Converting existing natural gas infrastructure for the transport of renewably produced hydrogen gas could aid in the energy transition away from fossil fuels. The Dampier-Bunbury Natural Gas Pipeline (DBNGP) in Western Australia, constructed from X65 pipeline steel, is a candidate for conversion to hydrogen transmission. However, it is possible that the pipeline steel exposed to hydrogen may have reduced material properties due to hydrogen embrittlement (HE).In this work, the HE susceptibility of X65 samples from the DBNGP (designated X65 D) was investigated. The foremost objective of this study was to investigate the effects of hydrogen on the mechanical properties of X65 D to inform the prospective introduction of hydrogen into the DBNGP and other pipelines. This included investigating the effects of hydrogen at multiple hydrogen concentrations and multiple locations in the pipeline wall. Other objectives of this study included (i) investigating the possibility of subcritical crack growth in X65 D, (ii) designing and constructing a gas permeation apparatus for evaluating hydrogen permeation at various partial pressures, temperatures, and blends of multiple gases, (iii) finding the resultant subsurface hydrogen concentrations in the pipeline wall from exposure to hydrogen, and (iv) investigating the effects of thermomechanical treatments during pipeline steel production to alter the HE susceptibility.The mechanical properties of X65 D samples were investigated using the linearly increasing stress test (LIST). Specimens were tested in-situ in an electrolytic cell containing 0.1 M NaOH at cathodic current densities of 9 and 25 mA/cm2. Specimens were taken from the weld metal, heat-affected zone (HAZ), and base metal. Base metal specimens were taken from three depths in the pipeline wall and two orientations (longitudinal and transverse). Hydrogen charging did not affect the strength of any region of the pipeline wall. No subcritical crack growth occurred. Hydrogen reduced the ductility of the weld, HAZ, and all three depths of the base metal by a similar amount. Transverse specimens from the base metal were more susceptible to HE than longitudinal specimens, which was attributed to anisotropic pearlite banding in the microstructure introduced by rolling during production. In air fracture initiated by ductile internal rupture, and in hydrogen fracture initiated from brittle surface cracks. These results indicated that hydrogen exposure should not affect the X65 D pipeline properties at constant stresses below the yield stress, but that hydrogen could reduce the ductility by enhancing crack initiation and propagation at high stresses. The reduction in ductility by hydrogen would be most severe for a longitudinally oriented crack on the internal surface of the pipeline.A gas permeation apparatus (GPA) was designed and constructed to investigate hydrogen permeation through X65 D at different hydrogen partial pressures. GPA tests on X65 D were conducted at hydrogen pressures up to 41 bar. The X65 D results were combined with literature results for gaseous hydrogen permeation through other pipeline steels to find a general equation for hydrogen concentration from hydrogen fugacity for pipeline steels. Electrochemical hydrogen permeation tests were conducted to find the hydrogen partial pressures which introduced an equivalent amount of hydrogen as the testing conditions for the LIST tests. A current density of 9 mA/cm2 was equivalent to a hydrogen partial pressure of 854 bar, and 25 mA/cm2 was equivalent to 1718 bar. These results indicated that the hydrogen concentrations in X65 D during LIST testing were significantly greater than those expected in a hydrogen transmission pipeline, and as such the HE results were highly conservative. Additionally, TDS tests of X65 D specimens were conducted to verify these results, and the results were similar to the permeation tests.To research possible methods to improve the resistance of pipeline steels to HE, two treatments were investigated: subcritical annealing and cold drawing. Previous research had found that these treatments have varying effects on the degree of HE for different steel grades. X65 D samples were subcritically annealed at 200, 400, or 600 °C for 2 h, or cold drawn to a 5% reduction in area. LIST tests of the treated X65 D found that, in air, subcritical annealing reduced the strength and increased the ductility, while cold drawing increased the strength and reduced the ductility. In hydrogen, both treatments were effective in reducing the severity of HE compared to the as-received material. Hydrogen did not reduce strength or cause subcritical crack growth for the treated X65 D. The reduction of HE by both subcritical annealing and cold working was tentatively attributed to a more normalised distribution of dislocations after treatment.