We had gone slightly off course with the Pirates of the Caribbean after learning about the role of the inert gas plant in the loading operations on ships (previous article) and now we will see something related to crude oil washing – the Sacred COW of the environment friendly people. The cargo tanks of the ship can be compared to a container in your kitchen which you use to store some particular product like say mustard oil one day, while the next week you might want to use the same container to store a different product say vegetable oil. You would normally want to wash your container thoroughly before putting in to the new product. But how are huge tanks on the ship cleansed of the previous cargo before loading a new cargo? Obviously hand washing is not an option unless you want to spend aeons at a port of call.
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.