LNG Cargo ship - YouTube

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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