Lng Risk Mitigation: a Comparison Between Active and Passive Barriers

In the last years, there has been a rapid increase in the proposals for regasification terminals to import Liquefied Natural Gas (LNG) mainly due to the global uncertainties of the energy market. Therefore, there has been a fast increase in the interest in the risk assessment of LNG regasification terminals. LNG is not poisonous; instead, its rapid evaporation together with the vapour phase flammability presents a non-negligible risk. The concentration range in which the gas-air mixture at ambient conditions is flammable is about 4.4%v/v (Lower Flammability Limit - LFL) to 15% v/v (Upper Flammability Limit - UFL). One of the major accidental scenarios, involved in an LNG regasification terminal, is the breakage of a pipeline carrying natural gas in the liquid phase. This would result in the release of large amounts of LNG leading to a fast-evaporating pool and, consequently, to a large flammable cloud and possibly to fires and explosions. Therefore, mitigation measures must be provided to reduce the risk up to an acceptable value; among the various mitigation measures, a protective barrier able to limit the hazardous distance related to a given accidental scenario (and therefore to protect sensible population living close to the regasification terminal) can be used. In their simplest configuration, passive mitigation barriers are high walls acting as obstacles on the cloud path, therefore enhancing the flammable cloud-air mixing. Unfortunately, to be effective passive barriers often must be quite high, possibly preventing their practical implementation. As an alternative, active barriers can be used where the flammable cloud-air mixing is enhanced not only thanks to the wake effect of the wall but also to the direct entrainment into the flammable cloud. This entrainment can be induced (for instance) either by high-velocity jets or by fans. Therefore, the main aim of this paper is to provide a comparison among the pros and contras of using passive vs. active barriers to reduce the hazardous distance related to an accidental scenario in an LNG regasification terminal. In particular, the various barrier configurations were investigated through Computational Fluid Dynamic (CFD) simulations using the Ansys Fluent 2023R2 suite of programs.