Structural integrity assessment of C-Mn pipeline steels exposed to sour environments

Oil and gas fields can contain significant amounts of hydrogen sulphide and the behaviour of C-Mn pipeline steels exposed to sour environments (i.e. those containing water and hydrogen sulphide) continues to be one of the most active areas of research in the oil and gas industry. This project is aimed at improving the procedures used to assess the significance of flaws in offshore pipelines and risers operating in such environments. Experimental work has focused on examining the behaviour of C-Mn pipeline steel in a sour environment with respect to both static and fatigue crack growth behaviour, for which there is a paucity of data. In particular, the critical influence of crack depth on the crack growth rate has been studied, in order to ensure that test methods and assessment procedures used in industry are appropriately conservative. Under cyclic loading conditions, an environmental crack depth effect has been demonstrated, whereby, shallow flaws appear to grow faster than deeper flaws at the same (low) value of ΔK. The observed behaviour is believed to be dominated by bulk hydrogen charging, i.e. hydrogen charging by absorption from the external surfaces of the specimen rather than at the crack tip, and a lower concentration of hydrogen exists in the centre of the specimen than at the edges. The novel data generated have been applied to real-life pipeline defect assessments to demonstrate the influence of the observed crack growth rate, with a view to developing an improved assessment method. Example engineering critical assessments have been performed for circumferential surface-breaking girth weld flaws located on the internal surface of a typical steel catenary riser, operating in a sour environment and subject to vortex induced vibration fatigue loads. Companies operating in the oil and gas sector will derive benefit from this research programme through the application of new validated test methods and the development of improved in-service assessment procedures.