Seismic vulnerability analysis of storage tanks for oil and gas industry

Petrochemical and oil processing plants are complex systems of numerous integrated components and processes, which can make them particularly vulnerable to natural hazard events, in particular, earthquakes. Steel fuel storage tanks are recognized as the most vulnerable equipment to the earthquakes, whose damage may result in the release of materials and thus the increase of overall damage to nearby areas. The seismic vulnerability of tanks is commonly expressed by fragility curves, which are conditional probability statements of potential levels of damage over a range of earthquake intensities. This paper aims to present an appropriate procedure for analytically deriving fragility curves of tanks. At first, the analysis of critical damage states of steel storage tanks observed during past earthquakes is presented. Possible numerical models of tanks subjected to earthquakes are then discussed. An overview of seismic fragility methodologies for tanks is next presented. The attention is paid to an analytical method, i.e., cloud method, which is conducted by using a probabilistic seismic demand model and nonlinear time history analyses. A broad tank, which is located in a refinery of Italy, is considered for the fragility evaluation. Resulting fragility curves for critical damage states of the tank such as the plastic rotation of the shell-to-bottom plate joint, the buckling of the bottom shell course, and the material yielding of the shell plate show a high seismic vulnerability of the tank.