Vortex-Induced Vibration Of Flexible Riser In Time Domain

Around seventy percent of the Earth surface is covered by water and huge amount of natural resources such as gas and oil are richly stored in the deep ocean. With the fast industry development, the sharp increasing demand of consuming energy, especially the oil and gas, has become bottle-neck issue for both developing and developed countries. Deep-water petroleum industry could be a good way to solve the global energy crisis in future. One of the technique challenges, related to deep-water petroleum industry, is the Vortex-Induced Vibration (VIV for short) of flexible riser.VIV happens while flow over the flexible riser and the structure is excited by the unsteady fluid fore which is generated by the periodic shedding vortex. The flexible riser will vibrate frequently with large amplitude under certain incoming flow conditions. Great damage to the riser would be resulted from the VIV due to the high vortex shedding frequency. Having a good understanding of the VIV mechanism could be helpful to reduce the damage effect on the riser, and also a quite kind strategy to the environments. However, VIV can be regarded as a complex fluid-structure coupling problem and the equations solving the flow domain and the structure bare strong non-linearity. The flow could turn into turbulence according to the high Reynolds number theory and how to revolve the turbulence accurately in a numerical way is still a big challenge world wild.The present research described in this thesis is a part of the project"Research on Dynamic Response of Platforms in Deep-water"supported by the National Natural Science Foundation of China under Grant No.50323004 and also supported by Shanghai Scientific Fund under Grant No. 05DJ14001. The purpose of the research is to study turbulence and to construct a numerical model to predict the VIV response of flexible riser under uniform and shear flow conditions in the time domain.Based on the latest achievements on turbulence research and VIV response of flexible riser, the present thesis focuses on the most interested points which include numerical tools for turbulence simulation and the VIV response of flexible riser under stepped and shear flow conditions. The main contents of the present thesis can be summarized as follows:1, A review on the turbulence research and the existed models for VIV response prediction has been conducted. A brief introduction to the numerical research tools on turbulence including RANS,LES and DNS is described, especially the DNS and RANS being used in the present thesis are compared in a detail way. Empirical model and time-domain model for VIV response are also introduced. Most of the latest experimental results on VIV of flexible riser are concluded and constructive suggestion is made for the future research.2, Three dimensional numerical simulation on shear flow over a circular cylinder has been studied by applying DNS. Shear effort on the three dimensional vorticity structure behind the circular cylinder has been intensively investigated. The physics of the fluid such as turbulence energy, kinetic energy, Reynolds stress, shedding frequency, boundary layer and pressure coefficient is also conducted and compared with the results of other sources. The discrepancy between the physical experiment and two dimensional simulations could be well resolved by applying the three dimensional numerical experiments.3, Two-degrees-freedom VIV of a spring constrained rigid circular cylinder with low mass and damping ratio has been analyzed using RANS solver combined with turbulence model for the Navier-Stoke equation. The vibration in streamwise and transverse direction is considered to have a limited effect on the peak transverse amplitude of the spring mounted circular cylinder with moderate mass ratio. It is interesting to find the critical mass ratio under which the supper upper branch could appear accordingly in the current numerical results. The time-trace circular cylinder movement and vorticity structure which obtained in physical experiments are also captured in the present simulation.4, Another numerical investigation is also conducted on VIV response of an elastic mounted circle cylinder under planar shear flow conditions. A planar shear ratio lock-in range can be concluded from the data analysis in transverse response, fluid force and time trace displacement at moderate Reynolds number. The figure"8"trace movement could be observed under small planar shear flow conditions while a completely new style time-trace circular cylinder movement is discovered as the planar shear ratio increases.5, Based on the strip theory and the combination of the Finite Volume Method and Dynamic Finite Element Method, a numerical model which can be used to predict the VIV response of a flexible riser in time-domain is successfully constructed. VIV response of a flexible riser under stepped and shear flow conditions is calculated and extensive investigation on the mechanics of the VIV is carried out by applying the research on the time-trace of the transverse response and fluid forces along the axis of the riser. The multi-modal response is obtained and two typical vortex shedding modes are observed at the same time in the spanwise direction as the riser is set in a stepped flow.6, The same numerical model is applied to predict the VIV response of a flexible riser under the axis shear flow condition. Compared to the results by other CFD model and empirical model, the present results are more rational and accuracy in agreement with the experimental results in the deforming modal shapes.7, The Reynolds number effect and mass-damping ratio effect on the peak transverse response amplitude of a freely vibrating circular cylinder is well studied based on the forced oscillation experimental data. A theory model which can be used to predict the peak transverse response is constructed. This model could be considered as an update to the empirical model for the prediction of the response of the flexible riser. According to the research work mentioned above, main conclusion can be drawn as follows:1, The vorticity structure such as Mode A and Mode B is successfully recorded under uniform inflow at low Reynolds numbers. Oblique vortex shedding in streamwise direction is observed by applying DNS calculation on planar shear flow over a circular cylinder. The vorticity energy corresponding to the higher velocity side becomes stronger and the vorticity energy in the other side will decay to be totally destroyed as it moves down in the far wake of the circular cylinder. The planar shear flow will affect the boundary layer and pressure coefficient around the surface of the circular cylinder, but no effect on the shedding frequency. Oblique vortex shedding in the spanwise direction and dislocations could be observed under the spanwise shear inflow over a circular cylinder. The two shear flow results could be regarded as a combination of the planar shear and the spanwise shear flow cases, and it can be inferred that the planar shear and the spanwise shear do affect the flow motion separately but without any interaction effect.2, DNS calculation is conducted to study the no-slip and free slip boundary conditions on the suppression effort in vortex shedding phenomenon under the planar shear flow over a circular cylinder. The discrepancy between the experimental results by Kiya et al (1980) and recently two dimensional numerical results by Kang (2007) is well explained according to the three dimensional DNS results. The no-slip boundary condition will have a great effect on vortex shedding suppression while very limited effect in free slip boundary condition3, The streamewise vibration has a limited effect on the peak transverse response of the circular cylinder with moderate mass ratio. Larger transverse response could be excited as the mass ratio is below the critical mass ratio m* cri= 3.5for the circular cylinder with two degree-freedom rather than the streamewise vibration is constrained. The vortex shedding mode, such as SS, 2S and 2P, corresponding to different transverse response branch is observed; especially the classical 2T mode appearing at the supper upper branch in the experiment by Jauvtis & Williamson (2003) is reproduced in the present numerical simulation. The figure"8"traces movement for the VIV response is also captured successfully.4, The results form the two dimensional planar shear flow over an elastic mounted circular cylinder infer that there exits a planar shear ratio lock-in range in which the transverse amplitude grows rapidly and the amplitude would decrease sharply as it goes out of the shear ratio lock-in range. The lock-in range becomes wider as the Reynolds number grows. The figure"8"time trace movement can be seen as the planar shear flow ratio is small, while a new time trace movement which is named as"water drop"shape is discovered.5, The multi-modal response of the flexible riser is obtained under the stepped inflow condition. The excited model for the long flexible riser can jump from one model to another one in the time domain while the deforming shape of the riser appears to be the dominated excited modal shape. It is can be inferred from the time trace contour of the lift or drag coefficient that the fluid force correction length along the axis of the riser could be given a strong supportive evidence for the present multi-strip numerical model. The added mass is not equally distributed along the axis of the riser but vibrates a lot according to the multi-modal response. It is suggested that the added mass distribution should take into account of the contributions separately from the main excited modals. The"2S"and"2P"vorticity structures are observed along the axis of the riser at the same time due to the small or large amplitude of the riser in various excited models. The switching phenomenon in the vortex shedding is observer in the present CFD model.6, The predicted VIV response of the riser agrees well with the experimental results as the riser is excited at low modal response in the spanwise shear flow. Compared to other multi-strip model or empirical model, the present CFD model could be recognized as a more accurate and rational multi-strip model for more strips are used to consider the fluid force correction along the axis the riser and the deforming mesh is applied to simulate the fluid-structure coupling movement in the current model. Furthermore, the non-linear global stiffness matrix is updated in the time domain as the riser deforms.7, Potential relationship between the freely vibration and the forced oscillation has been investigated and a theory model is successfully constructed. Reynolds number effect and mass-damping effect are considered to predict the peek transverse amplitude of a freely vibrating cylinder by applying the force oscillation experimental results. It is concluded that the Reynolds number ignored previously in the forced oscillation is a quite important parameter in the freely vibration case. The general idea in the current theory model could provide a strong background for the empirical model of VIV response of a flexible riser.In a word, the present paper starts with the simple case in flow over a circular cylinder, and then focuses on the VIV of a spring mounted circular to the VIV response of a flexible riser finally. A CFD model is successfully constructed and it can be used to predict the VIV response of a flexible riser in the time domain Most of the experimental results or vortex shedding phenomenon could be accurately reproduced in the current CFD model. It can be inferred that the main conclusions drawn from the present CFD model could provide a constructive way to improve the empirical model in VIV response prediction.