Fatigue Life Study On A Novel Deep Sea Cell-Truss Spar Platform And Risers

With the rapid tendency of the oil and gas exploitations to deep-water areas, more and more new types of floating units have been used to adapt the operation conditions in severe deep sea environment. Among these floating structures, Spar platform has excellent stability, relatively benign motion performance and easy to disassemble, which makes many international oil companies put great efforts on it. Spar platforms have been used successful in Gulf of Mexico for more than one decade which provides instructive conditions for its application in South China Sea for the similar climate conditions in the future.The present research in this thesis is a part of the projects of"Research on Dynamic Response of Deep-water Platforms"supported by the National Natural Science Foundation of China,"Theory and Experiment Research on Critical Dynamic Performance of Deep-water Spar Platform"supported by the Key Program for Basic Research of Shanghai Committee of Science and Technology and"Experimental Technology for Deep-water Oil and Gas Exploitations"supported by National High Technology Research and Development Program of China. The purpose of this research is to improve more understanding of fatigue behavior of the novel Cell-Truss Spar platform and the fatigue life prediction method for deep-water top tensioned risers of Spars due to vortex-induced vibrations.The present research combined theoretical research and engineering application effectively. The contents cover some main hot issues, including the theoretical research on fatigue life prediction approaches of ocean engineering, fatigue damage assessment of critical joints for Spar platform, vortex-induced vibration (VIV) response prediction and VIV fatigue analysis of deep-water risers. The main contents and contributions of this thesis may be summarized as follows:1. An overview of recent research and progress concerning Spar platform and its key technologies, fatigue life prediction approaches of ocean engineering, VIV response of deep-water risers and the fatigue damage due to VIV is presented. The new concept and its structural characteristic of a novel Cell-Truss Spar platform proposed by State Key Laboratory of Ocean Engineering of Shanghai Jiao Tong University are introduced.2. Fatigue spectral analysis and reliability assessment of the critical joints of Cell-Truss Spar is carried out based on S-N curve method. Sensitivity analysis of fatigue life with different ocean environments is also studied. The characteristic and regular patterns of fatigue damage are summarized.3. The limitations of traditional fatigue spectral analysis is pointed out based on the mathematical proof of the necessary and sufficient condition of the fatigue linear accumulation rules and the commentary on the crack propagation model proposed by vessel and ocean engineering rules, while a new time-frequency domain hybrid model for fatigue analysis is put forward to overcome the limitations. In this hybrid model, frequency information is obtained by spectral analysis and the time series of fatigue cyclic stress is simulated by wavelet transform, while a random crack propagation model is employed. The time-frequency domain hybrid model can solve the contradiction of the huge amount of calculation in full time domain analysis and neglecting the effects of load sequence in traditional spectral analysis method preferably. In addition, the hybrid model is also used to predict the fatigue life of Cell-Truss Spar.4. According to the characteristics of deep-water top tensioned risers with homogeneous mass property and diameter, a simplified model is presented to predict the multi-modal response of VIV in non-uniform flow based on the experimental data from self-excited and forced oscillations of rigid cylinders. The predicted values show reasonable agreements with several VIV tests of riser models. The simplified model can obviate the solving of hydrodynamic force, which can be used as a practical method in preliminary design of marine slender structures.5. A VIV finite element prediction method is presented based on the experimental information of forced oscillations test by de Wilde to overcome the limitation of VIV simplified prediction model, the VIV finite element prediction model can take account of the intrinsic characteristics of low mass ratio VIV system, and it is more universal which can provide detail analysis in engineering practice for complicated risers in shape.6. Based on the VIV simplified prediction model, a cross-flow single-mode VIV-induced fatigue damage prediction model for Top Tensioned Risers (TTRs) is presented and an empirical formula of fatigue damage is obtained under conditions of uniform flow. The effects of environment conditions and design parameters of the structure on VIV fatigue are studied, and the regular patterns of influence factors are summarized. The phenomenon of riser's axial resonant due to cross-flow VIV is also discussed.7. Compared with the large scale model test of deepwater risers, a reasonable prediction method for the multiple mode VIV-induced fatigue damage is recommended for engineering application based on the VIV finite element prediction model.8. The inappropriate of probabilistic model for VIV fatigue proposed by a few foreign scholars is pointed out, and a new Design Current Method for deepwater riser's VIV fatigue analysis is put forward based on the long-term distribution of current. Finally, fatigue life prediction for a TTR of the Cell-Truss Spar platform is carried out based on the new method.In conclusion, the theoretical research contents and many valuable conclusions obtained in this thesis will be helpful for the future work on fatigue assessment of Spar platforms and other equipments used in deep-water oil and gas exploitations.