Interactive behaviour of sandwiched submarine pipeline under tension–torsion combination

In deep-water oil and gas extraction, sandwiched pipelines experience complex stress states during installation and service, such as combined tension–torsion loading, making accurate assessment of their interactive behaviour essential for safety. Using a validated finite–element model, this study investigated the tension–torsion interaction of steel–concrete–steel submarine pipelines, focusing on the confinement effect under external hydrostatic and internal medium pressures. The parametric study was conducted, and a practical calculation method for tensile–torsional interaction was proposed. Key findings include: dual hydraulic pressures can improve the confinement effect for enhancing the torsional capacity; torsional capacity initially rises with an appropriate hollow ratio but declines as the axial tension ratio or hollow ratio increases – for example, increasing n_t from 0.1 to 0.3, 0.5 and 0.7 reduces the torsional capacity by 5.69%, 17.88% and 30.90%, respectively, at 800 m water depth; the correlation curve of axial tension and torsion considering the effect of dual pressures was proposed by adopting the averaged coefficients of 1.0 and 1.5 for axial tension and torsion, respectively.