By Feng Yuxiang, CCS Wuhan Rules & Research Institute

 Liu Shaoling, COSCO Shipping Heavy Industry (Dalian) Co., Ltd.

With the acceleration of decarbonization in the global shipping industry, ammonia, as a highly-anticipated green energy star, is gradually becoming a key driving force for the transformation and upgrading of the shipping industry by virtue of its environmental characteristics of near-zero carbon emissions and cost advantages. According to the relevant requirements of the CCS "Guidelines for Ships Using Ammonia Fuel", "the purpose of risk assessment is to ensure that necessary assessment is made on potential risks involving ammonia fuel, so as to eliminate or mitigate the adverse effects on personnel, environment, structural strength, or ship integrity. At this point, in order to guarantee the intrinsic safety and system safety of ammonia-fuelled vessels and optimize their operating procedures, this paper takes the design scheme of an ammonia-fuelled harbor workboat under construction as an example to conduct risk assessment on its system arrangement and process design scheme based on hazard identification (HAZID), hazard and operability analysis (HAZOP), as well as consequence quantitative analysis method, etc.

Case Overview

By taking the ammonia-fuelled system design scheme of a certain ammonia-fuelled harbor workboat as the analysis object, HAZID/HAZOP analysis is carried out based on the design drawing of the ship. The main parameters of the ship are shown in Table 1.

Table 1 Main Parameters of the Ammonia-fuelled Harbor Workboat Under Construction

The ammonia fuel is stored in a fully-pressurized liquid state at a storage pressure of about 1.6 MPa (working pressure) and a working temperature of -20 ℃~40 ℃. The ammonia fuel tank is arranged on both sides behind the main deck engine room casing, is equipped with injection, discharge, pressure release and other interfaces as required, and is provided with the functions of local monitoring of liquid level, pressure and temperature, as well as remote monitoring of pressure and temperature. For the convenience of handling of leaks at the joints of the fuel tank, a liquid collection tray is installed below all possible leakage points including joints, valves, etc. Each ammonia fuel tank is equipped with 2 sets of pressure relief valves (PRVs) with two interlocked isolation shut-off valves.

The gaseous ammonia fuel is allowed to enter the main engine, with a pressure of about 0.5~1.0MPa and a temperature of 20 ℃~40 ℃. The fuel supply system mainly consists of a pressure reducing valve set, an evaporator, and a buffer tank. The buffer tank needs to be insulated with a working pressure of 0.5-1.6 MPa. The heat source of the evaporator comes from the jacket water of the main engine, and a tubular heat exchanger is used. The pipelines for ammonia and jacket water are respectively introduced to the intermediate heat exchange medium to achieve heat exchange.

HAZID Analysis

1.    Node Division

For the convenience of HAZID analysis, the ammonia fuel power system equipment and arrangement of the ship are divided into five nodes for further analysis, namely: ① ammonia bunkering station (including filling joints, liquid phase pipelines, gas phase pipelines, control valves and accessories, etc.); ② ammonia fuel tank (fuel tank, related accessories, etc.); ③ ammonia fuel preparation room (pumps, heat exchangers, buffer tanks, etc.); ④ ammonia fuel engine compartment (main engine, post-treatment device); ⑤ ammonia release cabinet (sewage storage tank).

2.    HAZID Analysis Table

Based on the HAZID method, the causes, consequences, existing protective measures, risk levels, and recommendations for the hazards caused by the design layout of each node, process flow, equipment fault, human operation, etc. are analyzed regarding the equipment, process system, and layout of the ammonia-fuelled system of the ship. The HAZID analysis table (excerpted nodes of ammonia fuel bunkering station) is shown in Table 2.

Table 2 HAZID Analysis Table of the Ammonia-fuelled Harbor Workboat (Excerpted Nodes of Ammonia Fuel Bunkering Station)

HAZOP Analysis

1.    Nodes Division

For the convenience of HAZOP analysis, the power system process of the ship is divided into several nodes for further analysis. According to the working principle, it can be divided into the following nodes: ① liquid ammonia bunkering (ammonia fuel from the bunkering station to the fuel tank); ② BOG return gas (BOG from ammonia fuel tank to tank bunkering truck); ③ ammonia supply (ammonia in the fuel tank to the heat exchanger through its own pressure); ④ liquid ammonia supply (liquid ammonia in the fuel tank is pressurized by the pump group and then sent to the heat exchanger); ⑤ ammonia fuel gas (ammonia fuel passes through the heat exchanger to a buffer tank and then to the gas equipment); ⑥ blowing and ventilation (blowing and ventilation pipeline to ammonia release cabinet).

2.    HAZOP Analysis Table

Based on the HAZOP method, the causes, consequences, existing protective measures, risk levels, and recommendations for the hazards caused by equipment fault, human operation, etc. at each node, are analyzed regarding the ammonia bunkering system, ammonia supply system, etc. of the ship. The HAZOP analysis table---excerpted nodes of liquid ammonia supply (the liquid ammonia in fuel tank is pressurized by the pump group and then sent to the heat exchanger) is shown in Table 3.

Consequence Analysis of Leakage

Based on the above qualitative risk assessment results, combined with the technical conditions and functional characteristics of the ammonia-fuelled harbor workboat, it has been determined that ammonia fuel leakage in the fuel preparation room during ship operation and ammonia fuel leakage at the bunkering station during ship bunkering are high-risk scenarios. Quantitative calculation and risk assessment are conducted for the two high-risk scenarios.

This analysis will be conducted by using the computational fluid dynamics (CFD) software FLACS, which was developed with the funding and guidance from oil companies including BP, Conocophillips, ExxonMobil, Statoil, and three national legislative bodies including the Health and Safety Executive (HSE) of the UK, the Norwegian Petroleum Directorate (NPD), and the German Federal Ministry for Research and Technology (BMFT). FLACS is an explosion hazard assessment tool designated by major international oil and gas companies, which can meet the risk assessment requirements of relevant international standards in the oil and gas industry (such as ISO13702, NORSOKZ-013, ISO/DIS 19901-3, etc.).

Table 3 HAZOP Analysis Table — Excerpted Nodes of Liquid Ammonia Supply (the liquid ammonia in fuel tank is pressurized by the pump set and then sent to the heat exchanger) for the Ammonia-fuelled Harbor Workboat

Analysis is made by taking the scenario of small aperture liquid ammonia leakage at the flange connection of the ammonia fuel bunkering station during the bunkering operation as an example, with 50% of the ammonia explosion bottom line as the boundary. The diffusion range of combustible gases is shown in Fig. 1, and the diffusion range of toxic gases is shown in Fig. 2.

Fig. 1 Diffusion Range of Combustible Gases (8%~28%)

Fig. 2 Diffusion Range of Ammonia with a Concentration of300ppm After Leakage of the Bunkering Liquid Phase Flange

It can be seen that when there is leakage in the bunkering joint, a liquid pool is first formed in the liquid collection tray after the liquid ammonia leaks, with massive gasification into low-temperature heavy gas containing a small amount of liquid ammonia droplets. The combustible vapor range of the gas is mainly located in the liquid collection tray and adjacent lower areas, and is basically covered by the dangerous areas of the ship. The threshold for toxic vapors is based on the Chinese national occupational health standard GBZ2.1, IDLH (NH3)=300ppm, so the upper limit of the threshold is taken as 300ppm. In addition, the toxic vapor range covers some sensitive air inlets of the ship (such as the stern rudder engine room air inlet), and the maximum toxic gas diffusion range is about 133m in the length direction and 73m in the width direction of the ship.

Risk Assessment Conclusion and Recommendations

Based on the system design scheme of the ammonia-fuelled harbor workboat under construction, the following conclusions and recommendations are provided based on our risk assessment:

1.   Through HAZID analysis of 5 nodes and HAZOP analysis of 6 nodes, a total of 23 low-risk scenarios, 30 medium risk scenarios, and 4 high-risk scenarios are determined through assessment. Among them, the high-risk scenarios include damage and leakage at the ammonia fuel bunkering station and liquid ammonia leakage in the fuel preparation room. The main risk control measures are as follows: ① in case of damage/leakage at the ammonia fuel bunkering station, it is recommended that qualified professional operators should operate and wear PPE, during the bunkering operation, the bunkering personnel should carry portable ammonia vapor detectors; adjust the position of the disinfection spray head and eye washing facilities; in the case of significant bunkering temperature difference between the two parties, implement the measures such as verifying whether the bunkering pressure difference can be established, and reduce its risk level to an acceptable extent; ② for fuel preparation room leakage, it is recommended to implement such measures as setting up water sprays inside the fuel preparation room, triggering a leakage alarm, collecting sewage and discharging it into the sewage tank, etc. to reduce the risk level to an acceptable extent.

2.   Through further diffusion analysis of the consequences of the leakage, the results show that when leakage occurs at the bunkering station, the toxic gas may have a large impact range. It is recommended to divide the restricted operation areas according to the quantitative calculation results, and strictly control the entry of non-bunkering personnel within the restricted areas. The bunkering personnel should wear PPE and portable ammonia vapor detectors. At the same time, bunkering operations should be arranged according to the wind direction during on-site operations, and be carried out as much as possible when the wind direction is opposite to the bunkering dock. For the steering engine room inlet at the stern of the ship, the safe area air inlet at the bow of the ship, and the opening of the cab facing the bunkering station and the side of the ship, the diffusion range of toxic ammonia gas may cover the above positions in case of leakage. It is recommended that the above air inlets and openings be designed as closed types, and toxic gas detectors should be installed at the inlet and interlocked with the above air inlets and openings.