The control of a running ductile fracture in dense-phase CO2 carrying pipelines requires noticeably better fracture resistance than that typically required for the transport of lean or rich natural gas. The long saturation plateau of the decompression sustains a significant driving force at low fracture velocities.
Since 2012, at least four independent projects published data to better understand the applicability of the Battelle Two-Curve Method for CO2 transportation, provide insight on minimum toughness requirements and margins of safety. Nine full-scale propagation tests were executed across these projects. About 50 pipes had interactions with a running ductile fracture, 33 pipes supported the propagation of the fracture over their entire length, the other 17 pipes stopped the fracture.
The original BTCM is not considered applicable with dense-phase CO2. Despite the actual decompression velocity’s saturation plateau decreasing with velocity, and despite the pressure at the crack tip being typically 8 bar lower than predicted, the model can be significantly non-conservative. Correction factors on toughness and arrest pressure are required.
An empirical model for prediction of the minimum required toughness is proposed. It is supported by the data from the four aforementioned projects. The details and the limitations of the database are presented. The arrest boundary is expressed graphically in the frame commonly used to present the NG18 arrest pressure boundary.
A discussion on the location of the experimental data points relative to the arrest-propagation boundary is given. It supports the definition of three regions of interest: a region of likely propagation, a region of likely arrest, and a transition region between these two, where the boundary resides.
All current standard and recommended practices have seemingly similar gaps with respect to the control of a running ductile fracture. The empirical model brings along a set of recommendations and requirements to consider in the context of dense-phase CO2 applications.