Objective Mounded storage tanks are widely used in the petrochemical industry because the surrounding soil mound provides excellent external protection and the tanks offer high land-use efficiency. However, under complex, long-term loading, these tanks are susceptible to crack initiation in weld zones, which threatens their structural integrity. Therefore, investigating the mechanisms governing stress intensity for typical weld defects and clarifying how key factors influence these mechanisms is of significant engineering value for ensuring the life-cycle safety of these storage tanks.

Methods Based on a 3 500-m³ horizontal mounded storage tank, a 3D finite element simulation model was constructed that incorporates multiple coupled loads, including mound pressure, internal pressure, static pressure of the contained medium, and sand-bed support. For the foundation boundary conditions, an elastic foundation model and a soil-spring model were compared to evaluate their relative advantages and limitations. The Extended Finite Element Method (XFEM) was employed to preset cracks in complex geometric models, thereby simplifying mesh generation. The stress intensity factor (SIF) at the crack tip was extracted, and the failure assessment diagram (FAD) approach was applied. Conservatively accounting for property degradation in the heat-affected zone (HAZ), the stress intensity was compared between girth weld cracks in the cylindrical shell and fillet weld cracks at stiffener-to-shell connections. Finally, the effects of internal pressure fluctuations and material properties on the structural safety margin were systematically evaluated.

Results The two defect types exhibit markedly different failure driving forces. Girth weld cracks in cylindrical shell sections are dominated by membrane stress, resulting in substantially higher stress intensity factors, fracture ratios, and load ratios than those observed for fillet weld cracks at the connections between stiffening rings and the shell; consequently, circumferential cracks pose a greater risk of instability under static loads. Increasing internal pressure both raises the driving force for crack propagation and reduces propagation resistance, so the crack load ratio increases with operating pressure. Using higher-performance materials improves tank safety: substituting Q345R with Q690R reduces the load ratio for a given crack size. Girth weld cracks are principally membrane-stress driven and thus more prone to fracture under static loads, whereas fillet weld cracks are governed by local stress concentrations and are therefore more susceptible to fatigue damage. Accordingly, inspections of in-service tanks should prioritize detection of depth-direction cracks in girth welds of cylindrical shells, overpressure must be avoided in practice; high-strength materials should be used in tank design where process requirements permit.

Conclusion The failure mechanisms and parameter variations identified in this study provide a theoretical basis and practical guidance for targeted inspection, material optimization, and integrity assessment of mounded storage tanks.