| Considering many potential hazards in offshore platforms which have serviced for a long term, reliability analysis and remaining life prediction for aged platforms should be performed in order to guarantee their safety in succeed service. The dissertation focuses on life prediction approach for ageing offshore platforms, which is a part of the National Natural Science Foundation Program (50679083) of"Research on Life Prediction Methods and Management Theory of Aged Offshore Platforms for Life Extension"and the"863"High Technology Research and Development Program of China (No.2006AA09Z355) of"Research on Life Extension Technology for Aged Offshore Platforms". The theory and methods of life prediction and reliability for ageing offshore platforms are studied systematically, and then the theoretical basis for decision-making of whether continue to service, inspection, maintenance, or retirement is provided. The main works are summarized as follows:1. Research on infrared (IR) thermographic Experiment for Deterioration Mechanism of Damaged SpecimensThe IR thermography technique, as a nondestructive evaluation technique, is applied to visual monitor the damaged process of cracked specimens. The temperature profiles of three types of specimens made by Q235 steel are observed during tensile fracture tests. On the basis of thermomechanics, the relationship between the temperature and mechanical behavior is discussed, and the damage mechanism of the cracked specimen is revealed. The fracture process is modeled by performing finite element analysis using ANSYS software, and the propagation path of compound crack as well as the crack tip temperature field is simulated.Furthermore, the fatigue tests of high strength steel specimens are carried out under IR thermographic detection. The different phase during fatigue process is distinguished by temperature profile, and the fatigue damage mechanism is described qualitatively. At last, a fatigue life prediction method is proposed, based on the energy dissipation theory, which provides an idea of predicting fatigue life through temperature evolution.2. Strength Assessment and Fatigue Life Prediction for Ageing Offshore PlatformsThe tubular joint with crack and the platform structure is modeled using ANSYS software, and the influence of crack damage on the global structure response is investigated through FE analysis. The 3-D surface crack model on tubular T- joint is created, and the detailed local stress distribution of tubular joints is determined by applying finite element submodelling method and fracture mechanics theory. The stress intensity factors along surface crack fronts are obtained and the effects of crack size and pipe corrosion on the SIFs are analyzed. A dedicated analysis approach for safety assessment of cracked offshore platforms is provided.Furthermore, the method of predicting fatigue crack propagating life under random wave loads is studied. Based on P-M spectrum and response transfer function of structural member, the statistical parameter for stochastic stress is calculated, and then the crack propagating life is obtained by applying Paris equation.3. Reliability Analysis and Remaining Life Prediction of Ageing Platforms with Multi-Failure ModesConsidering possible multi failure modes of ageing platform members, including static strength failure, fatigue failure and corrosion failure. Each reliability model is established and the corresponding computing method is presented. The interaction between any two failure modes (i.e. static strength-fatigue coupled failure, static strength-corrosion coupled failure and fatigue-corrosion coupled failure) is analyzed through correlation factor. Then, two different methods are proposed to calculate the reliability of the component under 3 failure modes coupling: one of which is the O. Detlevsen's Narrow Bounds theory; and the other is introducing the overall resistance deterioration including fatigue crack and corrosion to static strength failure model. Furthermore, a more simple, practical computing method is developed for assessing the system reliability of ageing offshore platform in succeed service life. Finally, considering the uncertainty factors among loads, structural member configuration, material properties, and reliability computing models, the remaining life prediction technique based on reliability theory is investigated.4. Reliability Updating for Ageing Offshore PlatformsExisting offshore platform is an objective entity, which is much different from to-be-built platform structure. It requires using now available information to infer the behavior of structure in future service period, when analyzing the reliability of in-service structure. The reliability analysis method for existing ageing platform is introduced. In order to overcome the shortcoming of traditional environmental load description, a new assessment load prediction method is proposed, which may consider not only the stochasticity but also the time-varying of loads. Furthermore, on the basis of two types of historical information including experienced loads and inspection records, the reliability updating in future is analyzed. First, current measured crack size is used to obtain the resistance of the component, and then the reliability is updated with the Bayesian method; then, the dynamic reliability of offshore platforms in the subsequent service period is updated, based on dynamic time-dependent reliability theory, regarding the annual maximum wave loads in prior service time as proof loads, and considering the degradation of resistance for aging offshore platform members; finally, the reliability updating in future is analyzed according to inspection results.5. Extended Life Cycle Management for Ageing Offshore PlatformsConsidering the economical factor, the inspection and maintenance planning model in life extension period for aged offshore platforms is established based on the optimum cost and optimum cost-benefit ratio, respectively. Both the methods are required to ensure that the reliability in service life is larger than the minimum target reliability index. Among them, the former model makes the total expected cost of the ageing service period and life extension period minimum, while the latter makes the cost- benefit ratio minimum. A damaged component of offshore platform subjected to fatigue crack is set as an example to illustrate the proposed two models. Furthermore, some sensitivity studies are conducted to investigate the effects of basic parameters on the models. Finally, the optimum economic life prediction method is present for aging offshore platforms.
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