Research On Multi-Modal Vortex-Induced Vibration Bifurcation Of Marine Riser

Marine riser is one of the most frequently used equipment in ocean petroleum engineering.It not only bears the influence of external complex ocean environmental loads(such as currents,waves and internal waves)and the movement of the platform,but is also affected internally by high temperature and high pressure oil or natural gas flowing through it.The long-term periodic action of these loads and the coupled effect between them can cause cyclic reciprocating motion of the riser,which leads to fatigue damage of the riser structure.Vortex-induced vibration(VIV)of riser with multi-modal interaction is a key issue drawing people's attention at present.Existing experiments and CFD calculation show that such phenomena are ubiquitous.However,both methods are limited by costly computational demands when analyzing the effects of large-scale changes in parameters.It is difficult to fully reveal the nonlinear dynamic phenomena about multi-stable states and affects the correct security evaluation of the riser.Therefore,the study on the fluid-structure coupled dynamics of riser based on the empirical model of wake oscillator has important significance and value.It can not only reveal the prediction ability of this kind of model in multi-modal dynamic behavior of riser,but also support the analysis of dynamic phenomena obtained by experiments and CFD simulation.The thesis presented herein takes the top tension riser(TTR)with large aspect ratio of the Spar platform as the research object,and describes the action of fluid by wake oscillator.Nonlinear behaviors such as bifurcation and multi-stable behavior in VIV of riser are studied by using Poincarémap method,averaging method and singularity theory.The main achievements presented in this thesis are as follows.(1)Based on the third-order nonlinear variable lift coefficient hydrodynamic model and the single mode discrete results,the amplitude-frequency response equation of riser under 1:1 primary resonance excitation is deduced by averaging method.The singularity theory is used to obtain the analytical solution and discuss the influence imposed by main parameters in lift coefficient model on the riser VIV.It provides reference and guidance for the selection of empirical coefficients in empirical models.Several kinds of potentially destructive transient dynamic behaviors are revealed by vector field structure analysis,which deserves attention.(2)Based on the van der Pol wake oscillator model,the nonlinear dynamic model of TTR fluid-structure coupled system VIV is constructed.By comparing with the results of large-scale VIV experiments abroad,the accuracy of high-order Galerkin discrete scheme for riser VIV response calculation is verified.By solving eigenvalues of linearized Jacobian matrix of riser fluid-structure coupled system,the natural vibration characteristics of riser fluid-structure coupled system are analyzed.And the modal characteristics of riser system subject to uniform flow and shear flow are also compared.(3)Based on the 15^th-order Galerkin discrete results,the bifurcation and multi-stable behavior of the VIV cross-flow response of 1000 m long riser in linear shear flow are studied in the flow velocity ranges over 0 to 0.6 m/s.The nonlinear response characteristics of the riser VIV at typical flow velocities and the multi-modal interactions in the VIV response of the large aspect ratio riser are discussed in detail.The method about judging the tri-stable behavior of the riser VIV is summarized by bifurcation diagram and 3D spectrum diagram.(4)The VIV response of the riser subject to uniform flow is investigated.It is found that for the large aspect ratio riser system,although it exhibits more periodic motion phenomenon under the condition of uniform flow than the shear flow,the nonlinear vibration behaviors of high-order mode participation and multi-modal interaction are still significant and obvious.Even at the same flow velocity,there is the multi-stable behavior about four different steady state responses coexisted.For some multi-stable state behaviors,the response mode order and frequency of riser VIV are the same,but the amplitude and vibration shape are different.(5)The influence of parameter excitation and internal flow on the VIV of the riser has been analyzed,by comprehensively considering the effects of fluid flow inside the riser and platform heave motion on the riser.It is found that under the internal flow-parametric-vortex-induced coupled vibration,there is periodic motion with complex and varying phase diagrams in riser system.Typical motion patterns include periodic motion caused by the coexistence of 1/2 subharmonic resonance and1:3 internal resonance.There are some risks of divergence and instability about the riser system response under some conditions.Results found by this thesis show that multi-stable phenomena are common in multi-modal VIV.Therefore,reducing the potential hazard caused by multi-stable behavior is one of the important issues that need to be further solved in the design and safe operation of marine risers.