VORTEX-INDUCED VIBRATION OF DEEPWATER FLEXIBLE RISER EXPERENCING INTERNAL-WAVE-INDUCED SHEAR FLOW

The internal-wave-induced horizontal flow velocity distributes un-uniformly along the riser axis and is usually regarded as shear flow. The natural frequency of deepwater riser is low and crowded due to the high aspect ratio. Therefore, the prediction of the vortex-induce vibration (VIV) of deepwater riser suffered from multi-modes and higher-mode becomes more difficult. An approach based on modal energy is developed and subsequently is applied to the modified wake-oscillator model to predict VIV. At last, illustrative examples are given in which the VIV of flexible riser in the internal-wave-induced fluid field are calculated and compared with VIV in the sea current. The effects of internal-wave-induced flow velocity on VIV are explored. Our results show that both displacement and stress responses become larger with the increase of induced flow velocity; especially higher stress response companied with higher frequency vibration may significantly affect the fatigue life of structures.