Assessment of Storage Tank Thickness Obtained from the API 650 Design Procedure Through Nonlinear Dynamic Analysis, Accounting for Large Deformation Effects

This case study evaluates the design procedure for storage tanks proposed by API 650, using nonlinear dynamic analysis to consider large deformation effects. The investigation addresses both the seismic vulnerability of storage tanks that are going to be constructed in Naples, Italy, and their performance during seismic vibrations. Ten storage tanks with Height-to-Radius (H/R) ratios from 0.3 to 3.0 have been analysed. A target spectrum and 20 earthquake records are selected based on site-specific seismic characteristics. Initially, tank thicknesses were determined by manual calculations based on API 650 guidelines. These were then assessed through nonlinear time history analysis using SAP2000. Results indicate that the API 650-based thicknesses derived from linear average spectrum equations, do not perform adequately under actual seismic conditions. Subsequently, an optimum thickness for each storage tank was identified through trial-and-error methodology to ensure suitable performance across all seismic records. Key findings include the identification of significant mode participation variations, the prevalence of the elephant-foot buckling phenomenon in storage tanks with lower H/R ratios, and the inadequacy of linear spectral analysis for real earthquake scenarios. Due to the importance of storage tanks in the oil and gas industry, the study recommends incorporating time history analysis into the design process for enhanced structural integrity and performance under seismic loads.