Evaluation of frequency effects on corrosion-fatigue behaviour of offshore steel pipelines

During the operational lifespan, offshore steel pipelines are subject to cyclic loading condition introducing fatigue damage into the material in conjunction with corrosion damage as a result of operation in the marine environment. An important issue that needs to be understood in asset integrity management of offshore pipelines is the cyclic loading frequency effects on corrosion-fatigue cracking behaviour of the material. In this study, a newly proposed approach for corrosion-fatigue analysis of offshore steel structures has been employed to examine its suitability for evaluation of the cracking behaviour in offshore steel pipelines. For this purpose, the experimental corrosion-fatigue data on API 5L X56 steel at different frequencies were collated from the literature and re-analysed using a newly proposed time-dependent fracture mechanics parameter. Moreover, a predictive model was developed to estimate the corrosion-fatigue cracking trends at various frequencies using the high frequency test data in air. The results from this study show that while the frequency effects on corrosion-fatigue cracking behaviour of steels cannot be described using the traditional data analysis method, the new time-dependent approach facilitates accurate characterisation of corrosion-fatigue data at a wide range of frequencies. Also, knowing that corrosion-fatigue testing at low frequencies is time-consuming and costly, it has been shown in this study that the long-term cracking behaviour of the material can be predicted using much faster tests at higher frequencies in air. This study showcases the potential of the newly proposed corrosion-fatigue life prediction approach using a time-dependent fracture mechanics parameter for structural integrity assessment of steel pipelines. Last but not least, the results from this study highlight the need for employment of innovative approaches for long-term structural integrity assessment of offshore steel structures and pipelines.