Page 219 - International safety guide for oil tankers and terminals

SHIPBOARD OPERATIONS
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11.1.15.9
If there is a requirement to use shore supplied nitrogen, for
example for purging tanks, padding cargo or clearing lines, the
ship should be aware that this may be at high pressure (up to
10 bar) and at a high flow rate and that it can therefore be
potentially hazardous because of the risk of over-pressurisation of
incoming flow of nitrogen must be restricted to a rate equal to, or์นาวี
the cargo tanks. A risk assessment should be carried out and the
operation should only proceed if appropriate risk responses are in
place and operating. As a very minimum, the precautions detailed
in Section 7.2.2 must be observed.
One method of reducing the risk of over-pressure is to ensure
that the tank has vents with a greater flow rate capacity than the
vapour return line. Positive measures to ensure this should beย
inlet, so that the tank cannot be over-pressurised. Where vapour
control and emission regulations require closed operation, the
less than, the maximum flow of vapour possible through the
agreed. A small hose or reducer prior to the manifold can be used
to restrict the flow rate, but pressure must be controlled by the
terminal. A gauge will permit the ship to monitor the pressure.
It is not appropriate to attempt throttling a gas flow by using a
ship’s manifold valve that is designed to control liquid flow.
However, the manifold can, and should, be used as a rapid safety
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stop in an emergency. It should be noted that the effect of
pressure surge in a gas is not as violent as in a liquid.
Sensitive cargoes, for example some highly specialised lubricating
oils, may have to be carried under a pad or blanket of nitrogen
supplied from ashore. In such cases, it is preferable to purge the
entire cargo tank before loading. After such purging has been
completed, loading the cargo in a closed condition will create the
required pad within the tank. This significantly reduces the risk of
over-pressurisation that is present when padding with shore
supplied nitrogen as a separate procedure on completion of
loading.
Pigging
Pigging is a form of line clearing in which an object, most often in
the form of a rubber sphere or cylinder and known as a ‘pig’,is
pushed through the line by a liquid or by compressed gas. A pig
may be used to clear the line completely, in which case it will
usually be propelled by water or by compressed gas, or to follow a
previous grade to ensure that the pipeline remains as free of
product as possible, in which case it is likely to be propelled by
the next grade.
A common arrangement for catching the pig is for the shore
terminal to provide a pig receiver, which is mounted outboard of
the ship’s manifold, and from which the pig may be removed.
A pressure of about 2.7 bar (40 psi) is considered to be the
minimum necessary to drive the pig, but pressures of up to
7 bar (100 psi) may be used.
Before any pigging operations are carried out, the Responsible
Officer and the Terminal Representative should agree the
procedures and associated safeguards to be put in place. The
© ICS/OCIMF/IAPH 2006 185