Reliability of API, NGI, ICP and Fugro Axial Pile Capacity Calculation Methods
Authors: Lacasse Suzanne , Nadim Farrokh , Andersen Knut H. , Knudsen Siren , Eidsvig Unni K. , Yetginer Gülin Luis , Guttormsen Tom , Eide Asle 
Published: June 2014

The API RP2A (RP2GEO) and ISO 19902 guidelines include four CPT-methods for calculating the axial capacity of piles in sands. The guidelines require that if newer methods are to be implemented in design, the same level of safety shall be docu-mented for these methods as for existing methods. The designer is required to select an appropriate safety factor when using the newer design methods. The challenge lies in deciding which safety factor will ensure a consistent safety level for different soil conditions and pile dimensions. To evaluate the required material factor, the probability of failure was quantified in two case studies for piles designed with the API method and with the newer NGI, ICP and Fugro methods. A calibration of the required material factor for a target probability of failure of 10-4/yr was also performed. The results show that the annual reliability index and probability of failure vary with the axial pile capacity calculation method. The study provides a contribution to the discussion on the reliability of the API, the NGI, the ICP and the Fugro methods. The material factor needs to be associated with the characteristic soil parameters selected for design. A large number of case studies should be added to quantify the reliability and the required material factor for each pile capacity method. The findings on margin of safety and the definition of the characteristic shear strength have important implications for the design of piles offshore and can result in significant savings.

Introduction

Ensuring adequate reliability under severe loading conditions is a necessary consideration for offshore platforms, and the safety margin depends on the uncertainty in the parameters entering the analyses, in addition to the model uncertainty. The design engineer attempts to compensate for the uncertainties by introducing an appropriate " factor of safety" in design. There will always be a finite probability that the forces of the environ¬ment can cause damage, or the total collapse, of an offshore structure. Defining the level of finite probability that is tolerable is the challenge.

The API RP2A (RP2GEO) (1) and ISO 19902 (2) guidelines included four CPT-methods for calculating the axial capacity of piles in sands in 2007. The designer is required to select an appropriate safety factor when using the newer design methods. The difficulty lies in deciding which safety factor will ensure a consistent safety level for different soil conditions and pile dimensions. The axial capacity of tubular steel piles for offshore installations is frequently based on the Recommended Prac-tice of the American Petroleum Institute (API) (1). To evaluate the required material factor, the probability of failure was quantified for piles designed with the API method and with the NGI, ICP and Fugro methods. A calibration of the required material factors for a target probability of failure of 10-4/yr was also carried out. The paper presents the results of the reliability analyses and the calibration of the material factor for each axial pile capacity method. The approach is illustrated with two case studies of jacket on piles, the first on a mainly clay profile and the second on a very dense sand profile.