Active Control of Vertical Risers Undergoing Vortex-Induced Vibrations
Résumé
This paper presents a first control strategy to reduce the vortex induced vibrations (VIV). First, the system equations and an associated modal analysis are presented. This modal analysis shows that, for low damped resonant frequencies, there is a phase shift of ±90° between a periodic external force at the riser's top, and the generated vibration along the structure. This phase shift is used in the design of the control law to reduce the VIV along the structure. For some operating conditions, simulations show that the control system attenuates the VIV and reduces the vortex shedding synchronism along the structure. Simulations are presented with two different kinds of sea current profiles. The advantages of this strategy are the small external force required to reduce VIV, and the fact that no structural change is required along the structure submerged part. But a displacement sensor near the structure bottom is needed.