Natech scenarios in the process industries and reliability analysis: An inclusive approach to the seismic effect

Natural hazard triggering technological disasters (Natech) may give escalating hazardous events in industrial installations. As an example, domino effects and generalized failures may arise as the result of an initiating seismic event. Storage systems and equipment can experience severe damages and related risk quantification should be included in the Quantitative Risk Assessment procedure. Once such scenarios are included in conventional QRA, some knowledge gaps arise. These are linked to the availability and the reliability of models oriented to atypical scenario escalations. In fact, while usual failure rates that apply to plant components are readily available, statistics of seismic incidence on equipment integrity are still debated. This work deals with the inclusion of the seismic scenario among initiating events in a reliability analysis framework and a worked example on an atmospheric storage tank is proposed. The illustrated approach differentiates between a conventional failure scenario and that also induced by seismic causes. As a general conclusion, results show that depending on the equipment typology and seismic parameters, differences in the reliability profile in time are expected. The seismic action aggravates the overall unreliability of the system because the earthquake event modifies the relative contributions to the Minimal Cut Sets of a specified Top Event. In high seismicity areas, contributions up to 20-30 % are observed and are comparable to usual initiating causes (mechanical failure, instrumentation malfunctioning).