Abstract
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.
