Risk assessment of fire and explosion accidents in oil-hydrogen fueling station based on fault tree analysis

Owing to the coexistence of gasoline, diesel, and hydrogen, a specific and thorough risk assessment approach must be promptly implemented for oil-hydrogen fueling stations. Consequently, this study makes innovations in considering the safety issues of the joint construction of gas station and hydrogen refueling station, and undertaking the modeling and analysis of fire and explosion incidents at oil-hydrogen fueling stations based on fault tree analysis. The findings reveal that the risk level for fire and explosion at oil-hydrogen fueling stations is categorized as grade III. In our fault tree analysis model, there are 3240 minimum cut sets and 194 minimum path sets. The probability of fire and explosion incidents occurring at oil-hydrogen fueling stations is calculated to be 0.000265. This indicates that while the possibility of such accidents is low, their potential severity is exceedingly high. Ultimately, it is recommended that the safety of oil-hydrogen fueling stations be enhanced through the reinforcement of emergency management, customer safety education, and electrostatic discharge devices.


Introduction
As well known, hydrogen energy has been regarded as a clean, efficient and high-density energy with the greatest development potential by more and more countries [1].Moreover, the integration of transportation and hydrogen energy has become a general consensus to realization the low-carbon development of transportation sector.To guarantee the efficient operation of the hydrogen industry chain, hydrogen infrastructure is particularly important.
Oil-hydrogen fueling station is an important hydrogen energy infrastructure that can solve the problems of land area, construction cost and construction time, and it is a reasonable transition scheme in the early stage of hydrogen energy development [2].In the oil-hydrogen fueling station, all of gasoline, diesel and hydrogen are easy to burn, explore and leak [3].When fire happened in the oil pool, the temperature of the flame will have an impact on the hydrogen storage tank [4], then the number of victims of the accident may be much higher than that of the gas station and filling station alone [5].Therefore, A particular and intensive risk assessment method should be put forward in oil-hydrogen fueling station immediately.
Most of the existing studies focused on the risk assessment of hydrogen refueling stations.For example, Jixin Zhang et al. conducted a quantitative analysis using the DEMATEL methodology to determine the likelihood of unsafe control movements occurring in the hydrogen refueling station system, the study obtained the probability of such occurrences taking place [6].Ashi Chauhan et al. [7] put forward the integrated BN-BWM-HEART method, which can solve the problems of less data and strong subjectivity in assessing personnel's work reliability.Camila Correa-Jullian et al. [8] used traditional QRA modeling tools, like failure mode and impact analysis, event sequence diagram and fault tree analysis, to sort out the data of liquid hydrogen storage system required for quantitative risk assessment.Park and colleagues [9] evaluated the potential individual and social hazards linked to hydrogen refueling stations through an examination of the safety effectiveness of their protective measures.Haugom et al. [10] analyzed the influence of different leakage sizes on the final consequences based on the Bayesian model.
However, we can find that the evaluation of oil-hydrogen fueling stations seems very few.Especially, the risk factors will become more variable after the joint construction of hydrogen refueling station and oil fueling station.Therefore, this paper calculates the probability of fire and explosion accident, accident risk grade, and analyzes the sensitivity of basic event based on fault tree analysis of oil-hydrogen fueling station, then puts forward suggestions for the safety construction and management of the oil-hydrogen fueling station.

System Construction
The oil-hydrogen fueling station is the addition of facilities related hydrogen on the basis of the original oil station or the construction of two sets at the same time.The oil-hydrogen fueling station are divided into 8 parts according to the process function area, such as gas source system, oil unloading system, compression system, gas storage system, oil storage system, gas selling system, oil selling system and auxiliary system [11].The schematic representation of the oil-hydrogen refueling facility can be observed in Figure 1.The hydrogen gas stored in long tube trailer is discharged to the compression system, and stored in the high-pressure storage tank after pressure increase.The gasoline and diesel are transported to the unloading point by the tank truck, and stored in the oil tank through the unloading system.In addition, a gasoline and diesel recovery system with a certain slope pipeline is set up underground in the oil-hydrogen fueling station.The co-construction of hydrogen fueling station and oil station is an efficient way to speed up the popularization of hydrogen energy fuel vehicles.However, on the account of the coexistence of three dangerous chemicals, hydrogen, gasoline and diesel, in the event of an accident, it is crucial to consider not only the explosive combustion mechanism of one fuel substance, but also the interaction between different fuels.

Establishment of fault tree
Some scholars have concluded the risk and severity of fire and explosion accidents are the greatest [13].Therefore, "Fire and explosion accident of oil-hydrogen fueling station" is selected as the top event of this accident tree for analysis.With reference to the cases analysis [14], accident mechanism [15][16] and hydrogen properties [17], the logical connection between the top event and basic events can be established.Then, Fault tree can be figured out (shown in figure 2 and 3).Additionally, the name and probability of each code is shown in table 1.   Notes: D-designation; P-probability; R-reference.

Minimum cut set
The minimum cut set refers to the minimal combination of basic events that can lead to the occurrence of the top event, specifically the fire and explosion accident at an oil-hydrogen fueling station.If all the basic events within a minimum cut set take place, the top event is guaranteed to occur.The common method for finding the minimum cut set is Boolean algebraic simplification.First, according to the structure of the fault tree, the AND gate is replaced by multiplication, the OR gate is replaced by addition, then the structure function of the top event can be written down successively.The calculation process is shown in the following equation.T=M1M2M3 =(X1+M5+M4+M6)(M14+M15)(M12+M11) It is calculated that the structure function consists of the addition of 3240 terms, which means this study utilizes Boolean algebra calculations to identify a total of 3240 minimum cut sets associated with fire and explosion accidents at oil-hydrogen fueling stations.The quantity of minimum cut sets directly corresponds to the number of potential pathways leading to the "fire and explosion accidents of oilhydrogen fueling station."

Minimum path set
The minimum path set, in contrast to the minimum cut set, refers to the smallest combination of fundamental events that are necessary to prevent the occurrence of the top event, such as a fire and explosion accident at an oil-hydrogen fueling station.If any of the fundamental events in the minimum path set do not take place, the top event will not occur.To identify the minimum path set, it is essential to substitute the AND gate in the fault tree with the OR gate, and the OR gate with the AND gate to create the success tree.Subsequently, the minimum cut set in success tree corresponds to the minimum path set in fault tree.

Importance
Each basic event within the fault tree will exert varying degrees of influence on the top event.
(1) Structural importance.From the standpoint of fault tree structure, the structural importance denotes the impact on the top event given that the possibility of each basic event is uniform.The structural importance can be calculated utilizing the quadratic approximation method formula, as depicted in equation ( 1) [25].
(2) Probability importance.The effect of changing the possibility of a basic event on the top event.Probability importance can be calculated by taking the partial derivative of the probability function of the top event, as represented by equation (2) [25].
(3) Critical importance.The critical importance pertains to the rate at which the probability of the top event is influenced by the rate of change in the probability of a basic event, as represented by equation ( 3) [25].
In summary, poor emergency drills, personnel carrying fire, electronic equipment, medical delay and no emergency plan are the five main factors affecting the accident of oil-hydrogen fueling station.

Probability of the top event
When calculating the possibility of the occurrence of the top event, either the minimum cut set formula or the minimum path set formula can be used.Since there are 3240 minimum cut sets in the fire and explosion fault tree of oil-hydrogen fueling station, the calculation formula of the minimum cut set is complex and time-consuming.Therefore, the calculation formula of the minimum path set is used here, which is show as equation ( 4) [25].

Result analysis and discussion
By employing tree conversion and Boolean algebra, it has been determined that there are 194 minimum path sets associated with fire and explosion accidents at oil-hydrogen fueling stations.Each of these minimum path sets represents a specific pathway to mitigate the occurrence of the top event.
Consequently, there exist 194 distinct pathways for controlling the incidence of "fire and explosion accidents at oil-hydrogen fueling stations".Critical importance, which evaluates the significance of basic events based on sensitivity and probability, is deemed more rational and practical compared to structural and probability importance.Therefore, the subsequent analysis will utilize the results of critical importance as the primary basis, while considering the results of structural and probability importance as supplementary references, to assess the sensibility of each basic event.The findings of this analysis are presented in table 2. The aforementioned analysis demonstrates that the probability of fire and explosion incidents at the oilhydrogen fueling station is 0.000265, with a minimum cut set of 3240 and a minimum path set of 194.
The factors with the most significant influence on the occurrence of fire and explosion accidents include poor emergency drills, personnel carrying fire, electronic equipment, untimely medical treatment and no emergency plan.
Based on the available literature, the fault tree analysis of a solitary hydrogen refueling station indicates that the number of minimum cut sets is 144, while the number of minimum path sets is 16 [26].Upon the concurrent establishment of a gas station and a hydrogen refueling station, it is evident that both the minimum cut set and minimum path set numbers increase.This suggests a greater proliferation of accident occurrence and prevention mechanisms, albeit with a more pronounced rise in accident occurrence pathways.Consequently, it is apparent that the risk level escalates following the joint construction of gas stations and hydrogen refueling stations.
The concept of risk can be expressed as the multiplication of the probability and the severity.Relying solely on fault tree analysis to assess accident probability may not provide a comprehensive evaluation of the risk level associated with an oil-hydrogen fueling station.It is essential to also take into account the potential severity of accidents.Previous studies have indicated that fire and explosion accidents at oil-hydrogen fueling stations pose significant hazards and can result in substantial damage, warranting a classification of very serious severity.
The probability of an accident and the potential impact of its consequences are integrated into the risk matrix table depicted in Figure 4. Analysis reveals that the risk classification of the oil-hydrogen fueling station is situated within region III, indicating a significant level of risk.Several researchers have suggested various approaches to mitigate the risk associated with hydrogen refueling stations.These include the implementation of robust operational protocols, rational scheduling of employees' work hours, and enhanced maintenance of equipment [6].Additionally, strategies to reduce the risk level of gas stations have been proposed, such as eliminating mobile phone reception, minimizing mechanical collisions, and mitigating container static electricity [22].Ultimately, to mitigate the risk associated with oil-hydrogen stations, measures can be implemented across following dimensions.
(1) Strengthening emergency management.Strictly implementing the main responsibility of enterprise emergency management, so as to achieve comprehensive coordination, overall management, and special responsibility.Developing a scientific emergency plan, and ensuring that every staff in the station is familiar with the emergency plan.Increasing the investment in emergency management funds, improving emergency equipment and facilities, and regularly carrying out emergency drills.
(2) Strengthening customer safety education.Popularizing the safety knowledge of oil-hydrogen fueling station to customers through slogans, brochures and videos.Carrying out safety training for station staff to ensure correct guidance to customers.
(3) Adding electrostatic discharge device.Electrostatic discharge devices should be set next to highpressure hydrogen storage tanks, oil and gas storage tanks, hydrogenators and refueling machines.If Electrostatic discharge devices don't work, the injection cannot proceed, to avoid accidents caused by human static electricity and electronic equipment.

Conclusion
This paper study utilized the fault tree analysis method to evaluate the potential for fire and explosion incidents at oil-hydrogen fueling stations.The model generated 3240 minimum cut sets and 194 minimum path sets, ultimately yielding a calculated probability of 0.000265 for fire and explosion accidents.The most influential factors contributing to these incidents were identified as poor emergency drill, personnel carrying fire, electronic equipment, untimely medical treatment and no emergency plan.The analysis findings indicate that the risk level of oil-hydrogen fueling stations is classified as grade Ⅲ, signifying a low probability of occurrence but with severe potential consequences.

Figure 1 .
Figure 1.The plane layout diagram of the oil-hydrogen fueling station [12].

Figure 2 .
Figure 2. Fire and explosion fault tree in oil-hydrogen fueling station (without detailed M14).

Figure 4 .
Figure 4. Risk matrix[27].Several researchers have suggested various approaches to mitigate the risk associated with hydrogen refueling stations.These include the implementation of robust operational protocols, rational scheduling of employees' work hours, and enhanced maintenance of equipment[6].Additionally, strategies to reduce the risk level of gas stations have been proposed, such as eliminating mobile phone reception, minimizing mechanical collisions, and mitigating container static electricity[22].Ultimately, to mitigate the risk associated with oil-hydrogen stations, measures can be implemented across following dimensions.(1)Strengthening emergency management.Strictly implementing the main responsibility of enterprise emergency management, so as to achieve comprehensive coordination, overall management, and special responsibility.Developing a scientific emergency plan, and ensuring that every staff in the station is familiar with the emergency plan.Increasing the investment in emergency management funds, improving emergency equipment and facilities, and regularly carrying out emergency drills.(2)Strengthening customer safety education.Popularizing the safety knowledge of oil-hydrogen fueling station to customers through slogans, brochures and videos.Carrying out safety training for station staff to ensure correct guidance to customers.(3)Adding electrostatic discharge device.Electrostatic discharge devices should be set next to highpressure hydrogen storage tanks, oil and gas storage tanks, hydrogenators and refueling machines.If Electrostatic discharge devices don't work, the injection cannot proceed, to avoid accidents caused by human static electricity and electronic equipment.

Table 1 .
The possibility of the basic event.

Table 2 .
The top five basic events in terms of importance.