Casing System Risk Analysis Using Structural Reliability

Abstract

Abstract This paper describes a method for quantitative risk analysis (QRA) of casing/tubing systems, using a structural reliability approach. The technique was used to calculate risk-calibrated design factors for exploration and development wells, taking into account the nature and consequences of failure for each string, stress check type and load case. The methodology and results are a significant advance on previous work. Summary In 1991, BP Exploration commissioned a major design study to assist in the updating of their casing design manual. Its objectives were twofold:–to develop a general method for quantitative risk analysis (QRA) of the casing/tubing system;–to use the method to calculate risk-calibrated design factors for exploration and development wells, taking into account the nature and consequences of failure for each string, stress check type (eg, burst, tension, etc.) and load case. The first task was the more difficult of the two. In order to achieve the required functionality, it proved necessary to develop three complementary techniques - a FORM/SORM (First/Second Order Reliability Method) computer program and two simpler spreadsheet methods. The three methods were cross-validated against each other, with uniformly excellent results. In the second part of the study, the consequences of failure for each string, stress check type and load case were evaluated and divided into:–blowout events (target reliability = 10 -4);–repair events (target reliability = 10 -2) and the QRA method was then used to work backwards from the target reliability to the necessary design factor (DF) for each case. It was found that the maximum DFs for the exploration well were broadly in line with current industry practice, except for the tension DF which was rather lower (see Table 1). The minimum exploration well DFs were substantially lower than current values. The DFs for the development well were lower than those for the exploration well. It is therefore likely that substantial cost savings can be made by adopting risk-calibrated DFs for casing and tubing design. The QRA method is also useful for calibration of design methods, determination of least-risk operating practice, cost-benefit analysis, and QA/QC planning. RISK ANALYSIS - AN OVERVIEW Why risk analysis? The oil industry has, to date, designed casing using deterministic methods alone. As this approach has been largely successful, why complicate matters now? The answer lies in two areas, namely:–the limitations of the deterministic method;–the additional (and valuable) knowledge that QRA gives us. P. 169^