OTC 11998 Norwegian Deepwater Program: Damping of Vortex-Induced Vibrations

Abstract Vortex-induced vibration (VIV) of a long riser in sheared current is often considered as an energy balance problem: Excitation forces in the power-in region add an equal amount of energy to the system as is dissipated by damping forces outside this region and structural damping. A riser may have different excitation and damping regions depending on the actual oscillation frequency, cross-section properties and local flow velocity. A damping model must hence be able to handle higher and lower flow velocities than the excitation velocity range. In this paper the fluid damping models proposed by Venugopal [1] are compared with the experiments conducted by Gopalkrishnan [2] and Vikestad Introduction VIV is dependent on many factors such as the Reynolds number, flow velocity and turbulence of the incident current, surface roughness, cross-section shape, inclination, motion of the structure, etc. No solutions or models exist which can account for all these factors. The solution has therefore been to simplify the interaction problem as much as possible hopefully without loosing the information important for the prediction of the resulting VIV and fatigue life of the riser. One such simplification is to assume that each cross-section along the riser is oscillating harmonically in the cross-flow direction only and that the incident flow is constant in time. Then the fluid forces can be split into two parts; the inertia part in phase with the acceleration and the excitation/damping part in phase with the velocity of the cross-section. If the force in phase with velocity has the same sign as the velocity it is excitation, if it has opposite sign it is damping