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Potential hazards of a large liquefied natural gas spill during marine transportation

The potential hazards of a large LNG spill over water includes asphyxiation, cryogenic burns, and cryogenic damage to the ship from the very cold LNG, dispersion, fires, and explosions.

Based on expert review, the most likely hazards to people and property would be thermal hazards from an LNG fire. Cryogenic and fire damage to an LNG ship were also identified as concerns that could cause additional damage to LNG cargo tanks following an initial cargo tank breach, though the additional impact on public safety would be limited.

consequences of LNG spill over water
Fig: consequences of LNG spill over water

Risks and hazards from a potential marine LNG spill can be reduced through a combination of approaches, including reducing the potential for a spill, reducing the consequences of a spill, or improving LNG transportation safety equipment, security, or operations to prevent or mitigate a spill.

Explosions in confined spaces, either combustion events or events of rapid phase transition, may have the potential for causing secondary damage that could lead to further spillage of LNG.

Other proactive risk management approaches can help reduce both the potential for and hazards of such events. These include:

i) improvements in ship and terminal safety/security systems including improved surveillance, tank and insulation upgrades, tanker standoff protection systems;

ii) modifications and improvements in LNG tanker escorts, extension of vessel movement control zones, and safety operations near ports and terminals;

iii) improved surveillance and searches of tugs, ship crews, and vessels;

iv) redundant or offshore mooring and offloading systems; and

v) improved emergency response systems to reduce fire and dispersion hazards and improved emergency response coordination and communication.

LNG carrier at sea
Fig:LNG carrier underway

Risk prevention and mitigation techniques are especially useful in zones where the potential impact on public safety and property can be high. The hazards of brittle fracture, rapid phase transitions, and explosions in confined ship spaces, as well as cascading events that may result from the extreme fire exposure a ship would experience if a nominal 12,500 m3 spill on water around the ship was ignited, will require careful consideration. The definition of

The majority of liquefied gases are clean, non-polluting, products and create no danger to the marine environment. If however certain liquefied gases spill on to the sea you should be aware that they may:

The Data Sheets will give information on pollution, if any Pollution is most likely to occur during cargo or bunkering operations:
  • if the operation is not correctly monitored
  • if the cargo hose or loading arm connections are not properly made
  • when disconnecting cargo lines that have not been drained.
  • if moorings are not checked and excessive strain is placed on the cargo connections or the ship "breaks out" of the berth.
  • if cargo equipment is not properly maintained

  • Any spillage of LNG on any steelwork, unless stainless steel or wood-sheathed, will cause stresses and it is most likely serious brittle fractures will occur.

    As soon as any leak or spill of LNG is exposed to ambient temperatures, the liquid will vaporise or ‘boil-off’. This vaporisation will occur in two phases. Initially, for a period of from 20-30 seconds, there will normally be a high rate of boiling as the heat for vaporisation is taken from the liquid spill itself and the immediate surrounding areas. Secondly, the cold vaporised gas begins to insulate the liquid surface and the evaporation rate will level off at a lower steady rate depending on how quickly heat can be transferred to the LNG from the surrounding area. This vaporisation rate may be increased by:
    Thus, spraying an un-ignited spillage of LNG with water will speed up the vaporisation and reduce the hazards of cold fractures, fire or ignition. Alternatively, spraying with water on to LNG which has been ignited will increase the vaporisation rate and hence the burning rate. The use of solid water jets on LNG spills may cause splashing, leading to cold fractures or frost burns or, if ignited, may seriously aggravate the fire.

    Immediately after vaporisation, natural gas is 1·4 times heavier than air. As the gas warms, its density will decrease, becoming the same as air at approximately –120°C and reaching the value of 0·55 at 15oC. There may, therefore, be a tendency for cold vapours to form a layer around the spill in a similar manner to other hydrocarbon gases. Fortunately, this layering will normally be visible due to the condensation of atmospheric moisture.

    However, unlike other hydrocarbon gases, natural gas quickly becomes buoyant and, except in enclosed spaces, will rise and disperse rapidly as it warms. This dispersion is further aided by the very rapid diffusion properties of methane in air. Where spills may have entered enclosed spaces, it is important to recognise that gas pockets may become trapped near deckhead structures, etc.

    In the case of a leakage or spillage of LNG, the following general procedure should be followed:
    The exact procedure will depend upon the nature of the incident, inclusive of size of spill, location, ambient conditions and ignition risks.

    Where LNG spills onto water, Rapid Phase Transition may occur causing loud bangs similar to ones that may be heard during an explosion. However, there are no flames or explosion when this occurs.

    Related Information:

    1. Preventive measures against liquefied gas spillage

    2. Liquefied gas carrier safety training

    3. Leaks on the Cargo System, Continuous Flow - how to prevent

    4. Personal protective equipments for people working onboard gas carriers

    5. LNG tank leaks and immediate action by gas carriers

    6. Minor or major leaks from LNG tanks

    7. Initial Cool Down of cargo tanks

    8. Leaks from a Loading Arm due to Tidal or Current Effects

    Defining various gas carrier types

    Fuel flexibility of LNG ships

    Procedures for LNG cargo loading

    Liquefied gases - Properties And Hazards

    Safety guideline for changing previous cargo

    Reactivity of liquefied gas cargo

    Initial Cool Down of cargo tanks

    Procedures for LNG cargo loading

    Procedures for LNG cargo discharging

    External links :

    1. LNG industry online training company

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