How to Optimize the Design of Mechanical Crack Arrestors

Abstract

Ductile fractures in natural gas and other high-energy pipelines could be arrested by either toughness in the pipe body or by a crack arrestor device. Although numerous crack arrestor devices have been proposed and patented, the mechanical crack arrestor that surrounds the outside of the pipe (external sleeve type) is the most common type of arrestor. This paper presents an empirically based criteria developed for the optimization of the design of mechanical crack arrestors. The initial development was based on a significant number of steel sleeve crack arrestors with different radial spacings (with and without grouting) and axial lengths that had the same thickness and strength as the main-line pipe. That work was extended to circular cross-section (toroidal) arrestors with different mechanical connectors to eliminate the need for welding. These crack arrestor tests were on 152 and 304 mm (6 and 12-inch) diameter pipes pressurized with nitrogen, rich gas, and liquid carbon dioxide that produce radically different crack-driving forces. It will be shown that the arrestor size is related to the velocity of the ductile fracture as it enters the arrestor, i.e., the fracture velocity is a measure of the instability that needs to be overcome for arrest. A limited number of results from full-scale tests are also presented to validate the design guidelines from this project. Finally, it will be shown how the results could be expanded for composite arrestors.