A low-pressure gas fuel engine is normally used in four-stroke, trunk piston-type engines [11]. The gaseous fuel is injected at a low pressure of approximately 5 barg during the intake stroke of the engine. Once the suction into the cylinder is complete under an air-mixed condition, ignition and combustion of the gaseous fuel is achieved after the pilot oil is injected and ignite at the final stage of the compression stroke. The pilot fuel oil consists of commonly used MDOs. Generally, a low-pressure DF engine is applied as the power-generating engine [12].

A high-pressure gas fuel engine is generally used in two-stroke, low-speed, crosshead-type engines, unlike the four-stroke DF engine in shich the gas is introduced to the gas injection valve. The gas pressure is increased between 250–300 barg using a high-pressure gas compressor. Usually, a high-pressure DF engine is used as a direct-coupling type of propulsion engine.

The FGSS for DF engines is classified into two types: a low-pressure type for four-stroke engines and a high-pressure type for two-stroke engines [13].

The low-pressure LNG fuel system for DF engines considered in this study mainly consisted of a fuel tank, pressure build up (PBU), vaporizer, main gas heater, gas valve unit (GVU), bunkering station, and engines connected by valves and pipes [14]. Figure 1 shows the schematic diagram of a low-pressure FGSS. LNG fuel was gasified if required, and then heated before entering the GVU, that has certain inlet requirements. The GVU was connected to the engines, and it functioned as a control unit that regulated the fuel flow according to the condition that the engines required. A majority of the low-pressure fuel systems are equipped with an FGSS, as shown in Figure 1. The unit installed here was a closed-loop system in which the liquid fuel from the bottom of the fuel tank was vaporized using the PBU and was supplied to the top of the tank [15]. The vaporized fuel was injected into the tank to maintain the tank pressure at a required level if it dropped below the operating pressure. In general, there was a joint circuit with water and glycol as the vaporizer and main gas heater, respectively [16].

Figure 1: 
Low-pressure fuel gas supply system for a DF engine

To simulate the dynamic performance of the fuel gas system using Aspen HYSYS, data of the Econuri ship of the Incheon Port Authority was used. The ship has an LNG fuel tank of 20 m3 (IMO type C), a vaporizer, generators (Wartsila 9L20DF), GVU, and electric propulsive motors. The ship has the capability of achieving a voyage distance of 895 km in the gas fuel mode at nominal continuous rating (NCR) running. Table 1 presents the fuel consumption of the fuel gas and pilot oil when the generator loads to run the propulsion motors.

The gross calorific value is determined as 54.86 MJ/kg, in accordance with the LNG conversion factor taken from the Korea Gas Corporation (KOGAS) website. The gas consumption is also computed according to the manual of Wartsila’s 9L20DF engine. As noted in Table 1, at the operating conditions under a maximum load of 1,584 kW (100 %), the consumed gas is computed as 245.4 kg/h. The total amount of LNG consumed in the ship was provided by KOGAS. LNG may be obtained from different sources and may have different compositions, implying that the heat value is a variable. The main compositions of this gas coming from Qatar are listed in Table 2. Qatar LNG represents 22.9% of the import volume of KOGAS.

In case of diesel engines using a gas fuel, a pilot oil for ignition should also be supplied. Hence, to estimate the maximum sailing time of Econuri, consumption of the pilot oil is also an important factor. It was calculated based on Wartsila’s 20 DF engine guidebook. It is given in Table 1 that the hourly consumption of the pilot oil (D.O.) is 7.603 kg/h under the condition of MCR. The maximum bunkering of D.O. in Econuri is 6.9 ton (7.9 m3, at a specific gravity of 0.88) that can last up to 921 hours of operation. To examine the dynamic response to system disturbances, the models in Aspen HYSYS considered the fluid in the dynamic simulations, based on an equation of state for natural gas.

Figure 2 presents the diagram of the FGSS from the LNG fuel tank to the 9L20DF engine, and Table 3 gives the initial operating criteria of the system components (ISO/TC 8, 2014) [17]. The mass flow rate of the LNG fuel was calculated to determine the maximum fuel consumption of 245.4 kg/h. The initial running pressure in the LNG fuel tank was adjusted to 6.5 barg because the master gas valve for the DF engines requires a minimum inlet pressure of 5 barg at normal conditions, according to Wartsila’s 20 DF engine manual. Typically, the operation pressure was approximately 5-6 barg. The tank pressure pushed the LNG toward the LNG vaporizer. Usually all the tank connections and tank vaporizer are enclosed in a cold box as a secondary barrier. A fuel gas heater was employed to provide the vapor at the right temperature via a fuel gas master valve to the GVU. In addition, a 20 °C inlet temperature was applied to the master valve in the simulation model.

Figure 2: 
Schematic of the LNG tank and FGSS for the Econuri vessel

Figure 3 shows the FGSS of Econuri for supplying the LNG to the DF engine using the HYSYS program. The boil-off gas from the LNG storage tank was released if the pressure within the tank exceeded the design pressure with a safety margin. From the storage tank, the LNG flowed into the vaporizer where a water-glycol solution heated the LNG until it was fully vaporized to natural gas. The flow controller (FIC-100) in the system controlled and regulated the mass flow of the natural gas based on its temperature and pressure.

Figure 3: 
HYSYS model of the case study for low-pressure LNG fuel delivery system in Econuri

The conditions for the initial starting and normal load procedures were performed through a cooling down process as well as by an auxiliary apparatus including a glycol water pump. However, in the numerical process in HYSYS, the process of cooling down of the LNG fuel tank was not considered. During normal operating loading, fuel gas was supplied to the dual-fuel engine under the conditions of 250 kg/h and 5 barg.

Safety and stability in the fuel supply system of a DF engine were considered important factors in the case of supply gas cutoff and vaporizer shutdown [18]. In a supply shutdown, the supply gas valve was closed and the gas engine was isolated from the gas network. It was assumed that the GVU signal tripped because of a gas leak or malfunction of the control valve during navigation. In this case, the DF engines were shifted rapidly from the mode of simultaneous combustion of the gas and fuel oil to the mode of combustion of the fuel oil only. Here, the fuel oil was used not only for the ignition during gas combustion but also as the back-up fuel.

Therefore, we examined factors such as the fluctuation of supply pressure, change in flow rate, and the tank pressure for engine shutdown. Figure 4 shows the characteristic curves of the supply gas cutoff on closing of the GVU after approximately 100 min of operation with a 100% load. A drastic increase in the supply pressure is not detected because the valve is shut off. Particularly from the sub-systems of the GVU, the stability is more secure owing to the release of the gas into the atmosphere through purging. Even during emergency condition, as there is no pressure change in the upstream of the GVU, the result presents that the system stability of the fuel gas is certified.

Figure 4: 
Dynamic curves of the supply gas cutoff from closing of the gas valve unit (GVU) for emergency operation

Moreover, assuming the engine load is 50% during entry into a port or when leaving a port and by conducting sudden veering and steering when making a voyage, we assessed the reliability of the Econuri vessel for a safe stable sailing under the serious conditions. Under these conditions, the engine load was drastically diminished from 90 to 70 % or increased from 50 to 70 %. In a real voyage, as the load fluctuates often due to sudden steering, veering, and ship avoidance, the system stability is an extremely essential element for safety.

Figure 5 presents the dynamic response characteristics of the FGSS in the 9L20DF engine. There is a drastic variation in the gas flow rate when the GVU is opened, but the gas pressure does not show large fluctuations. Subsequently, to sail along the narrow channels for 2 hours, a sudden increase or decreases in the ship speed is required at approximately 50–70 % of the engine output. Thus, the gas supply pressure for the DF engine satisfies the requisite pressure of 6.0–6.5 barg without any variations.

Figure 5: 
System stability for frequent load variations. The load is drastically diminished from 90 to 70 % or increases from 50 to 70 %.

In addition, despite the drastic change in engine cut down from 90 to 70% of the load, it is proved that the pressure and flow rate of the fuel gas for DF engines maintain a stable reaction without critical fluctuations.

When sailing at the condition of engine loads at 75% or 100%, we estimated the maximum distance that the ship could sail without bunkering. The results can be used as reference data for establishing the navigation plan for the vessel.

Figure 6 and Figure 7 show the results of the navigation at 75% and 100% load, respectively. Our results reveal that 25 hours of operation are available under 100% load and 31 hours under 75% load without bunkering. Moreover, approximately 694.5 km navigation is possible at a speed of 15.0 knots with 100% engine load and 781 km at a speed of 13.62 knots with 75% engine load, respectively, according to the propeller law. Hence, navigation from the Incheon Port to Gageo Island of South Korea is possible in the absence of fouling and if the sea is calm.

Figure 6: 
Dynamic curves for 75% engine load operation

Figure 7: 
Dynamic curves for 100% engine load operation

It is shown that there is no problem in conducting the sailing for the maximum distance considering the sudden variations in pressure, flow rate, and other factors in the FGSS without bunkering at 30% volume [10].