As long as it remains in liquid form, liquefied natural gas (LNG) “will neither burn nor explode,” but the risks multiply when LNG vaporizes as it comes into contact with water and air, according to a highly technical report released by FERC Friday.

LNG vaporizes rapidly when exposed to water, producing 620 to 630 standard cubic feet of natural gas for each cubic foot of liquid, said the 128-page study, which FERC commissioned from ABSG Consulting Inc. The LNG initially produces a dense vapor cloud that stays close to the water. But as the cloud mixes with air, it warms up and disperses into the atmosphere.

LNG-sourced vapors are considered dangerously flammable when the concentration of gas in the air ranges from 5% to 15%, the report said. Below 5%, “the [vapor] cloud is too dilute for ignition,” and above 15%, “the cloud is too rich in LNG for ignition,” it said.

“If not ignited, the flammable vapor cloud would drift downwind until the effects of dispersion dilute the vapors below a flammable concentration,” the report noted. How far downwind the LNG vapors travel would depend on the “volume of LNG spilled, the rate of the spill and the prevailing weather conditions.”

The report, entitled “Consequence Assessment Methods for Incidents Involving Releases from Liquefied Natural Gas Carriers,” looked at the flammable vapor and thermal radiation hazards posed by unconfined LNG spills on water. It specifically focused on key models that are used to assess the dangers associated with LNG accidents.

The study did not evaluate the ignition probability of LNG. However, it noted that it would take a “significant energy source” to puncture an LNG tanker and cause a large release. “An event of sufficient magnitude to rupture an LNG cargo tank may [be enough to] provide ignition” for a flammable gas cloud. But the report said a flammable cloud once ignited would not likely travel a “significant distance over land.”

While LNG vapors “can explode…if ignited within a confined space, such as a building or structure, there is no evidence suggesting that LNG is explosive when ignited in unconfined open areas,” the Commission-directed study noted. This would seem to rule out or minimize the probability of explosions aboard LNG tankers that are in transit, but it doesn’t. The study noted that “confinement can be provided by spaces within the ship or nearby structures, such as a building onshore or another ship.”

Touting the fuel’s positives, the study noted that LNG was “less hazardous” than liquefied petroleum gas and liquefied ethylene, was not toxic and rapidly evaporated once exposed to the atmosphere. “Therefore, long-term environmental impacts from a release are negligible if there is no ignition of natural gas vapors.”

LNG has been transported at sea since 1959, and the tankers have a “remarkable safety record and provide an essential link in the movement of LNG from production locations” to consumer markets, the report noted. “However, stakeholders recognize that there are possibilities for some serious incidents involving LNG carriers, particularly in light of increased awareness and concern about potential terrorist actions.”

FERC said it commissioned the study to identify the appropriate models for estimating the distance at which populations would be affected by flammable vapors and thermal radiation hazards in the event of an LNG tanker spill.

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