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.

Casing accounts for 10% to 30% of the total well cost. The oil and gas industry has historically used working stress design (WSD) methods in the design of casing strings, This method requires the load to be defined as an estimate of the maximum anticipated load, while the strength is defined as a lower bound of the materials strength. The designer must assure that the minimum allowable stress of the material is not exceeded, by separating the load and the strength by a safety factor that is fixed by company guidelines. Load and Resistance Factor Design (LRFD) is a reliability based design philosophy, which explicitly takes into account the uncertainties that occur in the determination of loads and strengths. The LRFD format was first developed in the 1930's in USSR and Europe for use in the Civil Engineering industry. Its development and use has continued in Civil Engineering practices and is now widely accepted in many industry codes. As a reliability based design philosophy, the engineer is allowed to select a probability of failure that is commensurate with the consequence of failure. This allows the engineer to accept a probability of failure that maximizes the utilization of the casing design, thus improving the economic indicators on the investment. Beginning in the late 1980's and early 1990's, several efforts were undertaken to develop LRFD for casing design. This paper describes the basic LRFD philosophy and shows two examples of well designs developed using LRFD and compares them to the designs previously developed using WSD. The impact on the well cost and deliverability are discussed for each design. The issue of probability of failure and the impact on the well design are also discussed. The material selection criteria and the qualitysystem that were developed in concert with LRFD are also described Working Stress Design WSD is the traditional approach to design that ensures adequacy by computing elastic stresses under maximum anticipated load and comparing them with allowable stresses. The safety of the structure is assured by ensuring that for each element, elastically computed stresses do not exceed an allowable stress, that is a preset fraction of the yield strength of the material. The ratio of the yield strength to the actual stress is known as the safety factor. The allowable safety factor, is set in an empirical manner and reflects the companies policies with respect to safety and risk. Dueto the uncertainty in the loads, there is not a consistent definition of design loads or minimum factors of safety throughout the industry, except where mandated by government regulations. Although working stress is conceptually simple and has worked well in most cases, it is not without problems. Some examples of these problems are:it is overly conservative,it does not give the engineer any insight into the degree of risk or safety of the design,it has no risk balanced capabilities,it treats all wells the same,there is little justification in the safety factors.