Evolution of fixed offshore platforms for greater water depths, heavier topside loads, and more difficult soil conditions has increased the complexity of most platform components. Foundation systems are becoming more complex, utilizing various configurations of main piles, skirt and pin piles, and pile groups. Many current methods of analysis for the structure-foundation system, which have been sufficient for simpler configurations, are often inadequate of even not applicable for more complex systems.
An analysis method is briefly described in this paper which provides a three-dimensional, simultaneous solution of the complete structure-foundation system in a rigorous, economically feasible manner. Several types of complex foundation configurations are discussed in the context of design considerations necessary for a rational analysis, and the application of the present method of analysis to these problems is presented. Design considerations for mudslide conditions are also discussed in conjunction with the extension of the present analysis for these conditions.
The pile foundation systems of fixed offshore platforms are becoming increasingly complex due to a combination of factors generally not present or restrictive in earlier platform concepts. These factors include the move into deeper waters, increasing topside loads, and difficult soil-conditions often encountered at sites previously deemed not economically feasible. In addition, deep-water structures typically are restricted to compact base dimensions to reduce overall jacket weight, and meet fabrication and installation restrictions on size and weight of components. Foundation configurations designed to provide adequate support under these conditions have tended toward the use of more piles, and group pile arrangements. As foundations become more complex, the design/analysis procedures likewise increase in difficulty. Analysis techniques that are utilized warrant close scrutiny to assure the rationality and applicability of the involved assumptions.
In the past, analytical procedures have generally evolved in keeping with the complexity of the problem at hand, or problems foreseen in the near future. Structural analysis of platforms has evolved from tedious hand calculations such as moment distribution to sophisticated space-frame and finite-element computer programs. Similarly, pile foundation analysis has progressed from iterative, trial-and-error hand methods to nonlinear discrete, or finite-element pile and bent analysis computer programs. One element of the design process which has remained relatively unchanged until recent years has been the use of independent structure and foundation analyses in considering the complete structure-foundation system. Current design procedures, regardless of the sophistication of analysis tools, are performed in a manner similar to that idealized in Fig. 1. Foundation and structure analyses are executed independently in an iterative fashion until satisfactory compatibility between the two systems has been achieved. The number of iterations involved in this process generally depends upon the complexity of the system, experience of the design engineer, and the degree of incompatibility deemed acceptable. This procedure requires a significant engineering and computational effort to produce satisfactory results for other than relatively simple configurations. Significant progress has been made in recent years to implement the procedure in a systematic, automated manner.
