Impact of atmospheric dispersion of filled LNG cloud from NLNG’s operation

Recently, the demand for natural gas, both as a fuel and as a petrochemical feedstock, has been on the increase. Long distances between producing and consuming nations necessitate marine transportation. For economic reasons, this type of transportation is done with natural gas in liquefied state. NLNG (Nigeria Liquefied Natural Gas Ltd), situated in the western coast of Bonny Island in the Niger Delta area of Nigeria, is one of the industries engaged in this activity, with Bonny River as the take off point. Environmental implication of risks involved in marine transportation calls for quantification of such risk. There is a tendency for spill of LNG on water during the loading and shipping in the river; areas downwind of the spill point could be at risk since LNG is flammable at a certain concentration range in air (5% to 15%). This work identified the period in which these areas are in danger, the distance that needs to be allowed between the Bonny river and the AOCs (areas of concerns), and indoor concentrations of the vapourized gas. ALOHA (aerial location of hazardous atmospheres) and HEGADAS (heavy gas dispersion from area sources) packages were used. Simulations were carried out using two points on Bonny river as representative spill points (these are chosen on the basis of the wind directions and areas downwind). The two packages used predicted the maximum threat zones in agreement of between 50% to as high as 0.5%, as the wind speed increases. It was found that the AOCs could be at risk. A distance of 5.5 km should be the minimum distance between Bonny river and the AOCs. The maximum time for the cloud to reach the AOC, in the event of the rupture of a full tank load (14 500 000 kg) was predicted to be 22 minutes, but smaller masses would require smaller times. The indoor concentrations identified do not pose any threat to the households, as these values are below the flammability range of methane; they are also below the concentrations that can asphyxiate or cause freeze burns.