Analysis of mechanical behavior of deep-sea unbonded flexible risers under localized corrosion conditions

Unbonded flexible risers (UFRs) used in the intermediate transportation in seabed mining are prone to corrosion in seawater during long-term operation. In this study, 8-layer UFRs were equivalent to 5-layer UFRs, and the constitutive equations for the 5-layer UFRs were derived using the principle of stiffness equivalence. The length and width of the outer tensile armour layer and the effect of composite corrosion on the stiffness of the UFRs at different corrosion depths were investigated separately. Additionally, the displacement of the helical steel strip when buckling occurred and when it reached the yield limit under axial pressure owing to composite corrosion are discussed herein. The results indicated that when the corrosion depth was 1 mm, the stiffness reductions caused by the corrosion length were 3.92 %, 6.26 %, and 4.2 % for tensile, clockwise torsion, and counterclockwise torsion, respectively. The reductions caused by the corrosion width were 4.04 %, 5.41 %, and 3.29 %, with composite corrosion stiffness reductions of 48.21 %, 5.99 %, and 33.73 %, respectively. The buckling analysis of the UFRs under axial compression demonstrated that buckling failure occurred earlier in the corroded regions. When the corrosion depth reached 1 mm, the spiral steel strip exhibited signs of folding.