Improving the buckling capacity of the FRP-reinforced steel pipes under combined load of external pressure and torsion

This study presents a comprehensive numerical investigation into the linear and nonlinear buckling behavior of unreinforced and fiber-reinforced polymer (FRP)-strengthened thin-walled steel pipes (TWSPs) subjected to pure and combined external pressure and torsional loading. The effects of FRP reinforcement with different layering configurations ([0/90/+45/−45] stacking sequence, 1–4 layers) were studied for steel thicknesses of 0.4, 0.6, and 1.0 mm. Load interaction curves of critical pressure (Pcr) and torsion (Tcr) were developed, revealing that FRP confinement significantly enhances external pressure capacity (290–302% increase across all thicknesses) through increased circumferential stiffness and delayed ovalization. In contrast, torsional buckling improvements are modest (<3%) and thickness-dependent, as torsional resistance is governed primarily by steel shell stiffness. RIKS analysis revealed significant post-buckling reserve in thin pipes under torsion (up to 432% higher than eigenvalue predictions), highlighting the limitations of linear theory and the necessity of knock-down factors for plastic torsional capacity assessment. Under combined loading, FRP strengthening is most effective for thin TWSPs (t ≤ 0.6 mm), particularly when external pressure governs the design. The validated the finite element FE framework provides mechanics-based interaction curves that complement conservative code provisions, offering practical guidance for the design and rehabilitation of offshore pipelines subjected to multi-axial loading.