| As a slender and flexible offshore structure,Steep Wave Riser(SWR)has the capacity to accommodate larger platform motion and deeper operation environment because of its lower top tension and ability to adapt to large deformation comparing with the traditional Steel Catenary Riser(SCR).It is widely used in deepwater oil and gas exploitation.SWR is composed of a tensioned pipeline connected to the arch bend with greater buoyancy,and then connected to the floating oil production platform.Therefore,the whole riser from the seabed to the water surface consists of three segments: Decline segment,Buoyancy segment and Hang-off segment.Among them,the middle of the SWR is wrapped by several buoyancy modules.Buoyancy and gravity act together on the SWR which successfully decouples the motion relationship between the decline segment and hang-off segment.Thus,the static and dynamic problems of the SWR are different from those of top-tension riser and SCR.At present,the researches on the SWR mainly focus on its static response and the dynamic response for uniform current.The complex ocean loads such as top motion,wave-current interaction have not been taken into account,and the research on the vortex-induced vibration(VIV)of the riser induced by the external current needs to further be carried out.In view of this,based on the slender and flexible rod theory,this thesis establishes a nonlinear timedomain dynamic response model of the SWR considering the effect of the internal flow in the global coordinate system.The thesis focuses on the investigation of dynamic response characteristics on each segment under the effects of the top-end motion,the effect of internal and external currents and the regular waves.Then,the numerical model is coupled with a wake-oscillator model to analyze the unidirectional and bidirectional VIV characteristics of the SWR under the oblique currents.The main content includes the following aspects:In this thesis,the internal and external loads acting on the SWR during the operation are firstly investigated,which includes effective tension,effective gravity,wave,current and internal flow.According to the linear wave theory,the nonlinear free surface motion boundary condition and dynamic boundary condition are simplified to linear free surface boundary conditions,and the Morison equation is used to calculate the hydrodynamic load acting on the riser.Subcequently,based on the slender rod theory,the nonlinear time-domain dynamic response model of the SWR considering the internal flow is established in the global corrdinate system,and the governing equations are discretized by the Galerkin method based on Finite Element Method(FEM).Each buoyancy module installed in the middle section of the riser is equivalent to a riser unit owing the uniform diameter.A classical Newton-Raphson method is used to solve the nonlinear governing equations to obtain the static response of the riser.The comparisons against the existing numerical simulation results verify the accuracy of the present numerical model.The static responses of the SWR under different currents and the effect of the internal current are studied.Numerical simulatins are conducted to investigate the characteristics of the configuration,tension and bending moment of the SWR under the effect of ocean current velocity/direction and internal current density/velocity.Secondly,the static equalibrum configuration of the SWR is regarded as the initial condition of the dynamic analysis,and a Newmark-? method is used to solve the dynamic response of the SWR under the effect of top-end motion and wave-current interaction in time domain.The independency study of the time step is carried out to verify the accuracy of the numerical method.Then,the published results for the dynamic performance of SCR are used to validate the fesiblilty of the present numerical method.In this thesis,the floating platform motion is simplified to the harmonic motion of the top-end node of the SWR.The dynamic responses of the riser under top-end node motions in the x,y and z directions are investigated.The results show that the motions of the top-end node have a significant impact on the tension of the riser.Meanwhile,the effect of different buoyancy segment lengths on the dynamic response of the riser is studied.The results indicate that the increase of the buoyancy semgnet length would optimize the dynamic performance of the hang-off segment on the SWR and change the distribution of the tension avoiding the excessive tension at the top-end node.Subsequently,the numerical simulation is used to solve the dynamic response of the SWR under wave-current interaction.The independences of the tension,configuration,displacement and the bending moment of the each segment on current parameters are discussed systematically.Finally,the numerical simulation continues to solve the VIV problems of the SWR.The unbidirectional and bidirectional VIV phenomenons of the SWR under oblique currents are studied.A three-dimensional(3D)nonlinear time-domain FEM coupling with a wakeoscillator model is used to numerically simulate the VIV response of the riser.The slender rod theory is applied to establish the large deformation nonlinear motion equation of the riser in the global coordinate system.The classical Van der Pol equation simulates the fluid force.A Newmark-? method is used to solve the VIV coupling equations of the riser.The comparisons of the experimental results of VIV on the top-tension riser show that the present numerical method in the thesis can simulate the VIV of marine risers accurately.The thesis focuses on the investigations of time histories of displacements and the related power spectrum density(PSD)under the shear/uniform currents.The drag coefficient,excitation coefficient and lift force are systematically discussed.In addition,this thesis improves the coupled vibration equation of the SWR and conducts the numerical simulations of the bidirectional VIV under the oblique currents.The trajectories and PSD of the SWR VIV are illustrated under the oblique currents. |
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