The rise and rise of liquefied natural gas
WHAT IS LNG?
Natural gas comprises methane, ethane, propane and heavier hydrocarbons, plus small quantities of nitrogen, helium, carbon dioxide, sulphur compounds and water. Liquefied Natural Gas, or LNG, is produced by cooling natural gas to -160°C, once impurities such as water and carbon dioxide have been removed, which would otherwise solidify.
At this temperature, purified natural gas is a liquid at atmospheric pressure, occupying about 600 hundred times less volume than its gaseous form at room temperature. LNG is a clear, non-toxic, non-corrosive liquid with a density about half that of water.
Although cryogenic temperatures are required, LNG’s intrinsic properties make it a viable means of transporting natural gas across long distances by sea. As a result, LNG facilities fall into one of three categories: liquefaction plant, shipping and regasification plant.
Today, natural gas meets over 21% of the world’s energy needs, and growth in gas consumption is forecast to outstrip that of oil over the first quarter of this century reaching 26% by 2030 [Refs 1 & 2].
In OECD countries, gas-fired power generation is largely responsible for this growth, given the improved efficiencies of closed-cycle gas turbines and their cleaner emissions, compared to coal, for example. At the same time, countries once rich in gas reserves, such as the UK and US, are struggling to meet demand, while others, such as Nigeria and Qatar, are tapping into vast reserves. With rising gas prices, LNG offers an increasingly attractive method of transporting gas to market.
LNG already supplies about 7% of the global gas market, the majority of which goes to Japan and South Korea. This is set to rise, with investment in LNG facilities predicted to double in 2008 to over $20 billion per annum.
In the UK, for example, two major new LNG terminals are under development at Milford Haven (Dragon LNG and South Hook LNG). Similarly, in the US a new LNG terminal in New Jersey (Crown Landing) is planned to come on line at the end of 2007.
LNG HAZARDS AND SAFEGUARDS
LNG has been used worldwide for about 40 years. As such, there is a mature understanding of its associated hazards, gained in part from extensive operational experience [see Table 1]. Hazards fall into the following categories:
Extreme cold – Direct contact with LNG will cause immediate freezing of plant or people. Widespread exposure can cause serious injury and death. Secondary containment systems around LNG storage tanks are designed to contain tank contents, and in hazardous areas, personnel wear protective clothing and may have access to emergency showers.
Vapour cloud explosion – As uncontained LNG warms, it evolves methane gas. As this gas itself warms it disperses, mixing with air. Explosion occurs if a vapour cloud is confined and an ignition source is encountered within the range of flammability.
To prevent leakage, primary containment structures are designed from low temperature-resistant materials, such as high nickel content steel alloys, aluminium, stainless steels and reinforced concrete. Secondary containment limits leaks to areas where ignition sources are excluded or strictly controlled. Most modern facilities employ a full second containment structure capable of withstanding cryogenic temperatures, while older facilities use dikes, berms or dams. LNG ships are designed with double hulls to prevent leaks caused by grounding or collision.
Leak detection methods include monitoring of vapour pressure, temperature and liquid level, as well as direct sensing of gas outside primary containment. Upon leak detection, emergency shutdown systems can be called upon to limit leaks due to loading, unloading or process operations.
Multiple trains of cryogenic plant control LNG temperature. However, should a plant fail, pressure relief devices prevent a catastrophic loss of containment.
In the unlikely event of a release, the layout of LNG facilities seeks to minimise confinement and incorporates sufficient separation distances from the surrounding area to minimise loss of life and property.
Pool fire – Liquid leaks of LNG, when accompanied by an ignition source, can result in a pool fire similar in nature to other liquid hydrocarbon fires. In addition to containment and leak detection systems, smoke and fire detection is fitted, together with automated fixed fire fighting systems.
Rapid phase transition – If LNG is released onto water it floats and vapourises rapidly. In large amounts, with mixing between LNG and water, a rapid phase transition can occur capable of causing light structural damage. This hazard is particularly relevant for LNG ships, and is managed by using double hulls.
Rollover – When stored in large volumes LNG may become stratified in layers of different densities. For example, if bottom layers are warmed by natural heating, they become lighter than upper layers. The resulting liquid rollover of tank contents can result in substantial vapourisation above the capacity of pressure relief devices. To counter this, LNG tanks have rollover protection systems, which include distributed temperature detection and pumped mixing systems.
Table 1 Major LNG Incidents*
LNG is clearly set to continue and expand its role as an important global energy commodity. While not without its hazards, they are well understood and there are technologically mature solutions available to manage the associated risks.
1. International Energy Agency, Natural Gas Review 2006
2. Douglas-Westwood, The World LNG & GTL Report 2007-2011.
3. University of Houston Law Center, Institute for Energy, Law & Enterprise, LNG Safety & Security, October 2003.
4. California Energy Commission (www.energy.ca.gov/lng).
This article first appeared in RISKworld Issue 12.
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