Fig:LNG carrier underway
Cargo tank Material
Most cargo tanks are constructed of stainless steel which is a material, that retains its flexibility and strength characteristics over the temperature range being considered (-180ºC - 50ºC). However problems could occur if the material is subjected to very local and rapid cooling such as when a small droplet of LNG comes into contact with a warm tank wall. Because of the transfer of the heat from the wall into the liquid, the temperature at the particular point will decrease rapidly causing large thermal stresses to arise between the point and the surrounding material. This could lead to stress cracking.
Pipe tower construction
The tower which supports the pipe-work within the tank is constructed of stainless steel bars. If subjected to rapid cooling thermal stress within the material can be excessive, leading to the material cracking.
All three reasons are of equal importance as each, if not carefully controlled, can have a significant impact on the tank structure and overall safety of the vessel.
Lng tank cooling down with liquid from shore
After the cargo system has been gassed up the headers and tanks must be cooled down before
loading can commence. The cool down operation follows immediately after the completion of
gassing up using the LNG supplied from the terminal.
The rate of cool down is limited for the following reasons, note that although many of the reasons
remain similar between Membrane and Moss vessels the Moss vessels have additional
requirements that must be complied with.
To avoid excessive pump tower stresses.
Vapour generated during the cool down of the tank must remain within the capabilities
of the HD compressors, to maintain a tank pressure safely below that release pressure
of the safety valves.
On Membrane vessels to remain within the capacity of the Nitrogen system, to maintain
the primary and secondary insulation spaces at the required pressure.
To stay within the vertical thermal gradients and equatorial cool down rate as specified
by the tank manufacturers. This is particularly important on Moss vessels.
LNG is supplied from the terminal to the manifold cool down line and from there directly to the
spray header. The various spray valves are operated in order to produce a temperature profile in
line with, but not exceeding, the manufacturers’ instructions. Some tanks may require a minimum
equatorial temperature before bulk loading may commence; this temperature is to be strictly
During the cool down Nitrogen flow to the primary and secondary barriers (particularly on
Membrane vessels) will significantly increase. It is essential that the rate of cool down is controlled
in order to maintain the pressure in the primary and secondary barriers at the pressure determined
by the manufacturer.
Vapour is returned to the terminal via the HD compressors or, if requested in writing by the
terminal, may be consumed in the vessels boilers.