Enhanced Seismic Fragility Analysis of Unanchored Above-Ground Steel Liquid Storage Tanks

Earthquake damage in recent decades has revealed that storage tanks are one of the most vulnerable components in petrochemical and oil processing plants. Damage to tanks commonly associated with losses of containment, and thus results in the overall damage to nearby areas. Many of existing steel storage tanks were designed with outdated analysis methods and with underestimated seismic loads; therefore, various types of failure may occur during a strong ground shaking. This paper aims to present an appropriate methodology for the component fragility evaluation of existing storage tanks in a process plant, which will support for the determination of the loss of containment in terms of the ground motion intensity measure and finally the quantitative risk analysis of the plant and its nearby areas. In this respect, an unanchored oil storage tank, which is ideally located in Sicily (Italy), is selected as a case study. The significance of modeling parameters of the tank is first investigated with a screening study, which is based on nonlinear static pushover analyses of the tank using the ABAQUS software. The study aims to enhance the understanding of which modeling parameters significantly affect the seismic response of the tank and to reduce the number of analyses in the fragility evaluation. The fragility curves are then developed based on a lumped-mass model that is calibrated from the static pushover analysis results. Sources of uncertainty, related to significant parameters previously identified, are considered in the fragility analysis using a sampling procedure to generate statistically significant samples of the model.