Reliability-Based Assessment of Residual Collapse Strength for Worn Tubes Scanned with Ultrasonic Log

Summary In this work, we present a casing integrity study that begins with data collected from an ultrasonic log inspection and progresses to the estimation of the residual collapse strength of worn tubes, using a probabilistic-based finite element (FE) modeling. The goal is to provide a data-driven assessment of the structural integrity of well tubular components in severe environmental scenarios, as in the case of the Brazilian pre-salt region. The input data set contains measurement data from an ultrasonic log. For wear identification, we used a technique that seeks an ellipse geometry resembling the probable intact shape of the casing’s inner wall. Then, we estimated the residual collapse strength using nonlinear FE modeling. We performed the probabilistic analysis using the first-order reliability method (FORM), considering the FE modeling failure criteria as the limit state and the random variables associated with material and geometry of the tubulars, as well as wear parameters, such as maximum groove depth and its position. Geometries of ovalized and eccentric cross sections, associated with multiple wear grooves at various intensities and positions, were evaluated. The nonlinear FE modeling allows the capture of different collapse modes of the element, depending on its slenderness and wear configuration. Through the probabilistic approach, it became possible to account for inherent uncertainties associated with different design and wear parameters, enabling the calculation of the probability of element failure. In the conducted case study, the loads were set equal to the collapse pressures calculated with design equations from the literature. Given that the obtained probability of failure did not meet a predefined target, a search procedure was implemented to achieve an adequate collapse pressure for design purposes. Although this procedure is computationally expensive and only two critical cross sections were assessed, it mitigates simplifications observed in industry practice, supporting the evaluation of casing integrity, thus contributing to life extension assessment and well reuse analysis.