There are over twenty approved patent designs of containment vessel for LNG
ships, the majority of which fall into the membrane or independent tank
categories. Those types which have been or are more commonly found in service are
described below. A feature of LNG ships is their double hull construction
within which are fitted the cargo tanks and the secondary barrier system.
Independent Type A Tanks
Early LNG ships were fitted with self-supporting tanks of aluminium alloy
having centreline bulkheads. The balsa wood insulation system was attached to
the inner hull (secondary barrier) and each insulated hold contained three
tanks.
Independent Type B Tanks
The Kvaerner-Moss group have designed an independent Type B tank containment
system which has been well accepted and is installed in a good number of LNG
ships. Tank consist of either an aluminium alloy or 9 percent nickel steel
sphere welded to a vertical cylindrical skirt of the same material which is its
only connection to the full see Figure 25. The sphere expands and contracts
freely all movements being compensated for in the top half of the skirt. The
outer surface of the sphere and part of the skirt is covered with a
polyurethane foam insulation. The system is fitted with a partial secondary
barrier consisting of a drip tray under tank and splash shields at the sides.
Above deck the spheres are protected by substantial weather covers.
Semi-Pressurised (Or Semi-Refrigerated) Tanks
The capacity of pressurised ships ranges up to about 5000 m3 the
cargoes carried being similar to fully-pressurised ships. The independent Type
C tanks are generally constructed of ordinary grades of steel suitable for
temperature of –5°C and are designed for a maximum pressure of about 8 kg/cm2.
The outer surface of the tank is insulated and refrigeration or reliquification
plant cools the cargo and maintains the working pressure. Cargo tanks are often
horizontal cylinders mounted on the saddle supports and many designs (see
Figure 26) incorporate bio??? Tanks to better utilise the underdeck space and
improve payload.
Fully-Refrigerated Tanks
The capacity of fully-refrigerated ships ranges from 10,000 m3 to
100,000 m3 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 blocks so that surfaces are
accessible for inspection are located by anti-roll and pitch keys in such a
manner that expansion and contraction can take place relative to the ships
structure. Anti-floatation chocks are provided to prevent the tank floating off
the bearings if the hold were flooded. Tanks are constructed of a notch ductile
steel for the normal minimum operating temperature of –43°C the boiling of
propane.
Technigaz
The Gaz Transport system uses a 36 percent nickel-iron alloy called ‘Invar’
for both the primary and secondary barriers. Invar has a very low coefficient
or thermal expansion which makes any corrugations in the tank structure
unnecessary. The Invar sheet membrane used in only 0.5 to 0.7 mm thick which
makes for a very light structure. Insulation consists of plywood boxes filled
with pearlite (see Figure 24).
