Bistable Dynamic Analysis Of Vortex-induced Vibration Of Riser

With the development of the economy and the progress of the society,the demand for oil,gas and other resources is becoming more and more important in all countries.At the same time,the oil production on land had entered a steady and slow growth period.Therefore,the development of oil and gas of the ocean,especially the deep sea has become the inevitable trend of all countries.The development of production platform and equipment from shallow water to deep water is an inevitable trend.As a necessary equipment for drilling,infusion and exploration,risers are not only always subjected to ambient loads of wind,waves and waves but also inside the risers during service.The long-term,periodic loads and the coupling effect between them will lead to the occurrence of reciprocating motion of the riser.These will greatly reduce the fatigue life of the structure,shorter service time.It may even make the structure vibrate greatly,and the strength failure will occur directly.It may even make the structure vibrate greatly,and the strength failure will occur directly.Therefore,investigation of the dynamic characteristics of the riser has important engineering and theoretical value.The vortex-induced vibration of the riser has always been an important factor in the fatigue failure of structure.In this paper,the top-tensioned riser is taken as the research object.Based on the wake oscillator model,the multi steady state behavior of riser coupling system is studied,and the influence of stochastic factors on the system is also considered.The main contents are as follows:(1)In order to study the vortex-induced vibration of deep-sea riser,the square nonlinearity of fluid damping and the inertial coupling of the structure are considered in the fluid-solid nonlinear coupling dynamic model.Based on the first order Galerkin modal discrete equation,the influence of flow velocity on the stability of the system is analyzed by eigenvalue calculation.The influence of flow velocity on the response nonlinear feature(frequency and amplitude)is analyzed by Poincaré mapping method.The results show: coupled flutter region can be used to estimate the range of vortex induced vibration frequency-locked region;On both sides of the frequency-locked region,there are respectively two types of bistable phenomena: the coexistence of quasi-periodic motion and locking movement on the left side,the coexistence of small amplitude periodic motion and locking movement on the right side.There are structural modal and vortex modes simultaneously under small flow velocity,and the response is expressed as structural modal.At large flow velocity,the response only contains vortex excited modal.(2)With the increase of the length,in considering the vortex induced vibration of riser mode number will increase,which could lead to higher order vortex resonance phenomenon.According to the consideration of the maximum velocity of the riser under the vibration amplitude envelope convergence results,determine the Galerkin discrete system contains until 7th order model for simulation model.Taking the 1000 meter engineering pipeline as an example,the variation diagram of the response amplitude of the riser system with the flow velocity is obtained,the nonlinear characteristics of typical behavior are analyzed from time history,phase diagram and frequency diagram.It is found that due to the influence of nonlinear factors,the frequency of vortex induced resonant response is significantly different from the linear natural frequency of the system.Except for the first order vortex induced resonance region,there are second order and third order vortex induced resonance regions.The first order,the second order and the third order resonance interval overlap,forming the bistable region,the vibration amplitude of the riser in the bistable region is quite different.In engineering application,attention should be paid to in the surrounding environment and sudden large amplitude vibration caused by sudden interference and continuous random interference in the surrounding environment,so as to prevent the occurrence of disaster accidents.(3)In marine engineering,random excitation is an important factor affecting the stability of the system and induced fatigue damage.In this paper,the influence of the random factors on the vibration behavior of the riser is analyzed.Based on the response bifurcation diagram of deterministic system,according to the characteristics of its response,the velocity range is divided into 6 different regions,and the influence of noise intensity in each area on the system response is analyzed.The results show that the effect of random excitation on the response of bistable region and nearby velocity in deterministic system is obvious.