APPLICATION OF β-UNZIPPING METHOD IN OFFSHORE SYSTEM RELIABILITY ANALYSIS UNDER CHANGING LOAD PATTERN

摘要

This paper is concerned with the reliability assessment of a fixed platform operating in intermediate water depth under extreme environmental conditions. The structure is modelled by two-dimensional frame structure and subjected to quasi-static loads. Extreme wave loads acting on the platform in the unidirectioal mode are estimated together with their probabilistic characteristics. Fully plastic capacities of the members are also treated as random variables. The structure is modelled as a plane truss by using beam-column models and the plastic collapse of the structure is evaluated by using a linearized plasticity condition of the section under effect of axial force to generate the safety margins. For compressional elements, the strength reduction is assumed proportional to the yield stress and for the tension members, it is assumed that the material has a perfect elastic-plastic behavior. Both hydrodynamic drag/inertia forces and slamming loads have been considered in strength and reliabilitry analysis. Using a matrix method, probabilistically dominant collapse modes are selected by applying the β-unzipping method. The system reliability is evaluated by using wave fragility analysis (changing load pattern) and the results have been compared with the fixed load pattern.