In this study, the emission of ROK Navy's OOO-class DDH, a target vessel type for which activity data have been studied, is estimated in a bottom-up method. The target vessel is used not only for domestic missions but also for overseas missions such as joint force missions and overseas dispatch and cruise training for naval academy midshipmen. Table 1 shows the specifications of the target vessel.

The CODOG system, a propulsion system operated in the target vessel, is a system that produces propulsion with diesel engines in the low-speed part and gas turbines in the high-speed part. When the target vessel is operated, the prime mover is converted in the area of approximately 15~20 kts. In naval vessels that operate together with gas turbines, the operation ratio of diesel engines to the operation ratio of gas turbines is high.

The generator set operates 2 out of 4 generators at approximately 40% load, and each uses up to 70% load in high-temperature water areas. In addition, during combat or training, one spare generator is additionally operated in an idle state.

The fuel used in the ROK naval vessel is marine gas oil, which is commonly used for diesel engines, gas turbines and generator sets. Table 2 shows the results of the component analysis of MGO procured and used by the ROK navy.

The prime movers related to emissions from the target vessel are 2 diesel engines and 2 gas turbines for propulsion and 4 diesel engine generator sets. There is no boiler as used in merchant ships, and the heating water for the living is supplied using an electric heater, and the power required for this is produced by generator sets.

Among these elements, the engine for propulsion has the highest contribution period for emission, and the gas turbine engine has a high emission contribution per time unit. Emission by the diesel engine with a much higher usage time is expected to be dominant.

The vessel's operation profile must be used to estimate the vessel's emission because the vessel's operation profile corresponds to the values of speed from AIS and distance from AIS among the input data used when estimating the emission of a merchant ship. Although AIS is operated on vessels as an auxiliary means of navigation, uncertainty is high in activity data obtained through AIS because AIS is generally turned off during major operations and training. Therefore, speed data (operation profile), which was collected as an hourly log in the vessel's engine control room for one year, is regarded as activity data.

Figure 1 shows the navigation profile (speed) of the target vessel.

Figure 1: 
Navigation profile for the destroyers

In the case of ROKN's destroyer, the target vessel, which performs coastal operation with limitations in high-speed navigation, the diesel engine's usage proportion (16 kts or less) with low specific oil consumption is higher than the gas turbine's usage proportion (more than 16 kts). The overall trend is in the form of a negative skew with a high operating ratio of the diesel engine. For reference, the destroyer of the US Navy tends to be positively skewed compared to the target vessel due to the relatively high operating rate of gas turbines [5].

Cases in which gas turbines are operated at the speed range of 16 kts or less include special conditions such as failure of diesel engines or test operation of gas turbines; and cases in which diesel engines are operated at low load for a long time after setting the pitch angle of controllable pitch propellers to 0°, etc. There are some uncommon operation modes, but those cases are extremely rare, so they are excluded from discussion.

The annual operation hour of prime movers of the target vessel is approximately 4,820 h, and Gen. set was driven approximately 750 h additionally by harboring mode.

Table 3 shows the operation rate of the generator set operation mode of the target vessel. This data represents the average load and operating time according to each generator operation mode operated by the target vessel during the survey period, and the generator output at the corresponding load.

To estimate the emission inventories of vessels, the specific fuel oil consumption rate according to the load of the main emission sources, prime movers for propulsion and prime movers for generation, is essential. At this time, the required fuel oil consumption rate is the BSFC (brake-specific fuel oil consumption) considering transmission loss in the drive shaft and transmission efficiency of the propeller, not the indicated values measured in the engine. Using the BSFC of the prime mover measured in the vessel, the kW–BSFC empirical formula of each prime mover for propulsion is as follows. These empirical formulas are those derived by drawing a trend line from the survey results of actual fuel consumption per kW measured during the survey period for the target vessel.

P(kW) – BSFC empirical Equation for two diesel engines of the target vessel (empirical Equation (2)):

P(kW) – BSFC empirical Equation for two gas turbines of the target vessel (empirical Equation (3)):

In addition, based on the fuel consumption measured in the target vessel, the power (kW) of the diesel engine generator set –BSFC empirical Equation – is obtained as follows.

P(kW) – BSFC empirical Equation for a diesel generator of target vessel (empirical Equation (4)):

The emissions to be estimated in this study are CO₂, N₂O, SO₂, and PM₁₀. To estimate each emission, an emission factor according to the type of engine and fuel is needed, and in this study, the emission factor presented by the EPA was used. Table 4 shows the emission factors for each emission presented by the EPA [6].

The empirical formula related to the BSFC of each prime mover is substituted into Equations (5) to (7), and the EF calculated in this process is substituted into Equation (8) for each emission and used to estimate the emission emitted from the target vessel. The LLAF (low load adjustment factor) of Equation (8) should be applied to the result of analyzing the AIS activity data of naval vessels according to the EPA Guideline, but due to the limitations of information access for special purpose vessels, the value corresponding to ship Category 3 (C3) provided by the EPA was applied. C3 includes high-speed ferries and yachts, and the MTU diesel engine used in the target vessel is also the same medium-high-speed diesel engine and generator mainly used for high-speed ferries and yachts [7].

CCR (carbon content ratio of ROK naval MGO) and S_act (ROK naval MGO’s sulfur weight ratio) when applied to Equations (5) to (7) are values measured by commissioning MGO used in the actual navy to the Korea Petroleum Quality Management Institute. Figure 2 is a diagram showing a series of processes for estimating the emissions of the target vessel.

Figure 2: 
A simple diagram to estimate naval vessel emissions

Table 5 shows the estimated emission results. In addition, Figure 3 is a graph showing the GHG emissions of the Republic of Korea National Defense for 2020 estimated in a top-down method and classified by military forces and fuel use purpose. In the graph, the GHG emissions generated by vessels operated by the Navy are approximately 602 kt CO₂-eq.

Figure 3: 
Net GHG emissions in each military force [4]

CO₂ and N₂O are classified into six major GHGs. The total amount of GHG obtained by applying GWP to the amount of CO₂ and N₂O estimated in this study is approximately 8.66 kt CO₂-eq. This amount corresponds to 1.4% of the naval vessels' emissions (602 kt CO₂-eq), as shown in Figure 3. When estimated through empirical data (classified) of the operational scale (horsepower) of ROK Navy vessels, the fact that 1.4% of the GHG emissions of all ROK naval vessels per year are emitted from one destroyer has sufficient verification validity.