Abstract

Throughout much of the world, the demand for natural gas has been steadily increasing and is projected to double in the next 15 years. A large portion of the demand can only be met through long-distance marine transport, and for economic reasons, ships carry the product as liquefied natural gas (LNG). While the LNG industry lays claim to an exceptional safety record, safety concerns have threatened to delay or even prevent the expansion as regulators and general public remain unconvinced. In order to quantify associated hazards with LNG spillage we have concentrated, in this work, on one of the crucial factors that determines the formation and the future behaviour of the hazardous vapour cloud, namely the rate of vaporization of the LNG.

The spillage of LNG on water surfaces can lead, under certain circumstances, to a decrease in the surface temperature of water and subsequent freezing. A model for heat transfer from water to LNG is proposed and used to calculate the surface temperature of water and examine its influence on the vaporization rate of LNG. It was concluded that when LNG spills on a confined, shallow-water surface the ice layer will form. The formation of an ice layer, that will continue to grow for the duration of the spill, will have a profound effect upon the vaporization rate. The decreasing surface temperature of ice will decrease the temperature differential between LNG and ice that drives the heat transfer and will lead to a change of the boiling regime. The overall effect would be that the vaporization flux would first decrease during the film boiling; followed by an increase during the transition boiling and a steady decrease during the nucleate boiling.

The developed model reduces the uncertainty associated with predicting the crucial vaporization rate of LNG and allows for a more secure quantification, for the purposes of risk assessment, of hazards associated with accidental or terrorist-related LNG release.

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