Theoretical Investigation On Two-dimensional Nonlinear Vortex-induced Vibration Of Ocean Riser

The purpose of this paper is to deeply study the nonlinear dynamic characteristics of riser vortex-induced vibration in a complex ocean environment.According to the latest research trends on the nonlinear dynamic of riser vortex-induced vibration at home and abroad.In-depth research has been conducted on the nonlinear vibration characteristics of the flow pipe,the vortex-induced vibration characteristics of the riser under the action of shear flow,the vortexinduced vibration characteristics of the riser under the action of variable tension and vortexinduced vibration characteristics of riser under platform sway.The main research work of the thesis is as follows:The marine riser was simplified to Euler–Bernoulli beam,considering the combined influence of factors such as fluid velocity in the pipe,pipe weight,top tension,incoming flow velocity,structural damping,geometric nonlinearity,and boundary conditions.The Hamilton principle is used to establish the two-dimensional nonlinear motion equation of the top tension riser.The Van Der Pol wake oscillator model is used to imitation the wake.The fourth-order Runge-Kutta method is used to calculate the nonlinear equations.By comparing with existing experiments and Computational Fluid Dynamics calculation results,the correctness of the method in this paper is verified.The nonlinear dynamics model of the flow pipe is established,the Galerkin method is used to discretize the nonlinear motion equation of the flow pipe,and the fourth-order Runge-Kutta method is used to solve it.The effects of top tension,damping,and geometric defect amplitude on the nonlinear dynamic characteristics of fluid pipelines are studied.The result of research shows that when the fluid velocity in the pipe is less than the critical velocity,the flow pipe will not buckle instability;when the fluid velocity exceeds the critical velocity,the flow pipe will undergo buckling instability.In addition,damping,top tension,geometric defect amplitude,and geometric defect function have obvious effects on the stability of the flow pipe.By introducing the shear parameters into the riser model,the model is applied to calculate and analyze the vibration characteristics of the riser under different factors such as the fluid velocity in the pipe,the form of the external flow,the velocity of the flow and the top tension.The research results show that the response mode of the riser can be changed by the flow rate at the edge of the modal lock area.The modal locking region of the riser under shear flow becomes narrower,and the critical mid-span flow velocity decreases when a particular modal locking occurs.For a given mid-span flow velocity,the dominant modal order of the VIV response of the riser under shear flow is always higher than that of the uniform flow.The changes of the riser displacement,mode and motion trajectory under different top excitation modes are studied.The motion equation of the riser under the top excitation is established.The results show that the coupling response of the riser changes between adjacent modes with the change of top tension.The effects of alternating current excitation frequency and amplitude on riser vibration response were studied respectively.The changes of displacement,mode,trajectory and phase of the riser are discussed.The research results show that the coupling resonance effect is most obvious when the transverse excitation frequency is close to the natural frequency of the riser,and has obvious nonlinear phenomenon.By controlling the transverse excitation frequency at the top of the riser,the VIV displacement of the riser can be effectively reduced,thereby increasing the service life of the riser.