Case Study Of Offshore Pile System Reliability

Abstract

ABSTRACT This paper presents the results of research effort to evaluate the reliability of offshore pile foundation systems. It is an extension of previous work by the authors [14] dealing with the reliability of individual piles under axial and lateral loading. The reliability of the pile system is evaluated for three ease study platform designs, and the level of reserve between initial failure of an individual pile within the system and system collapse is studied. The results of this research are valuable in providing insight into the reliability achieved with current design procedures and the factors which affect the performance of the foundation system. The results should facilitate the development of reliability based design procedures which provide for a consistent level of safety between different site conditions, loading conditions and pile configurations. INTRODUCTION Satisfactory performance of pile foundations for offshore oil platforms is an important consideration in design, and a better understanding of the reliability of platform foundations is important as design practices move towards reliability based systems. Evaluation of pile system performance is influenced by various sources of uncertainty including uncertainties in system loads, pile properties and soil properties. In addition,. uncertainty is associated with the inability to predict performance using simplified models of actual behavior. These uncertainties have traditionally been accounted for through a variety of conservative design practices extreme loading conditions are use~ soil properties are conservatively selected; deterministic safety factors are applied to the design, and design is often based on failure of the most heavily loaded pile in the foundation, not failure of the entire pile system. AH is currently in the process of formulating a reliability based design procedure for fixed platforms. The primary advantage of this approach will be to achieve a consistent level of performance reliability in platform design between different site conditions, loading conditions and piIe configurations. In formulating a reliability based design procedure, the following questions arise:What level of reliability is achieved for an individual pile and for the pile system using traditional design methods?How do the various sources of uncertainty affect the reliability of the design?How much redundancy or reserve between initial failure and system collapse is built into the design? Since 1986, a series of research projects has been sponsored by API to address these questions. Findings from the first two phases, which deal with the reliability of individual piles under axial and lateral loading, are summarized in [14]. The third phase concentrates on the reliability of the pile system and the relationship between the performance of individual piles within the system and the performance of the system. Details of this research are reported in [15], and the results are summarized herein.