Gravity structures play a prominent role today in North Sea oil development. These structures are not supported by piles as are most ocean structures but rather sit directly on the ocean bottom, and depend on their foundation geometries and large weights to resist severe environmental loadings. A number of structural and foundation configurations have been proposed; however, attention will be restricted in this discussion to a general configuration typical of the most prominent structures being constructed at present (1975). An example of a gravity structure is illustrated in Fig. 1. The structure foundation consists of a large caisson placed directly on the unprepared seabed surface. The deck is supported by large columns extending from the caisson. Various combinations of steel and reinforced concrete have been proposed, but the majority of structures are being constructed almost entirely of reinforced and pre-stressed concrete.
One of the primary engineering concerns with these structures is foundation design. Because of the variability associated with the environmental forces as well as the basic soil properties, this problem lends itself well to modern probabilistic procedures. Such procedures provide a rational, quantitative means for evaluating uncertainties affecting appropriate design, even though a of subjectivity will always remain in any such evaluation. The probabilistic method requires the engineer to formally and consistently recognize the variability of many of the important design parameters. The method gives management and others responsible for setting design criteria an opportunity to appraise cost-benefits of design levels required for given reliability safety levels. It also quantifies reliability to permit direct camparison with other options.
This paper presents a method for analyzing the reliability of gravity structure foundations in terms of simple loading and resistance models. The sources of variability in estimating resistance to loads are discussed with particular emphasis placed on the nature of soil property variability and uncertainty. These concepts are illustrated through an analysis of a typical gravity structure foundation.
Reliability as defined in this paper is a measure of the probability of survival. For example a perfectly reliable foundation corresponds to a zero probability of failure. Because of uncertainties in environmental loads and soil properties plus the ever present uncertainties due to simplifying assumptions in the analytical methods, the chance of failure is always present although acceptably small for proper designs.
Characterizing the structure's resistance as a function R and the load acting on a structure as a function L then the failure probability Pf is simply the probability that L is greater than R. For structures of the type presented in Fig. 1 we can reasonably consider the load and resistance to be statistically independent so that
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