Research On VIV Prediction Methods Based On Modified Wake Oscillator Models And Their Applications

In the ocean engineering applications,most slender structures are exposed to vortexinduced vibration(VIV).VIV is influencd not only by the structure parameters,but also by the ocean flow.The parameters of marine slender structures vary greatly because the water depth of offshore oilfields changes from dozens of meters to thousands of meters and the exploitation quantities are different.Moreover,the form of ocean flow varys in different sea areas.Except for the ordinary steady flow,the oscillatory flow also leads to VIV.The above situations pose challenges to the accuracy of VIV prediction.VIV might result in fatigue accumulation damage.The time-varying characteristic of the ocean flow during the whole return period may influence the calculation efficiency and accuracy of the fatigue life.Although VIV results in fatigue damage,it also has the capacity to benefit us by harvesting erengy form ocean flow.When slender structures are attached by energy harvesting devices,several system parameters have oscillating characteristic in time domain.This makes the VIV response quite different from those without energy harvesting devices.Based on the generalization and summarization of previous researches on the above research scope,this paper focuses on not only theoretical research,but also engineering application.The modified wake oscillator models suitable for different sea conditions and working conditions are developed and investigated.And on this basis the application of the proposed models to fatigue and energy harvesting analysis is discussed.The present study has main features as follow:(1)The wake oscillator model is deduced and solved to calculate VIV response of slender structures under steady flow.The "Structure parameter-distinguish number-revision coefficient-reduced velocity" empirical parameters revision system is established by introducing the concept of "distinguish number" and "revision coefficient" to apply the model to various situations and acquire accurate results.The revision coefficients for empirical parameters are set as dependent variables of distinguish numbers and reduced velocities.In addition,the revision coefficients are matched by experimental data.The comparison between numerical and experimental results demonstrates that the established system has a better agreement.Moreover,it should be noted that the empirical parameters system for uniform and sheared flows should by different.(2)The oscillatory flow parameter and oscillatory damping parameter are introduced to express the influence of the change of flow velocity to the VIV calculation model under oscillatory flow.The modified wake oscillatory model is combined with the damping vibration model to calculate the developing proess and unique phenomenon of VIV under oscillatory flow in time dimain,including "building-up","lock-in","dying-out","mode transition" and "pure lock-in".A time-domain iterative method is adopted to calculate the initial values and oscillatory parameters in each time step.A good agreement is achieved in comparison between simulations and experiments.The results have shown that the important characteristics of VIV response in oscillatory flows are captured successfully.(3)The modified VIV prediction method based on wake oscillator model is developed to calculate the VIV fatigue in time-varying flows to improve the accuracy and efficiency of calculation model.Two Weibull functions are established to represent the probability density function(PDF)of flow velocities and fatigue stress in the whole return period.Next,a modified LHS method is developed to reduce the sampling number and calculated load by recognizing the "Samping free regin" based on the reduced velocity under given conditions.In addition,the RSM is employed to establish the "input parameters – damonant mode – truncation order" feedback calculation method,which aims at improving the calculation efficiency and accuracy.The result shows that the the accuracy and the efficiency of the proposed modified LHS model is better than that of ordinaty Monte Carlo method.The established "RSM+feedback calculation" method could meet the calculation requirement.In addition,a parametric study is conducted to investigate the effect of flow PDF parameters on the fatigue life.(4)The 1-DOF and 2-DOF calculation models for the coupled "fluid-slender structure-harvester" system are developed.The relationships between VIV response,energy harestering efficiency and design parameters of energy harvesting devices are investigated.In addition,the influence of energy harvesting devices on fatigue stress is studied.The result shows that the developed model is able to predict the influence of energy harvesting devices on VIV response.The "damping mode transition" phenomenon might be caused by the energy harvesting devices,which leads to sharp changes in energy harvesting efficiency curve and fatigue stress curve.The size parameter(LD)has a great influence on the maximum energy harvesting efficiency and energy harvesting amount.The energy harvesting efficiency under 2-DOF is larger than that of 1-DOF when other parameters are the same.The fatigue stress shows a dramatic decline with the increase of harverter number and damping coefficients.