## 4 Conclusions  
This study aims to examine the behavior of HBNG leakage in a pipeline compartment within utility tunnels and to analyze its leakage patterns under various ventilation conditions through simulation analysis. This study's findings reveal the most unfavorable working conditions associated with gas pipeline leakage from small holes and further investigate the accidental ventilation of sub-high-pressure gas pipelines under these conditions.  
This study demonstrated an increase in gas diffusion capacity with higher levels of HBR. In a mechanically ventilated setting, the ventilation airflow has a significant impact on gas diffusion. Insufficient wind speed could result in the buildup of gas downstream of the gas compartment, thereby posing a risk to pipeline safety. Furthermore, the calibration method for catalytic combustion-type combustible gas detectors in the pipeline corridor can continue to be used for methane even after the incorporation of hydrogen blending. It is advisable to install additional diversion facilities in the ventilation dead space of the gas compartment to improve safety measures.  
This study also indicated that the conditions least conducive to gas leaks from small holes in pipelines included elevated pipeline pressures, larger diameters of the leak holes, increased hydrogen blending ratios, and the proximity of the leaks to air supply outlets. At pressure levels of 0.4 MPa and 0.8 MPa, it is recommended that the gas compartment be ventilated accidentally at a frequency of 15 times per hour and 21 times per hour, respectively. However, the maximum unintentional ventilation wind velocity of 5 m per second, as specified in [14], needs to be revised to ensure the safety of the 1.6 MPa gas pipeline. Therefore, it is recommended to install an accidental interlocking device to promptly shut off and release a high-pressure pipeline in the event of a leakage accident.  
Conceptualization, Z.X., B.G., L.W. and S.C.; Methodology, Z.X. and B.G.; Software, M.L.; Validation, X.J.; Investigation, X.J.; Data curation, B.G.; Writing--original draft, Z.X. and L.W. All authors have read and agreed to the published version of the manuscript.  
Figure 15: Changes in the amount of gas present at monitoring point 15 of a sub-pressure HBNG pipeline leakage for various accidental venting strategies and pressure classes, (**a**) p = 0.4 MPa, (**b**) p = 0.8 MPa, (**c**) p = 1.6 MPa.  
**Funding:** This study was funded by the National Natural Science Foundation of China (No. 52104065), China Postdoctoral Science Foundation (No. 2022T150089), Natural Science Foundation of Heilongjiang Province of China (No. LH2021E019), China Postdoctoral Science Foundation (No. 2020M681064), Heil Long Jiang Postdoctoral Foundation (No. LBH-Z20101), National Natural Science Foundation of China (S2074090), Scientific Research Personnel Training Foundation of Northeast Petroleum University (No. NNYXLY202103), and Northeast Petroleum University Scientific Research Foundation (No. 2019 KQ54).  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data that support the findings of this study are available upon reasonable request from the corresponding authors. The data are not publicly available due to privacy.  
**Conflicts of Interest:** Author [PERSON] was employed by the company Gas Technology Institute of Petrochina Kunlun Gas Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.