Probabilistic assessment of material failure in anchored above ground storage tanks during flood events

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This study investigated material failures – yielding and rupture – in shell and bottom plates in anchored above ground storage tanks’ (ASTs) during flood events. Existing studies have assessed shell buckling in anchored ASTs during hurricanes and floods, however, material failure in ASTs – i.e., in tank shell and bottom plate - due to flood load has received limited attention. To this end, a finite element simulation-based approach was used wherein, first, design parameters such as tank diameter, relative density and height of stored contents, yield stress, inundation depth and bottom plate thickness were sampled using Latin Hypercube Sampling (LHS). For each design parameter combination, an AST was designed and maximum von Mises streses in bottom plate and tank shell were determined from finite element analysis for various flood depths and were compared against two different failure thresholds corresponding to material yielding and rupture. The results from finite element simulations revealed that even after considering buckling of the tank shell, bottom plate is more vulnerable to material yielding and rupture and would fail first before tank shell during flood events. These results were used in step wise logistic regression to develop parameterized fragility models. These models were applied to four case study tanks to estimate probability of failure, spill volumes, and required fill level to prevent failure. Bottom plate failure was found to initiate at low inundation depth for larger diameter tanks. Probability of failure of anchored bottom plate was found to be higher when compared with tank shell buckling. It was also found that anchoring the tanks, especially larger diameter tanks, was not effective in preventing spills during flood events due to bottom plate failure. Comparison of minimum fill levels to prevent flotation of unanchored tanks and bottom plate failure in anchored tanks against common rules of thumb showed that higher liquid levels might be needed to prevent bottom plate failure in case of larger diameter tanks. Results from this study suggested that tanks should be allowed to float in a controlled manner, and thus bottom plate failure and uncontrolled flotation induced spills could be prevented.