atmosphere kept safely inert, oxygen content maintained below the level that could support combustion, using the ship's inert gas system, and with strict procedural controls: washing sequence, machine pressure and duration follow an approved COW manual specific to the ship, tank atmosphere is monitored continuously, and the operation is coordinated closely with the discharge rate. Getting COW wrong, inert gas quality lapsing, or washing continuing after a tank atmosphere alarm, has been a factor in tanker explosions historically, which is exactly why the procedure is as tightly controlled as the cargo operation itself.