Numerical Simulation On The Non-Linear Dynamic Vibration Of Steel Catenary Risers Conveying Fluid

Steel catenary risers are the main connecter between a platform and the mouth of a well in the seabed and can be applied to floating platform, fixed platform and drilling shipping systems. The risers generally contain flowing oil or high pressure gas and bear the action of wave and current outside. When current or wave flows across the risers at certain speed, there will appear vortex shedding, and if the frequency of the risers is close to that of the vortex shedding, oscillation will make the vortex shedding frequency fixed nearby to the structural natural frequency and cause large amplitude vibration which is called 'lock-in' state. So the vortex-induced vibration is an important factor in the design of steelcatenary riser system. Otherwise some indicative examples include the riser-soil interaction effects in bottoms of practically unknown contour, the ill-understood vortex induced vibration (VIV) effects and the floater vortex induced motion (VIM) induced riser fatigue, extreme dynamic amplification of bending moments at the touch-down region, etc. Some of these issues are related to the dynamic response of the riser under imposed excitation applied at the top of the structure, which represents the motions of the floating vessel. Therefore the dynamic analysis of steel catenary risers is a key issue that requires deep and comprehensive investigation of the associated system, proper formulation of the theoretical model and finally, the use of an efficient solution method.When internal fluid travels along the curved path inside the deflected risers, it experiences centrifugal and coriolis accelerations because of the curvature of the risers and the relative motion of fluid to the time dependent risers motion, respectively. Those accelerations exert against the risers and, in turn, affect the dynamic behavior of the risers and cause additional vibrations. Steel catenary risers subjected to the vortex-induced vibration and the internal fluid may experience large response which gives rise to oscillatory stresses and cause fatigue damage. The numerical studies that were focused on the VIV considering internal flowing have not been carried out yet. So the numerical simulation on VIV considering internal flow flowing has both scientific and practical value.This paper presents a model formulation that can be used for analyzing the three-dimensional vibration behaviours of inclined extensible steel catenary risers. The virtual work-energy functional, which involves strain energy due to axial stretching and bending moment of the riser and virtual work done by the gravitational, inertial, and external hydrostatic and drag forces, is formulated. The coupled equations of motion in the Cartesian coordinates of global systems are obtained by taking into account the difference between Euler's equations and equilibrium equations. The method of Galerkin finite element is used to obtain the mass damping and stiffness matrices. Then the eigenvalue problem is solved to determine its natural frequencies and corresponding mode shapes. The numerical investigations are carried out to demonstrate the validity of the model and to explore in details the influence of various parameters on the behaviours of steel catenary risers.Try to simulate the fluid induced vibration of riser vortex force by Matteoluca wake oscillator model, solving riser vortex induced vibration. The motion of the floating body is simplified as sinusoidal excitation imposed on the riser at the top, solving the dynamic response of riser. And related research results were compared to verify the validity of this model. The program in Matlab was complied, detailed analysis of the dynamic response of riser influenced by the internal fluid velocity, the top tension and the external fluid. The results indicate that:steel catenary riser has different multi-modal coupling vortex induced vibrations under different external fluid velocity. Internal fluid velocity and top tension may change the riser vortex induced vibration frequency, amplitude, dynamic stress and bending moment. Top floating-body excitation frequency may change the distribution of dynamic stress and bending moment of the riser, and large changes in the top and bottom. Excitation amplitude, internal fluid velocity, top tension and the existence of external fluid may change the dynamic stress and bending moment of the response in different degrees.