A method for bending response assessment of non-metallic unbonded flexible pipes in deep-sea mining

Non-metallic unbonded flexible pipes, known for their significant advantages such as lightweight design, high wear resistance, and excellent load-bearing capacity, show broad application prospects in deep-sea hydraulic pipeline lifting mining systems. A finite element model with unidirectional fiber layup is established to investigate the effects of external pressure and interlayer friction coefficient on bending stiffness under monotonic and cyclic bending. Nodal stresses at the rectangular cross-section are analyzed. Results show that external pressure alters interlayer contact pressure, increasing the curvature required for strip slippage; after slippage, strain energy growth slows. Cyclic bending induces significant hysteresis from interlayer slip. Higher external pressure and friction coefficient enlarge the moment-curvature loop, increase the friction force for slip, and enhance energy dissipation. Additionally, external pressure modifies the strain state of helical strips in the tensile reinforcement layer, leading to hysteresis in nodal stresses, which vary with strip coordinates.