Natech QRA in natural gas processing plants

Natural calamities as floods, earthquakes or lightning strikes might lead to major accidents in chemical, petrochemical and process facilities. Such natural-technological events are termed as Natech events and occurred several times in the past. In the present study, a methodology for the quantitative risk assessment (QRA) of Natech scenarios triggered by floods, earthquakes and lightning based on the extension of the scheme used for conventional risk assessment is presented. However, specific steps and specific models and tools were required to extend the conventional QRA procedure to Natech risk assessment, as the characterization of the natural hazard, the use of vulnerability models to evaluate the equipment damage probability and the risk recomposition of credible combinations of simultaneous events. The QRA procedure was then applied to a typical natural gas processing plant located in Italy. After the application of the QRA procedure, the risk indexes were calculated for the case study, enabling a detailed insight of risk due to Natech events. In particular, the simultaneous failure of multiple items led to high severity Natech scenarios. This effect is clear analyzing the F-N curves obtained for the case of earthquake and flood induced Natech accidents, that showed some additional steps at high N values due to the presence of “combined scenarios” associated to the simultaneous failure of multiple units. However, some assumptions and simplifications were required, in particular for the extensions of vulnerability models in case of seismic events and for the application of the risk recombination tool used to evaluate the risk associated to combination of events evidencing that there are still some gaps that need to be filled in order to allow a more robust application of QRA to Natech scenarios, requiring future research efforts.