This paper emphasizes the concept of "as-built" reliability vs "as-designed" reliability related to fixed offshore platforms. After an offshore platform is initially designed in accordance with criteria based on probabilistic concepts, modifications may be introduced which can affect its intended structural reliability. These modifications can occur during the various phases of design, fabrication, and installation.
Several examples of such changes are cited. A brief outline of one approach to computing a platform's probability of failure is also presented and a computational example is included to illustrate the methodology of comparing the as-built reliability with the as-designed reliability.
In recent years probability theory has been applied to structural engineering in an attempt to develop a rational method of establishing consistent levels of safety and, hence, more economical structures. This is accomplished by characterizing the uncertainties involved in design and analysis in terms of probability distributions; i.e., the parameters involved in design and analysis are treated as random variables whose uncertainties can be described by probability distributions.
When probabilistic methods are used to design offshore platforms in accordance with structural reliability formats, estimates are made of the means and variances of design parameters. These means and variances are used in the fundamental reliability statement
to determine the structural reliability of offshore platforms,
RELIABILITY = PROBABILITY OF SURVIVAL =l-P[RESISTANCE < LOAD]...(2)
where load and resistance are considered random variables whose uncertainties are dependent on the uncertainty of the parameters of which they are a function. For example, in the offshore industry, uncertainty in the load includes uncertainty in storm-occurrence rates, wave heights, wind velocity, and the techniques used to translate the environmental state into loads on the structure, while uncertainty in the resistance includes uncertainties in the strength of jacket members, deck legs, piles, and connections, etc.
When an offshore platform initially is designed using prior estimates of means and variances of the design parameters, the structural reliability that it possesses can be termed the "as-designed" reliability. These prior estimates contain both variability and uncertainty. Variability describes the unavoidable deviations from sample to sample, deviations within the state of design and construction practice. Uncertainty reflects a lack of information; e.g., the designer may lack information about the mean yield strength of the steel that will be delivered to his job. However, before the structure goes into full service, information in the form of additional data and/or changes may become available that may alter the platform's structural reliability. This altered reliability can be called the as-built reliability.
The changes referred to above can occur in various aspects of design, fabrication, and installation. The purpose of this paper is to provide some insight into the concept of as-built vs as-designed reliabilities by giving several examples of changes that can occur and illustrating how these changes, in turn, can affect the structural reliability of offshore platforms.