Stability Analysis Of Deepwater Complaint Vertical Access Riser

With the progress of social economy and the development of the petroleum industry in recent years,the energy revolution of oil and gas development in deepwater is ready.As a new riser form used in deepwater,the deepwater complaint vertical access riser(CVAR)has vertical connection to the underwater wellhead,which can directly execute multiple operations for the well from the floating platform.During the operation process in the marine environment,risers not only bear the disturbance of the wave current,but also suffer from the vertical motion of the top floating platform,which is likely to cause parametric excitation.Therefore,the stability analysis of CVAR has important theoretical and engineering significance.In this paper,the principle of virtual work and the variational principle are used to derive the differential equations of motion of CVAR.Through the Galerkin method,the differential equation of CVAR is discrete to get the transverse vibration equation.Considering the influence of the riser's self weight,buoyancy block and gravity block on the axial effective tension,the MATLAB program is designed and programmed to get the modal information related to the natural frequency and modal shape of the vertical path.The factors that affect the natural frequency of the riser are also analyzed.Considering the parametric excitation,get and discrete the vibration equation of CVAR.Based on Floquet theory and state transition matrix of coupling and non-coupling,the stable and unstable region of the riser system is obtained by judging the eigenvalues of the state transfer matrix,and the riser response under different circumstances is obtained.Finally,the paper introduced the wake oscillator model of vortex induced vibration equation.By using the finite element discretization and Newmark methods in time domain,the response of the riser to the vortex induced vibration at different velocities is obtained by using the MATLAB program.At the same time,the stability of CVAR under the combined action of vortex induced vibration and parametric excitation is analyzed,and the corresponding measures to suppress instability are put forward.The research shows that: CVAR has large ratio of length and diameter,and its natural frequency is obviously smaller than that of shallow water riser.And the natural frequency will decrease when the diameter,wall thickness and length of riser increases.The parameter excitation of CVAR is mainly in the first order unstable region.Considering the modal coupling between the modes of the axial tension of the riser,more parameters are excited to stimulate the unstable region.Structural damping can reduce the unstable region of the riser to a certain extent.In the case of low current,the vortex induced vibration is easy to occur in CVAR.Considering the combined effect of parametric-vortex,vortex induced vibration is dominant in low current conditions,parametric excitation increases the displacement response of riser.When the current reaches a certain value,the response of the riser is dominated by the parameter excitation,and the parametric excitation will periodically increase the vortex induced vibration response of the riser.Therefore,it is of great engineering significance to study the excitation and vortex excitation combined excitation.