3-D Dynamic Analysis Of Deep-sea Flexible Beam Under The Vortex–induced And Parametric Excitations

The deep-sea flexible beam is widely used in offshore engineering,like risers and tether of the TLP.Its 3D dynamic responses are studied in this paper,considering the influence of the platform motion.At first,the research situation of the tension leg platform is summarized,it shows the coupled dynamic analysis of the TLP and deep-sea flexible structure is needed.Based on the theory of nonlinear elasticity mechanics,a theoretical nonlinear 3D coupling model is proposed.It can simulate the rise vibration under the influence of TLP heave motion.The Hamilton variational principle is used to obtain the governing equations of motion and the boundary conditions,the hydrodynamic effects are taken into account through Morison equation and vortex-induced forces to simulate the vortex-induced vibration with and without parametrically excitations.The non-linear and coupled equations of motion are solved using the finite difference approach and Runge-Kutta method,numerical results of three dimensional motion time history responses and the corresponding phase plot,Poincare map and power spectral densities are plotted and discussed.Special attention has been paid to the effect of the parametric excitation frequencies and the amplitude of the top tension.Finally the results are compared with experiments with same physical parameters carried out in Tianjin University.At last the 3-D dynamic analysis of deep-sea flexible beam under the wave and current is studied.The research shows 2/1 superharmonic responses were found when the beam is under vortex-induced excitations,and combination responses when vortex-induced and parametric excitations occurs.The heave motion increases the amplitude of the rises but does not influence equilibrium position.