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In the past decades the impact of natural events in industrial plants have triggered a number of technological accidents. Accidents may be followed by the release of relevant quantities of hazardous substances, with relevant consequences for the population and for the environment. Technological accidents triggered by natural events are very common in the industrial facilities, although only recently their specificity was recognized and they are now indicated as Natech (natural-technological) events. In particular, lightning strikes are a recognized hazard that causes the loss of large quantities of product and compromise the safety for chemical and oil industries. Past accident analysis evidenced that the most vulnerable installations to lightning strike are oil tank terminals of oil refineries. Since there is general agreement that a total protection from lighting hazard is impossible, quantitative risk assessment is needed to assess the possibility of technological accidents caused by lightning strike. The aim of the present study was the development of a dedicated methodology for the quantitative assessment of risk due to technological accidents triggered by lightning strikes in chemical and process industries. In particular the accidental scenarios have been identified and the frequencies for every scenario have been calculated. The method developed was applied to assess the risk reduction that can be achieved by the installation of air terminals to protect storage tanks in lightning prone zones. Different solutions are compared in order to select the best protection system for different storage tanks and lay-outs.

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.

The historical analysis of accidental scenarios triggered by lightning shows that the impact of a lightning on a process plant might be the initiating event of a severe accident. The analysis identifies the most common damages to the equipment as either immediate ignition of flammable atmospheres or structural damage with subsequent release. However, the available information on lighting damage to industrial equipment is fragmented and not very detailed. The aim of the present study is to define a failure model for reference equipment categories following lightning impact. Starting from the analysis of lightning damage, the possibility of equipment damage is assessed on the basis of lightning characteristics. The probability of failure is evaluated as the probability to find, in impacting lightning strikes, the minimum energy necessary to cause the loss of containment of the selected equipment. Probability distribution functions available for lightning characteristics, used together with the lightning capture frequency, allow the development of a failure probability model for reference equipment categories.