Application of probabilistic reliability methods to tubular designs

Standard tubular design methods are based on a specific margin (i.e., safety factor) between the maximum anticipated field load and the published rating of the tubular. This technique minimizes the risk of failure but promotes overdesign because of the conservativeness in the tubular rating and in the assumed high-load case. To quantify the safety of a particular design, we developed a new method that accounts for the variation of field loadings and tubular performance. Probability distributions for load and capacity are developed on the basis of assumed field-load histories and actual test results on tubular performance. Reliability design methods, originally used in civil engineering, are used to combine these distributions and to quantify the probability of failure. Results show that current design factors do not provide an effective reliability measurement. As a result of these limitations, true-cost/reliability decisions on designs cannot be made with conventional techniques. If reliability is quantified with the proposed method, decisions can be made that properly balance economics, safety, and uncertainity.