Reliability Analysis And Stochastic Finite Element Study Of Offshore Structure Foundations

With the rapid development of petroleum industry, platforms that take as the importance facilities, have been building continuously. The platform plays an important role in investigating, exploiting, producing, transporting and storing oil in offshore engineering.The designing method for upper structure of platform is based on the reliability theory. It has been found that the same procedure may not be easy to be adopted in foundation designing. This leads to an incoordination between the methodologies of designing upper structure and foundation of a platform. As a result, more attentions should be paid on studying the foundation design method of offshore structures. Modifications of these factors or development of new factors suitable for a different type of structural framing should be based on a rational system reliability analysis. In this thesis, the following work has been involved:1. When the reliability method is applied in foundation design, the statistic characteristics of subsoil have to be known. In this thesis, a database management system has been established, in order to manage and analyze the data of the geotechnical properties of foundation soils. This database includes almost all the borehole logs and properties of the soils of SZ36-1 Oil Field.2. Based on the database, the stationarity and ergodicity of a soil profile in SZ36-1 Oil Field have been examined and a random field modeling has been established. The correlation function, correlation distance and variation reduction function of SZ36-1 Oil Field have been simulated.3. A calculating model for shallow foundations has been established, on the basis of the established database and design experiences. As an example, reliability analysis for a foundation of self-raising platform has been performed. The calculated partial factors have been compared with those recommended by the API Criterion.4. In order to analyze the reliability of the shallow foundation of ocean structures, two-dimensional perturbation stochastic FEM(PSFEM) has been developed. As an example, a practical project, Bohai-6, has been calculated with this program. The reliability indexes calculated by PSFEM changing with safety factors have been compared with those calculated with the limit equilibrium method recommended by the Criterion for foundation design.The developed method may give a guidance for how to introduce the reliability theory in designing soil foundations of ocean structures.