Quantitative risk analysis of gas explosions in tunnels

Transportation of flammable liquefied gas in tunnels presents a significant risk of an accidental loss of containment leading to an explosion with major consequences. Possible scenarios include a BLEVE, a non-reactive gas expansion explosion and a reactive gas explosion. Quantification of the risk and consequences associated with such events is central in the design of tunnels and routing of dangerous goods. TNO previously developed a Quantitative Risk Analysis (QRA) method, which combines a probability assessment with state-of-the-art explosion effect and consequence models. The current article extends this model to combine the dispersion of a flammable cloud with its probability of ignition and the resulting physical effects such as overpressure. The model assumes an increasing probability of ignition with both the number and the duration of vehicles present within the flammable cloud. Various case studies are considered to illustrate the effect of different ignition probability parameters. These cases deal with instantaneous and continuous LPG releases with varying release rates including the effect of ventilation. They clearly show the capability to quantify the ignition probabilities and gas explosion load. The combination of the gas dispersion, gas explosion and ignition probability models are needed to derive design loads for tunnels, to perform tunnel risk assessments, and to develop safety measures. These models form the backbone for quantitative risk assessments.