This paper is intended to discuss the basic issues involved in structural reliability in conjunction with a limit state approach for design and analysis of offshore structures. Emphasis will be placed an a systematic semiprobabilistic approach suitable for strength evaluation of major structural elements. Problems arising from quantification of uncertainties of the pertinent random variables, load combination, and in the development of limit state functions involving both ultimate failure and fatigue, will also be discussed.


The goal of any structural design is to produce a sound structure that will be able to maintain its integrity for the duration of its intended service life and in a manner capable of performing its intended functions. To the extent that it is able to meet this goal, the structure is considered safe. Traditionally, the safety of structures and their components have been determined through a series of checks based on design criteria which, in essence, require that the structural resistance (strength) must be greater than the anticipated loading.

One such approach for evaluating structural safety, well known and widely adopted, is the working stress format. In a traditional working stress approach, the design loading may, for instance, be selected to simulate the most probable severe condition expected during the projected service life of a structure. The sizes and dimensions of the various structural components con then be determined by a set of stress limits based on the material properties and types of loading under consideration. properties and types of loading under consideration. Uncertainties associated with load predictions and strength assessments are presumably compensated for by introducing nominal safety factors derived primarily from post experience. Until the recent increased interest in tension leg platforms and other types of compliant structures, offshore structural design has also been primarily based on technology employing such traditional approaches.

It is to be recognized that such a traditional design procedure or offshore structures has some important shortcomings with regard to both loading and strength determination. On the loading side, the inherent assumptions usually result in a great deal of conservatism, since factors such as directionality and joint probabilities of occurrence are often not taken into consideration. probabilities of occurrence are often not taken into consideration. p. 9–1 p. 9–1

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