A Unified Technique for the Analysis of Structure Soil-Pile Systems
Authors: Brad Ernest J. , Fowler John N. , Easton Larry A. 
Published: October 2008

An efficient, computerized technique has been developed which allows a unified analysis of the interaction of a linear, statically-loaded structure with its nonlinear supporting pile foundation. All six degrees of freedom at each pile support may exhibit nonlinear force-deformation relationships. This technique is especially applicable to the analysis of fixed, pile-supported offshore platforms.

The method used consists of first modelling the linear structure and the supporting foundation systems as independent problems. Intermediate results from the analysis of these are then merged using an iterative procedure to obtain final equilibrium forces and deflections at the common structure-pilehead boundaries for each set of statically-applied loadings. Finally, the results derived at these boundaries are used to complete both the structure and pile analyses.

Computational effort is greatly simplified by using just the reduced structure stiffness and loads and the pile force-deformation relationships at the structure-pilehead interface during the iteration process.

The technique also incorporates several other new features. Among these is the ability to realign the local pilehead coordinate system so that pile lateral stiffness properties are described along axes corresponding to the direction of probable pile movement.

The principal advantage of this technique is the completeness of the analysis achieved. No longer is it necessary for the designer to make major simplifications and assumptions concerning the structure and pile foundation models and about their structural interaction with each other.

INTRODUCTION

The structural analysis of a fixed offshore structure and its soil-pile foundation is complicated by the fact that the foundation system is nonlinear with respect to its force deformation characteristics. Because of this, an iterative or trial-and error scheme is required to obtain simultaneous compatibility and equilibrium between the linear structure and its nonlinear foundation. In the past, this procedure has normally consisted of modelling the structure and soil-piling systems as independent problems into which were incorporated certain assumptions and approximations to describe the interaction effects. The results of these independent solutions were then compared and used as the basis for further model updates. This procedure continued until reasonable compatibility and equilibrium between the separate solutions occurred. Difficulties with the above scheme are primarily twofold:Multiple structure stiffness analyses are costly. Therefore, only two or three iterations are typically performed. This often leads to inadequate final results.The method of superimposing the interaction effects cannot be systematized easily. Even experienced design engineers often have difficulty properly simulating the boundary conditions for a problem so as to readily obtain compatible pile and structure solutions. The complexity of this operation can be appreciated when it is realized that the simulation for a given pile and its surrounding soil is not the same for all loading conditions in a given problem. Moreover, the various piles supporting a structure require different representations for even the same loading condition.