Study On Vortex-Induced Vibration Response Of Cylinders With Low Mass Ratio Considering The In-line Coupled

The vortex-induced vibration of marine risers caused by ocean currents can cause fatigue failure.The existed research results have shown that marine riser belongs to a class of low mass ratio structure.When the research is conducted on the riser vortex-induced vibration,vibration of in-line must be considered.Due to the complexity of the marine environment and increase of the length-aspect ratio of the riser,the vortex-induced vibration of riser is accompanied by traveling wave effect,and the traditional frequency model cannot predict the fatigue accurately.Therefore,itis great significance to accurately predict the response and fatigue damage of marine risers due to the consideration of the low-mass ratio vortex-induced vibration response under the coupling of in-line vibration.In this paper,the parameters of rigid cylinder under low mass ratio and the motion characteristics of flexible cylinder with large length-aspect ratio are simulated based on the coupled two DOFs cylindrical vortex-induced vibration coupled model.The main research work and conclusions are as follows:1.In this paper,the fluid force is obtained by coupled the In-line vibration and analyzing the wake oscillator based on the discrete-point vortex method,further,coupled model is extended to two degrees of freedom.The cylinder coupled vortex-induced vibration model considered In-line vibration is obtained.2.The coupled model is solved by the perturbation method,and the variation of dimensionless amplitude and phase in the cross-flow and in-line are obtained with the change of the reduction velocity.“Lock-in” appear in the range of 4-8 in reduced velocity.In this range,the phase of vibration in cross-flow and in-line are increasing sharply,and the phase angle of the vibration in the lock-in range decreases and then increases,but it is still less than 180°.3.The parameters of vortex-induced vibration responses of rigid cylinders are studied and discussed,such as reduced velocity,mass ratio and structural damping ratio.The increase of the mass ratio leads to the decrease of the dimensionless amplitude of the rigid cylinder.The increase of the damping ratio leads to the decrease of the dimensionless amplitude.The peak amplitude at reduced velocity is 6.4,and the main factor to change the trajectory shape is the reduced velocity.The influence of flow velocity,mass ratio and damping ratio on the vortex-induced vibration is analyzed.It is found that the in-line amplitude dominates at low damping ratio and the cross-flow amplitude dominates in lock-in range,and the cross-flow amplitude dominates under high damping ratio.4.A three-dimensional model for predicting the long-flexible riser proposed,and the three flow patterns,such as uniform flow,shear flow and step flow,are analyzed.It is found that the in-line mode number of the riser is 2 times of the cross-flow,similarly,the in-line frequency is 2 times of the cross-flow.The main frequency of the uniform flow is the highest,and shear flow is the lowest.The cross-flow vibration frequency of riser is constant along the axial direction,and the in-line vibration frequency will appear 2 times and 3 times high order of frequency.The vibration frequency of the uniform flow is stable with time,while the frequency will fluctuate with time in the case of ladder flow and shear flow.5.The responses of the four flow patterns of uniform flow,shear flow,ladder flow and semi-ladder flow at different velocity are calculated.The traveling wave,standing wave and power-in region are analyzed by distinguishing the motion trajectories of CC mode and C mode.The traveling wave effect mainly occurs in the in-line direction,and there are obvious traveling wave effects in the four kinds of flow patterns.At high flow velocity,the power-in region exists in the region of high flow velocity.At the low velocity,the power-in region exists in the region of the trajectory with CC mode.The traveling wave effect is transmitted from the power-in region to the power-out region.At high flow velocity,the standing wave is dominant near the boundary,the traveling wave effect is transmitted from the boundary to other regions.When the power-in region exists in the middle part of riser,the traveling wave transmit to the boundary.