Carrying liquefied natural gases by various type LNG ships
Liquefied Natural Gas (LNG) Carriers
LNG carriers in service are fitted with independent cargo tanks and with membrane tanks.
LNG carriers are generally specialised ships transporting
LNG at its atmospheric pressure boiling point of
approximately -162 degree C, depending on the cargo grade.
These ships are usually dedicated vessels, but some
smaller examples may also carry basic LPG cargoes. If
an LNG ship is capable of carrying basic LPG cargoes, a
reliquefaction plant is installed to handle the boil-off LPG
cargo vapours.
LNG carriers were typically in the range 80-135,000 m3
up until 2006.
In 2006 the first LNG ships of over 200 and 250,000 m3
were being constructed for the new LNG trains being
constructed in Qatar.
Temperature control
LNG is liquefied by refrigeration to -162°C and this
process is carried out ashore, before the cargo is loaded
onto the ship.
LNG carriers are fully insulated because it is not cost
effective to liquefy methane onboard (2006, though the
first vessels with reliquifaction plants may appear in the
next few years).
As the ship has no reliquifaction plant any boil-off
vapours are burned as fuel gas in the the engine room.
Gas carrier cargo containment systems
Fig:Various type LNG carrier
Construction
The cargo containment systems will generally be either:
LNG Carriers
- Membrane systems (Gaz Transport / Technigaz)
previously described.
(A full secondary barrier with inerted spaces is required
for the membrane system) This system has a primary
and secondary barrier that is constructed of a thin
material and an insulation layer.
- Type B (Moss Rosenberg)
(The Type B spherical tank requires only a partial
secondary barrier)
A full double-bottom and side tank ballast system is
fitted to all LNG ships.
Fig:LNG carrier membrane gaz transport
Membrane (Gaz Transport or Tecnigaz)
A liquefied gas tank design where the cargo is contained by a thin
stainless steel or nickel alloy flexible membrane.
There are two membrane systems in use. In both cases
the insulation is fitted directly into the inner hull and the
primary barrier consists of a thin metal membrane less
than one millimetre thick.
The Gaz Transport system uses two such membranes
constructed of ‘Invar’ (36% nickel-iron low expansion
alloy). One acts as the primary barrier and the other the
secondary barrier and they are separated by plywood
boxes of perlite insulation. Similar boxes are fitted
between the secondary barrier and the inner hull.
Loading is transmitted through the insulation to the ship
structure. No centreline division is possible in this type
of tank. The other system, developed by Technigaz, has
a stainless steel membrane as the primary barrier while
the secondary barrier is included in the insulation, which
consists of load bearing balsa and mineral woods.
Fig:LNG carrier moss tanks
Moss Tanks
Spherical tanks are generally produced in aluminium or
9% nickel steel. The sphere is welded to a steel skirt
that is connected to the hull of the ship and is then free
to expand and contract as necessary.
Insulation is fitted to the outside shell of the sphere but
no secondary barrier is regarded as necessary across the
upper part of the sphere. However, below the sphere,
an aluminium drip tray, together with splash plates,
provides secondary protection for the hull.
Fig:LNG carrier moss tanks cross section
( We have extracted gas carrier images and information from the publication ‘LNG Shipping Knowledge’ by Witherby Seamanship )
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