Conventional casing design utilizes a deterministic approach to compare a maximum load to a minimum performance property. The casing is deemed adequate if it meets a minimum design factor as specified by company practices. Occasionally, a design is proposed that does not meet the company guidelines, often due to material, procurement, or clearance constraints. The engineer must justify deviating from standard practices in order to convince management that the proposed design is still "safe".

Quantitative Risk Assessment (QRA) methods have been proposed as a way to quantify the probability of failure in casing design.1,2  The methods utilize mechanical properties to determine the range of performance that a string of pipe may exhibit rather than a single minimum value. Likewise, a QRA design can include a distribution of possible loads rather than just the worst case scenario. QRA is gaining acceptance in many companies as a way of optimizing casing strings for both cost and safety, especially in critical wells. This paper presents several real world examples where QRA is used to justify casing strings that do not meet conventional design requirements.

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