Preparation for loading LNG cargo -Operation procedures and precautions for gas carriers
Loading LNG cargo after dry docking
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 increases the internal pressure of the cargo tank and makes the atmosphere into a flammable condition.
In addition, the cargo tank is rapidly cooled, resulting tremendous thermal stress on cargo tank skins and cargo piping systems. To avoid such damages, the preparatory work for cargo loading after dry docking must be done in the following sequence.
During dry dock all the compartments of an LNG carrier are kept gas free. After leaving the dry dock the vessel has to be prepared to load cargo, for that the following points to be considered with priority.
Drying of Cargo Tank
During dry docking or inspection, cargo tanks which have been opened and contained humid air, must be dried to avoid the formation of ice when they are cooled down and the formation of corrosive agents if the humidity combines with sulfur and nitrogen oxides which might be present in excess in the inert gas.
The drying operation need not be performed independently by using dry air, instead during inerting operation by supplying dry inert gas, drying operation can be achieved. During such operation special attention is required to the delivery temperature of inert gas to prevent condensation of humid air inside the tank.
Dry air, with a dew of -70ºC to -40ºC, can be produced by the onboard IGG system.
i) It is essential that cargo tanks are thoroughly inspected for cleanliness, free of liquid, any loose objects and all fittings are properly secured. Once this inspection has been completed, the cargo tank should be securely closed and drying operation can be started
ii) During drying operation, measure the atmosphere at different levels at regular intervals. When the dew point of the cargo tank drops below than the planned temperature, finish the drying operation.
Fig:LNG carrier moss tanks
Drying of Hold Spaces
The drying operation of a hold space is carried out in order to prevent tank insulation damage due to condensation of moisture inside it prior to initial cool down operation and periodically during a voyage. Fresh air is dehumidified by the IGG and sent to a hold space as dry air with a dew point of -70ºC to -40ºC through its bottom section, humid air inside the hold space is released through the vent pipe provided in the upper portion of the tank. The hold space should be maintained at a higher pressure than the atmospheric pressure.
Operation procedures and precautions:
i) Before delivering dry air into a hold space, completely dry up the bottom section of the hold space, particularly the bilge well.
ii) When drying a hold space after completing the inerting operation of a cargo tank, purge relevant equipments and inerting/aerating lines with dry air to prevent the ingress of inert gas into the hold space. This is because the hold space holding dry air sent into it is kept almost sealed till the next dry docking and, in addition, about 15% CO2 gas is present in the inert gas, which may corrode aluminum cargo tanks and destroy insulation materials.
iii) During drying operation, measure the atmosphere at different levels at regular intervals. When the dew point of the hold space drops below than the planned temperature, finish the drying operation.
Inerting of Cargo Tanks
Before introducing the cargo into the tanks, the moisture content and oxygen content in the tanks shall be reduced simultaneously.
Cargo tanks filled with air shall be dried and inerted with inert gas supplied from the inert gas generator on board. Inert gas shall be led into the bottom of the cargo tank through the liquid filling line and displaced air shall be vented to the atmosphere through the vapour line and the vent mast. Drying and inerting shall be finished when the dew point and also the oxygen content in the cargo tank are less than the planned level.
The dew point and oxygen content shall be periodically measure by a portable instrument at the sampling lines in way of 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, insulation space or wedge space. Annular Space is inerted with nitrogen gas and continuously supplied from N2 generator through the N2 bleed line in service in order to ensure adequate path in the insulation space for the gas detection system.
A safety valve is installed in the N2 bleeding line of each hold in order to avoid over pressure of the insulation space.
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). The pressure in these spaces shall be regulated at a pressure slightly above atmospheric pressure in order to prevent any air ingress.
In normal operation, IBS and IS shall be purged with nitrogen in relation with atmospheric pressure variations and cooling or warming of the spaces during loading or unloading, and IBS should be continuously purged with nitrogen if gas is detected by micro-leakage of the membrane.
The Nitrogen provides a dry and inert medium for the following purposes:
i) To prevent formation of flammable mixture in the event of any LNG leak.
ii) To permit easy detection of an LNG leak through a barrier
iii) To prevent corrosion
In addition each space shall be protected against over pressure by two (2) pilot operated safety valves.
During cargo loaded, as normal procedure, maintain the IBS pressure at or below cargo tank pressure and maintain the IS pressure at or above the IBS pressure.
After lay-up or dry dock, the cargo tanks are filled with inert gas or nitrogen. 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 produces 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.
Supply of LNG for gassing up
LNG liquid is supplied from the terminal to the liquid manifold where it passes to the stripping/spray header via the appropriate ESDS liquid valve. It is then fed to the main vaporizer and the LNG vapour produced is passed at a temperature warmer than the dew point temperature existing within the cargo tanks through the vapor header and into each tank via the vapor suction fitted in the upper part of the tank. This method of gassing up is called “Piston Flow Method”. In this the lighter specific gravity LNG vapor is injected from top and the heavier IG is displaced from bottom.
Requirement to purge with Nitrogen
At the start of the operation, the piping system and main vaporizer contain inert gas. Consequently to avoid the formation of water condensate or solid CO2, the supply line to the vaporizer from the manifold has to be first purged with Nitrogen either by using the ships or shore supplying.
When 5% methane (% figure will be specified by the particular port authority) is detected at the vent mast riser, the exhaust gas is directed ashore.
The operation is considered complete when the methane content, as measured at the top of the cargo filling pipe, exceeds 98% by volume.
Due to local regulations on venting methane gas to the atmosphere, some port authorities may require the entire operation to be carried out with the exhaust gases being returned to shore facilities.
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. The cool down operation from an ambient temperature (from a condition after gassing up) to a planned temperature, is called ‘initial cool down’ and is to be differentiated from an ordinary cool down operation carried out on ballast voyage.
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. The reasons for this are as follows:
If LNG is introduced directly into warm tanks, the LNG will almost immediately turn into vapour. LNG has a liquid to gas expansion ratio 1: 600. Therefore, to enable the liquid to be loaded into the tank at a reasonable loading rate, necessity of large compressors would be required to remove the vapour generated in the process.
By reducing the cargo tank temperature, the amount of heat that is available to transfer into and heat the LNG is minimized. Consequently the amount of vapour generated can be maintained within reasonable limits.
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.
Before arrival at a terminal prior loading
The following checks and procedures are to be regarded as the minimum before cargo operations can commence.
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