Evaluation of Composite Crack Arrestor Design for Running Ductile Fractures on High Pressure Pipelines

Abstract The topic of running ductile fracture arrest has seen renewed emphasis in the transmission pipeline industry as high-pressure pipelines transporting natural gas or CO2, operating under a variety of conditions, can potentially be susceptible to running ductile fracture. This is less of a concern for pipelines transporting liquids (such as oil), where the content’s rapid decompression behavior does not leave much energy for driving a running ductile fracture. Pipelines repurposed for transportation of dense phase CO2 are particularly susceptible to the threat of running ductile fracture due to the specific properties of CO2 and the existing transmission lines. Further, high-pressure natural gas transmission lines, the focus of this effort, can also be vulnerable to running ductile fracture. A number of methodologies have historically been deployed to combat this threat, including the use of pipe material with sufficient toughness to arrest a running fracture or the installation of pipe joints with greater wall thicknesses at designated intervals along the pipeline. Another methodology involves the installation of externally applied crack arrestors. Using non-metallic composite materials for these externally applied arrestors has the potential to provide substantial cost advantages over other methods and has led to significant interest in employing this advancing technology. Despite the growing interest, the pipeline industry has not achieved consensus on the most appropriate design methodology for composite crack arrestors. This paper presents the results and findings of a study that reviewed several design approaches for composite crack arrestors. The primary goal was to achieve an optimized design that minimizes the material requirements of a composite crack arrestor, while still achieving fracture arrest. A large diameter gas transmission pipeline operating at an elevated MAOP was used as the operating asset. Two different methods were evaluated in the study. The first approach targeted a “soft” arrest that would slow the progression of the fracture and limit the potential for pipe “ring-off”. A second approach was examined that did not consider the mode of arrest. The Battelle Two-Curve Method (BTCM) was used to assess the likelihood of running ductile fracture for the subject pipeline. A parametric study was performed to assess the influence of various operating conditions on the decompression curve for the projected gas composition. Additionally, a literature review was conducted to compare existing arrestor designs, validation efforts, and appropriate BTCM modifiers. This research shows the feasibility of implementing composite crack arrestors to achieve either “soft” or “hard” arrest of a running ductile fracture in high-pressure pipelines. Both design methods considered possess notable advantages over traditional methods.