Seismic vulnerability assessment of storage tanks in terms of loss of containment

The chemical and process industries are increasingly subjected to NaTech events, technological disasters triggered by natural hazards such as earthquakes, floods or hurricanes. In this context, some of the most damaged equipment are storage tanks, which can be affected by high seismic vulnerability and containment of hazardous materials. In this regard, their failure often leads to the loss of containment (LoC) of hazardous substances, triggering domino effects such as explosions, environmental pollution, operational disruption, and business interruption. A review of past seismic events reveals that some of the most common damage modes affecting unanchored tanks are elephant foot buckling, uplifting, and sliding. Particular attention is given to the latter, due to the notable lack of effort dedicated to understanding and modelling this specific failure mechanism, which is strongly associated with the relative displacement between the equipment and the ground. Based on these premises, this study proposes a fragility analysis focused on LoC scenarios triggered by seismic events specifically associated with the sliding mechanism of unanchored storage tanks. A simplified probabilistic approach is adopted, where the selected engineering demand parameter (EDP) is represented by the relative displacement between tank and ground, which is then correlated to the liquid release. Fragility curves in terms of probability of leakage are formulated using the Average spectral acceleration (SaAV) as intensity measure. This methodology aims at providing a practical tool for quantifying the seismic risk of sliding-induced LoC events, supporting their integration into Quantitative Risk Assessment (QRA) frameworks.