A Modeling Method of Predicting Service Life at Crack Threshold Stage for Tensile Armor of Flexible Pipes Under SCC-CO2 Environment

Abstract Cracks triggered by Carbon Dioxide initiated Stress Corrosion Cracking (SCC-CO2) have been identified as a new failure mode in flexible pipes, yet a method of predicting its occurrence, especially the stage of crack initiation (T1) is lacked in the industry. The root cause of SCC cracks is complex, the experiment observation shows the cracking may initiate from different locations and grow along different paths. As a starting point, this paper introduces a methodology of T1 prediction on the basis of equivalent initial flaw size (EIFS) theory and also back-extension of Paris linear section. It implies the condition that SCC-CO2 cracks initiate from corrosion pits and propagate through grain boundaries. The influential factor from environment mainly includes the stress utilization factor (UF), CO2 fugacity and operational temperature. After the preliminary calibration using data from dissected pipes retrieved from different oil fields, the model is able to predict typical trends, agreeing with the most field observations. Investigation continues to understand the mechanisms leading to some specific deviations, and to explore potential solutions and improvements. The presented approach is anticipated to provide a reliable and consistent prediction of nucleation duration in determining SCC-CO2 service life. Furthermore, the model in this methodology will be further calibrated and validated against future tests and field experiences.