Reliability-Based Assessment of Tubular Collapse Integrity Under Subsurface Pressure Uncertainty for Energy Transition Wells

Abstract Collapse integrity of steel casing is increasingly important in geothermal, CO₂-storage, hydrogen-storage, and repurposed hydrocarbon wells, where uncertain pressure histories can challenge deterministic design margins. This study presents a revised reliability-based framework for tubular collapse using the classical four-regime API collapse formulation and Monte Carlo simulation. A 7-in, 26-lb/ft P110 casing with nominal wall thickness 0.362 inches is used as a transparent benchmark. Manufacturing variability in outside diameter, wall thickness, and yield strength is represented by Gaussian statistics reported from ISO/API manufacturing data, while external pressure is treated as an explicit scenario variable rather than as a field-calibrated distribution. For the baseline case with mean external pressure 4,500 psi and coefficient of variation (COV) 10%, 10⁶ Monte Carlo realizations produced a failure probability P_f = 1.088×10⁻³, reliability index β = 3.065, and 95% Wilson confidence interval of 1.025×10⁻³–1.155×10⁻³. The mean collapse resistance was 6,439 psi and the mean safety margin was 1,938 psi. Approximately 82.1% of realizations were governed by plastic collapse and 17.9% by transition collapse. Repeated simulations and a convergence study show that the previously used 1,000–7,000 realizations are insufficient for stable estimation of failure probabilities near 10⁻³. Pressure-uncertainty scenarios further show that increasing external-pressure COV from 5% to 20% increases P_f by more than three orders of magnitude. Rank-based sensitivity identifies wall thickness as the dominant resistance-side variable and external pressure as the dominant loading-side variable. The framework provides a reproducible probabilistic screening method while explicitly distinguishing the present static benchmark from future time-dependent integrity models required for cyclic thermal, chemical, and pressure conditions in energy-transition wells.