On the probabilistic assessment of casing applied to top hole design by FORM

This study employed reliability-based models to optimize the design of top-hole casing sections considering the uncertainties associated with soil behavior and casing manufacturing. The integrity of oil and gas wells significantly depends on the casing system throughout its life cycle, ensuring tightness, stability, and load support. Various load scenarios were analyzed to estimate the probability of occurrence of different soil-casing system failure modes. Analyses for various types of top-hole designs are included in this work. Reliability-based techniques have emerged as interesting tools for structural analyses and design. This research leverages soil characterization data from piezocone tests (CPTu) to statistically define the mechanical parameters crucial for conductor and surface casing design. Additionally, random variables linked to the material and geometrical properties of tubulars are incorporated, drawing from the casing manufacturing data outlined in API/TR 5C3 (2018). Probabilistic models are developed using the first-order reliability method (FORM), an efficient optimization-based procedure, and applied across multiple load scenarios to gauge the failure probability in the top-hole casing design. The analysis primarily focuses on the variability associated with undrained soil strength derived from CPTu data, which is deemed the most influential random variable owing to its spatial heterogeneity. The results underscore the viability and importance of estimating the probability of relevant failure modes aligned with internal regulations concerning the conductor casing load capacity, surface casing triaxial stress, and wellhead displacement. The work in progress considers random variables obtained from correlated soil test data and related to casing manufacturing (outer diameter, wall thickness, and yield strength) in a combined probability density function applied to the failure functions. The findings reveal that the conductor casing capacity is the critical failure mode, which is consistent with deterministic design practices. Moreover, the analysis highlighted that the outer diameter insignificantly influences the probabilistic response owing to its low dispersion. This novel approach combines soil statistics information and casing manufacturing data within a reliability-based framework, achieving a balance between cost and safety while aiding decision-making in top-hole design.