This article proposes a new methodology for evaluating the uniform corrosion depth growth model in the calculation of failure probability of subsea oil and gas pipelines or equipment aiming at supporting the application of the risk-based inspection process to control the burst failure. It was noted that variations in operating conditions can influence the corrosion rate, requiring additional analyzes in the evaluation of structural reliability. As a result of the probability of failure calculation, at low operating pressure levels, the depth of the uniform corrosion does not play a significant role in the probability of failure, limiting the use of the RBI methodology.
During the operational life of the pipes used in oil and gas offshore exploration, different failure mechanisms may arise, including degradation by corrosion. High inspection costs make it difficult to verify the integrity of these structures considering only the results of some sparse inspection reports. Optimizing time between inspections reduces the accidents risks but this optimization must rely on structural reliability analysis that shall indicate the evolution of the probability of failure during operational life. Structural integrity management requires corrosion rate estimates, typically using corrosion prediction software. In general, corrosion rate estimates do not consider variations in operating conditions over time, e.g., temperature and pressure. The present paper deals with a risk-based inspection (RBI) methodology to certify the structural integrity of offshore pipelines, introducing a new way of estimating the corrosion rate by considering the variations of operating conditions over time. To evaluate these variations, the time discretization was performed according to the shortest interval between the measurement records of the parameters that affect uniform corrosion, keeping the other parameters constant until the appearance of a new record. Thus, the pipeline thickness reduction is composed of the sum of the product of the corrosion rates and their respective time intervals. The failure probability calculation was performed using the AFOSM and MCS method based on the limit state equation for burst pressure considering the uniform corrosion failure mechanism referenced by the ASME B31G remaining life determination standard. Case studies are executed to present the proposed technique as well as verify the findings in the results and compare them with the direct use of corrosion rate estimation by the prediction software in determining the failure probability. Compared to conventional RBI, the proposed methodology allows evaluating the uncertainties of variations in operating conditions in determining the inspection time. The scope of this research is to introduce an efficient methodology to calculate the failure probability considering the variations of operating conditions that will be used in RBI practice, allowing to monitor and guarantee the integrity of the evaluated pipeline system.
