Research On The Prediction Model And Nonlinear Dynamic Response For Flow-induced Vibration Of Marine Riser

Flow-induced vibration(FIV)as one of the most important factors results in fatigue failure of marine riser.With the development of offshore oil from shallow sea to deep water,the aspect ratio of the marine riser is larger and larger,which can lead to the geometric large deformation under the action of internal and cross-flow fluids.In order to understand the FIV law of marine riser,this paper first studies the nonlinear response and prediction model of two-dimensional vortex-induced vibration(VIV).Then,the three-dimensional VIV models considering the geometrical nonlinearity of riser,including VIV model without internal fluid,VIV model under internal fluid and gas-liquid two-phase transporting riser model,are established based on the extended Hamilton's theory.The response characteristics of the model under different external excitations are also investigated.In contrast to previous studies,the present study has main features as follows:(1)The nonlinear responses of the elastic support cylindrical models,including the single-degree-of-freedom cross-flow vibration model and the two-degree-of-freedom vibration model,under the different initial excitation are studied based on two-way fluid-structure interaction(FSI).The results depict that the response of the riser under different initial excitations may be periodic,quasi-periodic and chaos.Based on the Van der Pol oscillator model,a two-dimensional VIV prediction model considering cylindrical geometric nonlinearity and hydrodynamic nonlinearity is also established.The accuracy of the model is verified by comparison with the experimental results.(2)Based on the extended Hamilton's theory,the nonlinear Euler-Bernoulli beam model is utilized to establish the three-dimensional VIV model of the riser considering the geometrical nonlinearity and the Kelvin-Voigt viscoelasticity.The finite element method(FEM)is adopted to directly analyze the strongly nonlinear FSI coupled equations.The effects of the empirical coefficient,the damping coefficient of the riser and the dimension of the model on the prediction accuracy are firstly investigated.Some cross-flow parameters,such as,the uniform cross-flow velocity,the gradient cross-flow velocity and the fluctuation cross-flow velocity,are then analyzed.Finally,the influence of the viscoelastic coefficient of the material on the natural frequency and time domain response of the riser is investigated.The results show that the excitations of cross-flow velocity will result in complex nonlinear responses for the riser,and the viscoelasticity coefficient has a certain suppression effect on the response of the riser.(3)The three-dimensional VIV nonlinear dynamic response models of the riser excited by top loads are established.The effects of top excitations,including constant tension,pulsating tension and cross flow direction to top excitation,on natural frequency and time domain response of riser are studied respectively.The response characteristics of different outer flow velocity are analyzed and compared,and the space-time responses of the displacements and stresses,cross-flow response modes,plane displacement trajectories and phase trajectories of risers are then analyzed in detail.It is concluded that different top excitations may enhance or suppress the response of the riser,and the response of the riser can be effectively reduced by applying appropriate external excitation.(4)Considering the geometric nonlinearity and the Kelvin-Voigt viscoelasticity,the three-dimensional VIV model of the riser under internal flow by adopting the Euler-Bernoulli beam prediction model is established based on the extended Hamilton principle.The influence of the internal fluid velocity on natural frequency and the space-time response characteristics of riser is investigated firstly.Then,the influence of the frequency and amplitude of the pulsation internal flow velocity on the displacement and stress is analyzed.The results show that the internal flow velocity will change the natural frequency of the riser,and the cross-flow response mode will be varied at the critical flow velocity under different cross-flow velocities.(5)The three-dimensional VIV model of riser considering gas-liquid two-phase flow for internal fluid is established based on the two-phase flow theoretical model of vertical riser.The influence of liquid volume flow rates and vapor volume fractions on natural frequency of the riser is firstly studied.The effect of gas volume fraction on displacement and stress of riser is also discussed.The results demonstrated that the natural frequency of the riser will be influenced by liquid volume flow rates and the vapor volume fraction,and the high order frequency will be seriously impacted.For the different cross-flow velocities,there is a critical gas volume fraction to change the cross-flow response mode.