Decarbonization Potential by Combining Slow Steaming and Wind-Assisted Propulsion Systems: A Case Study of a 6,500 DWT Tanker Operating in Indonesian Waters

2.1. Decarbonization Calculations / Izračuni dekarbonizacije

Carbon emissions (E) are determined by multiplying fuel consumption (FC) by a carbon factor (C_F), a coefficient that represents the amount of carbon dioxide released per unit of fuel burned by main engine, considering the fuel type and its specific combustion properties [21].

[TeX:] E = FC\ .\ C_{F}$E=FC.C_F$ (1)

Meanwhile, for decarbonization by implementing slow steaming ([TeX:] {\mathrm{\Delta}E}_{\text{SS}}${\Delta E}_{\mathrm{\text{SS}}}$), wind assisted propulsion ([TeX:] {\mathrm{\Delta}E}_{\text{WASP}}${\Delta E}_{\mathrm{\text{WASP}}}$), and combination of both stragies ([TeX:] {\mathrm{\Delta}E}_{SS + WASP}${\Delta E}_{SS+WASP}$) are calculated using equations (2) to equation (4),

[TeX:] {\mathrm{\Delta}E}_{\text{SS}} = \left( FC - \text{FC}_{\text{SS}} \right)\text{\ .\ }C_{F}${\Delta E}_{\mathrm{\text{SS}}}=(FC-{\mathrm{\text{FC}}}_{\mathrm{\text{SS}}})\mathrm{\text{.}}{C}_F$ (2)

[TeX:] {\mathrm{\Delta}E}_{\text{WASP}} = \left( FC - \text{FC}_{\text{WASP}} \right)\text{\ .\ }C_{F}${\Delta E}_{\mathrm{\text{WASP}}}=(FC-{\mathrm{\text{FC}}}_{\mathrm{\text{WASP}}})\mathrm{\text{.}}{C}_F$ (3)

[TeX:] {\mathrm{\Delta}E}_{SS + WASP} = \left( FC - \text{FC}_{SS + WASP} \right)\text{\ .\ }C_{F}${\Delta E}_{SS+WASP}=(FC-{\mathrm{\text{FC}}}_{SS+WASP})\mathrm{\text{.}}{C}_F$ (4)

where FC is fuel consumption under normal condition, FC_SS is fuel consumption under slow steaming condition, FC_WASP is fuel consumption with wind assisted propulsion system, and FC_SS+WASP is fuel consumption with combination of slow steaming and wind assisted propulsion system. The fuel oil consumption of each conditions are presented in equation (5) to equation (8).

[TeX:] \text{FC} = \sum_{i = 1}^{n}\left( \left( R_{\text{Ti}}\text{\ .\ v}_{\text{si}}/\eta_{p} \right)\ t_{i}\text{\ .\ SFC} \right)$\mathrm{\text{FC}}={\sum }_{i=1}^n\left(\left(R_{\mathrm{\text{Ti}}}{\mathrm{\text{. v}}}_{\mathrm{\text{si}}}/{\eta }_p\right)t_i\mathrm{\text{. SFC}}\right)$ (5)

where R_Ti represents the total resistance at segment-i route under normal operational conditions, v_si is ship speed at segment-i route under normal condition, t_i is ship sailing time duration at segment-i route under normal condition, η_p is the overall propulsion efficiency, and SFC is the specific fuel consumption of the main engine.

Under slow steaming condition, fuel consumption (FC_SS) is represented by equation (6), where R_TSSi represents the total resistance at segment-i route under slow steaming conditions, v_ssi is ship speed at segment-i route under slow steaming condition, and t_ssi is ship sailing time duration at segment-i route under slow steaming condition.

[TeX:] \text{FC}_{\text{SS}} = \sum_{i = 1}^{n}\left( \left( R_{\text{TSSi}}\text{\ .\ v}_{\text{SSi}}/\eta_{p} \right)\ t_{\text{ssi}}\text{\ .\ SFC} \right)${\mathrm{\text{FC}}}_{\mathrm{\text{SS}}}={\sum }_{i=1}^n\left(\left(R_{\mathrm{\text{TSSi}}}{\mathrm{\text{. v}}}_{\mathrm{\text{SSi}}}/{\eta }_p\right)t_{\mathrm{\text{ssi}}}\mathrm{\text{. SFC}}\right)$ (6)

While, fuel consumptions under normal condition with WAPS and under slow steaming condition with WASP are represented by equation (7) and (8) respectively.

[TeX:] \text{FC}_{\text{WAPS}} = \sum_{i = 1}^{n}\left( \left( \left( R_{\text{Ti}} - T_{i} \right)\text{\ v}_{\text{si}}/\eta_{p} \right)\ t_{i}\text{\ .\ SFC} \right)${\mathrm{\text{FC}}}_{\mathrm{\text{WAPS}}}={\sum }_{i=1}^n\left(\left((R_{\mathrm{\text{Ti}}}-T_i){\mathrm{\text{v}}}_{\mathrm{\text{si}}}/{\eta }_p\right)t_i\mathrm{\text{. SFC}}\right)$ (7)

[TeX:] \text{FC}_{SS + WAPS} = \sum_{i = 1}^{n}\left( \left( \left( R_{\text{TSSi}} - T_{i} \right)\text{\ v}_{\text{SSi}}/\eta_{p} \right)\ t_{\text{ssi}}\text{\ .\ SFC} \right)${\mathrm{\text{FC}}}_{SS+WAPS}={\sum }_{i=1}^n\left(\left((R_{\mathrm{\text{TSSi}}}-T_i){\mathrm{\text{v}}}_{\mathrm{\text{SSi}}}/{\eta }_p\right)t_{\mathrm{\text{ssi}}}\mathrm{\text{. SFC}}\right)$ (8)

where T_i is the sail thrust at segment-i route.

2.2.Sail Thrust Calculation / Izračun potiska jedra

The forces acting on the sail are illustrated in Figure 1. The thrust (T) generated by the sail is the component in ship direction of the resultant force (R) of lift (L) and drag (D). Additionally, the R also produces a side force (S).

L and D are generated by the sail at an angle α relative to the direction of the apparent wind with velocity v_a. The apparent wind velocity v_a is the resultant of the true wind velocity (v_w) and the ship's velocity (v_s). The angle between v_a and v_s is denoted as β, while the angle between v_w and v_s is represented as θ.

Figure 1 Sailing vessel forces

Slika 1. Sile koje djeluju na jedrilicu

T and S can be calculated by using equation (9) and equation (10).

[TeX:] T = L\text{.\ sin}{\beta - D\text{.\ cos}\beta}$T=L\mathrm{\text{. sin}}\beta -D\mathrm{\text{. cos}}\beta$ (9)

[TeX:] S = L\text{.\ cos}{\beta + D\text{.\ sin}\beta}$S=L\mathrm{\text{. cos}}\beta +D\mathrm{\text{. sin}}\beta$ (10)

where,

[TeX:] \beta = \cos^{- 1}\left( \left( v_{\text{w\ }}\text{.\ }\cos\theta + v_{s} \right)/v_{a} \right)$\beta ={cos}^{-1}\left((v_{\mathrm{\text{w}}}\mathrm{\text{.}}cos\theta +v_s)/v_a\right)$ (11)

[TeX:] v_{a} = \sqrt{\left( v_{w}^{2} + v_{s}^{2} + 2\text{\ .v}_{w}\text{\ .\ }v_{s}\cos\theta \right)}$v_a=\sqrt{(v_w^2+v_s^2+2{\mathrm{\text{.v}}}_w\mathrm{\text{.}}{v}_{s}cos\theta )}$ (12)

[TeX:] L = 0.5\ .\ \rho$L=0.5.\rho$[TeX:] \text{.\ v}_{a}^{2}\ \text{.\ A}_{s}\text{\ .\ }C_{L}${\mathrm{\text{. v}}}_a^2{\mathrm{\text{. A}}}_s\mathrm{\text{.}}{C}_L$ (13)

[TeX:] D = 0.5\ .\ \rho$D=0.5.\rho$[TeX:] \text{.\ v}_{a}^{2}\ \text{.\ A}_{s}\text{\ .\ }\left( C_{D} + C_{\text{Di}} \right)${\mathrm{\text{. v}}}_a^2{\mathrm{\text{. A}}}_s\mathrm{\text{.}}(C_D+C_{\mathrm{\text{Di}}})$ (14)

[TeX:] C_{\text{Di}} = C_{L}^{2}/\left( \text{π\ .\ AR\ .\ e} \right)$C_{\mathrm{\text{Di}}}=C_L^2/\left(\mathrm{\text{π . AR . e}}\right)$ (15)

A_s is sail area, C_L represent the lift coefficient of wingsails, C_D is drag coefficients of wingsails, and C_Di refers to the induced drag considering the three-dimensional shape of the wingsail. AR is sail aspect ratio and e is efficiency factor.

2.3. Slow Steaming Operational Conditions Set-Up / Postavljanje radnih uvjeta za sporu plovidbu

In this study, the variation of slow steaming is applied to a ship operating at normal speed, with speed reductions of up to 1 knot, decreasing in increments of 0.2 knots.

2.4. Limitation / Ograničenja

In this study, several limitations are applied. For the carbon emission calculation, emissions are considered only from the main engine. The weight of the wind-assisted propulsion system is not taken into account in its effect on the ship's resistance. Similarly, the side force generated by the ship is not considered in its impact on resistance. Additionally, the influence of the sail's construction and the side force produced by the wingsail on the ship's stability is not addressed in this study.

3.1. Case Study / Studija slučaja

3.1.1. Ship Particulars / Tehničke specifikacije broda

This paper presents a case study of a 6,500-ton DWT tanker, with the details provided in Table 1 and Figure 2. Additionally, the ship's resistance data is presented to support decarbonization calculations, as illustrated in Figure 3.

Table 1 Ship Particulars

Tablica 1.Tehničke specifikacije broda

Figure 2 A 6500-ton DWT tanker [22]

Slika 2. Tanker od 6.500 bruto nosivosti [22]

Figure 3 Total resistance (R_T) of 6500 DWT ton tanker with function of speed (v)

Slika 3. Ukupni otpor (RT) tankera od 6.500 bruto nosivosti u funkciji brzine (v)

3.1.2. Configuration of Wingsails as a Wind-Assisted Propulsion System / Konfiguracija krilnih jedara kao pogonskog sustava s pomoću vjetra

The wingsail configuration is derived from [23], featuring a three-wingsail design with a NACA-0012 airfoil cross-section and a rectangular shape. It has an aspect ratio (AR) of 2.5, and each wingsail covering an area (A_S) of 250 m² as presented in Figure 4.

Figure 4 Wingsails configuration

Slika 4. Konfiguracija krilnih jedara

The lift and drag coefficient of that configuration (C_L and C_D) as presented in Figure 5. Figure 5(a) shows the variation of C_L of each two-dimension wingsail (airfoil) with respect to the wingsail angle relative to the apparent wind direction (α). C_L for Airfoil 1 (black line) is the lift coefficient for the wingsail located at the front near the ship's bow, C_L for Airfoil 2 (red line) is the lift coefficient for the wingsail located in the middle, and C_L for Airfoil 3 (blue line) is the lift coefficient for the wingsail at the rear, near the ship's accommodation area. While Figure 5(b) illustrates the variation of C_D.

(a)

(b)

Figure 5 Lift and Drag Coefficient (a) C_L (b) C_D [22]

Slika 5. Koeficijenti uzgona i otpora: (a) CL (b) CD [22]

3.1.3. Ship Routes / Pomorski pravci

In this study, the ship is operating in Indonesian waters along a route around Sumatra Island over a one-year period, starting from July 21, 2023, to July 20, 2024. The ship's route are shown in Figure 6.

Figure 6 Ship’s routes

Slika 6. Pomorski pravci

Figure 5 illustrates the route, which is divided into two paths: the southern route of Sumatra Island (red line) and the northern route (blue line). The southern route is taken by the ship from July 21, 2023, until approximately April 20, 2024, while the northern route is followed from around April 21, 2024 to July 20, 2024.

3.1.4. Voyage Data / Podaci o plovidbi

To quantify the contribution of slow steaming and application of wind assisted propulsion system to the decarbonization, it is essential to consider the ship's voyage data which covers ship speed and its direction and also wind speed and its direction. For this case, the data which are collected starts from July 21, 2023, to July 20, 2024 [24,25].

Figure 7 illustrates the ship speed (solid blue circle) and wind speed (hollow orange triangle) over the course of one year, from July 21, 2023, to July 20, 2024. Generally, as shown in Figure 7, the ship's speed predominantly ranges from 10 knots to 12 knots, while the wind speed is mostly below 8 knots. In some data, it is observed that the ship's speed is 0 (zero), as is the wind speed. A ship speed of 0 (zero) indicates that the ship is either moored or at anchored.

Figure 8 shows the ship's direction (solid blue circle) and wind direction (hollow orange triangle) over the course of one year. Based on Figure 8, it can be observed that throughout the year, the ship primarily sails at angles between 300°-350° (heading northwest) and 100°-150° (heading southeast) for numerous round trips, both on the southern route of Sumatra Island (from July 21, 2023, to around April 20, 2024) and the northern route of Sumatra Island (from April 21 to July 21, 2024). Meanwhile, the wind direction is distributed across an angle range of 0° to 360°.

Figure 7 Ship and wind speed

Slika 7. Brzina broda i vjetra

Figure 8 Ship and wind direction

Slika 8. Smjer broda i vjetra

Particularly for wind speed and direction, the statistical probability of their occurrence is presented in Figure 9. Figure 9(a) illustrates the normal distribution of wind speed events over the course of one year of navigation, where the x-axis represents wind speed and the y-axis denotes probability density. Meanwhile, Figure 9(b) shows the normal distribution of wind direction events over the same period, with the x-axis representing wind direction and the y-axis indicating probability density. From Figure 9(a), it can be observed that the average wind speed over one year is approximately 2.4 m/s. As for wind direction, the average is around 200°, as depicted in Figure 9(b).

(a)

(b)

Figure 9 Normal distribution (a) wind speed (b) wind direction

Slika 9. Normalna distribucija: (a) brzina vjetra (b) smjer vjetra

3.2. Ship’s Carbon Emission Under Normal Operational Condition / Emisija ugljika pri uobičajenim radnim uvjetima

Based on the voyage data and emission calculation formulas, the carbon emission under normal conditions can be determined. Figure 10 shows the trends of carbon emissions, fuel consumption, and sailing days for each month, where the x-axis represents the division of each month starting from July 21, 2023, to July 20, 2024. The left y-axis displays fuel consumption and carbon emissions for each month, while the right y-axis shows the sailing days for each month.

Figure 10 Monthly CO₂e, fuel consumption, and sailing day

Slika 10. Mjesečne vrijednosti CO₂e, potrošnja goriva i dan plovidbe

From Figure 10, it can be seen that carbon emissions fluctuate in line with fuel oil consumption, while fuel oil consumption follows the duration of the sailing days. The highest emissions occur during the period from March 21 to April 20, 2024, reaching approximately 480 tons of CO₂e, with a fuel consumption of about 150 tons and 21 sailing days. The lowest emissions are observed between June 21 and July 20, 2024, with emissions around 140 tons of CO₂e, fuel consumption of approximately 44 tons, and 8 sailing days. The accumulation of carbon emissions, fuel oil consumption, and sailing days for the period of one year, from July 21, 2023, to July 20, 2024, can be seen in Figure 11.

Figure 11 presents the accumulation of carbon emissions (represented by the orange line) and fuel consumption (represented by the blue line), which are shown on the left ordinate. Meanwhile, the sailing days (represented by the green line) are shown on the right ordinate. Based on Figure 11, it can be seen that at the end of the year, specifically on July 20, 2024, the total carbon emissions amount to approximately 3,244 tons, total fuel consumption is around 1,014 tons, and the total sailing days of the ship over the year is approximately 173 days. Additionally, it is observed that the accumulation lines for both carbon emissions and fuel consumption are not smooth (they fluctuate), which is due to the varying sailing days (fluctuating) for each ship movement, as well as changes in the ship's speed, causing it to not remain constant, as can be seen in the ship's speed data in Figure 7.

Figure 11 Accumulation of CO₂e, fuel consumption, and sailing days during a year operation

Slika 11. Akumulacija CO₂e, potrošnja goriva i dan plovidbe tijekom jednogodišnjeg rada

The fluctuations in the accumulation of carbon emissions and fuel consumption reflect the dynamic nature of ship operations. The variability in sailing days throughout the year indicates that the ship’s operational patterns, such as the frequency and length of voyages, are not constant. This irregularity leads to inconsistent fuel usage and, consequently, fluctuating emissions. The ship's speed, which changes as seen in Figure 7, further impacts fuel efficiency, with higher speeds generally resulting in greater fuel consumption and higher emissions. These factors highlight the need for optimizing operational conditions, such as route planning and speed management, to reduce emissions and fuel consumption over time.

3.3. Decarbonization by Implementing Slow Steaming / Dekarbonizacija primjenom spore plovidbe

One of the strategies for reducing carbon emissions (decarbonization) is speed management, one of which involves reducing the ship's speed or implementing slow steaming. Figure 12 shows the annual carbon emissions (orange line), annual fuel consumption (blue line), and annual sailing days (green line). The x-axis represents the reduction in speed (slow steaming) down to 1 knot, with a 0.2 knot interval. The left y-axis indicates the annual carbon emissions and fuel consumption, while the right y-axis shows the annual sailing days.

Figure 12 indicates a clear trade-off between fuel consumption reduction, carbon emissions reduction, and the impact on sailing days as the speed is reduced. As the speed reduction increases, fuel consumption and carbon emissions decrease, but the time required to complete the journey increases.

Figure 12 Annual CO₂e, fuel consumption, and sailing day by implementing slow steaming

Slika 12. Godišnje vrijednosti CO₂e, potrošnja goriva i dan plovidbe primjenom spore plovidbe

As can be seen in Table 2, with a 0.2 knot speed reduction, fuel consumption is reduced by approximately 45.1 tons, leading to a 4.5% decrease in carbon emissions (144.5 tons). However, this reduction adds around 3.8 extra sailing days annually, demonstrating that while there are environmental benefits, the operational efficiency in terms of time is slightly compromised.

Table 2 Decarbonization by implementing slow steaming

Tablica 2. Dekarbonizacija primjenom spore plovidbe

Increasing the speed reduction to 0.4 knots results in a more significant decrease in fuel consumption (88.1 tons) and a greater reduction in emissions (8.7% or 282 tons). However, this comes with an additional 7.8 sailing days, indicating that the benefits in terms of emissions reduction are achieved at the less operational efficient of travel times.

A 0.6 knot reduction brings a notable decrease in fuel consumption (129 tons) and emissions (12.7% or 412.9 tons), but it also adds 12.4 extra sailing days. As the speed reduction increases, the benefits in emission reductions become more substantial, but the operational efficiency in terms of time decreases further.

Reducing the speed by 0.8 knots results in a fuel consumption reduction of 167.8 tons and an emission decrease of 16.5% (536.8 tons). This more significant reduction in emissions, however, leads to an increase of 14.7 sailing days, further highlighting the time trade-off associated with greater decarbonization efforts.

Finally, a 1 knot speed reduction leads to the highest reduction in fuel consumption (204.3 tons) and carbon emissions (20.1% or 653.7 tons). However, this comes at the lowest efficient in terms of additional sailing days (18.1 days), demonstrating that while the environmental benefits are the greatest, the impact on operational time is most significant.

3.4. Decarbonization by Implementing Wind Assisted Propulsion System / Dekarbonizacija primjenom pogonskog sustava s pomoću vjetra

In addition to slow steaming, decarbonization efforts can also be achieved by incorporating a Wind Assisted Propulsion System (WAPS). While slow steaming involves internal factors that can be directly controlled by reducing the ship's speed, the addition of WAPS relies not only on internal factors, such as the ship's speed, but also heavily depends on external conditions, particularly the wind speed and direction along the ship's route. In this case, the accumulated carbon emission over one year resulting from the addition of WAPS under normal sailing condition compared to the ship without WAPS can be seen in Figure 13.

Figure 13 Accumulation CO2e with and without WAPS

Slika 13. Akumulacija CO₂e s WAPS-om i bez njega

Figure 13 presents a comparison of the accumulated carbon emissions produced by the ship under normal operational conditions, both without WAPS (black line) and with WAPS (red line). As shown in Table 3, the accumulation of carbon emissions with WAPS is lower than without WAPS, where the carbon emissions without WAPS amount to 3244.3 tons, while with WAPS, the carbon emissions are 2909.8 tons. This indicates that the addition of WAPS results in a decarbonization of 334.5 tons, or a 10.3% reduction. It should be noted that the contribution of WAPS in this case is based on the assumption that, at certain points during the ship's journey, if the sails become ineffective or actually increase emissions, they will be deactivated. However, the technical details of deactivating the sails are not covered in this paper.

In addition, from Table 3, the 10.3% reduction in emissions (decarbonization) corresponds to a fuel reduction of 104.5 tons while maintaining the ship's speed under normal operational conditions, resulting in no change in sailing days, which remains at 173.4 days per year.

Table 3 Decarbonization by implementing WAPS

Tablica 3. Dekarbonizacija primjenom WAPS-a

From the above data reveals a clear benefit of incorporating Wind Assisted Propulsion System (WAPS) in terms of reducing carbon emissions. When compared to the ship operating under normal conditions without WAPS, the ship equipped with WAPS shows a significant reduction in accumulated carbon emissions. The decarbonization is achieved through the reduction in fuel consumption, while the ship's speed is maintained under normal operational conditions. A key advantage of this approach is that it results in no additional sailing days; the ship's operational efficiency remains unchanged. This suggests that the use of WAPS can provide significant environmental benefits without negatively impacting the time required for the ship's journey.

3.5. Decarbonization by Implementing Slow Steaming and WAPS / Dekarbonizacija primjenom spore plovidbe i WAPS-a

In this part, the combination of slow steaming and Wind Assisted Propulsion System (WAPS) is applied. Figure 14 presents a comparison between annual carbon emissions with the implementation of slow steaming only (orange line) and annual carbon emissions with the implementation of both slow steaming and WAPS (orange dashed line), annual fuel consumption with the implementation of slow steaming only (blue line) and annual fuel consumption with the implementation of both slow steaming and WAPS (blue dashed line), as well as sailing days (green line).

Figure 14 Annual CO₂e, fuel consumption, and sailing day by implementing SS and WAPS

Slika 14. Godišnje vrijednosti CO₂e, potrošnja goriva i dan plovidbe primjenom spore plovidbe (SS) i WAPS-a

Without changing the sailing days, it can be observed that carbon emissions and fuel consumption can be further reduced with the installation of WAPS. The reduction in emissions (decarbonization) achieved ranges from 10.3% to 29.7% (Table 3).

Table 4 Decarbonization by implementing slow steaming and WAPS

Tablica 4. Dekarbonizacija primjenom spore plovidbe i WAPS-a

The combination of slow steaming and WAPS provides valuable insights into how the integration of WAPS enhances the benefits of slow steaming, particularly in reducing both carbon emissions and fuel consumption, without impacting the sailing days. Comparing the annual carbon emissions and fuel consumption in both scenarios (slow steaming alone and slow steaming combined with WAP) clearly demonstrates the positive effect of WAPS. By incorporating WAPS into the slow steaming strategy, both carbon emissions and fuel consumption are significantly lowered.

The particularly noteworthy is that, despite the reduction in carbon emissions and fuel consumption with WAPS, the sailing days remain unchanged. This indicates that the use of WAPS does not require additional time for the ship's journey, making it an efficient solution for decarbonization without affecting operational schedules.