Local Ultimate Strength And Reliability Study Of Deep Water Platform

With the development of oil&gas exploitation in deep water area of South China Sea, it's the key problem that how to ensure the safety of deep-water offshore platforms in severe environments. The work in this paper is one important part of sub-program of the National High Technology Research and Development Program ("863"Program) "Safety evaluation technology of floating structures" (No. 2006AA09A109-5). The aim of the study is to perform a method on ultimate limit state assessment of local components of the floating structures. It's the base of safety evaluation by certain method and reliability method. This paper mainly contains following aspects:1. The characters of environment parameters (wind speed, wave height, current speed) in South China Sea are analyzed. The probability distribution functions and design values with a certain return period are determined.2. The ultimate strength of the stiffened plate is analyzed. The ultimate limit state(ULS) equations of the stiffened plate in every failure mode are established. Firstly, the structural component's ultimate strength is analyzed in the uniaxial load. Then based on these results, the ULS equations according to combined loads are established.3. The ultimate strength of the stiffened cylindrical shell is studied. According to the allowable stress equations of cylindrical shells in standard, the ULS equations of various local cylindrical shells are determined. In these equations, the different failure modes and loads are considered.4. Based on the finite element method(FEM), the method of analyzing ultimate strength of structure is established. Some samples are calculated using ANSYS. And the results are compared with theory ones. The comparisons show that the results are precise and that some parameters such as boundary condition, initial imperfection, load step order should be concerned carefully. The amplitude of initial deformation has little influence on results.5. The reliability analysis method for the local components is proposed. Based on the ULS equations, the reliability index and failure probability are computed using first order reliability method(FORM) when a single failure mode happens. Interval estimate method is adopted to calculate structure system failure probability.