| In the exploitation of offshore and deep sea oil and gas resources,the top tensioned risers have been widely used with the advantages of high transportation efficiency,good continuity and large volume.Since the top tensioned risers are placed in the sea,they are unavoidable to be affected by the external fluid flows such as ocean currents.When the external fluid flow has a certain velocity,some vortices will shed from the pipe and appear alternatively and periodically in the wake flow.As a result,the fluid pressure around the pipe will change dynamically and the pipe will be excited to vibrate,i.e.vortex-induced vibrations.The vortex-induced vibrations involve the fluid-structure interaction between the external fluid flow and the pipe structure,which has highly nonlinear characteristics.On the other hand,due to the limitations of technology and cost for the oil and gas separation in the subsea,the pipes are usually used to transport the oil and natural gas directly when they are produced by the wells.In this circumstance,the fluid flow inside the pipe consists of oil and natural gas for the same time,which is gas-liquid two-phase flow.When the oil and natural gas are transported inside the pipe,they are easy to deform,separate and get together.Consequently,the fluid mass and density will change with time and space,which will cause the pipe to vibrate.There is also a strong fluid-structure interaction between the internal fluid flow and the pipe structure,which is highly unstable and randomicity.Under the combined actions from the external fluid flow and the internal fluid flow,the vibrations of top tensioned risers are very complicated since the multi-filed coupling from the external flow,the pipe structure and the internal flow are involved.In order to ensure the pipe's safety,stability and durability,many experts and scholars from home and abroad have carried out in-depth studies and a lot of achievements have been obtained.However,in these studies,the internal fluid flow was usually simplified as a homogeneous single-phase flow or the two-phase flow and multi-phase flow was analyzed with the theory of single-phase flow,in which the change of fluid mass and fluid density inside the pipe with time and space has been neglected.In view of this,the dynamic characteristics of a top tensioned riser under the combined excitations from the external fluid flow and the internal fluid flow is studied in this dissertation,in which the change of fluid density of gas-liquid two-phase flow inside the pipe with time and space is mainly considered.In this study,the vortex-induced vibrations of a pipe caused by the external fluid flow is explored,the excitation on the pipe from the internal varying fluid density of gas-liquid two-phase flow is studied,and the dynamic responses of the pipe under the combined excitations from the external flow vortex excitation and internal flow varying density are analyzed.The specific research contents and conclusions are as follows:(1)The excitation on the pipe from the external fluid flow is individually analyzed.The hydrodynamic forces caused by the external fluid flow flowing across the pipe are analyzed,which contain the vortex-induced lift force and drag force.The influence of the instantaneous relative incoming flow velocity is considered.The lift force coefficient is modelled with a wake oscillator.Then,a hydrodynamic model of vortex-induced vibrations of a pipe excited by the external fluid flow is established.This model is used to predict the vortex-induced vibrations of an elastic-mounted rigid cylinder and the top tensioned riser which is subjected to uniform or shear flows.The predicted results are compared with experimental results,which demonstrates that the present theoretical model is reasonable and reliable.Furthermore,the vibration strain of the top tensioned riser is considered and the fatigue damage index is calculated.The calculation results are desirable.The present hydrodynamic model can be used to predict the vortex-induced vibrations of top tensioned risers in practical engineering and it can also be used to evaluate the fatigue damage of the riser.(2)The excitation on the pipe from the internal gas-liquid two-phase flow is individually analyzed.When the pipe transports oil and natural gas at the same time,the fluid flow inside the pipe is gas-liquid two-phase flow,which has many flow patterns such as bubble flow,slug flow,churn flow and annular flow.In these flow patterns,the pipe will vibrate violently when the fluid mass and fluid density change largely.The change of fluid density inside the pipe with time and space is simulated with an improved mathematical model,which has a travelling wave transmission feature,and it is more reasonable.When the rate of change of fluid mass in a micro segment control volume is deduced,it can be found that the improved fluid density model satisfies the mass conservation low of fluid flow.Then,based on the momentum theorem and the principle of force balance,a dynamic governing equation of a pipe transporting gas-liquid two-phase flow considering the change of fluid density is developed.The governing equation is nondimensionalized and then it is solved by the finite difference method and the Runge-Kutta method.When the present theoretical model is compared with experimental results,it can be found that the present model is reasonable and reliable.The present theoretical model is able to simulate the change of fluid density of gas-liquid two-phase flow inside the pipe and the corresponding governing equation is capable of predicting the vibration responses of the pipe excited by the internal varying fluid density of gas-liquid two-phase flow.(3)A further step to analyze the pipe excited by the internal varying fluid density of gas-liquid two-phase flow.With respect to the governing equation of the pipe,it can be found that the change of fluid density of the internal fluid flow with time will cause parametric excitation on the pipe.Applying the Galerkin's method,the governing equation of the pipe is discretized and then the order is reduced in order to obtain the first-order differential matrix equation,based on which,the characteristic complex frequency and natural frequency of the pipe system are solved.After this,the stability of the pipe parametrically excited by the internal varying fluid density is determined with the Floquet theory.When the occurrence conditions of parametric resonances are compared with experimental results,it can be found that the results of the present model are reliable.Based on the present theoretical model,the influences on the stability of the pipe system from the internal flow mass ratio,flow velocity,fluid pressure,axial force at the pipe end,viscous damping and viscoelastic damping of the pipe material are all analyzed in detail.The results show that the larger the mass ratio,the larger the flow velocity,the larger the fluid pressure,the smaller the axial force at the pipe end,the smaller the viscous damping and viscoelastic damping of the pipe material,the wider the unstable regions caused by the parametric excitation and the pipe system is more unstable.Therefore,some useful suggestions are given in order to prevent the occurrence of parametric resonances,such as increasing the axial force at the end of the pipe or improving the damping property of the pipe material.(4)The dynamic response characteristics of a top tensioned riser under the combined excitations from the external fluid flow and the internal varying fluid density of gas-liquid two-phase flow is analyzed.The hydrodynamic model is utilized to describe the vortex-induced vibrations of the pipe caused by the external fluid flow.The improved mathematical model of varying fluid density is used to simulate the change of fluid density of gas-liquid two-phase flow inside the pipe.According to the Hamilton principle,a dynamic equation of a top tensioned riser under the combined excitations from the external flow and the internal flow with varying fluid density is derived.The vibration equation is nondimensionalized and then numerically solved.When the present theoretical model is compared with experimental results,the comparison results demonstrates that the present model is reasonable.With the present model,the dynamic responses of the pipe under the combined excitation from the external flow vortex excitations and the internal flow parametric excitations are investigated.The results show that due to the excitations from the external flow and the internal flow,the vibration amplitudes of the pipe will become larger or smaller in the stable and unstable regions of parametric excitation.When the dominating mode of vortex-induced vibrations of the pipe is excited by the internal varying fluid density,the pipe's vibration responses will change largely.Because of the contributions of excited modes or the competitions between different excited modes,the time-space response displacements of the pipe will become nonuniform,irregular and aperiodic,and the vibrations of the pipe will have multiple response frequencies.Furthermore,the vibration fatigue damage of the pipe structure is analyzed.It can be found that the change of fluid density inside the pipe will increase the fatigue damage of the pipe,especially in parametric resonances.(5)As an example,a top tensioned riser is taken into account,in which the combined excitations from different external flows and the internal varying fluid density of gas-liquid two-phase flow are analyzed.Firstly,the pipe is considered as transporting homogeneous oil.The natural frequencies of the pipe system are calculated and the results are compared with the existing results.In this way,the present theoretical model is further verified.Secondly,the pipe is considered as transporting gas-liquid two-phase flow which contains oil and natural gas.The influences of the internal flow velocity,the internal mean fluid density and the top tension on the natural frequencies of the pipe system are analyzed.The results display that the larger the internal flow velocity,the larger the internal mean fluid density,the smaller the top tension,the smaller the natural frequencies of the pipe system.Furthermore,the varying fluid density inside the pipe is considered,which could cause parametric excitations on the pipe.According to the currents in the west Africa and in the northern south China sea,the external flow is taken as uniform flow and shear flow,respectively.Then,the influence of the internal varying fluid density on the vibration responses and fatigue damage of the pipe which is subjected to different external flows is explored.The research results demonstrate that when the fluid density inside the pipe changes with time and space,the vibration responses of the pipe under the action of external uniform flow or shear flow will become uneven,and the fatigue damage of the pipe structure will become larger.(6)A gas-liquid two-phase flow experimental device was designed and experiments of a pipe transporting gas-liquid two-phase flow were carried out.This experimental system mainly includes water supply system,gas supply system,test pipe,displacement measurement system and pressure measurement system.A high-precision laser displacement sensor was used to measure the vibration displacement of the pipe,a high-speed camera was utilized to film the flow state of the gas-liquid two-phase flow inside the pipe,and a high-precision pressure sensor was applied to measure the fluid pressure.At first,through the free decay experiments,the natural frequency and damping of the air pipe and the water-filled pipe were measured.It is found that when the pipe is filled with water,the natural frequency will decrease and the damping ratio will increase.Subsequently,the pipe carrying water was injected with more and more air and the change process of fluid from liquid to gas was explored.The experimental results show that: in the middle region of the change process from liquid to gas,the pipe will vibrate violently and the vibration frequency will become larger.With the increase of the gas flow,the fluid pressure inside the pipe will change more dramatically,and the average fluid pressure will become larger. |
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