This paper describes a study that was undertaken by Synercom Technology, Inc. to investigate the various considerations involved in the analysis of offshore platforms as large space frames with coupled interaction to non-linear pile foundations. STRAN, a large capacity structural analysis computer program developed by the company, was modified to consider non-linear boundary or support conditions. A general program interface was developed that permits 1inkage with most nonlinear pile analysis programs. Taking advantage of the solution by substructures approach to the stiffness method utilized in STRAN, matrix condensation methods were used to reduce the stiffness matrices to involve only the terms corresponding to pile support degrees of freedom. Techniques were developed to 1inearize the effects of the pH e foundation on the structure and an iterative procedure was used to determine convergence between structure and pile boundary conditions.
In the course of the study, the coupled system was used to analyze several representative offshore structures to determine the effects of varying different parameters. The offshore structures were analyzed as space frames for the effects of loads imposed by gravity, wind, wave, current and seismic forces. Parameters that were varied for investigation in the course of the study include structure size, soil conditions, pile batter, and structure loadings. The use of conductors as pile members and the convergence sensitivity of various pile-support degrees of freedom were also investigated. A description of the computer programs used and discussion of study findings are included in this paper.
The study determined that the methods for Coupled Interaction Analysis presented herein are feasible and provide good results for the range of structures and parameters considered. Convergence to a solution was generally achieved in approximately three to six iterative cycles and the required computer time was on the order of only twenty to thirty percent more than that normally required for linear analysis of the superstructure alone. A discussion of the significance of Coupled Interaction Analysis is included that compares results with those obtained by presently used manual methods of approximation of structure foundation interaction.
In the design and analysis of fixed-offshore structures, it has long been standard practice for designers to analyze the superstructure and the pile foundation as independent problems. The obvious reason for this practice is that the superstructure analysis commonly assumes a linear elastic system while the foundation analysis must consider the nonlinear nature of the pile-soil medium. As discussed in further detail in this paper, this practice involves assumptions and approximations that can lead to significant analysis inaccuracies. As offshore structures are designed for deeper water and more severe environmental conditions, it becomes ever more important to have as much numerical accuracy as possible in analysis methods. Therefore, to determine a compatible solution with a high degree of numerical exactitude, it is desirable for a designer to be able to analyze the pile, soil and structure as an integral system.
It is generally recognized that such a coupled analysis is possible.
