Three-dimensional finite element analysis of vortex-induced vibrations in deepwater steel catenary risers with large deformation

Resumo

Vortex-induced vibration (VIV) is an important phenomenon in fluid-structure interaction and one of the main challenges faced by slender offshore structures. Under the influence of ocean currents, long steel catenary risers (SCR) may exhibit nonlinear VIV dynamic response with various modes and frequencies, which occur simultaneously in three-dimensional space. These nonlinear dynamic response can lead to critical bending and longitudinal stresses, ultimately resulting in significant fatigue damage to offshore risers over time. This paper presents a large-deformation three-dimensional dynamic model for the SCR, which accounts for VIV and actual ocean loads, offering significant advantages over traditional small deformation beam or catenary theories. The model thoroughly considered the bending stiffness of the riser and the nonlinear effects of large deflections on the loads. It uses two Van der Pol wake oscillator equations to model the fluid dynamic impact of vortex shedding in both crossflow (CF) and in-line (IL) directions. Moreover, the model addresses the pipeline-soil interaction at the touchdown zone (TDZ). Employing the finite element method (FEM), each node was endowed with six degrees of freedom, enabling descriptions of vibrations in lateral, axial, and torsional directions for the riser. The structural dynamics and wake oscillator equations were solved using the Newmark-β and Runge-Kutta methods, respectively. The numerical model has been validated with published results and further simulations have been conducted to determine the dynamic behavior of the SCR. Subsequent comprehensive parameter analysis examined the effects of motion amplitude, current flow loads and the touchdown zone on the three-dimensional VIV responses of the SCR, which are crucial for understanding the dynamic behavior of the riser.