ANALYSIS AND PREDICTION OF COUPLED IN-LINE AND CROSS-FLOW VORTEX-INDUCED VIBRATION RESPONSE OF DEEP SEA REAL-SCALE TOP-TENSION RISER

It is widely accepted that the vortex-induced vibration (VIV) is a major concern in the design of deep sea top-tension riser, since it can result in large amplitude response in both in-line (IL) and cross-flow (CF) directions, and further lead to accumulation of fatigue damage within short time period. The purpose of this paper is to predict the coupled in-line and cross-flow VIV response of deep sea real-scale top-tension riser subjected to linear sheared flow in the non-locking region. Considering the effect of the riser in-line motion on vortex shedding frequency and hydrodynamic force, a modified three-dimensional time domain prediction model based on van der Pol wake oscillator has been proposed in the present study. The obtained nonlinear partial differential equations of the coupled dynamic model are constructed and simplified to a reduced-order model through the Galerkin-type discretization based on the Hermite cubic interpolation functions. The Newmark-fi method is adopted to solve the equations. The numerical simulation results based on the prediction model are compared with the experimental results. The comparisons show that the prediction model is reasonable in modeling some main features of the riser VIV response, such as vibration frequency, mode number and displacement amplitude. Finally, the in-line and cross-flow VIV response of a real scale top-tension riser using in the South China Sea is analyzed. The results indicate that the VIV response of the riser performs as a form of high order multi-modal vibration in the non-locking region, and the riser in-line and cross-flow VIV response mainly consists of travelling waves. The vibrations of the whole riser are complicated, and the in-line vibration response of the riser is much more irregular than the cross-flow vibration response. The effect of the sea current velocity and top-tension force on the riser VIV response has also been discussed in the present study. The results show that both the sea current velocity and the top-tension force all have great influence on the riser VIV response.