Page 241 - International safety guide for oil tankers and terminals

SHIPBOARD OPERATIONS
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11.7.2
งานห้องสมุด ศูนย์ฝกพาณิช
Ships should have written procedures available on board which indicate
the actions to be taken and the operations necessary for the safe transfer
of the cargo.
Entry into the tank should be prohibited until it is safe for entry and there
mentioned in 11.4.7) relating to the inerting of such tanks. Wheneverย์นาวี
is no further possibility of hydrocarbon ingress. However, if it is deemed
essential to enter the tank for any reason, such entry must be carried out
in accordance with Section 10.7.
Inerting Double Hull Tanks
The complexity of the structure in double hull and double bottom tanks
makes them more difficult to inert than conventional tanks. It is strongly
recommended that the operator use these guidelines as a basis for
developing procedures (similar to the guidelines and procedures
possible, these procedures should be developed in conjunction with the
shipbuilder and should be based on actual tests/experiments as well as
on calculation. They should describe for each tank the procedures to be
followed, the equipment to be used and its configuration, and the time
required to reduce the oxygen level in the tank to less than 8% by volume.
Where tanks are identical in structure and size, and where the method of
inerting is identical, the data can be obtained from tests on a
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representative tank. Otherwise, the tests referred to above should be
carried out for each tank.
The introduction of inert gas into a tank may give rise to electrostatic
charging. The compartmentalised structure of the tanks means that this
charge is unlikely to reach incendive levels. However, because there may
be a flammable atmosphere in certain areas within the tank (see Section
11.7.1) it is essential that all electrostatic precautions detailed in Sections
3.2 and 7.1.6.8 are complied with throughout the inerting process and for
30 minutes thereafter.
Flexible hoses used for inerting double hull tanks should be clearly
identified, be dedicated solely to this use and be stowed safely and
correctly. The hose string should be electrically continuous, and this
should be verified prior to putting hoses into service. It should be
confirmed that the string is properly earthed before inerting commences.
In order to minimise the transfer of hydrocarbon vapour from cargo tanks,
all cargo tank inert gas supply valves, where fitted, should be temporarily
closed. Prior to connecting the hoses, the inert gas line should be purged
with inert gas. The hoses should not be connected until required.
Once the tank has been inerted, consideration should be given to the
benefits of keeping it permanently connected to the inert gas system
(constant pressure monitoring, over-pressure protection via the deck
water breaker, ease of topping-up, for example) against any potential
problems of vapour transfer (vulnerability of the hose to heavy seas, for
example). If the hoses remain connected, then all the cargo tank inert
gas inlet valves must be re-opened. If the hoses are disconnected, the
inert gas system must be returned to its original status. If leaked oil is to
be transferred from a ballast space which has been inerted, it is important
to ensure that further inerting is carried out during the operation in order
to avoid the introduction of oxygen into the tank.
© ICS/OCIMF/IAPH 2006 207