By Niu Song CCS Shanghai Rules & Research Institute

This paper discusses the design basis, ship arrangement, safety protection, and other core elements of the application of ammonia fuel in ships by taking the practical application of ammonia fuel power system on ships as an example, in order to provide technical support and practical guidance for the promotion and application of ammonia-fuelled vessels in the shipping industry.

Design Basis

Ammonia-fuelled vessels involve multiple aspects including the characteristics of ammonia as fuel, the design of storage and supply system, the safety control during combustion, etc. The real ship design mainly considers the storage, supply, and combustion processes of ammonia fuel, the impact of ammonia as fuel on ship arrangement, as well as the safety of ammonia-fuelled vessels during operation.

The design of ammonia-fuelled vessels should first comply with relevant international and/or domestic ship design rules and standards (such as ISO 5771), and meet the corresponding ship type requirements. It is worth noting that for liquefied gas carriers, the IGC Code stipulates that "goods marked as toxic are not allowed to be used as fuel", which means that if liquid ammonia cargo is carried, it cannot be used as ship fuel. At present, considering the development trend of ammonia as marine fuel, IMO is also considering how to allow the use of cargo ammonia as marine fuel while ensuring the same level of safety (such as using natural gas as fuel). Based on this, the 9th meeting of the IMO Subcommittee on Containers and Cargoes (CCC) has established a post-meeting communication group to discuss the revision of the IGC Code requirements of not allowing the use of toxic goods as fuel (to be approved in 2026). In addition, at the 10th CCC meeting in September this year, the IMO Subcommittee on Containers and Cargoes (CCC) concluded that "ships that comply with IGC rules should follow IGC rules instead of IGF rules even if they use liquefied gas as fuel that does not carry cargoes". This also provides a reference for the design basis of liquefied gas as fuel for subsequent liquefied gas carriers.

Fig. 1: Schematic Diagram of Ammonia Fuel Tank Arrangement

For ammonia fuel, the applicable requirements of the IGF Code and the requirements of the codes/guidelines of all classification societies can also be applicable, such as the CCS "Guidelines for Ships Using Ammonia Fuel" (2022). In addition, IMO reviewed the "Interim Guidelines for Ships Using Ammonia Fuel" (hereinafter referred to as the "Interim Guidelines") at this year's CCC 10th meeting, which are to be submitted to MSC 109th meeting for approval. Therefore, the interim guidelines can also serve as a reference for design.

As a preliminary application of a new system and technology, ammonia-fuelled vessels still lack the experience of onboard application despite their high technological maturity. It is necessary to adopt acceptable and recognized risk analysis techniques to assess the potential risks involved in the design of ammonia-fuelled vessels, in order to eliminate or mitigate their adverse effects on onboard personnel, environment, structural strength, or ship integrity.

Ship Arrangement

The arrangement of ammonia-fuelled vessels is a complex and critical process that involves the overall design/layout plan of the ship, including the setting of ammonia fuel tanks, installation of ammonia fuel storage and supply systems, pipeline routing, provision of safety facilities, etc. In addition, with the addition of new equipment such as ammonia fuel storage tanks, storage supply pipelines, and safety devices, the spatial arrangement inside the ship also needs to be re-adjusted, which may affect the loading of goods, the layout of engine rooms, the division of living areas, etc.

Ammonia fuel tank, ammonia fuel preparation room, and other compartments should be arranged in areas where the hull structure is stable and not easily affected by external factors. Their capacity and quantity should be reasonably configured according to the ship's own voyage route. At the same time, the requirements for setting up isolated empty compartments in actual arrangement positions should also be considered to minimize the probability of damage in the event of collision or grounding of the ship (for example, to be arranged behind the collision bulkhead), in order to ensure their safety and stability.

Considering the physical and chemical properties of ammonia fuel (such as toxicity), the ammonia fuel tank and ammonia fuel preparation room should be arranged outside of Class A machinery/important machinery areas, and ammonia fuel tanks should not be arranged in the living areas. When the ammonia fuel tank is arranged on open decks, coaming, water spray system, and independent ammonia water release system should be installed to avoid direct discharge of aqueous solutions containing liquid ammonia or dissolved ammonia outside the ship. Meanwhile, the ammonia fuel tank should also ensure sufficient ventilation to prevent the accumulation of escaping gases.

In addition, the ammonia fuel containment system should be reasonably designed to ensure that the fuel storage risk reaches a level equivalent to that of conventional fuel-powered ships. Generally, the ammonia fuel tank joints, accessories, flanges, and valves should be enclosed within airtight fuel tank joints to reduce risks (except for those on open decks). At the same time, the structural strength of the ammonia fuel containment system should also have sufficient safety margin to withstand the shrinkage and expansion of the ammonia fuel tank caused by temperature change or ship deformation, as well as the risk of damage that may occur due to ship and/or cargo operation (mechanical protection).

A pressure release system should be equipped for the fuel tanks, inter-barriers, fuel tank joints, and isolation compartments for which the pressure endured during the operation of ships may exceed their design capacities. At least 2 sets of pressure relief valves should be installed in the ammonia fuel tank, and one set of pressure relief valve can be disconnected in case of malfunction or leakage. When the ammonia fuel tank may withstand external pressure exceeding its design pressure, a vacuum protection system should also be installed. The ventilation system of the ammonia fuel tank (only used for ventilation and degassing) should be connected to the top of the fuel tank for self-draining under normal operating conditions, and minimizing the harm caused by the diffusion of toxic and flammable vapors.

Since ammonia fuel is corrosive, it can cause corrosion to the materials in direct contact. Therefore, attention should be paid to the compatibility with ammonia when selecting materials. Ammonia fuel also has the characteristic of stress corrosion, which is more severe at high stress level. Therefore, in the design process, high-strength steel is usually avoided during the selection of materials, and appropriate surface treatment is performed on the materials to improve their ability to resist stress corrosion. At the same time, during the operation of ships, regular inspection and maintenance of system equipment and related structures should be strengthened to promptly detect and handle the stress corrosion cracks.

It is worth noting that the interim guidelines adopted at the CCC 10th meeting have put forward new requirements for ammonia fuel tank, ammonia fuel preparation room, ammonia fuel bunkering, ammonia fuel pipelines, ammonia fuel release/leakage treatment, toxicity prevention, and safe shelters:

(1) For ammonia fuel tank, it specifies that ammonia fuel should be stored at low temperature under atmospheric pressure, that is, fully refrigerated storage methods should be adopted. The original technological intention is to take into account the toxic hazards of ammonia, and fully refrigerated storage of ammonia fuel can ensure a lower ammonia evaporation gas generation rate to achieve a higher level of safety. In addition, the interim guidelines also mention that it is not allowed to use movable ammonia fuel tank.

(2) For the ammonia fuel preparation room, the interim guidelines explicitly specifies that the sharing of the ammonia fuel tank joint with the fuel preparation room is prohibited, but it specifically allows the ammonia fuel evaporator/heat ex-changer and the ammonia fuel tank submersible pump motor can be arranged at the ammonia fuel tank joint. At the same time, an instantly available water curtain should be installed at the inlet of the ammonia fuel preparation room (outside the ammonia fuel preparation room), and it should be able to be activated in the toxic safety zone outside the ammonia fuel preparation room. This water curtain is designed with main consideration to the good water solubility of gaseous ammonia fuel. When ammonia fuel leakage occurs in the ammonia fuel preparation room, the water curtain can be activated in a timely manner to absorb the leaked ammonia fuel, thereby avoiding the leaked ammonia from overflowing into the fuel preparation room. However, this requirement also means that the ship needs to be provided with corresponding ammonia fuel wastewater collection and storage tanks.

(3) For the bunkering of ammonia fuel, the interim guidelines stipulates that regardless of whether the bunkering station is in an open area, semi-enclosed area, or enclosed area, risk assessment must be made with consideration to such factors as the isolation of the bunkering station from other areas of the ship, the division of hazardous and toxic areas, ventilation requirements, leak detection and safe measures after leakage, setting of channels from non-hazardous areas to the bunkering station through airlocks, direct monitoring or closed-circuit television monitoring of the bunkering station, etc. Due to the toxicity of ammonia fuel, unlike LNG and methanol bunkering stations that are preferably arranged in well-ventilated areas such as open decks, it may be a better choice to adopt enclosed or semi-enclosed design for the ammonia fuel bunkering station.

(4) For ammonia fuel pipelines, the design pressure of liquid ammonia fuel pipelines should not be less than 18 bar, and the design pressure of gaseous ammonia fuel pipelines should not be less than 10 bar. All ammonia fuel pipelines should adopt secondary shielding in the form of double-walled pipes (except for breathable pipelines with open ends to the atmosphere), except for those at the ammonia fuel preparation room and the ammonia fuel tank joint. In addition, for double-walled pipes, since the ammonia release treatment system needs to be used to handle leaks in the annular space of the double-walled pipe inter layer, the interim guidelines do not put forward requirements for mechanical ventilation in the above space, but provide concentration monitoring requirements for ammonia fuel leaks in the double-walled pipe inner tube.

(5) For the treatment of ammonia fuel release/leakage, in order to minimize the possibility of toxic hazards to personnel and the environment caused by ammonia release/leakage, the interim guidelines requires that an ammonia release management system (ARMS) should be installed to collect and process various ammonia releases during normal operation and any foreseeable and controllable abnormal situations, thereby avoiding direct release of ammonia into the atmosphere (the listed release includes: ventilation of double shut-off vent valves in ammonia fuel pipelines, release of safety valve in the ammonia fuel system, release during ammonia fuel pipeline purging and discharging, etc.). In addition, the treatment standard for ammonia fuel release/leakage treatment system is that the ammonia concentration at the discharge end should be below 110ppm.

(6) For toxicity prevention, in order to avoid personnel exposure to toxic hazards after ammonia release, the interim guidelines has established the concepts of toxic areas and toxic locations. Among them, toxic areas are applicable to open spaces, while toxic locations are applicable to enclosed or semi-enclosed areas, and it has further clarified that gas safety areas (such as engine rooms) are not considered toxic areas. In terms of ammonia fuel release monitoring, the interim guidelines have set three-level monitoring limits of 25ppm, 110ppm, and 220ppm. Among them, 25ppm is the ammonia concentration monitoring display limit in the enclosed space of the double-walled tube inter-layer annular space; 110ppm is the ammonia concentration alarm value (without safety action requirements) in enclosed spaces such as double-walled tube inter-layer annular space and ammonia fuel tank joints, as well as the ammonia concentration standard value at the discharge end of the ammonia release treatment system; the concentration limit for ammonia fuel that requires corresponding safety actions is set at 220ppm.

In addition, the interim guidelines require that ammonia-fuelled vessels should have safe shelters in case of severe ammonia fuel leakage, which should accommodate all personnel on board. If necessary, the safe shelters should be in the positions where the vessels can be operated, and can (or after measures are taken) avoid exposure of personnel to the risk of ammonia leakage.

Safety Protection

Based on the characteristics of ammonia fuel, the safety protection requirements for ammonia-fuelled vessels are relatively higher than those for other conventional vessels.

Ammonia-fuelled vessels should be equipped with no less than 3 sets of safety protection equipment, and each should be able to provide sufficient personnel protection to allow entry into gas-filled areas for operation. Considering the nature of the fuel, each safety protection equipment should include at least: ① 1 self-contained positive pressure air respirator (including the entire mask) with a capacity of at least 1200L free air (without using stored oxygen); ② airtight protective clothing, boots, and gloves that meet industry standards; ③ steel core rescue rope with belt; ④ explosion-proof lamp. Meanwhile, the ship should also be equipped with the facilities that can provide sufficient compressed air, mainly including: at least one spare air tank filled with air should be equipped for every respirator required by the above safety protection equipment; 1 air compressor capable of supplying high-pressure air of the required purity, with sufficient capacity and continuous operation; 1 inflation valve box capable of inflating the spare air tank in "①".

In addition, ammonia-fuelled vessels should be equipped with sufficient respiratory masks and eye protection equipment for each personnel on board to use during emergency evacuation, and the following requirements must be satisfied: ① filter-type respiratory masks should not be used; ② self-contained respirators should have the capability to work continuously for at least 15 minutes; ③ the emergency escape gas masks should not be used for firefighting or other purposes, and this requirement should be indicated. For the medical emergency equipment (including oxygen resuscitation equipment and appropriate antidotes) on board, please refer to the requirements of the "Medical First Aid Guide for Use in Accidents Involving Dangerous Goods (MFAG)". 1 or more pollution removal sprinklers and eye wash devices with appropriate marks should be installed at suitable locations in the engine room, fuel preparation room, bunkering station, and deck, and they should be readily available.

Finally, safety facilities are also indispensable. Ships should be provided with sufficient quantities of fire-fighting equipment and leak handling equipment suitable for ammonia fuel, so that prompt response can be made in case of accidents. At the same time, ships should also be provided with obvious safety signs and warning signs to remind the crew members to pay attention to the safety precautions and operating procedures.