O projeto probabilístico e suas implicações no dimensionamento de revestimentos

Abstract

The integrity of oil wells heavily relies on the use of casing tubes. International standards provide equations and methodologies to calculate the resistance and loads that casing tubes may experience during their life cycle. API/TR 5C3, for instance, introduced in its 2008 revision reliability-based methodologies for casing design, addressing historical manufacturing data from different tube suppliers to statistically describe its mechanical and dimensional parameters. However, the adoption of these reliability tools in oil well casing projects remains limited to specific analyses, rather than being part of the designers’ routine. This research aims to investigate various loading scenarios on numerous casing tubes commonly employed in oil well projects, applying both the traditional deterministic methodology, widely known and used, and the more robust but less prevalent probabilistic methodology. Fifty-four tubes of varying slenderness and metallurgy are addressed, subject to loads corresponding to their elastic limit resistance, as typically considered in projects for calculating the traditional Safety Factor (SF) using Serviceability Limit State (SLS) equations. Through simulated data, a comparative study is conducted to establish the equivalence between the Safety Factor (SF) and the Probability of Failure (Pf ) by considering the resistance of tubular in the Ultimate Limit State (ULS). The results are presented in percentiles, offering a comprehensive overview of the Pf associated with the design load. Notably, for burst failures, the maximum Pf recorded was 1.62 · 10−5 , while the 50th percentile exhibited 4.27 · 10−10. In contrast, for collapse, the maximum Pf observed was 7.7610−2 , with the 50th percentile displaying 3.26x10−3 , confirming that collapse failure is critical in casing design. This study contributes to promoting the broader adoption of probabilistic methodologies by casing designers, supported by the equivalence with conventional deterministic responses based on resistance envelopes and safety factors. Leveraging probabilistic methods can yield cost benefits by increasing the likelihood of approval for bold projects, as well as enhancing safety standards by demanding improved quality control in the manufacturing and characterization of casing tubes used in well construction.