Limit states design is based on explicitly modeling the likely failure modes. Design margins ensure safety and durability by accounting for uncertainties in loads, strength, environmental exposure and idealization. This paper examines the idealization or model uncertainties. These uncertainties are particularly important when extending the limits of conventional design, such as HPHT well design.
The purpose of the paper is to discuss how such uncertainties could be handled for fracture mechanics based limit states. The paper provides a basic overview of commonly used FAD methods and some of its limitations. A survey of published verification tests was made to see whether this data could be used to estimate FAD idealization uncertainty.
The authors conclude that available data is not directly applicable to well design. A testing program is recommended to establish the model uncertainty for sour fracture of casing.