A Specialized Design and Research Tool for the Modeling of Near-Field Pile-Soil Interactions
Authors: Bea R.G. , Litton R.W. , Nour-Omid S. , Chang J.Y. 
Published: October 2008

A computer code and supporting documentation have been developed to permit analyses of dynamically loaded pile-soil systems. Advanced procedures have been incorporated into the code to enable analyses of cyclic, strain rate, gapping, nonlinear pile diameter and length, and damping (hysteretic, radiation) effects for piles embedded in cohesive and cohesion less soils (including layered profiles). The code has also incorporated standard industry procedures to facilitate highly automated design applications. Verification of the code with field pile load test data has again demonstrated the importance of user specification of soil properties consistent with the empirical bases of the industry procedures.

INTRODUCTION

In the last decade, the offshore platform engineering community has become concerned with design of structures in which dynamic loadings and responses are primary issues. This has led to a need for development of advanced analytical capabilities for modeling dynamically loaded pile-soil systems.

This need has been addressed by four oil companies (Chevron, Gulf, Mobil and Shell) in a project identified as NFEP, Near-Field Enhancement Project. The first phase of this project has focused on development of a general purpose design and research computer code that incorporates advanced procedures for analyses of cyclic, strain rate, gapping, non-linear pile diameter and length, and damping (hysteretic, radiation) for piles embedded in cohesive and cohesionless soils (including layered profiles). The code has been named PSAS, Pile Soil Analysis System.

CODE STRATEGIES

During NFEP, four key categories of code development strategies were evolved and implemented. These were:high degrees of user friendliness,generalized nonlinear element,state-of-practice (SOP) and state-of-art (SOA) algorithms for state determination of the generalized nonlinear element, andmultiple level verification.

Specific elements of user friendliness that were incorporated included extensive external and internal documentation of the code, documentation of user guidance for determination of input quantities, documentation of alternative procedures considered, reasons for selection of the specific algorithms, and coding with a focus on development of an essentially machine independent code.

Three options were provided for input and subsequent analyses:automated (design applications),semi-automated (development applications), andgeneralized (research applications). The automated input option requires a minimum of input and utilizes industry standard procedures for the analyses of laterally and axially loaded piles.

The semi-automated input option requires more extensive input and allows analyses of additional factors of importance to dynamically loaded pile foundations (e.g., strain rate effects, nonlinear diameter effects, cycle-by-cycle load-deformation behavior, radiation damping).

The generalized input option requires full specification of all points (states) and rules governing behavior of the nonlinear elements. This option provides a high degree of versatility to model unusual pile-soil interactions.