Research On Vortex-induced Vibration Bifurcation And Multi-mode Interaction Of Riser

With the rapid growth of global population and economy,the demand for oil and gas resources in the world is increasing day by day.The exploitation of marine resources has gradually developed from shallow sea to deep sea.Marine riser is the key equipment to connect the offshore platform and seabed mineral deposits,and it is also one of the weakest components in the offshore structure.There are high-temperature and high-pressure oil and gas passing through the marine riser,and the riser is subject to complex environment loads such as waves and currents.Under these long-term and periodic loads,the riser will vibrate and cause fatigue damage to the structure.Therefore,the research on the dynamic behavior of the riser has important theoretical value and engineering significance.With the increase of water depth,the vortex-induced vibration(VIV)of riser becomes more complex,which shows many new problems and phenomena.With the increase of slenderness ratio,the multi-modal vibration characteristics of riser become more obvious.In this paper,the top tension riser(TTR)with large aspect ratio in deep sea is studied.The riser is simplified as Euler Bernoulli beam model,the van der Pol wake oscillator is used to describe the effect of fluid.The nonlinear dynamic model of riser VIV is established.The harmonic balance method,Poincaré mapping method and Lyapunov exponent method are used to study the system dynamic bifurcation caused by the change of velocity.In addition,the multi-modal interaction mechanism of riser VIV is analyzed,and the following progress is made.(1)The dynamic equations of the system are discretized by the Galerkin method in two orders.Based on the harmonic balance method,the equations are solved semi analytically.Two solution branches are obtained and the stability solution is analyzed.The bifurcation phenomenon of the system with velocity increase is studied.The generalized coordinate amplitude ratio of the structure is calculated by the eigenvector of the linear system,and compared with the solution of nonlinear system.Presents a nonlinear modal prediction method of riser VIV.(2)In this paper,Poincaré mapping method is used to obtain the bifurcation diagram of the generalized coordinate amplitude of the riser,and the result is consistent with the solution of the harmonic balance method.The main frequency components obtained by numerical method and harmonic balance method are compared.The results show,the solution obtained by harmonic balance method can well predict the main frequency components in the quasi-periodic region.(3)It can be found from Poincaré map bifurcation diagram and Lyapunov exponent diagram that with the increase of velocity,the response of riser changes many times between periodic motion and quasi-periodic motion,but there is no chaotic motion.The transformation of VIV response from periodic to quasi-periodic belongs to the Hopf bifurcation of periodic solution rather than the frequency unlocking.The resonance mechanism and universality need further study.(4)Based on the fifth order Galerkin discrete results,the multi-modal interaction mechanism of riser VIV is analyzed.The influence of velocity on the modal weight of riser is discussed,and the mode switching law of riser VIV is analyzed.Compared with the single-mode discrete and the multi-mode discrete,it is found,in the frequency locked region,the single-mode discrete results can estimation the multi-mode discrete results approximately.