LNG - Alternative Fuel Spill Response Considerations

Figure 1: LNG vapour released during discharge operations in Barcelona in 2016 (Source: Tradewinds)

If released below the waterline, the LNG will rise to the water surface and rapidly flash to vapour, resulting in the same dense cloud appearing above the water before dispersing. The short period during which LNG exists as a cold vapour close to the surface is operationally significant because flammability hazards can peak during this window. During this period, typically minutes to tens of minutes, depending on breach size and wind conditions, the vapour can enter its flammable range, creating acute risks of vapour cloud or pool fires, rapid phase transition (RPT) overpressures and, potentially, a boiling liquid expanding vapour explosion (BLEVE). It is worth noting that these potentially flammable conditions can extend beyond the visible cloud/fog.

As LNG is insoluble in water, attenuation is driven primarily by evaporation and atmospheric dispersion, which generally occur more rapidly under warm, windy conditions (Figure 2). 

Figure 2: Expected fate and behaviour of LNG after release into the marine environment

3) Impacts on human health: Flammability and cryogenic damage

The predominant safety risk during an LNG incident is flammability, which is closely linked to whether vapour concentrations enter the flammable range (5.0–15.0% v/v in air) (Figure 3). In practical terms, responders and crews focus on the lower flammable limit (LFL), which can be monitored using portable gas detectors. Alarms are typically triggered at around 10% of the LFL. Above the LFL, flammable conditions exist unless concentrations exceed the upper flammable limit (UFL), at which point the atmosphere becomes too fuel-rich and oxygen-poor to sustain combustion.

Figure 3. Flammability ranges of alternative fuels, including LNG, ammonia and methanol, and some conventional fuels (diesel and gasoline)

 

In unconfined spaces, an LNG release from a tank or pipeline will rapidly mix and dissipate, meaning that only a small area near the leak is likely to reach concentrations capable of ignition. By contrast, in confined or poorly ventilated spaces, vapours cannot disperse and may stratify, creating a flammable vapour/air mixture at high points (e.g. near ceiling height). While the interior of a fuel storage tank contains almost pure LNG vapour and is therefore not within the flammable range, a leak or rupture that allows mixing with air can produce a cloud that enters the flammable range. If ignition occurs immediately, a pool fire may form and continue until the fuel is consumed. If ignition occurs after a cloud has formed, a vapour cloud fire may result, which can propagate back to the spill source and may cause a pool fire. Wind conditions can influence whether flame propagation back to the leak source occurs, but the key operational point is that the visible cloud does not define the full extent of the flammable zone.

A further hazard during the dispersion window is the possibility of rapid phase transition (RPT) overpressures, and potentially a boiling liquid expanding vapour explosion (BLEVE) if systems to prevent over-pressurisation fail. These escalation pathways are not the norm, but they are part of the risk considered when LNG is released under conditions of confinement, heat exposure or compromised pressure relief.

A secondary but important hazard is cryogenic injury from direct contact with liquid LNG or residual super-cold pools. Hands and feet are particularly vulnerable. Oxygen depletion and asphyxiation are also relevant in enclosed areas where vapours displace air.

4) Exposure control and operational implications

Given that fire and explosion hazards dominate, the response posture centres on ignition control, protective zones and gas monitoring. Responders should wear firefighting gear (bunker gear) when fire risk exists. Because LNG is non-toxic, gas-tight suits are not required and not appropriate when flammability is a hazard. Due to LNG’s cryogenic temperature, standard firefighting gloves and boots do not provide adequate protection where liquid exposure is possible. Therefore, thermally insulated gloves and boots designed for cryogenic conditions are required. Self-contained breathing apparatus (SCBA) is appropriate in unknown atmospheres or where oxygen depletion may occur. However, once the level of oxygen is confirmed as normal and conditions are safe, respiratory protection is not required.

5) Environmental effects and persistence

Because LNG is insoluble, there is no pathway for the substance to dissolve into the marine environment. Natural attenuation is therefore dominated by evaporation and atmospheric dispersion. Warmer, windier conditions generally reduce concentrations more rapidly than colder, less windy conditions.

Environmental effects are primarily associated with the cryogenic nature of LNG. The main environmental mechanism is rapid cooling of air and water at the release point, producing acute, short-lived, highly localised impacts. Unlike conventional oil, LNG components are insoluble and not bioavailable in the same way (e.g. via ingestion or absorption across gills or skin). However, if a vessel were to ground on sensitive habitats such as a coral reef, a sudden drop in water temperature could trigger cold-induced bleaching, metabolic disruption, tissue damage or mortality in corals. Water temperature is expected to return to normal quickly with increasing distance from the release site.

A secondary environmental pathway arises if ignition occurs, in which case, impacts would typically be confined to organisms at or near the surface and pelagic birds in the vicinity, with thermal exposure and direct flame contact being the primary injury mechanisms.

Overall, LNG releases are characterised by a rapid phase change, and a hazard profile dominated by flammability, cryogenic exposure and asphyxiation risk in confined spaces, with environmental impacts generally acute and localised.

Response considerations for releases of LNG

A response to an alternative fuel release is expected to differ significantly from a conventional oil spill response, with a broader and more specialised range of stakeholders likely to be involved. Fire and rescue services, for example, may be among the first notified, reflecting the acute hazards associated with flammable or toxic atmospheric plumes. However, as with oil spill response, preparedness remains essential to ensure that any intervention is effective, timely and well coordinated.

The presence of toxic or flammable vapour clouds may prevent on-scene operations until concentrations fall to safe levels through evaporation, dispersion or dilution.

In recognition of these unique challenges, many port and national authorities are increasingly developing dedicated hazardous and noxious substances (HNS) contingency plans or integrating HNS considerations into existing oil spill contingency frameworks. This reflects the growing presence of vessels carrying alternative fuels in their waters and the corresponding need to be prepared for potential response actions.

The primary action from the crew on board or those undertaking bunkering operations following a release is to isolate and stop the release by activating the Emergency Shutdown and Emergency Release Coupling systems, secure valves, establish safety and exclusion zones, and control ignition sources. Under Section 15.8 of the IGF Code, it is mandatory for LNG-fuelled vessels to have fixed gas detectors installed, and these will be augmented with the crew’s portable detectors. Gas detection will allow responders to understand whether the release has caused the atmospheric concentration of LNG to reach potential flammable or asphyxiative concentrations. If that is the case, plume and trajectory modelling can be used to define/adjust exclusion zones as appropriate. 

If a large release were to occur nearshore with metocean conditions pushing an LNG plume over a population centre, shelter-in-place orders may be required to ensure that exposure is limited. Evacuation measures could also be used; however, if these are not preplanned and regularly rehearsed (for areas near LNG terminals), these orders may result in panic and reduced effectiveness. Monitoring and evaluation should be undertaken until gas detection equipment demonstrates that there is no longer an increased risk as LNG concentrations have reduced to pre-spill levels. It should be noted that gas detection does not necessarily require human interaction, with UAVs/ROVs being mounted with detection equipment a possible solution. This reduces the risk to responders and allows remote assessment. However, for LNG incidents due to potential flammability and the static charge from this equipment, these devices need to be intrinsically safe or ATEX-certified.

As LNG rapidly vaporises at ambient conditions, recovery of the substance is not possible, unlike oil where traditional boom and skimmers are used. Therefore, priorities are source control, atmospheric monitoring and public safety. Despite recovery not being possible, control measures can be undertaken, such as the use of water spray/curtains. These act as barriers to ‘steer’ and ‘dilute’ cold vapours away from ignition sources and protected locations (e.g. local population centres) or to avoid creating confined vapour pockets that could lead to asphyxiative or flammable conditions. In addition, the use of water spray can warm the LNG pool faster, increasing its buoyancy faster and reducing flammability hazards. Due to LNG’s insolubility, firefighting runoff will not be contaminated and therefore stringent management is not necessary, unlike for other alternative fuels such as ammonia.

As many of these actions are necessary immediately after a release and are specialised, it is essential that operators and responders are trained regularly to undertake these response measures and that roles and responsibilities are clearly included in a contingency plan. 

Special thanks are extended to ITOPF for their invaluable expertise and contribution to the research and development of this article.

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[1] GIIGNL. 2023. “LNG Information Paper #1 – Basic Properties of LNG”. 2019, GIIGNL. Neuilly-sur-Seine, France

[2] NOAA. 1999. “Liquefied Natural Gas – CAMEO chemicals profile”. June 1999, NOAA. Washington DC, USA