A Modelling Framework for Describing the Corrosion Fatigue Behaviour of Carbon Steel Pipelines and Risers

The fatigue performance of pipeline or riser girth welds is often a critical factor during design. Under corrosive conditions the detrimental influence of an environment on anticipated fatigue lives needs to be taken into account. Fatigue design codes provide advice for the case of loading in seawater at typical wave frequencies, but in other environments, or at other cyclic loading frequencies, the required approach is less certain, and laboratory testing is often needed to provide suitable guidance. In addition to the time and expense of such an exercise, in some cases there are physical limits to the nature of data that such programmes can provide. The provision of data associated with very low cyclic loading frequencies is one such case, as in this instance the time taken for each test becomes unmanageable. This leads to the need for extrapolation of test data into the region of interest, for instance by carrying out tests at either a higher cyclic loading frequency, or a higher stress range, than that anticipated in service. Such extrapolation requires careful consideration in order to avoid potential non-conservatism. It is suggested that the availability of a model which accounted for the influence of these two key parameters (frequency and stress range) would greatly improve confidence when extrapolation is needed, and more broadly would provide a rational basis for designing experimental testing programmes and applying determined data to pipeline design. Such a model should be based on a description of the perceived underlying mechanisms of a material’s interaction with the environment. This paper provides a review of relevant test data and describes the basis for such a model. A generalised framework is developed and fitted to numerous published test data for carbon steels in seawater or sour environment.