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Intertanko procedure can limit emissions
When crude oil is transported
in tankers there is always a build up of gas in the tanks. Normal procedure
today is to release these gases into the atmosphere.
 However,
the vessel’s command does not know when or at what pressure a manually
controlled release should be stopped. The result is that excess cargo
vapours can be unnecessarily released causing loss of cargo and air pollution.
To
remedy this problem the Intertanko Safety, Technical and Environmental
Committee (ISTEC) has developed a Shipboard Procedure for the Control
of Atmospheric Pollution by Volatile Organic Compounds and Reducing Loss
of Cargo. This procedure has been well received by owners and operators
of crude oil tankers.
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Figure 1
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Two
different types of gases are present. They are the unsaturated gas from
the inert gas supply and the saturated hydrocarbon vapours from the crude
oil cargo.
The
procedure uses the current knowledge of the different behaviour of the
two gases to determine when the manually controlled released should be
stopped in order to avoid unnecessary release of these vapours into the
atmosphere.
What
happens when the procedures are used is that the command is required to
monitor and record the pressure drop during a release of gas from the
cargo tank vapour system. This can be done with the use of the inert gas
pressure gauge in the cargo control room or on the vessel’s bridge.
The
two diagrams graphically illustrate what happens. Figure 1 records a typical
pressure release pressure drop profile when a P/V (Pressure/Vacuum) valve
is used, while Figure 2 depicts a similar profile when the mast riser
is used.
The VOCON Procedure
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Figure 2
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The procedure, which is commonly called the VOCON Procedure works as
follows (from the Intertanko literature on the matter):
1.
Before opening either the mast riser or the P/V valve on deck, note the
pressure in the inert gas pipeline system.
2.
Open the pressure release valve and record/monitor the pressure within
the inert gas pipeline at regular short intervals – every 30 seconds
for the riser release or every one minute for the P/V Valve release.
3.
Plot the pressure drop profile. This can be done either manually or by
use of the inert gas oxygene and pressure recorder in the cargo control
room, but an increase in the recorder paper feed rate will be required
to achieve definition of the plot.
4.
When rate of pressure drop becomes constant – after a more rapid
fall – then the gas release should be stopped and the valve closed.
5.
Monitor the tank gas pressure after completion of the controlled release
in order to check the final pressure obtained within the vapour/inert
gas system.
It
all makes sense and it will contribute in no small way to reduce emissions
from crude oil tankers at a time when authorities, and the public at large,
are focusing on shipping (quite unfairly) as a major source of pollution
at sea.
// Petter Arentz
Back to SSG 6, June 14
Latest update 18-10-2006 8:49
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