Investigation On Vortex-induced Vibration Of Deep-sea Risers With Internal Flow

The development of economy in our country increases the demand for oil and gas greatly,which leads to the oil and gas exploration in the deep ocean.The crisis of lack of oil and gas in our society can be mitigated by exploring oil and gas in the deep sea.However,the unpredictable climate and environment of the ocean bring great challenges to the exploration.When subjected to current and wave,the risers,the main part of transporting oil and gas from the seabed to the platforms,can perform complicated dynamics,such as vortex-induced vibration(VIV).Furthermore,the dynamical behavior can be more complicated while severe slugging occurs in the risers.Therefore,it is of scientific and practical significance to investigate the VIV of risers with internal flow.Based on the aforesaid situation,the review of study on VIV and severe slugging is firstly summarized.Then the dynamics of the risers is focused.By using the wake oscillator model and semi-empirical time domain model,the dynamical VIV behavior of the risers with internal uniform flow and severe slugging is investigated.Moreover,a transient theoretical model based on fewer experimental parameters is proposed for predicting flow parameter characteristics of the gas-liquid severe slugging in a pipeline-catenary riser system,which can lay a foundation for researching the VIV of the catenary risers with severe slugging.The main work of the doctoral dissertation is as follows:In Chapter 3,the VIV of the riser with internal uniform flow is preliminarily studied based on wake oscillator model.A set of nonlinear 3-D equations modeling both the structural motion and the vortex-induced forces are discretized via the Galerkin method.Then the transformed partial differential equations are solved by a fourth order Runge-Kutta scheme.And the first and second responses of VIV of the riser with the internal flow velocities in the subcritical and supercritical regimes are focused.The results indicate that the riser performs more complicated VIV dynamics while the internal flow is considered.When the internal flow velocity is in the subcritical regime,there exist two response bands in the in-line(IL)direction while only one in the cross-flow(CF)direction.With the increase of the internal flow velocity in the subcritical regime,the lock-in domains expand for both ILand CF responses and the IL and CF amplitudes decrease.Moreover,for the dynamics in the lock-in condition,3-D periodic,quasi-periodic and chaotic vibrations are observed.For the cases studied in the supercritical regime,corresponding to different values of the first buckled instability of the riser,the responses are obviously distinct from each other.Also,the dynamics shows a great diversity compared with that in the subcritical regime.Various dynamical responses,which include 3-D periodic,quasi-periodic as well as chaotic motions,are found while both CF and IL responses are coupled when the internal flow velocity is beyond the critical value.In Chapter 4,the VIV of a top tensioned riser considering both internal and external flows is investigated numerically based on the semi-empirical time domain model.Correspondingly,typical VIV characteristics such as the dominating modes and frequencies in both IL and CF directions are analyzed,as well as the Root Mean Square(RMS)of the amplitudes,standing and traveling waves for the IL and CF responses.The newmark-? method is firstly utilized to solve the governing equations after they are discretized by the FEM method.The model is verified by the comparison of the experimental data,numerical results based on wake oscillator model and the semi-empirical time domain model respectively.The results show that the IL and CF dominating modes are obviously influenced by the internal flow while the dominating frequencies and RMS in both IL and CF directions are slightly changed during the investigation.The dominating modes for both IL and CF responses increase with the internal flow velocity increasing.Moreover,conspicuous traveling wave response is detected for both IL and CF responses while the internal flow interacts with the external flow for VIV.In Chapter 5,the VIV dynamics of a top tensioned riser considering severe slugging is investigated based on the semi-empirical time domain model and FEM method.Firstly,the severe slugging occurring in a vertical riser is simulated by using the transient theoretical model in order to obtain the transient gas and liquid mass in the riser.And the VIV dynamics of risers with Type I and II of severe slugging is researched.The results indicate that severe slugging has an evident effect on the IL and CF vibrating equilibrium positions and amplitudes of the riser,especially when the external flow velocity value is small.Furthermore,the riser vibrates with multi frequencies in the IL and CF directions.And one of the IL and CF frequencies is dominated by severe slugging.But the IL and CF dominating modes are not influenced by severe slugging eevidently.In Chapter 6,a theoretical model including simplified transient model and phase distribution model is proposed for investigating the characteristics of severe slugging in a pipeline-catenary-riser system.Moreover,comparisons are made between the numerical and experimental results.It is proved that the model can not only repeat the transient fluctuation of the pressure fluctuation,period,void fraction and superficial velocity in the pipeline and the riser,but also predict the type of the severe slugging in the pipeline-catenary-riser system and the flow regime in the riser according to different gas and liquid velocities.Besides,a good agreement is obtained between the experimental and numerical results,which indicates that the theoretical model can provide theoretical support for the research on VIV of catenary risers with severe slugging.