The U.S. Department of Energy has recently announced decarbonization targets for 2030 of 50% to 52% and a net-zero target for 2050. To achieve these goals, the future energy mix will consist of numerous carbon-free sources including existing technologies such as solar, wind, and nuclear energy. In addition to these, emergent technologies such as biofuels and hydrogen fuel will play a key role. A significant amount of work has been undertaken over the last decade to understand the behavior of materials that will be used in the hydrogen economy. For hydrogen to significantly impact decarburization and become widely used by consumers, it will need to be transported from generation sites to end-users. One proposed solution is the utilization of the existing pipeline infrastructure. However, there is still uncertainty on how to safely retrofit and operate pipelines constructed from vintage carbon steels. A tool was developed to estimate remaining life utilizing recently published hydrogen fatigue crack growth rate models published in ASME B31.12 and by Sandia National Labs. At low stress intensity ranges (ΔK), below about 5 to 6 MPa-m0.5, these values are at or below in-air crack growth rates. Pressure variations which account for these low ΔK values account for a large percentage of gas pipeline pressure fluctuations. The tool evaluates remaining life based of a pipeline based on the models, anticipated pressure cycles, flaw size, and fracture toughness behavior. A focus of this work was exploring behavior in the low ΔK range by applying different factors of safety to the in-air data (3 to 10 times). The outcome of the effort will not only provide a remaining life estimation, but it will identify gaps in the existing knowledge that can be the focus of future research and industry efforts.