Study On The Non-linear Response Of Flexible Offshore Structures Under Stochastic Loading

The geometrical nonlinearity plays a key role in the stochastic vibration of flexible structures. A novel method for the random vibration analysis of geometrically nonlinear structures is proposed in this thesis. In addition, the method applied to the analyses of a mini Tension Leg Platform (TLP) and a seabed suspended span pipeline. The major contents are summarized as follows:1. The nonlinear equivalent system for flexible structure is established. Using the modal analysis, a multiple-degree-of-freedom (MDOF) physical system is translated into a modal system, which can be solved easily. The implicit geometrical nonlinearity system is represented as an explicit equivalent nonlinear system, which is based on the combination of modal coordinates. In conjunction with the nonlinear finite element technique, the relative unknown parameters are confirmed.2 . Based on the nonlinear equivalent system and stochastic equivalent linearization technique, the method of random vibration of geometrically nonlinear structures is proposed. Incorporating the pseudo-excitation method and the precise time-integration method, the restriction of the stationary excitation is removed and the computational efficiency is greatly enhanced. The method can be applied to practical structures.3. The nonlinear random vibration analysis software for flexible structures is developed. There are plentiful complicated calculations in the analysis. For the convenience of the application, a software package is developed, which is called GNRV (Geometrically Nonlinear Random Vibration Analysis).4. The stochastic response and vibration comfort of the deepwater TLP is studied. Considering the particularity of TLP, the nonlinear random vibration analysis method is applied. Based on the stochastic response and combined with the vibration annoyance rate model, the vibration comfort assessment approach is proposed.According to the design parameters of one deepwater new style TLP (mini TLP), the random response subjected to wave excitation is analyzed. The results are compared favorably with the measured results. The accuracy of the method is further validated. The difference between the linear and nonlinear results is relatively distinct. And the influence of water depth, wave height and direction, pretension and different wave spectrums are considered.5. The stochastic vibration and fatigue reliability of the seabed suspended span pipeline is studied. According to the design parameters and environmental data of one submarine pipeline in Bohai, the vortex-induced vibration and the fatigue reliability are analyzed. And the influence of geometrical nonlinearity, length of the suspended span, wave height, water depth, diameter and residual stress are considered.This research work is sponsored by National Hi-Tech Research and Development Program of China (863) (Grant No. 2006AA09A109-5) and Natural Science Foundation of China (Grant No.50278085).