ITOPF: Consequences of an LNG spill in the marine environment

Fate and behaviour of LNG spilled in the environment

The Standard European Behaviour Classification (SEBC) categorises LNG as a gas (G). During an incident, LNG’s hazards will be the drivers for the first actions and emergency response, followed by specific actions linked to its behaviour classification.

In its transported and stored form as a liquid, when the storage tank temperature rises above LNG’s boiling point or when liquid LNG is exposed to ambient conditions, it vaporises. If the liquid LNG spills on or above the waterline, it will first float and, depending on the quantity spilled, may form a shallow cryogenic pool on the water surface before vaporising. There is also potential for some seawater in the immediate vicinity of the release to freeze due to LNG’s low temperature, causing localised ice patches.

If released below the waterline, due to its specific gravity, it will rise to the water surface before rapidly boiling and volatilising into the atmosphere as its temperature increases in contact with seawater and the atmosphere. When vaporising, the cold LNG vapours are heavier than air and stay close to the ground or sea. These vapours condense the moisture in the air, forming a visible white cloud of water vapour (or fog), mixed with gaseous LNG, close to the ground or sea. As the gas mixture warms up and its overall specific gravity decreases, it rises and dissipates into the atmosphere within a short period of time.

It is likely that the water vapour cloud will persist for longer than the LNG cloud. The footprint and height of the vapour cloud depends on the metocean conditions at the time of the release. In low wind conditions (<5 mph), the vapour cloud footprint is smaller on the sea surface and is expected to dissipate at a higher altitude, whereas in moderate wind conditions (>5 mph), the vapour cloud plume is likely to be knocked down on the sea surface, therefore being more low-lying and spreading over a larger footprint on the sea surface.

Previous studies have indicated that vapour clouds spread roughly at the same rate as wind speed and are likely to persist in the order of tens of minutes, depending on multiple factors such as hull puncture size, location of the breach, discharge rate and environmental conditions at the time of the spill.

Hazards of LNG spilled in the environment

LNG’s hazards can lead to direct impacts on health and safety, mainly through its flammability and cryogenic temperature.

Flammability

Liquid LNG is not flammable; however, its vapour has a flammability range of 5 – 15 (v/v) %, which is detectable with specialised equipment. Outside of this range, the LNG/air mixture is not flammable. This is greater than conventional fuels such as diesel, which has a smaller flammability range of 0.6 – 5.5 (v/v) %.
In an unconfined space, a release of LNG from a tank or pipeline will rapidly mix and dissipate to reach concentrations below the 5% lower flammability limit (LFL). Only a small area near the immediate leak would likely reach the concentration that would allow LNG to ignite.

In a confined space, where flammable vapours cannot dissipate, a small leak would result in a stratified vapour layer at high points in the space (e.g. at ceiling height), which could lead to a flammable vapour/air mixture. Note that inside a fuel storage tank, the percentage of LNG is almost pure and, as such, the conditions are not in its flammable range. However, following a leak/rupture, a vapour cloud will form and once it has mixed with air, when its concentration reaches between the LFL (5% volume) and the upper flammability limit (UFL) (15% volume), the mixture can sustain a flame if ignited. If ignition occurs immediately, a pool fire will result and continue until all the fuel is consumed.

If flammable vapours come into contact with an ignition source, a vapour cloud fire can result, which may propagate back to the spill point and may cause a pool fire. The flame burns slowly and therefore, if the wind speed is moderate (> 10 mph) the flame might not be able to propagate back to the leak source and will burn downwind from the ignition point, until all LNG is consumed below the LFL or is extinguished. The shape of the visible cloud gives an indication of the wind direction and the vapour dispersion. However, it does not define the boundary of the flammable cloud, particularly in low relative humidity environments.

Explosivity

When exposed to ambient conditions, spilled LNG may result in a rapid phase transition (RPT), which is a flameless overpressure due to a very fast change of phase, in this instance from liquid to gas. The vapour cloud expands so quickly that a sonic boom and localised overpressure occurs. These have been observed in the past but are unpredictable and do not release significant amounts of energy. Measured overpressures due to RPT have been found not to be sufficient to cause more than minor damage to either vessels or port structures.

In particular conditions, LNG could potentially undergo a boiling liquid expanding vapor explosion (BLEVE), which is an explosion caused by the rupture of a tank containing a pressurised liquid that has reached a temperature above its boiling point, in the case of LNG, -162 °C. This would be the case if the temperature of the tank were to raise and gas release systems were to fail.

Asphyxiant

Like with any gas in a confined environment, high concentration of LNG vapours displace oxygen in the air, decreasing oxygen availability, leading to asphyxiation to those present in these confined environments.

Temperature

The cryogenic temperature of liquid LNG will result in the freezing of any tissue (plant or animal) upon contact and can cause materials to become brittle and lose their strength or functionality.

LNG incidents and claims

LNG’s short residence time in the marine environment and high volatility means that claims arising from incidents involving this alternative fuel would greatly contrast those associated with conventional persistent hydrocarbon oil spills.

Clean-up and preventive measures

Claims from clean-up and preventive measures are expected to arise from different measures, such as source control, fire-fighting measures, monitoring via expert modelling or sensors mounted on UAVs/ROVs, and possible bunker fuel removal. Traditional clean-up measures will not be possible and therefore, claims from a protracted spill clean-up operation will not arise.

Personal injury and loss of life

However, personal injury and loss of life claims may be significant. Risks from fire, explosions, cryogenic damage, and asphyxiating vapours could lead to death or life-altering injuries to crew, passengers, nearby operators, and members of the public.

Environmental damage claims

Claims arising from environmental damage are likely to be geographically confined in comparison to damage from oil spills. Post-spill studies may be undertaken, in certain circumstances, to establish the severity and extent of damage. Restoration measures are likely to be minimal and confined to a small area.

Property damage and structural repair

Rather than property damage claims involving cleaning and cosmetic repair of oiled property, LNG claims are likely to be a result of fire/explosion or cryogenic damage. Therefore, structural repair or replacement may be required, which would likely be more costly and potentially time-consuming.

Economic losses

Economic loss claims resulting from a fire/explosion could include port closure/disruption and associated demurrage costs, losses from damaged/destroyed property, local aquaculture losses from mortality of stock, and local losses resulting from fishing bans. Impacts to commercial water intakes and tourism may also occur.