Page 269 - International safety guide for oil tankers and terminals

SPECIAL SHIP TYPES

such as a U-bend or a special valve, but the possibility of a blockage
should always be suspected after a rough voyage. A blockage may also
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occur if the vessel has been in very hot weather which has caused the
cargo to expand above the gas line outlet.

งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
Drains are normally fitted in each gas line and these should be checked
before commencing cargo operations, in order to ensure that the cargo
hold is able to ‘breathe’. These drains may become blocked, particularly
during the carriage of high pour point cargoes, and gas lines should be
blown through with inert gas to ensure they are clear.

During the carriage of dry bulk cargoes, the holds should be sealed from
the main oil cargo pumping and gas venting systems and alternative
venting systems utilised as required. Wing tanks should be maintained
either in a gas free or an inert condition.

14.1.7 Inert Gas

The basic principles of inerting are the same for a combination carrier as
for a tanker. However, differences in design and operation give rise to
certain particular considerations for combination carriers.
It is particularly important for combination carriers to maintain their holds
in an inert condition. These holds may extend to the full width of the ship
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and, even at small angles of roll, agitation of clean or dirty ballast in a
slack hold may result in the generation of static electricity. Slack holds
should be avoided whenever possible.
The cargo holds of combination carriers are adjacent to ballast and void
spaces. Leakages in pipelines or ducts in these spaces, or a fracture in
the boundary plating, may cause oil, inert gas or hydrocarbon gas to leak
into the ballast and void spaces. Consequently, gas pockets may form
which, because of the complex structure of these spaces, could be
difficult to disperse. Personnel should be alerted to this hazard.
If slops are retained on board due to the lack of reception facilities, the
slop tank or tanks should be maintained in an inert condition and at a
minimum pressure of 100 mm WG at all times. These tanks should be
checked at intervals of not more than 2 days to ensure that the oxygen
level does not exceed 8% by volume. The tanks should be re-inerted if
the oxygen level is found to be more than 8%.
When the ship is in a ‘dry’ trade, the slop tanks containing oil or oil
residues should be isolated from other tanks by blank flanges which
should remain in position at all times when cargoes other than oil are
being carried. (See the IMO ‘Guidelines for Inert Gas Systems’.)

14.1.8 Hatch Covers

The hatches of combination carriers are much larger than on oil tankers,
but are required to remain gas and liquid tight at all times when carrying
oil cargoes.
Regular attention should be paid to the closing devices, for example by
adjusting the tension on them evenly and by lubrication of screw threads.

When closing hatch covers, the closing devices should be evenly and
progressively pulled down in the correct sequence in accordance with the
manufacturer’s instructions.

© ICS/OCIMF/IAPH 2006 235