Corrosion Fatigue Behaviour of Welded Risers and Pipelines

The fatigue design of pipelines or risers in deepwater oil and gas developments, is often critically dependent on quantifying the extent to which aggressive service environments affect performance. Girth welds in these structures are often exposed to seawater on the external surface, and sweet or sour production fluids on the internal surface. All of these environments can lead to higher rates of fatigue crack growth and lower overall life compared to performance in air. The seawater environment has been studied in some depth, and design codes provide advice on how steel structures are likely to behave under conditions of either free corrosion or cathodic protection. However, it is important to note that there are limits to how widely these guidelines can be applied, and for more complex environments, such as production fluids which are inevitably project specific, design guidance is rarely available. Laboratory testing provides a means of quantifying material behaviour in a simulated service environment, and allows the impact of various environmental variables to be explored. This is important as parameters such as temperature, H2 S concentration or loading frequency can have a significant effect on the extent to which performance is affected. This paper provides a review of published information and recent research data, and highlights particular areas where existing data are limited and design challenges remain.Copyright © 2007 by TWI Ltd.