Numerical study on leakage diffusion characteristics and hazard boundaries of buried hydrogen-blended natural gas risers

Leakage from buried hydrogen blended natural gas (HBNG) pipelines can cause gas migration through porous soil and accumulation at the ground surface, creating fire and explosion hazards. Vertical risers, owing to stress concentration and distinct leakage patterns, exhibit hazard evolution different from horizontal pipelines. A three dimensional computational fluid dynamics (CFD) model of a wall adjacent buried HBNG riser was developed to investigate the effects of hydrogen blending ratio (HBR), pipeline pressure, soil type, and leakage hole number, shape, and spacing on dispersion and hazard boundaries. Four indicators, namely first danger time, duration of danger, ground danger range, and farthest danger range, were used to characterize spatiotemporal leakage hazards. Results show that obstruction by the pipeline and adjacent wall produces an asymmetric semi elliptical HBNG distribution in the axial plane and a dome shaped distribution in the transverse plane. Pipeline pressure exhibits a nonmonotonic risk pattern, with faster dispersion at high pressure but longer lasting risk at low pressure. Crack shaped leakage holes enhance the jet effect, causing HBNG to reach the lower flammability limit earlier and significantly enlarging the ground danger range. Among the investigated cases, a leakage hole spacing of 50 mm promotes faster horizontal dispersion. Compared with studies on horizontal pipelines, this study reveals the asymmetric dispersion of wall adjacent riser leakage, clarifies the nonmonotonic evolution of fire and explosion risk with pressure, and quantifies risk differences among leakage hole configurations. These findings provide a theoretical basis for risk prevention and emergency response for buried HBNG risers.