Natural Gas

Natural gas constituents vary according to to the region. Typically natural gas consists primarily of methane with low percentages of other hydrocarbons such as ethane, propane and butane. Other gases such as nitrogen, oxygen and carbon dioxide may also be present.

Liquid Natural Gas (LNG)

LNG is natural gas stored at -162^oC. Reduction to this temperature requires the removal of compounds which would freeze and other contaminants which would cause corrosion. This results in the percentage methane being increased from around 80% to 85-95%. Small amounts of nitrogen and lighter hydrocarbons such as ethane, propane and butane remain.

LNG Bunkering

LNG is bunkered at -162^oC and stored in a cryogenic state. This requires special bunkering arrangements on board. Approximately 1.8 times the volume of heavy fuel oil is required to give the same range.

Although the storage tanks on board are highly insulated there is still heat leakage in to the tanks. This results in boil off of some LNG to gas. The storage tank pressure and temperatures will rise if the boil off gas is not drawn off the tank. Relief valves are fitted to mitigate the risk of over-pressure. In practice the boil off gas is burned in engines and boilers, thus maintaining the low pressure and temperature in the tanks.

Bunkering of LNG is a far more complex process than that required for diesel or heavy fuel oil. The fact that bunkers will be taken as a cryogenic liquid presents a hazard to personnel and to the ship structure if any leakage occurs. The flammability of any vapour released also presents a hazard.

The bunkering procedure must therefore include the following:

Communication testing between bunker supplier and ship. Intrinsically safe radio equipment only.
Ensuring appropriate firefighting capacity is available
Setup of Emergency Shutdown (ESD) capable of operation by both supplier and receiving ship.
Protection of hull from effects of leakage of LNG - drip trays and water curtain(s)
Inerting and Purging - ensuring system is inert before and after bunkering, purging lines with warm gas and slowly cooling lines with LNG. Lines must be purged of LNG and inerted after bunkering is completed.
Ensuring electrical isolation between supplier and receiving ship

LNG Storage

Handling of the LNG storage tanks is a separate but related problem to the actual bunkering process.

A small volume of LNG (5%) is normally left in the tank to keep it cool - this small volume is known as the 'tank heel'.

If tanks have actually been emptied then they must be inerted, purged and cooled down before bunkers are admitted to the system. Cold gas or LNG is at a very low flow rate is used. The cooldown process is slow and may take over 12 hours.

Because the LNG being bunkered may have different density to the LNG already in the tank there may be a risk that the two liquids may initially stratify in the tank. However this is not a stable condition and the two LNG layers in the tank may later mix rapidly - this is known as 'rollover' and it can result in excessive boil off. This can be avoided by filling in such a way as to cause mixing - using the bottom filling connection if lower density LNG is being bunkered and the top filling connection if higher density LNG is being bunkered.

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Further Reading

ABS have produced an LNG Bunkering Advisory