An incremental loading approach to structural reliability analysis is presented and illustrated. The structure is progressively "unzipped" as successive members reach their capacity until overall structural collapse-occurs. An inherent advantage of this approach is that the variance of structural capacity is extracted in addition to the mean value, with a minimum of computational effort. The method accounts for different types of member behavior such as brittle and ductile, different structural arrangements such as members in parallel and series, and statistical and mechanical correlation between structural elements.


The reliability assessment of offshore platform structures depends on an understanding of complex environmental 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 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 down hole pollution control devices.

The importance of making reliability assessments, especially for the purpose of making comparative 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, e.g., Ref. 1. A comprehensive study of structural reliability theory as applied to rationalization of design safety factors was completed recently in the United Kingdom2,3. Codified design procedures are reviewed in the study and a comparison of the provisions of various British, Norwegian, Danish, Canadian, and American structure design codes is made, with reference to the partial safety factor format recommended in ISO 23944 and CEB Bulletin 1115. In view of the widespread research and development efforts in reliability engineering, it is natural that these developments have found useful application in offshore structural engineering, e.g., Refs. 6-13.

Recent research in structural engineering has pointed the way for applying reliability theory to offshore 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 which 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 as has been the general rule in 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 which develops before a structure undergoes large deflections or collapses.

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