TWI Industrial Member Report Summary 1000/2011
By M Cheaitani and W He
As worldwide demand for energy continues to rise, oil and gas companies are increasingly exploring and developing new reserves in more challenging environments such as offshore in deeper waters or on land in Arctic or near-Arctic locations. Pipelines installed in such regions may be subjected to axial plastic straining combined with internal pressure, eg due to lateral buckling, frost heave or seismic ground movements. Offshore pipelines may also experience axial plastic straining during installation eg by reeling or other methods where the welded pipe strings are bent around reels or formers as part of the laying process.
The need to build pipelines that can withstand significant axial plastic straining with adequate safety margins against structural failures, such that the risk to the environment and public is minimised, has led to substantial worldwide research which began nearly a decade ago and is still ongoing. This research is aimed at developing improved design tools for pipelines including, in particular, robust fracture assessment procedures for pipeline girth welds subjected to axial plastic straining.
This report provides a brief overview of approaches adopted in a number of recent research programmes for the development of fracture assessment procedures, with an emphasis on those intended for circumferential girth weld flaws subjected to axial plastic straining. Then, attention is focused on a TWI approach for strain-based fracture assessment, which is based on a re-formulation in terms of strain, of the widely used stress-based reference-stress J estimation scheme and corresponding failure assessment diagram (FAD) approach. The TWI assessment model builds on a general framework for strain-based fracture assessment, formulated as a strain-based FAD (SB-FAD), that was developed by Budden and co-workers, at British Energy Ltd, for inclusion in R6 (BEGL, 2001). Although the procedures developed by Budden and TWI can potentially be used to assess any flawed component, attention is focused on surface circumferential girth weld flaws subjected to axial straining due to tension, bending or combined pressure and bending. Both proposed approaches are evaluated using data from elastic-plastic finite element (FE) analyses of pipe models containing surface circumferential flaws, representative of those found typically in pipeline girth welds.
- Develop a strain-based approach for estimating the driving force for circumferential surface flaws in pipeline girth welds subjected to axial straining with or without internal pressure.
- Compare the above approach with a method proposed by Budden and co-workers at British Energy Ltd.