Numerical modelling of a steel catenary riser section in the touchdown zone under cyclic loading

The fatigue life of steel catenary risers is significantly influenced by cyclic riser-seabed-water interaction in the touchdown zone. In this study, the penetration and extraction of a shallow embedded riser section, subjected to cyclic vertical loading, are simulated using a computational fluid dynamics approach with ANSYS CFX. An empirical strength degradation model is proposed for soil softening due to undrained remoulding and clay-water interaction in the highly sheared interface, termed 'shear wetting.' The combined effects of strain rate and softening on the mobilized shear strength of deepwater offshore clay are implemented in CFX. A sufficiently large number of loading cycles is simulated using this computationally efficient numerical technique to achieve a stable response. A significantly large reduction of vertical resistance of a shallowly embedded riser section due to cyclic loading, as observed in physical model tests, is obtained using the proposed strength degradation model with shear wetting, which cannot be explained simply by undrained remoulding.