Probabilistic Investigation of Foundation Design for Offshore Gravity Structures

This paper lays the groundwork for establishing foundation safety criteria for offshore gravity structures. The concepts are explained in terms of first- and second-order uncertainty analyses. Various uncertainties associated with foundation analyses are identified and applications are illustrated with examples. Introduction 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 m resist severe environmental loadings. A number of structural and foundation configurations have been proposed; however, attention is restricted here to a general configuration typical of the most prominent structures presently being constructed. prominent structures presently being constructed. An example of a gravity structure is illustrated in Fig. 1. The structure foundation consists of a large caisson placed directly on the unprepared sea-bed surface. The deck is supported by large columns extending from the caisson. Various combinations of steel and reinforced concrete have been proposed, but most structures are being constructed almost entirely of reinforced and prestressed concrete. prestressed 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 modem probabilistic problem lends itself well to modem probabilistic procedures. Such procedures provide a rational, procedures. Such procedures provide a rational, quantitative means for evaluating uncertainties affecting appropriate design, even though a degree of subjectivity will always remain in any such evaluation. The probabilistic method requires the engineer to formally and consistently recognize die variability of many of the important design parameters. The method gives management and 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 levels. It also quantifies reliability to permit direct comparison 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 on the nature of soil-property variability and uncertainty. These concepts are illustrated through an analysis of a typical gravity-structure foundation. SPEJ P. 97