Reliability Analysis Format for Offshore Structures

Abstract

An incremental loading approach to structural reliability analysis presented and illustrated. The structure is progressively "unzipped" as presented and illustrated. The structure is progressively "unzipped" as successive members reach their capacity, until overall structural collapse occurs. The method accounts for different types of member behavior, different structural arrangements, and statistical and mechanical correlation between structural elements. Introduction The reliability assessment of offshore platform structures depends on an understanding of complex environmental phenomena, such as storm and seismic loadings, and an phenomena, such as storm and seismic loadings, and an accurate analysis of structural capacity. The increasing demands for producing in hostile environments and the huge economic investments in even a single offshore platform structure suggest that application of structural platform structure suggest that application of structural reliability theory and design for target risks should be considered. The ultimate goal is an optimum utilization of economic resources properly divided among the demands of structural capacity, materials inspection, construction quality, and safety equipment such as downhole pollution control devices.The importance of making reliability assessments, especially for comparing design judgments, has received recognition in the last few years. Reliability theory has been used extensively in analysis and design of bridges, buildings, transmission towers, nuclear power plants, and in development of general purpose structural design codes. A comprehensive study of structural reliability theory as applied to rationalization of design safety factors was completed recently in the U.K. Codified design procedures are reviewed in the study, and the provisions of various British, Norwegian, Danish, provisions of various British, Norwegian, Danish, Canadian, and American structure design codes are compared with reference to the partial safety-factor format recommended in ISO 2394 and CEB Bulletin 111. In view of the widespread research anddevelopment efforts in reliability engineering, it is natural that these developments have found useful application in offshore structural engineering. Recent research in structural engineering has pointed the way for applying reliability theory to offshore platforms. Environmental and load effect uncertainties platforms. Environmental and load effect uncertainties have been modeled and combined with material and strength capacity uncertainties to obtain a measure of the risk of failure that occurs when load demand exceeds structure capacity. Although such research efforts have not reached the stage where full design implementation is recommended, a number of important developments have occurred. This paper applies these reliability developments to offshore structures. Reliability modeling of complete structural systems is presented, rather than modeling of only single elements, which has been the general rule during development of probability-based design codes. A platform framework is modeled as an assemblage of numerous structural elements including brace members, connections, piles, etc. Actual behavior of the elements is required to predict the true resistance or capacity that develops before a structure undergoes large deflections or collapses.A general approach to resistance prediction is described for assessing capacity. This method progressively "unzips" the structure with load increments as progressively "unzips" the structure with load increments as successive members reach their capacity until failure occurs. JPT P. 347