In-situ studies of corrosion fatigue of high strength steel for offshore oil and gas applications

The corrosion fatigue behaviour of high strength carbon steel wires that are used in flexible risers has been explored. An investigation of the corrosion fatigue failure mechanisms for two different sets of corrosion fatigue tested high strength steel wires has been carried out. This included undertaking statistical analysis of fatigue crack and corrosion pit data to establish the effects of environment, applied stress, R-ratio and degree of cold drawing on the corrosion fatigue behaviour. This analysis identified that a significant amount of localised corrosion pitting was present on the surface of the samples and that many corrosion fatigue cracks had initiated from these geometric discontinuities. In-situ studies of high strength steel wire in a seawater environment were carried out to establish how the surface strain fields develop around corrosion pits which ultimately leads to crack initiation. A method was developed in order to apply an optical image correlation technique to a sample immersed in seawater. The results of the surface strain mapping show that the interaction between corrosion pits demonstrates a significant increase in surface strain when compared to a single surface pit acting alone. The results also show that a small single stress raiser can demonstrate a high surface stress concentration when compared to a larger one. Further to this the effects of residual stress on crack nucleation were considered. Fatigue cracks initiate at the surface of the high strength steel wire therefore surface measurements were carried out to establish the effects of environment and applied load on the development of residual stress fields.