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
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.
