Impact of hydrogen blending ratio on quantitative risk assessment of natural gas pipelines

Hydrogen blending into existing natural gas (NG) transmission networks has emerged as a promising pathway for reducing greenhouse gas (GHG) emissions while enabling the transition towards a hydrogen-based energy system. This study presents a Quantitative Risk Assessment (QRA) of a high-pressure hydrogen-natural gas (H 2 /NG) transmission pipeline considering hydrogen blending ratios ranging from 0% to 100%. Consequence modeling was performed using EFFECTS, while individual and societal risks were evaluated using RISKCURVES under consistent pipeline, meteorological, and population conditions. Increasing hydrogen content reduced the mass discharge rate and the thermal radiation hazard distances associated with jet fires, while substantially increasing the overpressure distances associated with flash fire/VCE scenarios, particularly for large leaks and full-bore rupture (FBR) conditions. The resulting risk showed a non-monotonic dependence on hydrogen content. The maximum individual risk was obtained for the 50% H 2 blend, reaching 2.33 × 10 −6 yr −1 , compared with 1.35 × 10 −6 yr −1 for pure NG and 1.63 × 10 −6 yr −1 for pure hydrogen. In contrast, the societal risk assessment showed a more pronounced effect for pure hydrogen, with the F-N curve extending to substantially higher numbers of potential fatalities than the other blending ratios. For 100% H 2 , the maximum Guide Ratio (GR) occurred at 47 fatalities, compared with 15-24 fatalities for the 0-50% H 2 cases, and the FBR flash fire scenario accounted for 93% of the cumulative frequency at 50 fatalities. All investigated cases remained below the adopted upper societal risk tolerability criterion. These results demonstrate that increasing hydrogen content does not produce a monotonic change in overall pipeline risk, but instead shifts the relative importance and spatial extent of competing accident consequences. The findings highlight the need for comprehensive QRA and appropriate prevention and mitigation measures when assessing the repurposing of natural gas transmission infrastructure for hydrogen transport.