2.3.1 During the voyage, the temperature of the
gases/vapours in the ullage space of the cargo tanks and the liquid
cargo varies. The gas phase consists of a mixture of unsaturated gases
(Inert Gas – for tank safety and protection) and saturated vapours
(evolved hydrocarbon vapours from the cargo). The temperature of the
gas phase of the tank varies diurnally with its maximum temperature
being achieved by mid afternoon and its coolest temperature in the
early hours of the morning. The liquid phase temperature varies very
much slower and is dependent upon both the hull design and the temperature
of the surrounding seawater.
2.3.2 Figure 2.3 below records, as an example,
the vapour pressure and cargo temperature data of a reported voyage
for a single hulled (but segregated ballast) tanker. The graph records
on the “X” axis the days of the voyage whereas the “Y”
axis records both the cargo temperature (°C) and the pressure
(mmWG) within the vapour phase of the cargo tank system. Superimposed
upon the graph is both the normal operational release pressure as
well as the P/V valve opening pressure levels. The vapour pressure
readings were recorded every four hours whereas the cargo liquid temperature
readings (blue) were recorded daily.
Figure 2.3 Temperature and Pressure profile for a crude oil voyage
2.3.3 The double hulled construction of a crude
oil tanker has a void/ballast space located between the cargo tank
and the outer hull, this causes the temperature of the liquid cargo
to remain closer to the temperature of the cargo upon loading for
a longer period due to the so called “Thermos Effect”
or heat loss insulation created by the void or empty ballast space.
The cargo temperature profile, as shown in Figure 2.3, reflects the
expected changes to temperature for a cargo carried on board a single
hulled vessel where the impact of the seawater temperature upon the
cargo is more apparent. This aspect can be more clearly seen in Figure
2.3 for the early/interim days of the 47-day voyage from North Sea
to the Far East.