Question.
Describe briefly the tank system in LNG
vessel.
Liquefied Natural Gas Ships
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
Explain how the boil off from LNG is handled
With LNG reliquifaction is
not economically viable. It is a requirement by class that a suitable method be
installed for the handling of this gas
One common method is to utilise
the gas as fuel for the propulsion plant. A suitable method of disposing with
excess energy should be fitted. Typically for a steam powered vessel this would
take the form of a steam dumping arrangement.
Alternately, the gas may be vented although port
restrictions mean it may not always be possible.
Boil off occurs during loading of LPG cargos, whether
this is passed to shore via the vapour line or must
be liquified on board is very much dependent on the
shore facility. Generally shore facilities much prefer it to be done on board
ship therefore the liquifaction plant must be
maintained in top condition.
During the loaded passage a quantity of the Cargo will
boil off removing heat that has leached past the insulation. This gas is used
in the propulsion plant as fuel. Although progress has been made with fitting
gas burning gear to large diesel plants, the vast majority of LNG tankers are
steam powered.
The boil off gas is fed to the propulsion plant by a
low duty compressor via a heater. The gas pipe entering the engineroom
is surrounded by a gas tight enclosure whose atmosphere is monitored for
hydrocarbons. The spaces around the burner platform are positively ventilated
and again monitored for hydrocarbons.
The gas flame in the boiler is extremely clean
reducing sootblowing from a daily to a fortnightly
requirement. However, the flame is clear blue requiring either a special type
of flame detector or for a small quantity of fuel to be burnt which can be seen
by normal flame detectors.
The gas burner normally takes the form of a simple
open ended pipe.
Discharge
The Liquid header is cooled down before arrival whilst
still gas burning. To do this a spray pump is started filling the spray line
then the liquid header via the cross-over. Boil off and excess liquid flow back
to the tank via the branch pipe.
[In early
stages of LNG ships the excess methane gas was vented astern of ship and burned
Reliquification of gas was not economical
but use of this gas in main engine as fuel is a major improvement in fuel
technology.
Motor vessel along with normal injection equipment a equipment used to inject the boil off gas is fitted
It is hydraulically operated gas injection valve in
cylinder head which blows the combustion gas at pressure of 3bar and against scavenge air pressure
The gas line is fitted with relief valve to avoid over
pressurization of the line
The boil off gas cannot be ignited with compression
pressure created by the internal combustion engine so that diesel fuel supply
is also simultaneously on and which produces flame, which is then transferred
to burn the boil off gas
As the gas pressure falls below 10%, then diesel fuel
supply through injector starts, also if gas pressure falls below 15% then gas
flow is stopped and diesel fuel injection is continued
Therefore while starting and manoeuvring of engine diesel oil is used and when engine runs at full load boil off gas is used]