Investigation on Standing and Traveling Wave Response Patterns in Long Flexible Risers

Abstract

Due to the complexity involved in the vortex-induced vibration (VIV) of long offshore risers, the fundamental understanding of the coupled kinematics and dynamics of the standing and traveling waves is not well established. In the present contribution, a systematic numerical study on slender flexible riser immersed in a turbulent flow is performed on a flexible riser pinned at both the ends to investigate the standing and traveling wave responses. This wake-body resonance problem requires a stable coupling of the Navier-Stokes equation with the low mass flexible riser structure subjected to strong inertial effects from the surrounding fluid flow. A partitioned iterative scheme that relies on the nonlinear interface force corrections is employed for the modeling of coupled fluid-riser problem. The study here includes a flexible cylindrical riser considered as a long tensioned beam via linear modal analysis. Full three-dimensional simulations are performed on the flexible riser exposed to two different inflow conditions: uniform and linearly sheared. At first, the response characteristics of the riser model are validated with experimental measurements under pinned-pinned condition for uniform current. A detailed analysis is performed on the response characteristics and vorticity dynamics at various locations along the span of the flexible riser. Our simulations show that for uniform inflow condition, the flexible riser exhibits a standing wave-like phenomenon. On the other hand, for linearly sheared inflow, a traveling wave response is observed for both cross-flow and inline oscillations. These traveling waves travel from the top point to the bottom point.