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Gas carriers have a similar overall arrangement to tankers in that their
machinery and accommodation are aft and the cargo containment is spread over
the rest of the ship to forward where the forecastle is fitted.
Specific gravity of LPG cargoes can vary from 0.58 to 0.97 whilst LNG ships
are often designed for a cargo specific gravity of 0.5 so that a characteristic
of LNG ships in particular and most PLG ships is their low draft and high
freeboards. Water ballast cannot be carried in the cargo tanks so adequate
provision is made for it within the double hull spaces, double bottom, bilge
tank, and upper wing tank spaces.
The double hull feature of LNG carriers and many LPG ships is a required
safety feature and the tanks of LPG ships which do not have this feature are
required to be a minimum distance inboard of the shell.
Fore end and aft end structure is similar to that for the other ships. The
cargo section is transversely or longitudinally frame depending primarily on
size in the same manner as other cargo ships, the inner hull receiving special
consideration where it is required to support the containment system.
All gas ships have spaces around the tanks which are monitored for gas leaks
and in many ships these spaces also inerted, an inert gas system being fitted
aboard the ship. Liquid gas cargoes are carried under positive pressure at all
times so that no air can enter the tanks and create a flammable mixture.
Liquefaction equipment is provided aboard LPG ships, ‘boil off’ vapor from
the tanks due to any heat ingress is drawn into the liquefaction plant and
returned to the tank. Boil off vapour from LNG ship tanks can be utilized as a
boiler fuel in steam ships, otherwise it is vented to atmosphere, although this
is not permitted in many ports, and several other solutions have been developed
to overcome this problems.
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