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Although the lateral oscillation of steel catenary risers (SCRs) contributes to trench production, traditional riser-seabed models cannot simulate the process. This study proposes a modified lateral riser-seabed interaction model and integrates it into the SCR model based on the vector form intrinsic finite element (VFIFE) method to simulate the trench formation process. The effects of the loading history, extra soil berm resistance, and cyclic vertical riser-soil interactions were included in the soil model. A 3D trench could be developed under the cyclic out-of-plane movement of the SCR. The trench formation process, seabed resistance distribution, axial tension, bending moment, and stress range distribution of the SCR were discussed in detail to illustrate the effects of the soil model. Different loading amplitudes and nonlinear soil parameters were chosen to determine their influence on the trench shape and distribution of the horizontal seabed resistance. The developed trenches were then inserted into the fresh SCR model before executing the dynamic analysis, in which only the cyclic heave motion was applied to the hang-off point. The simulation results showed that the trenches produced by cyclic sway motions beneficially influenced the fatigue life of the SCR. However, the effect would differ with the profiles. • A modified lateral riser-seabed interaction model was proposed and integrated into the VFIFE model. • A 3D trench can be developed under the cyclic out-of-plane movement of the SCR. • The trench formation process and the mechanical behavior of SCR were analyzed. • Sway motion amplitude and soil parameters can affect the trench shape. • Sway-motion-induced trenches beneficially influenced the fatigue performance of the SCR. [ABSTRACT FROM AUTHOR]

As a frequently occurring and widely distributed environmental load, an internal solitary wave (ISW) always appears with a large amplitude and may result in the instability of deepwater structures and offshore accidents. In this study, a self-developed program was established to describe the dynamic behavior of a coupled platform–mooring–riser system. The response of the platform is based on the rigid body dynamics theory, that of the risers/mooring lines is obtained by the lump mass method, and the coupled effect between them is achieved by a semi-coupled analysis. Two types of riser configurations, steel catenary risers (SCRs) and lazy-wave steel catenary risers (LWRs), in the coupled system were compared under only ISWs and combined excitation of ISWs and random waves. The numerical results were proven to be reliable through validation against previous literature. The results show that different riser configurations have little influence on the platform response or mooring top tension. Both riser configurations exhibit a second-order deformation mode along the riser length and a half-wave shape with time. Additionally, a mutation in the horizontal tension, shear force, and bending moment occurs near the ISW interface. Compared with SCRs, LWRs always produce larger motion characteristics and responses with time differences owing to their complex configuration. • A coupled platform-mooring-riser numerical model has been described through a self-developed program. • The responses under only ISW and combined excitation of ISW and RW are calculated. • Comparisons have been made between two kinds of riser configurations in the coupled system: SCR and LWR. • The mechanism of tension, shear force and bending moment in both riser configurations have been discussed in detail. [ABSTRACT FROM AUTHOR]

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 pipe-soil interaction models exert a significant influence on the penetration depth and the variation of the bending moment in the TDZ. • Dynamic analysis of SCRs under multiple ISWs considering soil stiffness degradation and trench formation. • A nonlinear numerical model based on Vector form intrinsic finite elements and an improved pipe-soil interaction model. • Quantitative analysis of displacements, pipe-soil interactions, tensions, and bending moments of the touchdown zone. • Impact analysis of ISW amplitudes, seabed stiffnesses, pipe-soil models, and riser configurations. [ABSTRACT FROM AUTHOR]