Crude Oil Washing
Crude Oil Washing (COW) is a method of cleaning the cargo tanks of a crude-oil tanker using the crude oil cargo itself. During discharge, high-pressure jets spray crude oil onto the tank structure, dissolving and washing down the heavy oil that clings to the surfaces so that more cargo is discharged and less remains as residue.
COW reduces the oil left in the tanks (clingage), so it both increases the cargo actually delivered and cuts the amount of oily residue and the need for water washing — which reduces pollution. It is required by MARPOL for larger crude-oil tankers and is always carried out under an inert-gas atmosphere for safety.
Crude oil washing delivers more of the cargo and dramatically cuts the oily residue and slops that would otherwise have to be dealt with, reducing both cargo loss and pollution. It is a MARPOL requirement on larger crude tankers precisely because it is one of the biggest reductions in operational oil pollution available.
Why is crude oil washing done under inert gas?
Spraying oil creates a mist and static that could ignite in an oxygen-rich atmosphere; washing is only carried out with the tank inerted by the inert gas system to keep the atmosphere non-flammable.