This paper describes a rigorous analysis method, PASS (Pile And Structure Solution), for pile supported structures such as fixed offshore platforms. PASS considers the total structure-pile-soil system in a consistent, systematic, and fully three-dimensional manner, and is believed to require the least possible manpower and computer effort for rigorous solution of any given problem. The analysis is restricted to linearly elastic behavior of the superstructure and pile members but the nonlinearity of the axial, lateral, and tensional soil displacement- reaction relations is included. Complete interaction and compatibility are enforced at the structure-pile interface for the six possible displacement components.
Efficiency of the analysis is achieved by condensation of both the superstructure and pile foundation to the structure-pile interface, and by the capability to obtain the foundation behavior for multiple load cases with only a single space-frame analysis. PASS may also be combined with many existing, extensively developed frame analysis programs to provide a total solution package.
The ability to obtain a rigorous, fully three dimensional solution for the complete, coupled system frees the designer from reliance on unnecessary approximations regarding the mechanics of the structural solution process. More of the designer's effort may then be devoted to some of the other complex aspects of the design process.
The superstructure of a fixed offshore platform is generally a complex, highly indeterminate frame which transfers the applied loads to the pile foundation where the soil reactions place the structure in equilibrium. The superstructure is usually considered to behave in a linearly elastic manner, while the axial, tensional, and lateral displacement-reaction relations of the foundation soil are invariably nonlinear. Nonsymmetrical geometry of the structure or of the applied loading leads to three-dimensional behavior of both the superstructure and the pile foundation.
Development of analysis methods suitable for the individual components (superstructure and piles) of an offshore platform has been the subject of considerable effort as evidenced by the numerous frame analysis methods (Refs 4,6,8,9,10,11,18) and a lesser number of pile analyses (Refs 5,12,15) which are currently available. Three-dimensional behavior of the structure is generally considered in the superstructure (frame analysis) but is not considered in the pile foundation analysis, that is, the pile member and supporting soil (e.g., p-y curves) are restricted to planar behavior.
The flowchart in Fig 1 describes an idealized current method of platform design in which the superstructure and pile foundation are analyzed more or less independently in an iterative scheme until satisfactory compatibility is achieved between the structure and the pile foundation. This method is often not economically feasible since it requires multiple space-frame analyses and significant external control by the designer. Economic and time restrictions typically may lead to acceptance of less than satisfactory compatibility requirements.
Recent efforts have been directed' toward application of basic structural analysis concepts in a systematic and more efficient manner.
