Elastic shakedown limit analysis of sleeve-reinforced 90° back-to-back pipe bends with local wall thinning under cyclic in-plane bending moment and steady internal pressures

This study presents the elastic shakedown limit of a sleeve-reinforced 90° back-to-back pipe bends structure with local wall thinning, using the finite element method. In this study, the sleeve-reinforcement is inspired by double-walled piping systems that have been popularly applied to pipelines carrying hazardous fluids. A three-dimensional finite element model is created to investigate the effects of the sleeve reinforcement. The model incorporates the double-walled system over the pipe bends area with localized wall thinning at the extrados, intrados, and crown. From cyclic plastic analyses using a direct numerical analysis method, the elastic shakedown limit is established for the pipe bends structure both with and without the sleeve reinforcement. Comprehensive numerical parametric studies are conducted on the elastic shakedown limit, for different wall thinning rates of 20 %, 40 %, and 60 % at the three designated wall thinning locations. The study results demonstrate that the sleeve reinforcement can enhance the structural integrity of the 90° back-to-back pipe bends structure. The safety margins under cyclic bending moments and internal pressure are increased by up to 133 % and 18 %, respectively. These findings highlight the effectiveness of sleeve reinforcement as a practical solution to improve the reliability of pipelines with local wall thinning under combined loading conditions.