LQR control on multimode vortex-induced vibration of flexible riser undergoing shear flow

Under the actions of ocean currents and/or waves, deep-sea flexible risers are often subject to vortex-induced vibration (VIV). The VIV can lead severe fatigue and structural safety issues caused by oscillatory periodic stress large-amplitude displacement. As have natural modes with lower frequency higher density, a multimode is likely occur in under action currents, which considered as shear flow. To decrease response level riser actively, control approach that uses bending moment at top end via an LQR optimal controller developed this study. dynamic equations including flow established both time state-space domains. controllers then designed optimize objective function, indicates minimum cost riser's input energy based on Riccati equation closed-loop system assumption lift coefficient distribution constant. Finally, responses original systems different velocities examined through numerical simulations. results demonstrate active approaches effectively reduce displacement/angle approximately 71%–89% compared system. Further, for control, presented mode-weighted more effective than mode-averaged control; displacement 1.13 times control. Owing increase velocity higher-order excited, decreases slightly while gradually increases. For system, becomes smaller complicated, efficiency certain velocity.