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The proposed project concerns an investigation into typical high strength steels (particular interest is on offshore pipelines with surface breaking defects and embedded cracks) used in arctic environments under conditions of loss of crack-tip constraint. The loss of crack-tip constraint is expected to counteract the negative effects of thick sections and arctic temperatures on fracture toughness, and so quantifying it is extremely useful. The fitness-for-service standards e.g. BS 7910, include methods to account for crack-tip constraint, but these methods are not fully developed, and would benefit from refinement for these applications.
The research aims to present a systematic approach to understand the mechanics involved in the fracture of offshore pipelines transporting high pressure fluids, the effects of internal pressure and other residual stresses on the flaws. For this purpose, the thesis needs to present a substantial test programme involving low temperature fracture toughness tests linked to a detailed finite element-based stress analysis with different specimen and pipe configurations.
The methods developed for the base-metal should be able to be applied adequately to welds as well (with suitable corrections for crack driving force applied as part of a defect tolerance analysis), without the need for additional complications associated with testing and analysis of welds. Suitable guidance should be developed such that it can be incorporated into the existing fracture mechanics assessment procedures (primarily, BS 7910).
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