By M Cheaitani and W He
Some degree of axial misalignment across pipeline girth welds is inevitable, especially in pipelines constructed using seamless pipe where the wall thickness can vary significantly around the pipe circumference and across the girth weld. Axial misalignment can also occur due to out-of-roundness in the region containing the longitudinal seam in pipelines constructed from seam-welded pipe. The presence of misalignment generates an additional through-wall bending stress, Pb', which causes an increase in the total stress, on the side where the bending stress is tensile, and a decrease in the total stress on the other side where the bending stress is compressive. Consequently, misalignment influences the crack driving force for flaws located in misaligned welds, which in turn can have a significant influence on the behaviour of the weld under cyclic and static loading conditions.
This report is concerned with guidance given in BS 7910 (BSI, 2005), DNV-OS-F101 (DNV, 2007) and DNV-RP-F108 (DNV, 2006) on treatment of axial misalignment in fracture assessment of pipeline girth welds using the failure assessment diagram (FAD) approach.
A key objective is to determine whether the local bending stress due to axial misalignment in girth welded pipes should be considered primary (contributing to the FAD fracture and collapse parameters), secondary (contributing to the FAD fracture parameter only), or partially primary (ie only a proportion contributing to the FAD collapse parameter). The classification of misalignment stresses as primary or secondary is particularly important for fracture assessment of pipeline girth welds where the applied axial stress is close to or exceeds yield. Such loading conditions can occur during installation of offshore pipelines, just before the pipeline is lowered into the water, or during operation for example due to lateral buckling. Under such loading, the collapse parameter for a given fabrication girth weld flaw could exceed the maximum allowable value if there is significant axial misalignment across the girth weld and the local bending stress due to this misalignment is treated (or classified) as a primary stress. If such a classification leads to overestimating the driving force, it can result in overly conservative assessments, which may lead to unnecessary and costly weld repairs. If on the other hand, classification of the misalignment stress as secondary leads to underestimating the driving force, it can result in potentially unsafe assessments and may undermine the integrity of the misaligned weld.
Another objective of this work is to evaluate the Neuber method, as used in FAD-based fracture assessment of pipelines subjected to significant plastic straining, to allow for the effects of axial misalignment (DNV, 2006). In some cases, eg for materials with weak strain hardening or a yield discontinuity, the application of Neuber's method can imply that axial misalignment has a relatively little effect on the elastic-plastic driving force, which is counter intuitive. If the application of the Neuber method to allow for the effects of axial misalignment can, in some cases, lead to underestimating the elastic-plastic driving force, this can potentially lead to unsafe assessments of the tolerance to weld flaws in axially-misaligned girth welds.
The work described in this report is aimed at achieving the above objectives with the aid of results from elastic-plastic finite element analyses on a range of pipe and flaw geometries with and without axial misalignment.
- Evaluate methods for the treatment of axial misalignment in FAD-based fracture assessment of pipeline girth welds with regard to whether the local bending stress due to misalignment should be considered primary or secondary.