Development of a Statistical Model for Laterally Loaded Pile Analysis to Reduce Uncertainty in an Offshore Field

INTRODUCTION
The purpose of this study is to develop a statistical model for analysis of lateral pile capacity that incorporates regional geology. The model is intended for use to design piles for fixed offshore platforms. Relevant soil properties for analyses were obtained from geotechnical site investigation reports performed in the study area over a period of 30 years by different companies.
The model developed for lateral pile capacity analysis is field-specific. However, the methodology for developing the model is general and can be applied to other fields.
1.1 Background
The conventional investigation program for designing offshore foundations (steel pipe piles in this study) is to drill a soil boring at the site of the foundation. The shear strength profile for the site is the primary quantity used for pile design. The design shear strength profile is determined by shear strength tests performed on soil samples obtained from borings. In current practice, design strength profiles are developed from unconsolidated undrained triaxial compression tests performed on samples obtained from borings using pushed sampling techniques. However, the final platform location may be relatively distant from where the soil boring or borings were made. For example, facilities can be moved after the geotechnical 2 investigation, but before construction due to reservoir considerations or geohazards. As a result, it is of interest to the designer to be able to assess the spatial variability in soil properties and pile capacities across the field.
The model developed herein attempts to predict the design soil shear strength profile for lateral pile capacity analysis at sites where data from unconsolidated-undrained triaxial tests performed on samples from borings with pushed sampling techniques are not available. The same study was done by Gambino and Gilbert (1998): A database of soil properties was compiled and statistical models for axial pile capacities were developed for an offshore area. The study presented in this thesis will follow a similar approach and organization.
1.2 Organization of Thesis
This thesis is organized into nine chapters with tables and figures placed at the end of each chapter. Chapter 2 presents a brief description of the database and type of data entered into the database. A brief description and the geology of the study area are presented in Chapter 3. Also, in this chapter, a conceptual geological model is developed for shallow soil deposits (0-300 feet). The conceptual geological model is developed based on the available information on geology, geotechnical engineering properties of soils and shallow geophysical investigation reports. The finite difference method using p-y curves for analysis of laterally loaded piles is described in Chapter 4. The required soil parameters for the lateral pile capacity analyses were obtained mostly from the geotechnical investigation reports, and the soil and pile parameters used in the analyses are presented in Chapter 5. Computation procedures for lateral pile capacity analyses and theresults of the analyses are presented in Chapter 6. In Chapter 7, equivalent simplified soil profiles for each boring location are developed and the variability in these profiles across the field is analyzed. A model to predict the design shear strength profile at a new site with either no boring or a boring using driven sampling techniques is developed and calibrated in Chapter 8.