Research On VIV Induced Fatigue Damage For Deepwater Riser Using Pseudo-excitation Method

With the exhaustion of the energy on land, more and more people turn eyes to the ocean and furthermore the exploitation field tends toward deep water. Riser is one of important subsystems connecting undersea drilling with offshore structures. Although the ocean environment is very complex, the main factors that cause damages of riser are the wave and current. When the current exerts on the risers, the vortex-induced vibration (VIV) may occur. The fatigue damages to the riser indued by the VIV may have serious influences on natural gas and oil exploitation. Therefore, in recent years the vortex-induced vibration has become a hotspot in marine engineering. For catering for the actual project needs, the random vibration method is used to analyze the prolem of vortex-induced vibration of deepwater riser and investigate the fatigue damage to the riser induced by vortex-induced vibration.Firstly, the current research status of vortex-induced vibration of riser at home and abroad is summarized and the background and its significance of the research are introduced. Secondly, the phenomenon and features of vortex-induced vibrations are reviewed and some new results obtained by the recent research are introduced. Subsequently the random vibration theory is elucidated. The basic principles and numerical implementaion of pseudo-excitation method for VIV calculation are presented in detail, providing the theoretical support for the analysis on vortex-induced vibration of riserPrior to implementation of pseudo-excitation method for vortex-induced vibration of the riser, a load spectrum and structural model are prepared. Due to the characteristics of the large slenderness ratio of deepwater riser, the cable model is adopted to calculate the nonlinear stiffness matrix. The correctness of our model is validated by the experimental results.The discrete vortex method in this paper accords the convenience to the calculation of fluid problems of high Reynolds number. The vortex induced vibration is considered as a stationarity random process. The fluid-structure interaction of the discrete nodes has been considered in our model. For evaluating the prolem of the random vibration, the vortex induced lift of two-dimensional riser is calculated using discrete vortex method to obtain the load spectrum for the calculation of Pseudo-excitation. Therefore the load spectrum is used to construct Pseudo-excitation。 Eventually, the vortex induced vibration is considered as a stationarity random process. The response of riser to vortex induced excitation is calculated using pseudo-excitation method. An experimental model of slender ratio 1700 is used to validate the feasibility and reliability of pseudo-excitation method applied to numerical calculation of vortex-induced vibration. Taking a length of 2360m deepwater riser as a practical engineering example, the characteritics of the riser are analyzed. The fatigue damage rate of the riser per year is evaluated and the service life of the riser can be predicted based on the P-M criterion.