3










 

3    
Cargo Containment
Systems



 

 

 

3.1                    
Independent tanks

These types of tanks are
completely self-supporting and do not form part of the shipłs hull and do not
contribute to the hull strength. Depending mainly on design pressure, there are
three different types of independent tanks for gas carriers, Types A, B and C

 

3.1.1               
Independent tanks,
type A (MARVS < 0. 7 bar)

Independent tanks of type A are prismatic and
supported on insulation-bearing blocks and located by anti-roll chocks and
anti-flotation chocks. The tanks are
normally divided along their centreline by a liquid-tight bulkhead; by this
feature, together with the chamfered upper part of the tank, the free liquid surface
is reduced and the stability is increased.
When these cargo tanks are designed to carry LPG (at -50oC),
the tank is constructed of fine-grained low-carbon manganese steel.

The Conch design has been
developed for carriage of LNG (at-163oC). The material for these cargo tanks has to be either 9% nickel
steel or aluminium.

 



Figure 3.1 Prismatic self-supporting Type A tank
for a fully refrigerated LPG carrier

 

 

3.1.2               
Independent tanks,
type B (MARVS < 0. 7 bar)

Independent tanks of type B are normally spherical
and welded to a vertical cylindrical skirt, which is the only connection to the
ship's hull. The hold space in this
design is normally filled with dry inert gas but may be ventilated with air
provided that inerting of the spaces can be achieved in the event of the vapour
detection system detecting cargo leakage. A protective steel dome covers the
primary barrier above deck level, and insulation is applied to the outside of
the primary barrier surface. This containment system has been used for carriage
of LNG. The material of construction is
either 9% nickel steel or aluminium.

 

 

 

 

 

 

 



Figure 3.2 Self-supporting spherical Type B tank

 

 

3.1.3               
Independent tanks,
type C (MAR VS< 0. 7 bar)

Independent tanks of type C are cylindrical
pressure tanks mounted horizontally on two or more cradle-shaped
foundations. The tanks may be fitted
on, below or partly below deck and be both longitudinally and transversely
located. To improve the poor
utilization of the hull volume, lobe-type tanks are commonly used at the forward
end of the ship. This containment
system is used for LPG and LEG. The
material, if used for the construction of tanks designed to carry ethylene, is
5% nickel steel.

 

 



Figure 3.3 Type C tanks as found on fully
pressurised gas carriers

 

 



Figure 3.4
Type C tanks as utilised on semi-pressured/fully refrigerated gas carriers

 

 

 

3.2                    
Membrane tanks (MARVS normally < 0.25 bar)

Membrane tanks are not self-supporting tanks; they
consist of a thin layer (membrane), normally not exceeding 1 mm thick,
supported through insulation by the adjacent hull structure. The membrane is designed in such a way that
thermal and other expansion or contraction is compensated for, and there is no
undue stressing of it. The membrane
design has been developed for carriage of LNG.
The material of construction is lnvar steel (36% nickel steel) or 9%
nickel steel.

 

 

3.3                    
Semi-membrane tanks (MARVS normally < 0.25 bar)

Semi-membrane tanks are not self-supporting; they
consist of a layer which is supported through insulation by the adjacent hull
structure. The rounded parts of the
layer are designed to accommodate thermal expansion and contraction, and other
types thereof. The semi-membrane design
has been developed for carriage of LNG, and the material of construction is 9%
nickel steel or aluminium.

 

 

3.4                    
Integral tanks (MA RVS normally< 0.25 bar)

Integral tanks form a structural part of the ship's
hull and are influenced by the same loads which stress the adjacent hull
structure, and in the same manner. This
form of cargo containment is not normally allowed if the cargo temperature is
below -1 0 OC. Today, this containment
system is partly used on some LPG ships dedicated to the carriage of butane.

 

 

 

 

3.5                    
Internal insulation tanks

Thermal insulation must be
fitted to refrigerated cargo tanks for the following reasons:

a)   
To minimise heat flow
into cargo tanks and thus reduce boil-off.

b)   
To protect the
general ship structure around the cargo tanks from the effects of low
temperature

 

For use aboard gas
carriers insulation materials should process the following characteristics:

i)                 
Low thermal conductivity.

ii)              
Non-flammable or self-extinguishing.

iii)            
Ability to bear loads.

iv)            
Ability to withstand mechanical damage.

v)               
Light weight.

vi)            
Material should not be affected by cargo
liquid or vapour

The materialłs
vapour-sealing properties to prevent ingress of water or water vapour is very
important. Not only can ingress of moisture result in loss of insulation
efficiency but progressive condensation and freezing can cause extensive damage
to the insulation. Humidity conditions must therefore be kept as low as
possible in hold spaces.