Reduction of natech risk due to lightning by the use of protection systems

Several accidents that occurred in the last decades evidenced that the impact of natural events in industrial plants may trigger accidental scenarios involving the release of relevant quantities of hazardous substances. Severe scenarios typical of the process industry, as fires, explosions, toxic releases, and water pollution were reported as the consequence of natural events in industrial areas. The specific features of technological accidents triggered by natural events were recently recognized, and these scenarios are now indicated as NaTech (natural-technological) accidents. The screening of past accident databases points out that NaTech scenarios are quite common in industrial facilities. Past accident analysis evidences that lightning is one of the most common initiator event for fires in oil refineries and tank farms. In most facilities, simple lightning protection measures, such as grounding or lighting rods, are implemented to reduce the risk due to lightning strikes. However, several accidents suggest that these measures may not be sufficient to retain the structural integrity of the equipment. In the present study a procedure for the determination of the mitigation effect due to lightning rods on storage tank farms is proposed. A statistical method was developed to evaluate the failure frequency of tanks due to lightning strikes. The model includes both the assessment of lightning impact probability and of tank damage conditional probability given the lightning impact. The method then considers the effect of lightning rods placed at given distances from the equipment items and calculates the reduction of the failure frequency due to attraction competition between the rods and the equipment. The procedure was implemented in a GIS-based software tool for the calculation of individual and societal risk. The method allows the assessment of the effect of lightning protection in terms of individual risk reduction.