ABSTRACT
A large percentage of merchant fleets have considerable tonnage in the way of tanker-type vessels, the majority of which are carriers of crude oil and the associated oil products. Oil cargoes are usually carried in very large crude carriers (VLCCs) up to 350,000 deadweight tonnage (DWT). Mainly smaller tankers up to 50,000 tonnes, these are divided into carriers of clean oils such as motor spirit, naphtha, kerosene and gas oil; and carriers of black oils such as fuel oil, diesel oils and furnace oils. The most dangerous condition of an oil tank is when the cargo has been discharged, and before any tank cleaning and gas freeing has been carried out. When a cargo tank is full, the possibility of fire is present but explosion risk is quite small since the air/hydrocarbon vapour atmosphere above the oil is small. When the tank is empty, however, the air/hydrocarbon vapour atmosphere is at its maximum.
