Crude Oil Washing - is something of a dying art with the phase-out of single hull tankers. It was introduced in parallel with inert gas systems to reduce the volume of unpumpable residues, or ROBs, in crude oil tankers. Crude oil is not an homogeneous liquid, containing wax and sediments which drop out of suspension at sea. Research into reducing ROB's concluded that the best solvent was the crude oil itself and that washing out the tanks during discharge was the most effective way of reducing clinkage and sedimentation.
COWing can only be done in an inert tank atmosphere due the high pressures and static charge generated. Tanks are discharged in bulk until about one metre remains in each tank. If the slop tanks have carried cargo, these are discharged and then recharged with 'fresh' crude. Fixed tank washing guns and eductors are driven from a cargo pump at 10 bar or 150 psi. The washing guns spray the tanks with crude oil, the eductor strips the washings back to the slop tank in a closed cycle.
Working progressively back from forward to aft, each tank is hosed down from top to bottom for about 90 to 120 minutes per tank. On large tanks, it's sometimes necessary to reprogramme the washing guns to wash the tanktop in a final sweep. COW reduces the amount of residual wax and sediment lost if a ship has to clean (with sea water) to load the next cargo. The downside of COW is that volatile fractions, valuable in the refining process, are released into the inert gas and purged into the atmosphere.
COW'ing is a technically demanding operation to plan and execute. It has become less frequently practiced with the development of full double hull tankers and the use of pour point depressant additives, which keep the solids in suspension. Charterers of tankers trading continuously in crude oil prefer to 'load on top' of previous cargoes, accepting swings and roundabouts gains and losses rather than losing the volatiles from COW. Deck Officers have to maintain a logbook of COW discharges and tankers are required to undertake a number of COW operations each year to maintain their certification.
I would like to hear from any serving tankermen on how often they COW, or alternatively how often Charterers instruct "No COW" during discharge.