Safety risk modeling and major accidents analysis of hydrogen and natural gas releases: A comprehensive risk analysis framework

• A comprehensive risk analysis methodology for hydrogen gas release is proposed.
• Process safety of hydrogen production in an oil refinery is analyzed.
• Risk and consequences of hydrogen are qualitatively investigated and quantitatively modeled.
• Maximum vulnerability range of jet fire, flash fire and vapor cloud explosion is calculated.
• A comparison between hydrogen plant F–N curve and societal risk criteria is provided.

The potential safety risk of hydrogen production is often the most important element to achieve authority approval and public acceptance. Safe application of hydrogen, especially in a large scale, will require adopting adequate risk control, which requires investment on reliable risk analysis methodology. In the present study, first of all, a reliable and comprehensive safety risk analysis methodology was developed for a hydrogen production plant in an oil refinery, that consists of two qualitative methods: Hazard and Operability (HAZOP) and Preliminary Risk Analysis (PRA), a hybrid method: Event Tree Analysis (ETA) and a quantitative method: Quantitative Risk Assessment (QRA) along with a risk and consequence simulator. A HAZOP study along with the PRA technique was used for determining main hazardous sources and carrying out a qualitative risk analysis. The incident outcomes of the identified high risk scenarios were modeled using the PHAST 6.7 simulator and the frequencies of the initial events and incident outcomes were calculated using risk assessment data directory of International Association Oil & Gas Producers (OGP) and ETA, respectively. Finally, the vulnerability areas of the incident outcomes were determined and the societal risk of hydrogen plant was shown using a ‘Frequency vs. Number of fatality’ graph, known as ‘F–N’ curves. The findings show that the maximum vulnerability distance is caused by the vapor cloud explosion (280 m, at 0.01 bar) and the jet fire (275 m, at 4 kW/m2), respectively. The societal risk of the plant fell in the As Low As Reasonable Practical (ALARP) and intolerable regions according to the F–N curve of UK HSE (Health, Safety Executive) The reformer were the highest and the heat exchanger was the lowest contributor to the total risk. Therefore, the ALARP principle should be applied to indicate the appropriate ways to reduce risks and, for the intolerable risks, the system must be modified structurally, functionally, or organizationally.

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