Innovative Nondestructive Method Determines Fracture Toughness of In-Service Pipelines

Abstract

Applications of the innovative, patented Stress-Strain Microprobe (SSM) system, that utilizes an in-situ nondestructive Automated Ball Indentation (ABI) test technique to determine fracture toughness of in-service steel pipelines, are described in this paper. The ABI test provides the actual/current values of fracture toughness properties for base metal, welds, and heat-affected-zones. The ABI-measured key mechanical properties are used with other nondestructive measurements, such as crack/defect sizes (determined from in-line smart pigs or from on-line ultrasound instruments), to determine the safe operating pressure of the pipeline or to necessitate certain rehabilitation actions. The ABI test is based on progressive indentation with intermediate partial unloadings until the desired/required maximum depth (maximum strain) is reached, and then the indenter is fully unloaded. The ABI test is fully automated (using a notebook computer, data acquisition system, and a servo motor), and a single test is completed in less than two minutes. This paper describes two recent field investigations. The first investigation assessed a catastrophic failure that occurred in a natural gas plant on a cold winter night shortly following the leak of liquid natural gas into a natural gas pipeline. The combination of cold temperature and high strain rate near a crack resulted in the destruction of approximately a 12-meter section of a 508-mm (20-inch) diameter pipeline into several hundred small pieces. Since the remaining pieces from the exploded pipeline section were not sufficient to machine destructive tensile and fracture toughness specimens, the SSM system was used to measure the tensile and fracture toughness properties from multiple ABI tests on several pipeline pieces. The ABI-measured tensile and fracture toughness results provided the basis for the fitness-for-service assessment of the remaining pipeline sections of the natural gas plant. The second application involved a fire that occurred due to a leak from a 356-mm (14-inch) diameter Kerosene pipeline. The fire-damaged section of the pipeline was cut out and replaced. As part of the effort to prevent future accidents, the entire 7-km pipeline needed a structural integrity assessment. In-Situ ABI tests were conducted to measure the tensile and fracture toughness properies, from each ABI test, for the fitness-for-service assessment since the carbon steel pipeline had undocumented grade.