2.2.1 Displacement of crude oil cargo vapours
at the loading port continues to occur. The reasons for the existence
of these volumes of this displaced, but co-mingledfootnote, vapour must be subdivided and attributed
to two discrete tanker operations; namely existing vapour in the cargo
tank system before loading and, the evolved vapour created during
the loading programme.
2.2.2 The first portion of the vapour displaced
from the cargo tanks to be considered is that from the evolved vapour
generated during the previous discharge programme and in particular
that vapour generated as a result of the Crude Oil Washing of the
cargo tanks. The concentration of this proportion of vapour within
the co-mingled gas mixture within a cargo tank can be determined prior
to commencement of the loading process. The second portion of vapour
displaced is that that develops or evolves during the loading programme
itself. This vapour evolves as a result of, both, the turbulence generated
in the cargo tanks due to the volumetric rate of loading and the pressure
differentials within the loading pipeline system creating a degree
of “flashing” of the vapour from the incoming crude oil.
2.2.3 To illustrate the extent of these gases
within a cargo tank system on a tanker during a loading process, Figure
2.1 below shows the measurements of hydrocarbon vapour concentrations
as taken from a tanker during its loading programme. The “X”
axis of the graph records the percent status of loading of the tanker
whereas the “Y” axis records the percentage of hydrocarbon
vapour (VOC) concentration. The graph primarily records the total
hydrocarbon gas concentration at the differing percentages of loading
of the cargo tanks. However, this total figure is then mathematically
proportioned and subdivided, taking into consideration the diminishing
size of the vapour volume in the cargo tanks, into the two concentrations
of vapours, namely those present at the commencement of loading (in
the event approximately 4% of the total tank vapour volume) and the
concentration of vapours that evolve as a result of the loading process.
2.2.4 These vapours are displaced by the incoming
cargo volumes, throughout the loading period, and released through
the ship’s vapour pipeline system (inert gas pipeline) to atmosphere
via the ship’s mast riser. In order to prevent excess pressures
within the cargo tank system the isolation/control valve to the mast
riser is fully opened at the commencement of loading and remains opened
until completion of loading. Once the mast riser valve is shut and
loading is completed, the necessary “in tank” positive
pressure is achieved to prevent any form of air/oxygen entry into
the cargo tank vapour system as is required by the SOLAS regulations.
Figure 2.1 Hydrocarbon vapour concentration in the vapour phase during a
loading
2.2.5 In Figure 2.2 below, a photograph shows
the deck of a tanker and highlights the relevant pressure control
and release mechanisms, namely the vessel’s mast riser, the
individual tank Pressure/Vacuum (P/V) valves and the secondary safety
mechanism of the P/V breaker. These mechanisms will be explained further
in this section.
Figure 2.2 Main Cargo Deck of a Crude Oil Tanker
2.2.6 Typically a normal loading programme will
take about 24 hours for a VLCC with a volumetric rate of loading of
up to 20,000 m3/hour. The mast riser is normally used during
loading for tank vapour pressure control. Its exit location, being
at least 6 metres above the deck, allows for the free flow of the
vapours displaced from the cargo tanks by the incoming liquid crude
oil at the rate of loading of the cargo. The rate of displacement
of VOC vapours from the cargo tank system will be the same as the
loading rate but the concentration of VOC vapours in the displaced
stream will be greater dependent upon the extent and rate of evolution
of VOC vapours (vapour growth) from the incoming cargo that would
add to the volume of gas/vapour mixture already existent in the cargo
tank prior to loading, as shown in Figure 2.1 above.