Research On Reliability Assessment Method For Structural Strength Of Semi-submersible Platform

The safety and reliability of offshore platform structures is one of the most important design indexes for its normal production, operation and survival. Along with the development of ocean exploitation to deep water, platform structural safety problems have been drawing more and more concerns from related industries. Therefore, in this paper, extensive explorations and researches are carried out for semi-submersibles in the various aspects of wave loads prediction, ultimate strength assessment, the uncertainties analysis of both wave load and structural capacity, the reliability analysis of whole platform and its main partial structures.Firstly, the importance of deep-sea marine structural safety assessment is emphasized, the development of loads prediction and structural ultimate strength reliability evaluation for semi-submersible platform are reviewed.Secondly, the short-term wave load prediction, long-term prediction and extreme value analysis are carried out, the effects of the wet surface mesh size and horizontal support pipes in simulation scheme on wave loads are analyzed. The effects of different sea state statistics on long-term prediction results are further investigated; the influences of wave directions and the number of sea states involved on the long term extreme distribution of wave loads are also studied.Thirdly, the structural ultimate strength is evaluated. Based on simplified progressive collapse method, a self-programming software, which is verified and validated by comparing the predicted extreme strength with other calculated results for various models, is applied to calculate the hogging and sagging transversal ultimate strength of semi-submersible platform. On the other hand, the strength capabilities of horizontal support pipes of the platform are obtained according to analytical formulas in the codes and regulations.Fourthly, the uncertainties of basic parameters that affect structural capabilities are investigated through extensive literatures survey and analysis, including the structural sizes, material properties and calculation models, etc. The statistical characteristics of structural capabilities is analyzed by Rosenbluth method; In the uncertainty analysis of wave loads, the effects of the wave load transfer function, wave spectrum type and wave height on wave loads are studied intensively. Three approaches are established to account for the effects of short-term uncertainties on long-term values.Finally, the reliability of semi-submersible platform transversal structures and its horizontal support pipes is researched by Check Point Method. Through the reasonably chosen characteristic value of wave loads and structural capabilities, the partial safety factors of structural capabilities and loads are given by Load and Resistance Factor Design (LRFD) method.The achievements arrived will provide technical supports for design and safety evaluation for semi-submersible structures with LRFD method based on reliability principle.