In recent years, considerable doubt has arisen over the prediction of the level of toughness required to arrest a propagating fracture in higher-strength line pipe. It has been clear for many years that the most widely used traditional approach, the Two-Curve Method (TCM) developed at Battelle in the early 1970s, could not be applied directly when the required toughness, expressed as full-size Charpy energy, exceeded about 80–90 J. Initially, this issue was addressed by the adoption of empirical correction factors, but more recently, there have been indications that this approach is no longer effective for modern, high-strength materials. Additional information, which in general can only be derived from well-characterized burst tests, is essential to furthering understanding of the fracture arrest problem under conditions that are typical of modern, long-distance, large-diameter pipeline design.
In the context of the Coastal GasLink (CGL) project, TransCanada has carried out a program of full-scale burst testing at the Spadeadam test site of DNV GL. The tests were supported by LNG Canada and the TransCanada Technology Management Program. These tests are described in another paper at this conference [1]. Though most of the testing was directed towards the assessment of different crack arrestor designs, one half of one test contained a run of four pipes of progressively increasing Charpy energy, up to a very high level (over 450 J). The fracture was observed to run through all four pipes, before being arrested by a crack arrestor fitted to a fifth pipe having lower toughness.
Nearly all approaches to determining requirements for fracture arrest depend, directly or indirectly, on relationships between fracture velocity (for given levels of fracture resistance) and the driving force, generally considered to be directly related to the pressure in the plane of the crack tip. By comparing measured fracture velocity with the crack tip pressure determined either directly at pressure transducer locations or by comparison with propagation velocities within the expansion wave, conclusions can be drawn regarding the accuracy of existing relationships. Most previous work regarding correction factors has been based simply on discrepancies between predicted and observed propagation and arrest behaviour. Direct comparisons of observed and predicted fracture speed potentially provide much more data and focus more clearly on where model deficiencies may lie. The current analysis focuses on comparisons with the predictions of the traditional TCM and those of a transient model developed by JFE. While data from the present work are clearly limited, this approach appears to present a way of recalibrating fracture velocity formulations that may extend the range over which traditional, Charpy-based approaches can be applied. For the future, the incorporation of additional results from other recent, well-characterized burst tests would be extremely valuable in this respect.
