The chemical and physical characteristics of LNG give rise to a variety of risks and hazards during bunkering in ports as well as in other steps of the supply chain. Like other fuels, an incident with LNG can have an impact on humans and the environment. Bunkering involves the following potential risks:
- incorrect disconnection of hoses
- overfilling and overpressure of fuel tanks
- external impacts
- leaks from LNG pumps, pipes, hoses and tanks
Hazards related to the cryogenic characteristics of LNG
The cryogenic characteristics of LNG may pose a threat to humans and materials. Skin contact results in injuries comparable with thermal burns and contact with sensitive areas like the eyes may lead to serious harm. In the case of skin contact, the extremely low temperatures may cause hypothermia: a decrease in body temperature to below that required for normal metabolism and bodily functions.
There are also risks associated with LNG contact with steel, which on contact with LNG becomes brittle and prone to fracture. Such damage has the potential to cause cracking of deck surfaces and damage to metal equipment and therefore requires standard ship steel to be protected and insulated to avoid exposure to LNG.
Hazards associated with a flammable concentration of gas
LNG does not burn or explode in the liquid phase, but if the liquid is released at ambient temperatures, the LNG will evaporate and form a vapour cloud. At methane concentrations of between 4.5% and 16.5%, such vapour clouds may result in a fire or explosion. Methane only ignites when an ignition source is present, however; and while a methane explosion may occur in a enclosed space (within a vessel), it is unlikely to occur in the open air.
What type of fire occurs will depend on the following factors:
- immediate or delayed ignition
- spill within or outside a confined area
- release of LNG on land, on ship, or on water
When a gas cloud disperses and meets an ignition source, it may be ignited. An ignited cloud of gas burning without generating any significant overpressure is called a flash fire, because the ignited cloud will ‘flash back’ across its entire flammable mass (the mass within the flammable range of methane). The cloud will continue to burn at the upper flammable limit boundary (15%) until the methane is entirely consumed. The duration of the flash fire is relatively short, but it may stabilize as a continuing jet fire or pool fire (see below) from the leak origin.
A jet fire occurs in the event of a continuous outflow of LNG above its boiling point being directly ignited. Jet fires are typically related to high pressures over 2 bar, such as releases from high-pressure equipment.
Hazards associated with overpressure as result of phase transitions
If LNG vapour is ignited in a confined space, a vapour cloud explosion may occur. Several situations with overpressure may lead to such dangerous situations.
Problems with pressure could arise if LNG is trapped somewhere along the transfer line. Transfer systems should be so laid out that entrapment can not occur.
At higher temperatures (> 537°C) the gas can also auto-ignite, which means it can ignite without the need of an ignition source. Other fuels like petrol, diesel and LPG can auto-ignite at lower temperatures.