Fluid Dynamic Models Application in Risk Assessment

Abstract

Risk is a common name for the probability of a hazard turning into a disaster. Vulnerability and hazard are not dangerous in and of themselves, but if they come together, they generate a risk. However, risk can be reduced and managed. If we are careful about how we treat the environment, and if we are aware of our weaknesses and vulnerabilities to existing hazards, then we can take measures to make sure that hazards do not turn into disasters. Hazard from LNG (Liquefied Natural Gas) cargo begins in the first processing stage of natural gas liquefaction and loading the substance into LNG tankers. The transport itself over the sea is the safest part of the distribution process, as is demonstrated by the statistic of nautical accidents in the past 40 years (DNV, 2007, Perkovic et al., 2010 & Gucma, 2007). A review of a Rand Corporation document (Murray et al.) published in 1976 indicates a high level of safety for LNG tankers. The document indicates that in the initial 16-year history (from 1959 up to 1974) there had been no significant accidents. It should be noted, though, that in 1974 the world LNG fleet included only 14 vessels; by November, 2009, there were 327 vessels, a figure expected to increase to 350 vessels sometime in 2010 (LNG Journal, 2008). The DNV (Det Norske Veritas) counts 185 nautical accidents involving LNG tankers, all without severe consequences for the crew. The frequency of LNG tanker accidents is therefore 5.6 x 10-2 per ship year. The findings of the DNV (2007) furthermore demonstrate that the potential loss of life for the LNG crew member is 9.74 x 10-3 or less, considering the occupational fatality rate onboard gas tankers is 4.9 x 10-4. The analysis of the northern Adriatic Sea (Petelin et al. 2009), or, precisely, the gulf of Trieste, demonstrates that nautical accidents should occur with a frequency of 1.25 x 10-2 per year, assuming the current traffic density, and increases to 2.62 x 10-2 if the ship traffic increases by 100%. The hazard associated with LNG is mainly in its potential to cause severe fires resulting in heat radiation. If a large quantity of LNG is spilled into a pool, the cloud that is formed as it evaporates is a mixture of natural gas, water vapour, and air. Initially the cloud is heavier than air (due to its low storage temperature) and remains close to the ground. The buoyancy moves the natural gas upward at a gas temperature of around 170 K (-1030C), as experimentally demonstrated by ioMosaic (2007). The major influences on natural gas diffusion are environmental conditions. The cloud moves in the direction of the wind and the wind causes the cloud to mix with more air. If the concentration of gas in the air is between 5% and 15% it is flammable and burns if it contacts any ignition source. A concentration of gas smaller than 5% will not ignite and if the concentration is over 15% the air becomes saturated. The explosion of natural gas is not possible in open spaces because