(55ao) Seismic Evaluation of Storage Tanks Under New State Regulations

Storage tanks and transfer piping properly designed to current codes and standards have an excellent track record when subjected to strong seismic events. However, legacy storage tanks and their piping systems designed to older standards and codes may not have considered any level of earthquake ground motion. Also, after several decades of deterioration and corrosion, the remaining material thickness is smaller than the as-built tanks. Thus, these tanks may not satisfy the current design standards and codes. Some states passed new regulations which require proof that these legacy tanks can meet the seismic load according to current design codes. Therefore, some advanced techniques need to be adopted to analyze the performance of these tanks under the design seismic load.

A methodology is proposed in this study to evaluate the fitness-for-service of storage tanks under design ground motions. The seismic evaluation comprises extensive Level 2, Level 3, piping, and nozzle analyses. A case study based on a real-world project in the State of Washington is conducted to demonstrate the methodology. A scaled ground motion is developed based on historical hazards at the tank location and the tank’s target response spectrum. An FEA analysis of the tank under the ground motion is conducted, incorporating the fluid-structure interaction to investigate the shell buckling, stress and strain, bottom uplift, and shell radial deformation. The model results are used to derive the fitness-for-service of the piping system and the nozzles. Finally, the results of the analyses are used to provide risk-informed hazard mitigation of the studied storage tank.