Effect of Grade on Ductile Fracture Arrest Criteria for Gas Pipelines

Abstract

Several different criteria have been proposed over the years to predict the minimum toughness for arrest of an axial propagating crack for natural gas pipelines. The initial ones were empirically based. The Battelle Two-Curve Method (TCM) was subsequently developed and was somewhat less empirical. The TCM is still used frequently today. Nevertheless, all of these criteria use the Charpy energy as a measure of the material’s ductile fracture resistance. As higher-grade steels have been developed, it has been found from full-scale tests that a multiplier was needed on the predicted minimum Charpy arrest energy value as calculated from the original TCM. Several researchers have also suggested that a correction factor was needed on the Charpy energy as the Charpy energy value increased above a certain level. This was a nonlinear correction factor that essentially showed that as the Charpy energy value surpassed a certain level, the effective energy for ductile fracture arrest is less than the total energy from the Charpy test. This paper presents background information on several of these toughness correction factors, as well as statistical analyses of full-scale pipe burst tests on 186 lengths of X60 to X100 grade pipes using these methods. The results show the effects of grade level on not only the original TCM predictions, but also several other modifications for high Charpy energy levels. Additionally, a method has also been developed where the DWTT energy was used instead of the Charpy energy in the Battelle TCM. The results of the statistical analyses showed that all the Charpy-energy-based criteria required an increasing correction factor as the grade level increased. The one DWTT energy criterion was statistically constant with grade level. This difference between the Charpy criteria and the DWTT criterion was traced back to a changing relationship between the Charpy and DWTT energy values as the grade of the steel increases.

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“…For modern higher grade steels, if the predicted CVN value is greater than ~95 J [57], then the CVN value should be corrected using a certain correction factor to match the results of full-scale burst tests [58,59]. The Australian Standard (AS 2885.1), states that the predicted toughness should be multiplied by a factor of at least 1.4.…”

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“…Recently, Wilkowski et al [55] showed that the pipeline grade has a significant effect on the correlation between CVN and DWTT energies. The slope of the linear function continues to decrease from 2.94 ($3.00 as obtained in Eq.…”

“…On this basis, they obtained the so-called Wilkowski 1977 prediction of CVN energy from the standard DWTT tests: In order to apply this nonlinear DWTT-CVN relationship to pipeline grades up to X70, a statistical factor 1.3 was inserted before (E/A) DWTT in the square brackets of Eq. (11), which leads to another correlation equation called the Wilkowski 2000 prediction of the CVN energy from the standard DWTT tests [55]. Note that these correlations between DWTT and CVN energies are not corrections to relate actual to predicted arrest CVN toughness.…”

“…In order to describe a nonlinear relationship between the DWTT and CVN energies, Wilkowski et al [55] proposed two new nonlinear correlations. Note that historically the standard PN-DWTT specimens were used to develop the linear relationship between the PN-DWTT and CVN energies [37], and a brittle-notched (BN) DWTT specimen was employed to develop a nonlinear relationship between PN-DWTT and BN-DWTT energies [53,55].…”

“…Note that historically the standard PN-DWTT specimens were used to develop the linear relationship between the PN-DWTT and CVN energies [37], and a brittle-notched (BN) DWTT specimen was employed to develop a nonlinear relationship between PN-DWTT and BN-DWTT energies [53,55]. These authors assumed that the PN-DWTT specimen and the BN-DWTT specimen were equivalent and able to absorb the same impact energy, because both specimens have less initiation energy compared to the total absorbed energy for vintage pipeline steels.…”