Sandia assesses hazards of LNG transport

By OGJ Editors

HOUSTON, Dec. 21 -- Damage from an accidental or intentional breach of an LNG carrier would extend as far as 1,600 m from the damaged ship for a large spill but pose little threat beyond that, says a new federal study.

In an assessment of LNG mishaps over water for the US Department of Energy's Office of Fossil Energy, Sandia National Laboratories says that beyond about 750 m for small spills and 1,600 m for large spills, "the impacts on public safety should generally be low for most potential spills."

A spill large enough to threaten public safety out to 1,600 m probably would result from an intentional breach such as from a terrorist attack, the study says.

The Sandia study assumes that a spill from an intentional attack would be larger than that from an accident because it would involve greater damage to cargo tanks. It estimates that the most likely accidents would create breaches smaller than 2 sq m. It estimates breach sizes resulting from intentional attacks at 2-12 sq m, with 5-7 sq m most likely.

For an accident creating a tank hole of 0.5-1.5 sq m, the risk of which Sandia calls "generally low," the high threat to public safety and property would lie within 250 m of the spill, with some damage possible out to 750 m. An example of this type of event would be a high-speed collision damaging an LNG cargo tank and causing an LNG spill and small fire.

In an intentional attack creating a larger breach to a cargo tank, the risk of damage to public safety and property would be high within 500 m of the spill and present but less severe out to 1,600 m. Effects would be much lower beyond about 1,600 m, Sandia says, "even for very large spills."

Although fire likely would result from an intentional breach, under some circumstances a cloud of unignited vapor might form and extend the threat of fire to 2,500 m from the spill.

An intentional attack might ignite LNG vapor inside the vessel and upset structural integrity or extend damage to cargo tanks other than the one breached, Sandia says. But extended damage of this kind would affect no more than two or three cargo tanks. Its main effect would be extending duration of the fire.

Sandia notes that rapid phase transition, an explosive combination of hot and cold liquids, might occur in a large spill. But its effects would be confined to the area near the spill source "and should not cause extensive structural damage."

Because hazards are high near a potential spill site and diminish with distance, Sandia adjusts its recommendations accordingly.

It recommends the strictest safeguards against accidental spills for places where LNG shipments cross narrow harbors or channels, pass under major bridges or over tunnels, or come within 250 m of people or major infrastructure.

For management against the risk of intentional LNG spills, it extends the strictest-safeguard distance to 500 m. Within that range of a large spill, Sandia says, "thermal radiation poses a severe public safety and property hazard and can damage or significantly disrupt critical infrastructures."

Recommendations for these areas include strategies for dealing with vapor dispersion and fire hazards and include "the most rigorous deterrent measures, such as vessel security zones, waterway traffic management, and establishment of positive control over vessels."

Coordination is crucial among everyone involved with port security in high-hazard zones, Sandia says, urging careful evaluation of emergency response measures to ensure that enough firefighting and salvage resources are on hand.

Sandia's recommended safeguards are less strict at distances of 250-750 m from locations of potential accidental spills and of 500-1,700 m from sites of potential intentional attacks. Again focusing on the management of vapor dispersion and fire hazards, "strategies should include incident management and emergency response measures, such as ensuring areas of refuge are available, development of community warning signals, and community education programs to ensure persons know what precautions to take."

Risk management can focus on vapor cloud dispersal in large bays and open water where LNG shipments pass no closer than 750 m to population areas or infrastructure, for precautions against accidental spills, and no closer than 1,600 m for intentional breaches.

Strategies in such areas can concentrate on incident management and responses to vapor cloud dispersion. Measures should include provision of refuge areas and community education for dealing with vapor clouds, Sandia said.

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