LNG safety

The key safety issues for LNG relate to the consequences of an LNG spill. Most significant is the potential for ignition of an LNG pool or vapor cloud. Models indicate that under the right conditions, a fire igniting an LNG pool or vapor cloud could cause extensive damage to life and property. Other less significant issues include the potential for freeze burns, cracking of metals, explosions due to leaking natural gas, and flameless explosion. Let’s start by looking at the physical properties of LNG.

Physical properties of LNG
LNG is simply natural gas that is cooled below –260°F (-162°C). When allowed to warm above this temperature, LNG will revert to its natural gaseous state with properties no different from natural gas that has never been liquefied. Natural gas is lighter than air, so when released it will rise and disperse unless it is confined within a physical enclosed space. LNG is heavier than air but lighter than water, and if spilled on water it will float until it warms and returns to its gaseous state. Small amounts of LNG vaporize very rapidly when they encounter warmer air or water, which is why we use the term "boil-off" to describe LNG becoming gaseous. LNG spills are not toxic and leave no residue once the gas has dispersed.

LNG is not combustible in liquid state but in gaseous state can ignite when concentration of the gas (i.e., gas/air ratio) is between 5% and 15%. If the gas is in a confined space at this gas/air ratio and is ignited, an explosion can occur. If not confined, the gas will simply burn until there is no longer sufficient fuel and/or air to maintain the 5% to 15% concentration.

Specific safety concerns
Much of the concern for LNG accidents is focused on the potential for a rapid spill of large volumes. In such cases, the physical properties of LNG may prevent harmless vaporization and dissipation into the atmosphere. Following a large spill, the LNG would pool on the ground or water until it warmed to the point where it became gas. As the top surface warmed, the pool would begin to give off LNG vapor. The LNG vapor would be heavy because it is cool. Mixtures of dry air and cold methane vapor are heavier than air, and in the absence of either heat from the ground or water or humidity in the air, the vapor remains heavier than air. This would cause it to float just above the ground or water. Thus, if the LNG vapor remained heavier than the surrounding air, a vapor cloud similar to ground fog would tend to form. This vapor cloud would spread slowly in the absence of wind, or more rapidly if wind is present. 

There are a number of specific hazards that could result from an LNG spill, as well as a few other areas of concern. Each is discussed below.

• Pool fires — If LNG is spilled near an ignition source, the potential exists for the vaporizing gas to immediately ignite and burn above the LNG pool. If the LNG pool were not confined, it could spread quickly – especially on water. This could result in a pool fire expanding over a large area. This fire would burn at a much higher temperature than oil or gasoline fires and could not be extinguished. It would simply continue to burn until all the fuel had been consumed. The thermal radiation from such a fire could injure or kill people and damage property at a distance far from the actual fire itself.

• Vapor cloud fires — There is also a second type of fire possible. If LNG spills but does not immediately ignite, then a vapor cloud may form. This vapor cloud would drift from the spill site according to wind conditions. If the vapor cloud were to come in contact with an ignition source while the gas/air ratio is between 5% to 15%, then the cloud would catch fire. The fire would likely be smaller and less intense than a pool fire because only a portion of the fuel within the cloud would be at a combustible gas/air mix. However, the fire could burn its way back to the LNG spill where the cloud originated, in which case the fire could act as an ignition source for the remaining LNG pool, resulting in the vapor cloud fire becoming a pool fire. Because vapor cloud fires are smaller than pool fires, the distance at which thermal radiation effects would be a concern is much smaller. But because vapor cloud fires could move rapidly in heavy winds, the distance from the fire at risk for a hazardous situation is much greater. As with a pool fire, a moving vapor cloud fire would continue to burn until the combustible fuel-air mix no longer existed.         
  
  

• Rapid phase transition explosion — A third theoretical danger is known as a rapid phase transition explosion or "flameless explosion" resulting from LNG heating up very quickly and converting to gaseous state so rapidly that pressure changes result in a shock wave. Studies to date indicate that such an explosion is highly unlikely, and that if it were to occur it would affect a much smaller area than either pool or vapor cloud fires. A rapid phase transition explosion could cause some injury to individuals directly in the area and/or localized structural damage, although impacts would likely be minor.
  
  
• Additional hazards of leaking or spilled LNG — Other hazards of spilled LNG include the potential for asphyxiation, freeze burns, and cracking of metals. While LNG is not poisonous, an individual trapped in the center of a vapor cloud could asphyxiate due to lack of oxygen. Because of its temperature, contact with skin can result in severe freeze burns. And contact with certain metals, will cause them to crack. LNG that leaks and becomes regasified and then migrates to an enclosure can result in explosion if the gas/air concentration is at the right level and a source of ignition is present. However, this danger is no more significant than the danger of explosion from any natural gas source.

Due to these potential hazards, LNG tankers and facilities are designed to prevent spills and to contain damage were a spill to occur. Tankers are designed with numerous safety standards including double-walled hulls. Storage tanks are secured within physical berms sized to contain more than 100% of the LNG tank volume. Catch basins are located throughout LNG facilities so that liquids from ruptured pipes would flow and accumulate where they would evaporate. And all LNG facilities include emergency shutdown systems, monitoring systems, and security procedures.