Assessing the Effectiveness of Stiffener Rings in Mitigating the Surge Buckling Fragility of Above-Ground Storage Tanks

Above ground storage tanks (ASTs) are thin walled structures that often store hazardous substances. In the United States, a large number of these ASTs are situated near the coast, such as the 4200 tanks in the Houston Ship Channel. Proximity to the coast makes these tanks vulnerable to the multiple threats posed by hurricanes, such as wind and storm surge. During hurricanes Katrina and Rita, several AST failures were reported leading to spillage of over seven million gallons of petrochemical substances. Such spills have catastrophic impact on the surrounding environment and habitat. Reconnaissance reports identify floatation and wind buckling of ASTs as the primary modes of failure. Since floatation failure of ASTs may cause catastrophic spills, one of the solutions is to anchor the tanks to their foundations. This solution may prevent floatation of tanks; however, a recent study, by the authors, suggests that large external hydrostatic pressure due to storm surge may buckle the thin shells of ASTs containing a low level of produce. Buckling of the tank shell may cause delays in resuming operations of facilities and may even rupture the shell leading to a spill. In order to prevent such failures, this study aims to assess the effect of stiffener rings, which are known to be effective in preventing wind buckling but have yet to be evaluated for their potential to mitigate surge buckling probability. For this purpose, surge buckling fragility of a representative tank from the Houston Ship Channel will be evaluated with and without stiffener rings. Such fragility models depict the conditional probability of buckling given surge and internal liquid level. In order to derive the fragilities, buckling simulations in the finite element software package LS-DYNA will be conducted across a range of surge elevations and internal liquid levels, while propagating uncertainty in other modeling parameters. The results of this study aid in assessing a potential surge buckling mitigation strategy which may prevent catastrophic spills and reduce delays in resuming operations in industrial facilities after hurricanes. Future work will evaluate the surge risk to a portfolio of tanks and explore the application of stiffener rings along with other structural and regional protective strategies for mitigating risks across the region.