| Based on the theory of structural optimum design, decision-making, engineering fuzzy set and the method of structural reliability analysis, the optimization theory and approach of offshore platforms are studied in this thesis. The major contents are summarized as follows:1. The nonlinear equivalent springs are used to simulate the action of soil on piles with the consideration of the nonlinear characteristic of pile-soil interaction and the finite element analysis is carried out on the pile. The Dual-pole Fuzzy Pattern Optimum Selection Model is adopted to perform synthetical decision-making for the 5 schemes of pile penetration depth and 4 indexes are selected as the evaluation criterion. With this model, the selected scheme can meet the objectives such as economy and safety during the construction and operation of the offshore platform at the same time. Finally, the pile penetration depth is obtained to the given loads and seabed geological conditions using the model presented.2. The multi-objective lectotype optimum model based on water depth and geological condition is presented for offshore platforms taking into account multiple items such as the original investment, safety during construction and operation, maintenance cost, the ability of resisting fatigue and corrosion, construction period, threat to the environment, and so on. On the base of Three-Scale Analytical Hierarchy Process (T-AHP), the Three-Scale Fuzzy Analytical Hierarchy Process (T-FAHP) is proposed by introducing the trapezoid fuzzy number. Aimed at the characteristics of our country oil field development, water depth is divided into 6 depth intervals and different platform alternatives are supplied to each interval. Simultaneously, the seabed condition is classified as 4 types. Through the analysis of a practical instance with this model, it can select the optimal platform type suitable for the specified water depth interval and geological condition and provide references for the primary design of offshore platforms.3. The features of reinforcement optimum design of prestressed concrete offshore platform are studied and an optimization model with mixed discrete variable for reinforcement design is established subsequently. The number of vertical and circumferential bars and their section areas or diameters of prestress wire and regular reinforcement per meter are regarded as discrete variables, while the ring wall thickness is regarded as continuous variable. The objective function of the optimum design is the total material cost of the concrete tank and the constraint conditions are set up with those design requirements specified by current codes and standards, such as the strength, crack resistance, dimensional requirement, and so on.4. The structural characteristic of the concrete-steel combined platform is analyzed. Because it involves so many kinds of material as concrete, reinforcing bars and steel, the traditional dimension optimization method will meet great hardness, even it cannot be implemented at all.The multi-level optimization method is applied to the optimal design of concrete-steel combined platform, which is formulated as a system-level optimum design problem and a set of uncoupled substructure-level optimum design problems. The system-level optimization and substructure-level optimizations employ the continuous variable programming technique and mixed discrete variable optimum design method, respectively. The optimization process is to perform iterative computation between system-level optimization and substructure-level optimizations until the objective function converges. An illustrative example indicates that this model is effective in the optimum design of concrete-steel combined platform. 5. The intensive corrosion action of ocean environment is an extraordinary hazard to the steel jacket platform. The existing optimum design methods only aim to achieve the minimum initial investment or the lightest weight. In fact, result obtained with these methods is the lowest level design that satisfies...
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