Risk assessment of accidents induced by lightning strike on storage tanks

Lightning strikes emerge as one important cause for the Loss Of Containment (LOC) of storage vessels containing hazardous materials if past accident databases are analyzed. The release of these substances may cause severe accidents in refineries, storage sites, processing sites and other industrial facilities that may involve both workers and nearby residential areas. Although lightning hazard is well known and it is usually considered in the risk analysis of chemical and process plants, well accepted quantitative procedures to assess the contribution of accidents triggered by lightning to industrial risk are still lacking. In particular, the approaches to the assessment of lightning strike probability and to the damage caused by lightning strike are mainly qualitative or semi-quantitative and they are mostly based on expert judgment. In the present study, a quantitative methodology for the assessment of the risk due to major accidents triggered by lightning is presented. The methodology takes advantage of the relevant progress made in recent years in the assessment of the expected frequency and of the expected structural damage due to lightning. The starting point of the methodology is the assessment of the frequency and severity of the lightning event. The standard parameter for the frequency is the isokeraunic number given by meteorological studies. The lightning severity is quantified by means of probability distribution functions of two parameters: peak current intensity and lightning charge. Through the application of a Monte Carlo simulation of lightning strikes on different process equipments the overall frequency and intensity of the LOC is thus determined. These results are then adapted in order to obtain an analytical relationship between the equipment features and the release probability. The application of the conventional risk recomposition procedure allows the calculation of individual and societal risk figures.