Three dimensional large deformation VIV of a steel catenary riser

Vortex-induced vibration (VIV) is a critical phenomenon in fluid–structure interaction and a major challenge for slender offshore structures. Under ocean currents, steel catenary risers (SCRs) exhibit three-dimensional nonlinear responses, which may induce stress accumulation and lead to significant fatigue damage. This work presents a three-dimensional nonlinear dynamic finite element model for accurately predicting the in-line (IL) and cross-flow (CF) VIV responses of SCRs under large deformation conditions. The model employs a co-rotational coordinate framework with quaternions to capture large displacements and rotations, thereby enhancing numerical stability, while simultaneously accounting for axial, bending, and torsional deformations. Hydrodynamic loads are modeled by using a double wake oscillator, with drag and lift coefficients given by two distributed Van der Pol oscillators. The coupled structural dynamics and wake oscillator equations are solved iteratively using the Newmark- β and the Newton–Raphson algorithms. Results show that spanwise geometric nonlinearity triggers significant modal competition, while lock in regions migrate systematically with current velocity and display distinct directional differences. The near-seabed segment exhibits high curvature standing-wave patterns that substantially amplify bending moments and strains, and torsional effects should also be accounted for when evaluating fatigue initiation and accumulation in this critical region. These findings deepen the understanding of nonlinear VIV of SCRs in complex ocean currents and provide essential reference for the design and fatigue life assessment of deepwater risers.