Abstract
Fracture propagation in gas transmission pipelines is a major concern for the operation of high pressure lines. Therefore, the resistance against propagating fractures is one of the main requirements for these pipes. To date, ductile fracture resistance is commonly measured with Charpy impact tests and the most widely applied concept to predict pipe behaviour is the Battelle Two Curve (BTC) method which was developed in the 1980s on basis of the material available at that time. Limitations of this methodology have been shown in the last decade, especially on grades exceeding X80, for which it was not possible to yield reliable predictions. Recent full scale tests on X80 for arctic conditions underlined this once again. Alternative testing methods as well as refined concepts to predict structural behaviour have been investigated by researchers around the world. Nevertheless, there is no clear indication as to which modification of either testing methods or concepts may solve the problem. Within this paper, a review of the state of the art is given and the most promising alternatives are highlighted. Based on this, a thorough assessment of the results of a fracture propagation test on grade X80 is conducted. Calculations with BTC for the test set-up with air pressurisation at a temperature of −10°C lead to a Charpy impact requirement below 250 J. The test itself revealed that pipes with CVN energy above 300 J could not arrest the propagating fracture. On the other hand, the energy measured in BDWT tests showed a better correlation to the arrest properties of the pipes. Different specimen preparation methods in terms of notch insertion were compared to identify the most suitably setup to correlate with full scale test results.
Highlights
Fracture propagation in gas transmission pipelines is a major concern for the operators of high pressure gas pipelines
These formulae were calibrated on lower strength lower toughness steels with impact energies below 100 J where laboratory test results were correlated to the results of full scale fracture propagation tests
Moving outside the database of full scale fracture propagation tests against which the empirical or semi-empirical models were calibrated increases the probability of receiving non-conservative predictions
Summary
Fracture propagation in gas transmission pipelines is a major concern for the operators of high pressure gas pipelines. Problems with meaningful laboratory specimens; limitations of Charpy impact test for high strength high toughness steel (specimens commonly do not break when impact energies exceed around 200 – 250 J; Charpy impact test does not discriminate between initiation and propagation energy). To overcome this situation, considerable amount of research was directed towards alternative test methods to measure the resistance against a propagating fracture in a laboratory scale and reÞning existing methods
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