Oil tankers carry various hydrocarbons in their cargo tanks, from crude oil to high octane spirits such as petroleum.
These cargos are loaded by shore-side pumps at different ports of oil producing countries or oil refineries throughout the world. When the destination port is reached the tankers own cargo pumps located at the bottom of the pump-room, discharge the oil.
The pumps discharge the cargo ashore through the chiksan arms; a specialized component connecting the ships discharge pipes to the jetty pipes. (Some modern tankers as the one shown in the Introduction, have these arms incorporated in the tanker deck)
Meanwhile inert gas is supplied to the air space left by the oil and this expels the gases to atmosphere or a specialist gas treatment plant where the gas is treated before admittance to atmosphere.
On completion of cargo discharge the tanker leaves port for its next cargo and in the old days once well clear of land the mates (Deck Officers) always wanted to start gas freeing and clean the tanks. I was an Engineer Officer with Esso Tankers for many years and this was the worst part of any voyage. The whole ship stank of oil fumes and, the mates used our mess-room for meals bringing the reek of oil fumes with them enough to put any ships engineer of his beer - well nearly!
However, I diverge. The tanks are gas-freed using inert gas processed from the main engine, boiler fume exhausts or nowadays from a dedicated inert gas plant. These gases are forced through the cargo tanks by fans and once the oxygen content is at the required level, the cleaning commenced.
Tank cleaning methods have changed over the years the modern one uses pressurized crude oil to wash down the internal surfaces, so gas freeing is sometimes not required. However it is worth examining this process as the tanks still require gas freeing before inspection for corrosion.
This is another article in my series on marine engineering and we shall examine the use of inert gas in preventing explosions and gas-freeing of oil tanker cargo tanks. We will also have a look at the methods to clean cargo tanks when I was at sea, and the current methods.
We begin then with the operation of a typical inert gas system.