Fatigue crack growth tests were conducted on two API 5L X52 pipeline steel alloys. One alloy was from a new pipe that was installed for hydrogen service in 2011. The other alloy was from a vintage pipe that first saw natural gas service in 1964. Baseline tests were conducted in air, and those results were compared with tests conducted in pressurized hydrogen gas. All tests were run at (load ratio) R = 0.5 and a frequency of 1 Hz. Tests were conducted at hydrogen pressures of 5.5 MPa and 34 MPa. A new method that tests 10 compact tension specimens...