Application of β-Unzipping Method in Offshore System Reliability Analysis Under Changing Load Pattern

Abstract

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.

System reliability assessment of ductile frame structures using methods of moment

“…The traditional procedure for the system reliability assessment of ductile frame structures includes two steps, that is, identifying important failure modes and the computation of system probability of failure stemmed from these important modes. A number of different methods have been developed to identify the important failure modes (or sequences) of ductile frame structures, such as incremental load approach (e.g., Moses, 1982; Feng, 1988), β -unzipping method (e.g., Thoft-Christensen and Murotsu, 1986; Thoft-Christensen, 1990; Daghigh and Makouie, 2003; Yang et al, 2014), truncated enumeration method (e.g., Melchers and Tang, 1984; Nafday, 1987), branch-and-bound approach (e.g., Murotsu et al, 1984; Mahadevan and Raghothamachar, 2000; Lee and Song, 2011), stable configuration approach (Ang and Ma, 1981; Bennett and Ang, 1986), and stochastic optimization techniques (Zimmerman et al, 1993). Although some of these methods provide elegant techniques for the identification of the important failure modes, they tend to be uneconomical or even unresolvable when applied to large structures.…”