Liquefied Petroleum Gas Ships

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Ships carrying LPG are categorised by their cargo containment system.

Fully Pressurised Tanks

The capacity of fully pressurised ships is usually less than 2000 m3b
or propane, butane or anhydrous ammonia carried in two to six uninsulated
horizontal cylindrical pressure vessels arranged below or partly below deck.
These independent tanks of Type C are normally designed for working pressures
up to 17.5 kg/cm2 which corresponds to the vapour pressure at 45°C,
the maximum ambient temperature the vessel is likely to operate in.

The ship has a double hull extending over the bottom and bilge area, the
secondary barrier being provided by low temperature (notch ductile) steel at
the inner bottom, sloping bilge tank, part side shell, and sloping bottom of
topside tank. Transverse bulkheads may be single or double plate (cofferdam)
type between cargo holds. Insulation can be either on the tank or the secondary
barrier for this type of ship.

Semi-Pressurised (Or Semi-Refrigerated) Tanks

The capacity of semi-pressurized ships ranges up to about 5000m³ the cargoes
carried being similar to fully pressurized ships. The independent Type C tanks
are generally constructed of ordinary grades of steel suitable for a
temperature of -5°C and are designed for a maximum pressure of about 8kg/cm².
The outer surface of the tank is insulated and refrigeration or reliquefaction
plant cools the cargo and maintains the working pressure. Cargo tanks are often
horizontal cylinders mounted on two saddle supports and many designs (see
Figure 26) incorporate bio-lobe tanks to better utilize the under deck space
and improve payload.

Fully-Refrigerated Tanks

The capacity of fully-refrigerated ships ranges from 10 000m³ to 100 000m³
the smaller ships in the range being multi-product carriers whilst the larger
vessels tend to be single product carriers on a permanent route, Tanks fall
almost exclusively into the prismatic, independent Type A category with tops
sloped to reduce free surface and bottom corners sloped to suit the bilge
structure. In most cases they are subdivided along the centreline by a
liquid-tight bulkhead, which extends to the underside of the dome projecting
through the deck, which is used for access and piping connections, etc. The
tanks sit on insulated bearing chocks, attached to brackets in the following
areas, double bottom tank tops, top sloping sides against the saddle ballast
tanks and the flat sides, there is a clearance between the chocks and tank to
allow thermal movement, the clearance must not be excessive, otherwise in heavy
weather, the tank will bounce and generate fractures in the surrounding
structures, the tank surfaces are accessible for inspection, the chocks prevent
the tank floating off if the hold is flooded. Tanks are constructed of notch
ductile low temperature steel for the minimum operating temperature of -43°C
the boiling point of propane.
The ship has a double hull extending over the bottom and bilge area, the
secondary barrier being provided by notch ductile low temperature steel at the
inner bottom, sloping bilge tank, part side shell and sloping bottom of topside
tank. Transverse bulkheads may be single or double plate (cofferdam) type
between cargo holds. Insulation can be either on the tank or the secondary
barrier (Void Space) for this type of ship.
When the liquid gas is inflammable, the void space will be purged through with
dry inert gas to an oxygen level of <1%, and slightly pressurised. This is a
safety factor in the event of a collision, also reduces corrosion in the
surrounding steel work.

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