Carrying and handling liquefied gas cargo onboard ships poses significant potential
hazards to the crew and the environment.
Each person working on a gas carrier and terminal ashore needs to understand the risks involved,
obtain the necessary training and take all the needed precautions.
There is always a possibility of the presence of gas in the atmosphere, particularly:
during loading and discharging of liquefied gases
when the ship is gassing-up or being gas-freed when a pipeline or cargo pump is opened up
for maintenance in compressor rooms
within ballast tanks and void spaces and double bottom tanks adjacent to cargo tanks
LNG or liquefied natural gas, as the name suggests, is the liquid form of natural gas
stored at controlled temperature.
LNG carriers are specialised ships that transport LNG at its atmospheric pressure boiling point
of approximately -162 degree C.
Before we dive further into LNG tank construction and cargo operations, please do subscribe
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Most cargo tanks are constructed from stainless steel, which is a material that retains its
flexibility and strength characteristics over the temperature range between (-180ºC - 50ºC).
There are different types of LNG cargo tanks, and we will discuss two important types -
Membrane -
The tank consists of very thin layers called membranes of special steel - INVAR.
It minimally expands and contracts and the layer of steel is about 1mm thick.
There is a second membrane providing a secondary barrier.
The space between these two layers has a plywood box filled with perlite insulation to support
and insulate the tanks. Usually these spaces are supplied with positive pressure nitrogen.
Moss tank -
The moss system uses spherical aluminium tanks that have a working pressure of 22 kPa (3.2Psi)
which can be increased in case needed. They are self supporting and are secured around
the equatorial ring.
All pipes and equipment are carried in a central column.
There is a drip tray provided at the bottom to catch any leakages and avoid damage to
the hull. Ballast is carried below and alongside the cargo hold.
Cargo Operation preparation
Usually shore terminal and LNG tanker loading systems are linked by loading arms or by pipes.
All the flange connections details are checked and have valid certificates.
LNG is a cryogenic substance and its main component is methane.
It gasifies violently when directly introduced into a cargo tank at ambient temperature,
rapidly increasing the internal pressure of the cargo tank and making the atmosphere into
a flammable condition.
Inerting of Cargo Tanks
Before introducing cargo into the tanks, the moisture content and oxygen content in the
tanks shall be reduced simultaneously.
Cargo tanks filled with air are dried and inerted with inert gas supplied from the inert
gas generator on board.
Inert gas is led into the bottom of the cargo tank through the liquid filling line and displaced
air is vented to the atmosphere through the vapour line and the vent mast.
The dew point and oxygen content are periodically measured by a portable instrument at the sampling
lines in the cargo tank dome.
Inerting of Annular Space for Moss type vessels
The space between the surface of a cargo tank and insulation is called annular space.
Annular Space is inserted with nitrogen gas and continuously supplied from N2 generator
through the N2 bleed line
Inerting Inter Barrier Spaces (IBS) and Insulation Spaces (IS) for Membrane type vessels
The space between the primary and the secondary barrier is called inter-barrier space (IBS).
The space between the secondary barrier and the inner hull is called insulation space
(IS).
In normal operation, IBS and IS shall be purged with nitrogen.
During cargo loading, as normal procedure, IBS pressure is maintained at or below cargo
tank pressure and IS pressure is maintained at or above the IBS pressure.
Gassing-up
If the purging has been done with inert gas, the cargo tanks have to be gassed up and cooled
down when the vessel arrives at the loading terminal.
This is because inert gas contains about 14% carbon-dioxide, which will freeze at around
-60ºC and produce a white powder which can block valves, filters and nozzles.
During gassing up, the inert gas in the cargo tanks is replaced with warm LNG vapor. This
is done to remove carbon dioxide and to complete drying of the tanks.
Initial Cool Down
Cool down is an operation to pre-cool cargo tanks and lines required before taking on
cryogenic LNG.
Cargo tank cool down is carried out by spraying LNG through the spray nozzles of each cargo
tank, using LNG received from the shore terminal.
Before LNG can be introduced into the cargo system of an LNG vessel, the system, and in
particular the cargo tanks, have to be cooled down to a temperature close to that of the
LNG which is to be loaded.
CARGO OPERATION
Air purge of loading arms
After the connection of loading arms, air should be purged from the loading arms and
the tips of manifold pipes.
N2 gas is led into the loading arms from injection lines connected to the arms, and then pressurized
up to about 4 to 6 kg/cm2G.
After pressurization, the ship’s liquid manifold vent valve and vapor manifold vent/drain
valve are opened to release air and N2 gas into the atmosphere.
Loading Arms Cool Down
The cool down of the loading arms is performed from shore side by use of a small capacity
pump.
At a discharge port, the arms are cooled down by sending in LNG by ship’s spray pump.
Loading Operation
LNG is loaded via the loading manifolds to the liquid header and then to each tank filling
line.
The boil-off and displaced vapour leave each tank via the vapour suction to the vapour
header.
The vapour is initially free-flowed to shore via vapour crossover manifold and, as tank
pressure rises, one compressor is brought into operation to increase the gas flow to
shore and limit the vapour main and cargo tank pressure.
As the loading rate increases, it is important to monitor the tank pressures and to start
one HD compressor.
If the compressors are unable to cope with the volume of boil-off and displaced gas,
it will be necessary to reduce the loading rate.
It is important to maintain a water curtain below the loading pipes to avoid hull structure
damage if there is any cargo leakage .
The ice also needs to be washed off from LNG loading pipes.
Bulk loading
When all lines and valves are fully cooled the vessel can commence ramping up the loading
rate in the sequence agreed with the terminal.
Deballasting should be commenced in accordance with the cargo plan. The cargo should be evenly
distributed during the loading.
BOG and Reliquification:
The Boil off gases during the loading operation is cooled down. This is done by using a reliquification
plant.
BOG is removed from the cargo tanks by means of a two stage centrifugal compressor.
The BOG is cooled and condensed to LNG in a cryogenic heat exchanger (cold box) which
liquefies the BOG and returns the LNG back to the cargo tanks.
Precautions during cargo operations be vigilant at all times, ­ especially during
cargo operations and maintenance of cargo equipment
If there is contact with toxic vapour or liquid , remove contaminated clothing immediately
and wash with large quantities of water. Inform an officer
do not eat, drink or smoke unless you have thoroughly washed
always prevent hands from coming into contact with your mouth
be sure you know where decontamination showers are located.
wear a protective suit and breathing apparatus, when necessary
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