Load and Resistance Factor Design for Oil Country Tubular Good
Authors: Lewis D.B. , Brand P.R. , Whitney W.S. , Hood M.G. , Maes M.A. 
Published: October 2008

This paper describes the development of a reliability based design format, Load and Resistance Factor Design (LRFD), for casing used in oil and gas wells. Although LRFD methods have provided a basis for design in a number of industries, it has not been widely applied to the design of casing. Load and resistance factors in LRFD account for the influences in the variability of design loads, steel properties and geometrical tolerances on the casing design for a particular application. These factors are analogous to experience based safety factors in Working Stress Design (WSD). However, they are determined by comparing the reliability estimated design by an explicit consideration of the design uncertainties with the tubular design obtained by the specified design check equations. Historically all wells have been designed using WSD. WSD assumes historically based safety factors in the design process. Little documentation and insight into the degree of safety is allowed with WSD. The LRFD approach considers and quantifies uncertainties, risk, and economics with the well design. With LRFD the engineer selects the level of risk which one wishes to design based on the well?s economics, safety, and environmental concerns. LRFD allows the engineer to treat wells differently. The engineer can take different risk levels based on the well type (exploration versus production)as well as what limit state the engineer is designing for (burst versus collapse). LRFD allows the design to fit the application.

Two years ago a commitment was made to a multi-million dollar, multi-discipline quantitative risk assessment based research and development project with the objective of 1) increasing reliability and decreasing down time, 2) minimizing environmental impact, and 3) maximizing well economics for OCTG. This project has been completed, is functional, and is based on the Load and Resistance Factor Design methodology. This overview paper will discuss the limitations of previous methods, the hurdles which were required to be overcome in bringing the LRFD method operational, and the advantages of the new method.

INTRODUCTION

It is important to realize that LRFD is more than a design philosophy. Material and quality requirements are required to adequately support the design. It does not matter how elaborate or sophisticated a design is, if the material requirements are not met and the quality system is not adequate to support the material requirements, the design will not perform as required. In short, LRFD is a complete package of design, material specifications, and quality systems which cannot be separated. Figure 1 shows how the reliability based design concept is a compete and integrated package. The Design logo shows how load and strength are not deterministic values but are stochastically based. As long as strength is greater than load, no failure will occur. The area of overlap shows the probability of failure. The Material logo shows a failure assessment diagram which is used to determine if the material is acceptable for the given load and well conditions. The third and final logo, Quality Systems, shows the supplier switching rules for assuring good quality material.