TWI Industrial Member Report Summary 983/2011
By S D Smith
Engineering critical assessments (ECAs) are undertaken to provide confidence that structures are safe, even in the presence of possible flaws. These methods, exemplified by BS 7910, determine the criticality of flaws by comparing the loading on the crack (ie crack driving force) with that needed to make the crack grow and become unstable (fracture toughness).
It is important that the fracture toughness parameter is unambiguous and can be determined experimentally. These issues have largely been solved for ECAs in which the nominal stresses are below the material yield strength. Both J-integral and crack tip opening displacement (CTOD) have wide acceptance under these conditions. However, when nominal applied strains are above the material yield strain, current assessment procedures become inaccurate and potentially provide non-conservative estimates of crack driving force (J-integral and CTOD).
It is generally accepted that, when the applied strain is low, a finite element analysis (FEA) conducted of a component containing a crack under small-strain analysis conditions, where geometric non-linearities are ignored (ie the original component shape is maintained) is sufficient for ECAs. However, when geometric non-linearities are included in the model, as would occur at large applied strains (ie changes in shape are considered), the procedure for crack modelling is not straightforward. A trade-off of the computing efficiency, convergence and accuracy is usually needed.
These choices have become particularly important when assessing the integrity of circumferential flaws in high-performance pipelines subjected to plastic straining during installation or service. Compared with pipelines subjected to elastic loading, the tolerance to flaws will be lower and the likelihood of failure higher. With subsea pipelines, plastic straining can occur because of the installation method, such as reeling, or during service because of lateral buckling. With overland lines, plastic straining can occur during installation when the pipe is lowered into the trench and during service because of ground movement.
The primary concern when pipelines are under plastic straining is the integrity of the girth welds as these may contain imperfections arising from the pipeline welding procedure. The essentially stress-based procedures, as exemplified by BS 7910 (2005) which currently form the basis for most flaw assessment methods, are inadequate for high strain situations. Correction factors are often applied to ensure that results are realistically conservative. These corrections are based on FEA methods. Furthermore, until a strain-based assessment procedure has been fully developed for high strain high integrity applications of pipelines, currently the only viable approach is to carry out FEA. The choice of whether pipe shape changes (ie geometric non-linearities are considered) or original shape is retained (small strain analysis) in the FEA can have a significant effect on the evaluation of the crack driving force. In addition, the choice of the crack driving force parameter, J-integral or CTOD, to be used raises questions on their equivalence and which is more appropriate when assessing pipelines under high strain. A number of workers in the field have tended to characterise crack driving force in terms of CTOD rather than J-integral but the reasons for this are not clearly explained.
The present work was initiated from a 2009/2010 TWI Group Sponsored Project developing a procedure for the assessment of circumferential flaws at pipeline girth welds subjected to plastic straining with and without internal pressure. The present project is intended to provide guidance on crack modelling for ECAs of pipelines at large deformation level by comparing results from different modelling approaches.
Provide guidance on finite element (FE) modelling of circumferential cracks in steel pipelines at large applied strains.