Crude oil washing .What is Cow ? COW onboard Ships 

Crude oil washing (COW) is washing out the residue from the oil tanker using the crude oil cargo itself, after the cargo tanks have been emptied. Crude oil is pumped back and preheated in the slop tanks, then sprayed back via high pressure nozzles in the cargo tanks onto the walls of the tank. Due to the sticky nature of the crude oil, the oil clings to the tank walls, and such oil adds to the cargo 'remaining on board' (the ROB). By COWing the tanks, the amount of ROB is significantly reduced, and with the current high cost of oil, the financial savings are significant, both for the Charterer and the Shipowner. If the cargo ROB is deemed as 'liquid and pumpable' then the charterers can claim from the owner for any cargo loss for normally between 0.3% up to 0.5%. It replaced the load on top and seawater washing systems, both of which involved discharging oil-contaminated water into the sea. MARPOL 73/78 made this mandatory equipment for oil tankers of 20,000 tons or greater deadweight.