Study on axial compressive buckling of unbonded flexible risers with local corrosion defects in tensile armor layer

Unbonded flexible risers (UFRs) are critical for deepwater oil and gas transportation. Corrosion in tensile armor layers weakens the circumferential confinement, potentially triggering axial compressive buckling. This study developed a 3D finite element (FE) model with geometric nonlinearity and interlayer contact to examine three corrosion cases: corrosion of the inner tensile armor layer, the outer tensile armor layer, and simultaneous corrosion in both layers. By comparing different corrosion defect lengths in three cases, results show that inner layer corrosion reduces the peak load capacity by 20.2% to 53.1% compared to an intact riser. Outer layer corrosion decreases the peak capacity from 176 to 118 kN and induces a double birdcage buckling. Corrosion in both layers causes a further reduction in capacity. A synergistic degradation coefficient is proposed to quantify coupled system instability, supporting a shift from local damage assessment to system-level evaluation.