Storm surge fragility assessment of above ground storage tanks

This study proposes a dual layer metamodel based approach to develop parameterized fragility functions for above ground storage tanks (ASTs) subjected to hurricane induced storm surge. ASTs are extensively used in petrochemical facilities for storing large volumes of hazardous substances. Failure of ASTs due to storm surge may lead to spills that can cause severe environmental damage and considerable economic loss. A significant number of ASTs are located in coastal areas which are susceptible to hurricanes, such as in the Houston Ship Channel, Texas. However, tank design guidelines are deficient in addressing prevention of surge related failures. Although their vulnerability has been exposed during past hurricanes, the literature lacks studies on performance analysis of ASTs during storm surge events. In this light, a method is presented to derive fragility functions for the most important failure modes of ASTs - flotation and buckling - in addition to the system fragility, considered as series system of failure modes. For this purpose, a novel dual layer metamodel based approach is proposed where a limited number of simulations are used to train the first binary classifier, which predicts failure of tanks, upon which the second metamodel is trained; the final metamodel is used to derive parameterized fragility functions. This approach significantly reduces the number of limit state evaluations, which may require costly finite element simulations, and enables accurate fragility assessment to capture the nonlinear behavior of tanks under surge loading, while also considering the correlation between failure modes during system fragility modeling. Results indicated that the fragility estimates of a typical tank obtained with the dual layer metamodel compare well with those derived by high fidelity methods such as Monte Carlo Simulations. In order to demonstrate the application of the parameterized fragility functions to study the effect of variation in design and construction parameters, fragilities of four case study tanks are evaluated. The results highlight the effect of parameter variation on the fragilities and offer insights into the influence of alternative design impacts on tank vulnerability. For example, anchoring tanks significantly reduces the probability of flotation; however, anchoring leads to buckling dominated failures.