Structural Design Optimization Of Offshore Platforms: Methods And Software

Offshore platforms are main structures for marine petroleum and natural gas exploitation. They are also the platforms of production and life on the sea. The design .period and quality affect directly the cost and efficiency of marine exploitation. The application of advanced technology of design optimization is important means for raising design quality, shorting design period and ensuring design security and reliability. Thus, it is very important to research the method and software of design optimization for offshore platforms. The platforms are under the action of complex ocean environment including wind, wave, current and ice, which lead to structural static and dynamic responses intricate. A significant matter is to develop effective methods for structural response analysis, sensitivity analysis of responses with respect to shape and size design variables, and modeling and solving the problems of design optimization.The theoretical and numerical methods of structural design optimization for offshore platforms are studied in this thesis. A design optimization model of jacket platforms with multiple design criterions and multiple constraints is established. The numerical calculation methods of structural static, dynamic, random response and sensitivity analysis of structural response under the action of complex ocean environment are presented, and the formulation and solving methods of layout, shape and size optimization for platforms are studied according design criterions. The software of design optimization for offshore platforms is developed based on CAE and JIFEX software system. With exploitation of the Bohai Sea oil field in the background, several actual jacket platforms are optimized. The major contents in this thesis are summarized as follows:1. An integrated layout, shape and size design optimization method for the jacket platforms is studied. The structural characteristic of the jacket platforms and ocean environment are analyzed and multi-load cases including wind, wave, current and ice from different directions are considered in the formulation. The reasonable finite element models for various structural mechanical responses are established. According to the design criterions, the model of layout optimization (that is the union between topology optimization and size optimization) is presented. Various kinds of constraints including stress, displacement, stability, transverse carrying capacity of pile foundations and shearing strength of tube joints are considered. The process of layout optimization for platform structure includes two steps: firstly, the ground structure method is used to find the topology optimum. Then the size and shape of the structures are optimized. Coupling between various design variables possibly cause trouble in convergence, two-level optimization scheme is adopted to deal with the problem. After layout optimization, a new offshore platform model is obtained that can satisfy various design requirements such as structural stability, safety service time, the lowest cost and so on. The design optimization of an actual jacket platform is presented to show the effect of improving mechanical performance and reducing cost.2. The methods of structural transient response analysis and design optimization for jacket platforms under ice force are studied respectively. The ice force is simulated as cycling load. The problem of transient displacement response is solved by the method of finite element numerical analysis. In order to control vibration of platforms under the action of ice force, the model of dynamic design optimization for platforms is established considering the constraints of transient displacement response and structural inherent frequencies. The cross sectional sizes and nodal coordinates are taken as design variables. The derivatives are coded directly in the sensitivity analysis of structural transient response, which based on theNewmark integral method. Sequential Linear/Quadratic Programming (SLQP) is used to solve the problem of design optimization.