For more than two decades, CSA Z662 Annex K has provided a method for developing alternative acceptance criteria for weld flaws in mechanized welded pipelines. Increasingly, over the years, fracture mechanics practitioners have found the method overly conservative and restrictive with respect to brittle fracture criteria when compared to other accepted fracture mechanics-based engineering critical assessment ECA codes and methods. These limitations rendered the CSA Annex K method difficult to implement on pipelines constructed with materials not possessing optimal toughness and in cases requiring consideration of fracture toughness at temperatures lower than the typical minimum design metal temperature (MDMT) of −5°C. This paper presents experiences implementing CSA Z662-15 Annex K Option 2 methodology on a 610 mm diameter liquids pipeline and compares and contrasts the utility and benefits of the code revision. This pipeline required consideration for installation during winter months, necessitating installation temperatures as low as −30°C. In addition to evaluation of actual ECA results, analytical evaluations of the Option 2 methodology were also conducted considering parameters outside those used on the project.
The new Annex K Option 2 method was found to be of considerable benefit in preparation of a practical ECA. Since fracture toughness testing was conducted at the anticipated lowest installation temperature, the flaw criteria were, as expected, principally controlled by elastic/plastic crack growth consideration. The failure assessment diagram implemented into the CSA Z662-15 Annex K Option 2 provided tolerance for both longer and deeper flaws than that afforded by Option 1 (which resorts to the former 2011 Annex K method). Furthermore, the reduced restriction to the surface interaction ligament (p distance) offers additional advantages including increased flexibility in weld profile design and weld pass sequencing.
Fracture toughness (CTOD) testing of TMP pipeline steels used in the project at −30°C often produced transitional fracture toughness results. It was found that the particular project materials were quite sensitive to the level of test specimen pre-compression (an acceptable plastic straining method to reduce residual stress gradients) applied to the CTOD specimens to enhance fatigue crack-front straightness. It was found that optimizing the level of pre-compression (to achieve acceptable pre-crack straightness while minimizing plastic pre-strain) achieved a balance between fully satisfying testing requirements, providing a conservative assessment of CTOD, and facilitating a functional Annex K ECA.
Verification of Brittle Fracture Criteria for Bimaterial Structures