A Novel Elastic Analytical Model for API 650 Liquid Storage Tanks: Stress and Deformation Analysis under Hydrostatic Loads

Abstract This research presents a novel analytical model for above ground liquid storage tanks. Using elasticity theory, the tank is modeled as an axisymmetric structure with a multi-course cylindrical wall welded to a flat bottom floor under hydrostatic pressure loads. While all courses satisfy compatibility and equilibrium conditions when the tank is filled with liquid, a unique flexible constraint is introduced at the bottom connection with the floor at the corner joint using equivalent torsional and radial springs. The spring parameters are obtained by solving the moving boundary equations for the floor critical zone and its vicinity region on foundation under hydrostatic pressure. The constraint equations are transformed into matrix form for computational implementation. The analytical model provides a comprehensive and rigorous solution for the entire tank structure under various liquid fill heights. Deformation, stresses, and strains are calculated through iterations. Results from the analytical model are validated by finite element analysis on different tanks and liquid load conditions. The innovative method enables the relation of minimum required component wall thicknesses to hydrostatic load. Especially, the method provides an analytical procedure for the lower course and bottom floor critical zone as part of the solution for the first time, which allows the two components to be reclassified from Type C to Type B components in API 579. The method is intended to be used as a procedure for design analysis or fitness-for-service assessment of liquid storage tanks in relevant industrial codes and standards.