1. Introduction
Ships emit various pollutants, such as nitrogen oxides, sulfur oxides, particulate matter, and carbon dioxide. Among them, CO
2 emissions from ships account for approximately 2.5% of global emissions. The International Maritime Organization (IMO) adopted initial greenhouse gas (GHG) strategies that specified GHG reduction goals and measures in 2018 to reduce GHG emissions (
Ye et al., 2022). These initial strategies aimed to reduce GHG emissions by at least 50% compared to the 2008 levels by 2050 and ultimately to net zero. The initial strategies included short-term (2018–2023), mid-term (2023–2030), and long-term (2023–2030) measures. The short-term measures involve technical/operational energy efficiency measures and speed reduction regulations for new and existing ships. The mid-to-long-term measures involve technical measures that regulate the overall GHG emissions by forcing operational efficiency, market-based measures that forcibly increase the price of fossil fuels or impose a levy on GHG emissions, and technology/market combination measures that combine the two measures (
Chircop, 2019). The 80
th meeting of the Marine Environment Protection Committee strengthened the regulations on GHG emissions further by adopting strategies for net-zero GHG emissions by 2050 (
Zhang et al., 2024).
With strengthened regulations on GHG emissions, the shipping industry is accelerating the introduction of zero-carbon fuels, such as hydrogen and ammonia. Ammonia has been considered a fuel for large ships because of its high storage efficiency compared to hydrogen and the fuel supply and demand benefit. According to
Valera-Medina et al. (2018), the storage cost of ammonia was more than three times lower than that of hydrogen. Ammonia has already secured the technology and infrastructure required for the production and transport of fuel (
Ishaq and Crawford, 2024). Nevertheless, caution is needed when handling ammonia because it is toxic and corrosive (
Lan and Tao, 2014). Ammonia-fueled ships have been developed worldwide. The core of technology development is the ammonia engine and fuel supply system (FSS). MAN Energy Solutions, a German ship engine maker, succeeded in a combustion test of an ammonia engine in 2023 (
MAN Energy Solutions, 2023).
The company also set the basic concept of ammonia FSS to be similar to that of the LPG low-flashpoint fuel supply system (LFSS) and added an ammonia catch system to treat ammonia that is inevitably released. WinGD, a Swiss ship engine maker, revealed its plan to supply ammonia engines starting in 2025. Alfa Laval, a Swedish marine equipment company, announced it would supply an ammonia FSS for engine testing (
Alfa Laval, 2023). Hyundai Heavy Industries (HHI) developed an ammonia FSS equipped with a dual leakage prevention gas treatment system. The FSS can remove the nitrogen oxides in the ammonia engine exhaust gas using the ammonia released from it (
HHI, 2021). Japan Engine Corporation conducted ammonia fuel tests under various conditions. Mitsubishi Shipbuilding developed and supplied ammonia FSS for large and low-speed two-stroke ship engines (
Mitsubishi, 2023).
Various classification societies published regulations and guidelines for using ammonia as a ship fuel. The published rules and guidelines commonly presented key design considerations based on the International Code of Safety for Ships using gases or other low-flash point fuels (IGF code). The IGF code is the international standard for ships that use gases or other low-flash point fuels and provides essential criteria for arrangement, installation, and control to minimize the risks for the crew and environment. Each classification considered the characteristics of ammonia (toxicity and corrosiveness) and presented detailed safety considerations. Bureau Veritas (BV) enacted temporary rules for ammonia in 2021 and published a revised edition in September 2024. The company set the allowable ammonia exposure concentration and the concentration at which the water mist system operates to treat ammonia to 30 ppm and required at least 30 times of ventilation per hour for the ventilation system capacity in the space with the ammonia exposure risk (
BV, 2024). The Korean Register (KR) published a guide for ammonia-fueled ships in 2021. It also published a revised edition in June 2023, which included standard concentrations, toxic areas, safety measures, and gas treatment systems. It set the allowable ammonia exposure concentration to 25 ppm, which is lower than BV, and the upper exposure limit concentration at which the ammonia treatment system operates to 300 ppm. It requires a ventilation system with a capacity that is at least 30 times ventilation per hour (
KR, 2023). ClassNK added ammonia fuel to the guidelines on ships that use low-flash point fuels in September 2021, with a revised edition published in June 2022. It set the allowable ammonia exposure concentration for sounding an alarm to 25 ppm and the concentration at which the master valve and double block and bleed (DBB) valve are blocked to 300 ppm. It requires a ventilation system with a capacity of at least 30 times ventilation per hour in the space where gas can accumulate and at least 45 times per hour for areas where ammonia-related equipment is installed and people can enter during operation (
ClassNK, 2021). The American Bureau of Shipping (ABS) published a guide for ammonia-fueled ships in 2021. It also published a Requirement in September 2023 by adding essential items for issuing a classification society certificate. It limits the ammonia concentration before being released into the atmosphere through the vent mast and vent riser to 25 ppm and requires a ventilation system with a capacity that is at least 30 times ventilation per hour (
ABS, 2023). Det Norske Veritas (DNV) published revised rules in October 2023 by adding requirements, such as atmospheric emission limits, diffusion analysis for emergency scenarios, ammonia leakage-related systems, and fire safety, to the rules published in 2021. It set the ammonia concentration released into the atmosphere to 300 ppm and the ammonia concentration in ventilated space at which a water mist system automatically operates to 350 ppm. Moreover, the ventilation system must operate at least 30 times per hour in the fuel preparation room and have sufficient space to handle fuel (
DNV, 2023).
Various studies have also been conducted to ensure the safety of ammonia-fueled ships. In Japan, hazard identification (HAZID) was performed to propose the risk of ammonia-fueled ships. Fuel leaks and the loss of equipment control signals were estimated to be the most significant risk factors, but they need to be examined at the ship design, arrangement, and operation stages. Research was also conducted on the risks that may occur when fuel is supplied to a ship during the bunkering process. Surbana Jurong, Singapore’s state-owned consulting firm, and Singapore Maritime Academy (SMA), a college, conducted research on ammonia ship-to-ship (STS) bunkering concept design and HAZID/hazard and operability study (HAZOP)/ quantitative risk assessment (QRA) (
DNV, 2022).
Duong et al. (2023) analyzed a diffusion analysis software program using FLACS to compare the safety zones for leakage accidents during the NH
3 and liquefied natural gas (LNG) STS bunkering processes. NH
3 has a wider diffusion range than LNG when leaked under the same conditions because of its wider explosion range and toxicity. NH
3 at − 33°C (1 bar) lasts longer in the diffusion area than LNG at −163°C (1 bar) because of the weak convection phenomenon caused by the insignificant temperature difference from the atmosphere.
Ng et al. (2023) analyzed the effects of operational and meteorological conditions on atmospheric diffusion in the event of an NH
3 STS bunkering leakage accident using Phast software. They mentioned semi-refrigerated (SR), non-refrigerated (NR), and fully refrigerated (FR) types as tank storage conditions and reported that FR is safer than SR and NR. They recommended that the flow rate should be distributed and transported using multiple hoses because the accident area increases in the event of an accident as the bunkering flow rate increases.
Kim et al. (2022) analyzed various accident scenarios using the Phast software to determine safety zones during the truck-to-ship (TTS) bunkering of ammonia-fueled ships. They proposed population-dependent analysis (PDA) to consider the effect of population density on the level of damage. The hazard range increased as the leak hole size increased, and the hazard radius was presented within 10, 57, and 373 m in the event of one accident in 20,000, 100,000, and 200,000 years.
The engine room of a ship is a dangerous zone that contains all three elements of fire, i.e., oxygen, heat (ignition source), and fuel. Research has been actively conducted on the fire risk of the engine room.
Pomonis et al. (2022) conducted a risk assessment for engine room fire scenarios when ammonia fuel was used compared to conventional fuels (LNG/marine diesel oil (MDO)) using PyroSim, a fire simulation software program. They reported that ammonia has less equipment damage and a longer fire evacuation time because of the low flame temperature compared to conventional fuels, but the risk from its toxicity in the event of direct leakage is relatively high.
Yadav and Jeong (2022) analyzed the risks of fuel leaks inside the engine room of ammonia carriers using a Solidworks flow simulation. They reported that the results differed according to the leakage direction (transverse, vertical, and longitudinal) when the fuel leak from pipes began. The fuel leak in the longitudinal direction exhibited the highest risk as it spread within 30 seconds and reached the range set for alarms. The range was not reached in the transverse and vertical directions, but evacuation in less than two minutes was recommended.
Various studies have been conducted to determine the toxicity and fire risks of ammonia FSS and fuel, but no study has presented a risk assessment and measures to prevent and mitigate the risk for FSS, a key system of ammonia-fueled ships. The application of ammonia fuel to ships requires an integrated examination and analysis, including FSS. This paper presents preventive/mitigation measures to minimize the risk by performing HAZID for ammonia FSS. The contents of this paper are as follows. The concepts of ammonia carriers and FSS are proposed in Chapter 2, and the risk assessment methodology is described in Chapter 3. Each result and discussion are presented in Chapters 4 and 5. Finally, conclusions are drawn in Chapter 6.
3. Methodology of Hazard Identification
A risk assessment is a method of identifying and evaluating potential hazards that may occur in a system. It aims to prevent accidents by removing hazards and performing necessary measures. Risk assessment methods can be divided mainly into quantitative and qualitative assessment methods. Quantitative assessment methods include QRA/toxicological risk assessment (TRA), and qualitative assessment methods include failure mode and effects analysis (FMEA), HAZID, and HAZOP. QRA, a method of determining the impact on the risk of the system, is used in process design, safety, inspection, maintenance, planning, and operational management (
Acikalin, 2009). TRA, a method of evaluating the risk related to exposure to potentially hazardous substances, is used to evaluate the toxicological risks in the early stages of the system (
Torres and Bobst, 2015).
FMEA, a methodology for identifying the results caused by the errors of components and determining the importance of each error mode regarding the system performance, is used in the safety and reliability analysis and design stages. HAZID, a qualitative analysis method, is performed using a brainstorming technique that involves experts from various fields to identify risks (
Kim, 2022). HAZOP is a technique to analyze and evaluate hazards systematically in the design review process and risks for operability (
Siddiqui et al., 2014).
The study was conducted based on HAZID, which can identify existing risks at the initial design stage and assess all potential hazards inside the ship through brainstorming. HAZID is performed based on the procedure shown in
Fig. 3 and is described below (
KRISO, 2021).
3.1 HAZID Procedure
3.1.1 Identification of HAZID nodes
The target system was divided into nodes to examine its hazards. The size and number of nodes vary depending on the complexity of the process and the discretion of the risk assessment team.
3.1.2 Node briefing
The nodes used for risk assessment and the operating procedure were explained to the members participating in risk assessment. The members were composed of experts related to the target system, and HAZID was performed using brainstorming to identify potential risks.
Table 2 lists the expert group.
3.1.3 Identification of hazards and hazardous events
Through drawings and information documents, hazards are identified by determining if certain situations and equipment may cause risks.
3.1.4 Identification of causes & consequences
All potential causes for each identified accident and the magnitude of damage in terms of people, assets, and the environment were considered. Double jeopardy, which applies two independent events simultaneously, was not considered.
3.1.5 Identification of preventive and mitigating safeguards
This study examined whether the safeguards considered were designed to prevent accidents or reduce the magnitude of damage.
3.1.6 Risk ranking
A risk assessment was performed by combining the frequency of accidents and the magnitude of damage using the matrix technique. The matrix technique was derived numerically, and it can determine the priorities of risks.
3.1.7 Identification of recommendations
Further recommendations were identified by judging the devices and recommendations reflected in the current system to minimize and prevent risks and considering the risk ranking.
3.2 Risk Ranking
The risk ranking was calculated by identifying the hazards of the system and determining the frequency of the risk and the magnitude of the damage. The risk ranking for each identified accident scenario was determined (
Table 5) considering the likelihood rating listed in
Table 3 and the consequence rating shown in
Table 4. The matrix consisted of seven likelihoods and five consequences, making it possible to estimate the risk with 35 measures through combinations according to
Eq. (1).
Frequency ratings can be distinguished for each scenario, and the likelihood ratings were assigned based on the criteria shown in
Table 3. The magnitude of damage was calculated according to the five ratings distinguished in
Table 4, and damage influence levels for people, assets, and the environment were considered.
Based on the risk matrix, the risks mentioned in
Table 5 were classified according to the frequency of accidents and the magnitude of damage.
- Low risk (green): It is considered acceptable and requires no additional preventive or mitigation measures.
- Medium risk (yellow): The risk must be reduced in the applicable range, and mitigation measures must be implemented.
- High risk (red): The risk is unacceptable, and mitigation measures must be implemented to reduce it below the medium level.