Nonlinear dynamic response of touchdown zone for steel catenary riser under multiple internal solitary waves

Frequent and intense internal solitary waves (ISWs) and ISW packets pose serious threats to the in-service safety of deep-water risers in the South China Sea. In this study, a deep-water steel catenary riser (SCR) under multiple ISWs was investigated along with the dynamic characteristics of the touchdown zone (TDZ), including displacements, pipe-soil interactions, tensions, and bending moments. A nonlinear numerical model was developed with a novel vector form intrinsic finite element method and improved pipe-soil interaction model that considers soil stiffness degradation and trench formation. The ISW flow velocity field was solved using the mixed Korteweg-de Vries (mKdV) equation and the hydrodynamic forces were determined using Morison's equation. The effects of the ISW amplitudes, riser configurations, seabed stiffnesses, and pipe-soil models on the structural dynamic responses were investigated. The results showed that the ISW packets rapidly increased the penetration depth of the TDZ. Trench formation changes the bending moment response of the TDZ in subsequent ISWs, increasing the response of the front section and decreasing the response of the latter section. However, its effect on the riser tension is limited. The steel lazy wave riser (SLWR) has significantly larger displacement and bending moment responses in the TDZ than SCR because of the different riser tension and configuration. Different pipesoil interaction models exert a significant influence on the penetration depth and the variation of the bending moment in the TDZ.