Crude Oil Washing – The Sacred COW
Crude oil tends to stick around the walls and surfaces of the ship tanks and it needs to be cleaned after every discharge of cargo. In the earlier days sea water was used to clean the tank and where was the water put after washing the tank? Yes you guessed it right – back to the ocean from where it came, but this time mixed with the oil which led to increasing oil pollution. Hence load on top systems were developed which used hot water which was later stored in slop tanks and it was tried to separate the oil from water to the maximum possible extent.
The above measures were not fully satisfactory hence the crude oil washing systems were developed wherein heated cargo was used to wash the tanks and the residue could be pumped out with the cargo rather than throwing it as a waste. We will study the details of the crude oil washing in some separate article but for now we are concerned with the inert gas conditions during the operation.
During the crude oil operation the pressure in the tank should be kept positive as well as in the inert condition. The level of oxygen in the tanks during this operation is normally measured at locations of 1 meter depth from the deck level and midway in the tank depth. In case the inert conditions are not found at the relevant spaces, the operation of COW should be suspended immediately and should only be commenced once the atmosphere inside reaches the fully inert condition.
The installation of COW equipment has been made mandatory for all tankers above 20,000 tons deadweight by MARPOL Annex I Regulation 13 (6). There have been several other resolutions and amendments to the same since then, and the reader is advised to visit the website of International Maritime Organisation for more information.
It is all very well to clean the tanks and remove the maximum possible cargo using the crude oil washing technique but since the washing is carried out at a positive inert pressure, the tanks are still not safe to be entered for the purposes of repair or inspection after this operation is over. That needs another operation known as gas freeing which is necessary before any person could enter the tank. We shall learn about this in our next article.