Development of a general framework for the quantitative assessment of risk related to Na-Tech accidents

It is well known that the impact of natural events on chemical and process plants may cause severe accidents involving the release of relevant quantities of hazardous substances. However, although several of these events, referred to as “Na-Tech” accidents, are reported in the literature, scarce attention was devoted to the assessment of risk due to major accidents triggered by natural events. A further complication comes from the different impacts that may be related to different categories of natural events, that result in important differences in the expected damage to process equipment. As a matter of fact, most of the previous studies only afforded the analysis of specific interactions between natural events and potential damages to process plants or storage sites having relevant inventories of hazardous substances. In the present study, a general framework was developed for the assessment of Na-Tech events. An approach towards the quantitative assessment of risk due to these events was also defined. The approach was based on the characterization of the expected damage states of process equipment following the impact of natural events, and on the subsequent assessment of an expected loss intensity. Available data on hazardous substance inventory and on equipment operating conditions may thus be used to assess accident consequences. The methodology also allows the assessment of the possible simultaneous scenarios that may arise from the damage of several equipment items. The quantitative assessment of individual and societal risk may also be estimated, but this requires the availability of specific simplified equipment vulnerability models to estimate the damage probability of process equipment caused by the natural event of concern. The procedure was applied to the analysis of specific case-studies aimed to the assessment of risk due to seismic events and to floods in chemical plants. The implementation of a specific software package for the calculation of individual and societal risk indexes allowed the analysis of extended plant lay-outs with a limited effort and using a limited amount of additional data with respect to those required for conventional plant safety reports. The results evidenced that the additional risk due to Na-Tech events is not negligible, mainly due to the rather high frequencies that are expected for the impact of natural events on process plants. The procedure developed proved to provide useful data for the identification of critical process equipment and for emergency planning.