Structural Investigation of Fatigue Behaviors in Corroded Subsea Pipelines for CO2 Transportation Using the Strain–Life Approach

Abstract

The subsea CO2 transport pipelines, which are closely related to environmentally sound technology under Sustainable Development Goal (SDG) 17, are subjected to cyclic loading from ocean currents, wave-induced forces, internal pressure fluctuations, and start–stop operating cycles. In harsh marine environments, corrosion may reduce pipe-wall strength and create local defects that promote fatigue crack initiation. These cracks may subsequently propagate under cyclic loading, leading to structural failure. This study investigates the fatigue crack initiation life of corroded subsea steel pipelines for CO2 transportation using an integrated local strain–life (ɛ–N) framework. The framework combines nonlinear finite element analysis, the Coffin–Manson–Basquin relation, and the Smith–Watson–Topper mean-stress correction. The modeled pipeline has an outer diameter of 300 mm, a wall thickness of 20 mm, and a length of 1,800 mm, with API 5L X65 steel adopted as the pipeline material. Rectangular corrosion defects with varying depth, length, and width are considered, while the internal pressure ranges from 0 to 20 MPa. Fully reversed cyclic bending is applied with a stress ratio of R = −1. The finite element results are used to extract local stress–strain responses at the corrosion defect, which are then used to predict fatigue crack initiation life. The results show that corrosion defects produce pronounced strain localization, in contrast to the relatively uniform strain distribution in intact pipelines. Corrosion depth is the most critical geometric parameter because it directly reduces the effective wall thickness and increases the local cyclic strain amplitude. Corrosion length affects fatigue life through strain redistribution: short defects behave as severe strain concentrators, whereas longer defects reduce peak strain under displacement-controlled bending. Corrosion width has a secondary effect by modifying the circumferential constraint. Internal pressure further reduces fatigue life by introducing tensile membrane stress and a non-zero local mean stress. Overall, fatigue crack initiation in corroded subsea CO2 pipelines is governed by the coupled effects of thickness reduction, strain localization, circumferential constraint, and pressure-induced mean stress. These findings highlight the importance of local strain-based assessment for reliable pipeline integrity evaluation.