Dynamic Response Of Marine Riser Transporting Fluid In Internal Waves Field

Marine risers are important equipment of deepwater oil/gas exploration and development and generally serving offshore structural system as the link between the platform and the well head on the seabed. As exploration activities move into deepwater, the long slender risers tend to undergo a large amplitude motion and risk in high tension subject to the harsh and complex environmental conditions. Moreover, due to wind forcing or tidal currents, complex terrain under water and serious density stratified, internal waves always exist in the South China Sea, and it is believed to be responsible for a great deal damage, meanwhile, large amplitude internal waves can create enormous local loads and bending moments in offshore structures. To complete the gap in marine risers design, it is important to study the dynamic response of marine risers under internal waves or combined internal waves and background currents.In this paper, the governing differential equation of internal waves is obtained from the basic hydrodynamic equation, and discussing the difference when background currents exist or not exist. Then the TH method is used to solve the dispersion relationship and vertical structure of internal waves, thus velocity distributions of flow are obtained by linear formula as the frequency and amplitude of internal waves are given. Based on the virtual work-energy functional of marine risers, the developed structural model developed consists of the strain energy due to axial deformation, strain energy due to bending, virtual works due to effective weight, inertial force, structure resistance and hydrodynamic force. Herein, the forces on risers are computed by the Morison equation, then using the finite element approximation to derive the matrix equation of equilibrium, and the Newmark-β method combined with simple iteration is used to solve the system of equation in time domain, at last programming in Matlab. In addition, wake oscillator model is taken to study the vortex induced vibration(VIV) of marine risers in combined internal waves and currents.It is found that the internal waves will result in horizontal shear currents in the ocean, and there different modes of internal waves exist at the same time. Internal waves corresponding to the first two modes will lead to considerable effect on the dynamic response of marine risers, and rapidly weaken in higher modes. The response amplitude of risers would increase as the internal flow rate grow, but its natural frequency decrease in some degree; meanwhile, the increases of top tension would diminish the response of riser largely when top tension is low, while this effect is weaken in large top tension. Results analysis are applied and it is found that the response amplitude of risers rise as a quadratic function with the amplitude of internal waves, moreover, the response amplitudes increase as the frequency of internal waves reduce, it is more obvious in high frequencies but almost not change as moving into low frequencies. When the background currents exist, whether the internal waves propagate in or against the direction of background currents, the response amplitude of risers increase largely, it may results in crash. In addition, the results of VIV analysis show that although the internal waves will weaken the VIV response in some frequency, it has developed multi-modal vortex resonant and extended to high modes, which will low the fatigue life of riser.