Introduction

Ships depend mainly on the fossil fuels to produce the required propulsion power. Unfortunately, all these fuels present a source of adverse pollutant emissions (El Gohary et al. 2015; Jakub et al. 2016; Kim et al. 2015). The International Maritime Organization (IMO) in their latest studies implied that the maritime sector is responsible for 6.6%, 0.24%, and 2.6% of the global nitrogen oxides (NOx), hydrocarbons (HC), and carbon dioxide (CO2) emissions, respectively (IMO, 2014a; Johansson et al. 2017). Moreover, El Gohary et al. (2015) indicated that the international shipping sector is responsible for about 4% of all global SOx emissions with expectation of continuous increment. In a step to eliminate the adverse effect result from ship emissions; the IMO issued regulations 13 and 14 for reducing NOx and SOx emissions, respectively. These regulations can be found in the international convention of marine pollution prevention (MARPOL), annex VI (Ammar and Seddiek 2018; Seddiek 2016; Seddiek and Elgohary 2014; Yang et al. 2012). Another regulation was issued to reduce CO2 emissions, through applying energy efficiency indexes (Ammar 2018; Papanikolaou et al. 2016; Rehmatulla et al. 2017). These indexes are the Energy Efficiency Design Index (EEDI) and the Energy Efficiency Operational Indicator (EEOI). The main target of applying these indexes is to reduce CO2 emissions from ships by 30% at the end of 2023 (Rehmatulla et al. 2017). The statistics of the uppermost ten ship types which have the most influence with reference to the fuel consumption and CO2 emissions can be shown in Fig. 1. It reveals that the container ships are responsible for about 205 million tons of emitted CO2, due to burning of about 66,000 kt of marine fuels, which presents about 26.28% of the total annual marine fuel consumption (IMO, 2014a; UNCTAD 2019).

Fig. 1
figure 1

Top ten ships in CO2 emissions and fuel consumption

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At international level, the IMO implemented some procedures which are intended to improve the energy efficiency of shipping. The measurements of ship’s energy efficiency include mainly ship’s hull, propellers, ship’s engines, and ship’s systems. The effect of ship’s hull could be achieved by bow optimization, hull coating, and air lubrication (Guangrong et al. 2013). Propellers and rudders can be contributing in energy improvement using ducted propeller, contra-rotating propellers, and twisted rudder (Sarasquete et al. 2011). Ship’s engines and systems consider the most sector which can be used through main engine de-rating, common rail upgrade, waste heat recovery systems, and renewable power sources (Benvenuto et al. 2014; Diab et al. 2016). The percentage of contribution of the previous technologies regarding energy improvement varies depending mainly on ship type as some technologies are restricted to special ships such as solar-powered ships. Moreover, the selection of the optimum energy-efficient concept for a specific new building is a comprehensive task. Therefore, it is important that all parameters which have an influence on the energy efficiency index such as economy, operation characteristics, and environmental effects are analyzed and evaluated before the final decision is made.

Containerized trade overview

Statistics provided by maritime transport researches and inventories indicate that container ships contribute by valuable percentage of the total cargo transported by year and this value is in continuous increment. The share percentage of container ship in the international seaborne trade is continuously increasing and it reached 17.13% in 2017 (Brent-Petersen and Rex 2018; UNCTAD 2011), as the container ships were responsible for transport of 1834 million tons of cargo out of 10,702 million tons which present the international seaborne trade worldwide.

One of the steps toward reducing the gap between the transported cargo and its associated emissions is most of companies start to depend on the medium and mega containerized size in order to reduce the ratio of fuel consumption per ton transported. The main driver for ordering bigger vessels is to reduce the energy needed to transport each individual container. More energy efficiency lowers costs and helps to minimize CO2 emissions, which improves profitability and reduces the environmental impact of global supply chains (Martin et al. 2015). The previous objective was achieved through the modification of container ship design to increase the loading area.

In the same contrast, ship economies are factors that have a direct impact on the growth of the global economy. The freight rate of container ships is estimated according to the type of container 20 or 40 ft and then according to the place to which they will be transported, which is estimated as dollars per 20/40 ft equivalent unit. As Shanghai port is one of the world’s largest ports in receiving ships that carry containers, the port can be taken for a real statistic. Figure 2 shows the cost of transporting containers from Shanghai port to four major container ports worldwide, including Shanghai-United State West Coast, Shanghai-Northern European, Shanghai-Mediterranean, and Shanghai-West Africa (Brent-Petersen and Rex 2018; Deloitte 2019; UNCTAD 2011).

Fig. 2
figure 2

Container ship freight rate $/TEU

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With reference to the development in the container ships and its environmental impacts especially CO2 emissions, the aim of the current paper is to study the effect of using emission reduction strategies onboard these ships with emphasis on improving ship energy efficiency indexes from environmental and economic points of view. In addition, the effect of using waste heat recovery, shaft generator/motor, slow steaming, and dual-fuel engines (DFE) on the ship energy efficiency indexes will be investigated. As a case study, container ships of different TEUs classified as A19 and A7 container ships are investigated.

Environmental and economic modeling of ship emissions control

In this section, an environmental model for ship emissions is presented with emphasis on the calculation of the Energy Efficiency Design Index (EEDI) and the Energy Efficiency Operational Indicator (EEOI). One of the short-run solutions for improving both container ship emissions and energy efficiency is to reduce ship speed. Therefore, the impacts of ship speed from environmental and economic points of view are modeled. Finally, an economic modeling for applying emission control methods onboard ships is discussed.

Environmental modeling

From an environmental point of view, the total ship emissions per trip (me) in tons can be calculated using Eq. (1).

$$ {m}_{\mathrm{e}}=P\cdotp L\cdotp T\cdotp {E}_{\mathrm{f}} $$
(1)

where P is the main engine output power (kW); L is the main engine load; T is the trip time in hours; Ef is the fuel emission factor in g/kWh. The emission factors for the slow speed marine diesel engine operated with ULSHFO (0.1% S), HFO, and NG at cruise are shown in Table 1 (Ammar 2019a; Ammar and Seddiek 2017; Banawan et al. 2010; EEA 2000; ICF 2009a; Seddiek and Elgohary 2014).

Table 1 Emission factors for slow speed marine diesel engines
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On one hand, the permissible emission quantity regarding NOx and SOx is addressed in regulations 13 and 14 of the MARPOL annex 6. On the other hand, CO2 permissible emission can be evaluated using EEDI and EEOI from ship design and operational points of view. Two values of EEDI can be calculated for ships, the reference and the calculated (attained) rates (ABS. 2013; Ančić and Šestan 2015; Bøckmann and Steen 2016). The calculated value should not exceed IMO reference value for a definite vessel type (IMO 2014b). EEDI reference values set by IMO (EEDIref.) can be calculated using Eq. (2).

$$ {\mathrm{EEDI}}_{\mathrm{ref}.}=\left(\frac{a}{{\mathrm{Capacity}}^c}\right)\left(1-\frac{X}{100}\right) $$
(2)

where a and c are the ship parameters which can be determined using the fitted EEDI regression curve values at different ship capacities. Their values are 174.22 and 0.201, respectively, for container ships. Capacity is the deadweight tonnage (DWT). X is the required reduction percent in EEDI reference value, specified according to IMO requirements. It increases from 10% in 2015 and 20% in 2020 to 30% in 2025 (GL 2013; IRCLASS 2013).

The calculated EEDI (EEDIcal.) can be estimated in (gCO2/ton-NM) as expressed in Eq. (3) (Alisafaki and Papanikolaou 2015; Ammar 2019a; IMO 2014b; Zhu et al. 2017).

$$ {\mathrm{EEDI}}_{\mathrm{cal}.}=\frac{\left[\prod \limits_{j=1}^M{\mathrm{f}}_{\mathrm{j}}\left(\sum \limits_{i=1}^{\mathrm{nME}}{P}_{\mathrm{ME}(i)}\cdotp {\mathrm{SFC}}_{\mathrm{ME}(i)}\cdotp {C}_{\mathrm{FME}(i)}\right)\right]+\left[{\mathrm{SFC}}_{\mathrm{AE}}\cdotp {C}_{\mathrm{FAE}}\cdotp {P}_{\mathrm{AE}}\right]}{\mathrm{Transport}\ \mathrm{work}} + \frac{\left[\ \left(\prod \limits_{j=1}^n{\mathrm{f}}_{\mathrm{j}}\cdotp \sum \limits_{i=1}^{\mathrm{nPTI}}{P}_{\mathrm{PTI}(i)}-\sum \limits_{n=1}^{n_{\mathrm{eff}}}{\mathrm{f}}_{\mathrm{eff}(i)}\cdotp {P}_{\mathrm{AE}\mathrm{eff}(i)}\right){C}_{\mathrm{FAE}}\cdotp {\mathrm{SFC}}_{\mathrm{AE}}\ \right]-\left[\ \sum \limits_{i=1}^{n_{\mathrm{eff}}}{f}_{\mathrm{eff}(i)}\cdotp {P}_{\mathrm{eff}(i)}\cdotp {C}_{\mathrm{FME}}\cdotp {\mathrm{SFC}}_{\mathrm{ME}}\ \right]}{\mathrm{Transport}\ \mathrm{work}} $$
(3)

where PME(i) and PAE are the output powers of each main engine (i) and the auxiliary engine; SFCME(i) and SFCAE are the specific fuel consumption of the main and auxiliary engines; CFAE is the conversion factor of the auxiliary engine fuel to CO2 emission. CFME(i) and CFAE are the fuel conversion factors of engines to CO2 emissions (Tran 2017). Shaft motors, innovative technologies, and waste heat recovery improves the EEDI based on the following variables: PPTI(i) is the shaft motor power divided by the shaft generator efficiency; Peff(i) and PAEff(i) are the reduction in the main and auxiliary engines power due to using mechanical and innovative technologies with an availability factor of feff(i) for each method.

In Eq. (3), the auxiliary engine power (PAE) can be calculated using the maximum continuous rating power (MCR) of the main engine as expressed in Eqs. (4) and (5).

$$ {P}_{\mathrm{AE}\left(\mathrm{MCR}\left(\mathrm{ME}\right)>\mathrm{10,000}\ \mathrm{kW}\right)}=\left[0.025.\left(\sum \limits_{i=1}^{\mathrm{nME}}{\mathrm{MCR}}_{\mathrm{ME}}+\frac{\sum \limits_{i=1}^{\mathrm{nPTI}}{P}_{\mathrm{PTI}(i)}}{0.75}\right)\right]+250 $$
(4)
$$ {P}_{\mathrm{AE}\left(\mathrm{MCR}\left(\mathrm{ME}\right)<\mathrm{10,000}\ \mathrm{kW}\right)}=0.05.\left({\mathrm{MCR}}_{\mathrm{ME}}+\frac{\sum \limits_{i=1}^{\mathrm{nPTI}}{\mathrm{P}}_{\mathrm{P}\mathrm{TI}(i)}}{0.75}\right) $$
(5)

In addition, transport work can be calculated using Eq. (6).

$$ \mathrm{Transport}\ \mathrm{work}={v}_{\mathrm{ref}}\cdotp \mathrm{Capacity}\cdotp {f}_{\mathrm{i}}\cdotp {f}_1\cdotp {f}_{\mathrm{w}}\cdotp {f}_{\mathrm{c}} $$
(6)

where Vref is the ship speed at the design load condition expressed in knots. The deadweight tonnage (DWT) is used as the capacity measure for most of cargo ships. fi, f1, fw, and fc are correction factors to account for specific ship types and elements. Their value if not applied is set to 1.0 (GL 2013). fw is a non-dimensional factor representing the reduction in container ship speed due to weather and environmental conditions. It can be calculated using Eq. (7) (IMO 2014b).

$$ {f}_{\mathrm{W}}=0.0208\ast \mathrm{Ln}\ \left(\mathrm{Capacity}\right)+0.633 $$
(7)

Moreover, the energy efficiency of a ship during operation can be measured using EEOI in terms of tone CO2/transport work. The EEOI value changes for each ship voyage based on the working circumstances (IMO 2009; Tran 2017). The average EEOI, expressed in Eq. (8), is calculated annually where more efficient ships will have smaller index values.

$$ {\mathrm{EEOI}}_{\mathrm{average}}=\frac{\sum \limits_i\sum \limits_j\mathrm{M}{\mathrm{F}}_{ij}\cdotp {C}_{\mathrm{F}(j)}}{\sum \limits_i{M}_{\mathrm{cargo},i}\cdotp {D}_i} $$
(8)

where the subscripts i and j refer to the voyage number and the fuel type; MF is the fuel consumption in tones; CF is the fuel conversion factor to CO2 emissions. Mcargo is the cargo mass in tones; D is the distance of the ship voyage in nautical miles.

For a certain condition, if there is need for applying slow steaming concept, the fuel consumption at the reduced ship speed (mfsr) can be calculated each trip using the cubic law of ship operational and design speeds ratio (Vo/Vd) (Ammar 2018; Banawan et al. 2013; Chang and Wang 2014; Stopford 2009) as expressed in Eq. (9)

$$ {m}_{\mathrm{fsr}}=\left[{m}_{\mathrm{ME}}\cdotp {\left(\frac{V_{\mathrm{o}}}{V_{\mathrm{d}}}\right)}^3+{m}_{\mathrm{AE}}\right]\cdotp \frac{D_{\mathrm{t}}}{24\cdotp {V}_{\mathrm{o}}} $$
(9)

where mME and mAE are the fuel consumption of the main and auxiliary engines per day; and Dt is the distance per trip in nautical miles.

Economic modeling

From economic point of view, the annual fuel saving costs (FSC) for each ship speed reduction can be calculated using Eq. (10) (Ammar and Seddiek 2018).

$$ \mathrm{FSC}={m}_{\mathrm{sav}}\cdotp {\mathrm{C}}_{\mathrm{fuel}}\cdotp {\left[1\pm \mathrm{IP}\right]}^n $$
(10)

where msav is the mass of fuel saving per trip in tones; IP is the annual fuel price increment; n is the expected years of ship operation.

To quantify the annual CO2 emission reduction as a result of ship speed reduction in terms of saved fuel costs, the cost-effectiveness for reducing CO2 emissions (CERCO2) in $/ton CO2 is calculated as shown in Eq. (11) (Ammar 2018; Bishop et al. 2014; Eide et al. 2009).

$$ {\mathrm{CER}}_{{\mathrm{CO}}_2}=\frac{P_{\mathrm{loss}}-{C}_{\mathrm{oth}.}}{R_{{\mathrm{CO}}_2}} $$
(11)

where Ploss is the loss in ship profit due to slow steaming; Coth.is the economic benefits except emission reduction during the remaining ship life; RCO2 is the reduction amount in the CO2 emissions in ton.

The annual ship profit (ASP) after each speed reduction can be calculated using Eq. (12) (Chang and Wang 2014; Corbett et al. 2009).

$$ \mathrm{ASP}=\left[{F}_{\mathrm{e}}-\left({C}_{\mathrm{f}}+{C}_{\mathrm{pc}}+{C}_{\mathrm{f}\mathrm{uel}}\times {m}_{\mathrm{f}\mathrm{uel}}\right)\right]\cdotp {N}_{\mathrm{t}} $$
(12)

where Fe is the freight expenses for the transport containers per trip in $/trip, Cf is the ship fixed costs per trip, Cpc is the cost of passing through navigational canals per trip, mfuel is the fuel consumption per trip in tons, Cfuel is the price of fuel in $/ton, and Nt is the annual number of trips.

In case of using one of the emission control methods, the added annual costs (AAC) for the ship operating and maintenance costs can be calculated using Eq. (13) (Ammar and Seddiek 2017; Hunt and Butman 1995). These methods include Selective Catalytic Reduction (SCR), Seawater Scrubber (SWS), and using low sulfur fuel for reducing NOx and SOx emissions, respectively.

$$ \mathrm{AAC}=\mathrm{IC}\times \frac{i{\left(1+i\right)}^n}{{\left(1+i\right)}^n-1}+\mathrm{MOC}+\Delta \mathrm{FPI} $$
(13)

where IC is the initial cost value, i is the annual interest rate, MOC is the annual maintenance and operating costs, and ΔFPI is the annual fuel price increment in case of using low sulfur fuel.

Finally, the annual cost-effectiveness (ACE) for using emission reduction method can be calculated for the annual emission reduction (AER) using Eq. (14) (Ammar and Seddiek 2017; ICF 2009b). ACE is calculated separately for each reduction in pollutant emissions.

$$ \mathrm{ACE}=\frac{\mathrm{AAC}}{\mathrm{AER}} $$
(14)

Case study: Container ship classes A19 and A7

As a case study, two different ships are taken to evaluate factors affecting the improvement of ship’s energy efficiency, especially EEDI and EEOI calculations. The chosen ships belong to Hapag-LIoyd, a company which contributes by about 6.1% of the world container market through 217 container ships with a total of 1,550,874 TEU (Hapag 2019b). The two ships are classified A7 and A19. A7 sailing under the flag of Liberia, port of registry Monrovia, has a capacity of 6921 TEU. A19 is currently sailing under the flag of Malta, port of registry Valletta, which has a capacity of 19,870 TEU. Table 2 summarizes the main particulars of the two case study ships (FleetMon 2019; Hapag 2019a; Marine Traffic 2019).

Table 2 Ships main particulars
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Technically, with reference to the power plant, A19 and A7 class ships depend on two stroke diesel engines as a prime mover using ultra-low sulfur fuel oil (ULSFO) of 0.1% S and heavy fuel oil (HFO), respectively. A19 is equipped with shaft generator and turbo generator as a waste heat recovery system. In addition, A19 is built with the installed LNG infrastructure onboard, but did not use LNG as fuel until now. The ships transport containers from/to different ports and covered United State West coast, Northern European, Mediterranean Sea, and East Asia zones. The route of Shanghai-Northern European is taken as a datum through the environmental and economic calculations.

Results and discussion

In this section, the environmental effects for using emission reduction methods onboard A19 and A7 class container ships are discussed. Moreover, the eco-environmental results for applying energy efficiency principal are discussed. The calculated EEDI values are compared with the required IMO values and their application phases. The energy and cost-effectiveness results of the improved energy efficiency of A7 class container ship using slow steaming respectively are presented. Finally, the effect of using dual diesel-natural gas fuels onboard A19 container ship is discussed from environmental and economic points of view.

The results are based on a group of assumptions. The container ships travel between Shanghai and Northern European ports. These ports are considered one of the largest ports receiving containers worldwide (Brent-Petersen and Rex 2018; Deloitte 2019; UNCTAD 2011). Therefore, the distance traveled by the ships is 13,000 nautical miles. The currently used fuels for A19 and A7 class container ships are ULSHFO and HFO, respectively. In order to improve the environmental effects, the two ships are assumed to operate with dual-fuel engine and ULSHFO, respectively. The trip time depends on ship specifications especially the operational speed. For ease of calculations, the current study assumes the average number of trips per year depends on ship speed. In addition, the auxiliary diesel generators for A19 container ship will not be operated during ship cruise (PAE = 0.0). Moreover, the selective catalytic reduction (SCR) method is installed only onboard A19 container ship to reduce NOx emissions. The electric power will be supplied using shaft generators. In EEDI equation, the operating ship speed is used as the reference speed (Vref) (Perera and Mo 2016).

Environmental results

During ship cruise, the only source for the exhaust gas emissions for A19 container ship is the main engine which provides the mechanical torque to propel the ship and operates shaft generator for required electric power onboard. In contrast, A7 uses both main and auxiliary diesel engines during ship cruise. In addition, the fuel consumption for the case study can be considered a good indication for the rate of the emitted exhaust gas emissions from the ship (Ammar 2019b). Table 3 shows the different fuel consumptions for the tabulated container ship classes A19 and A7. It can be noticed that, although A7 class container ship has lower cargo capacity, its diesel fuel consumption is higher than A19. This is because of the high power of the main engine (54,612 kW), compared with the other ship, to drive the ship at a speed of 24.0 knots. The annual auxiliary engine fuel consumptions are 1908 and 406 ton/year for A7 and A19 ship classes, respectively. The proposed dual-fuel engine for A19 container ship is assumed to operate with a mixture of 95% natural gas and 5% diesel oil (Ammar and Seddiek 2017). It can be noticed that the fuel storage volumes are increased in case of using the dual-fuel engine.

Table 3 Fuel consumption for A19 and A7 container ships
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In order to calculate the dual-fuel emission factors (EDFDE), the engine emission factor (Ef) in Eq. (8) should be modified. EDFDE can be calculated using the emission factors of the diesel and natural gas engines (eD and eNG), as expressed in Eq. (15), assuming the dual fuel is composed of 95% natural gas and 5% diesel oil.

$$ {E}_{\mathrm{D}\mathrm{FDE}}=\kern0.5em 0.05\cdotp {e}_{\mathrm{D}}+0.95\cdotp {e}_{\mathrm{NG}} $$
(15)

One of the main advantages for the container ships built after 2011, like A19 class container ship, is the use of selective catalytic reduction (SCR) method for reducing NOx emissions (Seddiek 2016). Figure 3 shows NOx emissions in kg/min for the dual-fuel engine using different natural gas fuel percentages. These rates are compared with IMO 2016 tier III limits. Starting from natural gas percentages of 89% would comply with the IMO limits without using SCR for reducing NOx emissions. This will save the operating and maintenance costs for SCR, as will be discussed in the economic results. In addition, using dual natural gas fuel engine will comply with the SOx emission limits set by IMO (Ammar and Seddiek 2017).

Fig. 3
figure 3

Effect of using dual fuel on NOx emissions for the A19 class container ship

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On the other hand, A19 and A7 class container ships are currently operated with ULSHFO and HFO, respectively. In order to comply with emission rates, set by IMO, SCR, and seawater scrubber (SWS) measures should be installed onboard the ship to reduce NOx and SOx emissions, respectively. The use of SCR system will decrease NOx emissions by 90% (Wärtsilä 2016). In addition, installing SWS system will reduce SOx and PM emissions by 98% and 70%, respectively (Contini et al. 2015; Doudnikoff et al. 2014). As a case of comparison, A7 is proposed to operate using ULSHFO with the installed SCR method. Figure 4 shows the result of the proposed methods for decreasing SOx, NOx, PM, and CO2 pollutant emissions. These results belong to applying SCR+SWS and SCR+ULSHFO for A7 and dual fuel for A19 class container ship.

Fig. 4
figure 4

Emission reduction percentages using the proposed control methods

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The different emission rates from A19 and A7 class container ships can be calculated using the calculated fuel consumption values presented in Table 3 considering the emission reduction percentages shown in Fig. 4. A19 class container ship operated with ULSHFO and dual-fuel engines has the lowest NOx and SOx emissions per trip where the SCR is used only in case of using ULSHFO, as shown in Table 4. On the other hand, A7 class container ship operated with ULSHFO results in lower NOx emissions rates per trip than using HFO. In order to show a good indication for the annual emission rates per transported cargo, the emission rates are calculated per transported TEU. A19 class container ship operated with DFE is the lowest in CO2 emission rates per transported cargo. This is a result of its high capacity of 19,870 TEU and the lower speed compared with A7 container ships.

Table 4 Environmental analysis for the container ships after installing SCR and SWS
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SOx and NOx emissions from ships should be compared with the latest IMO emission-limit rates. These rates can be calculated based on fuel sulfur content (0.5%) and engine speed (rpm), respectively. Figure 5 shows a comparison between NOx and SOx emissions with IMO limits for A19 and A7 class container ships. It can be observed that SOx emissions for the case studies where the ship is operated with ULSHFO are compliant with the IMO limits. This is due to the used ULSFO of 0.1% S. The ship emission rates in kg/min depend mainly on engine power (kW) and fuel emission factor (g/kWh). Practically, during ship operations, the rated engine power in maneuvering is lower than in cruise mode. Therefore, the rate of ship emissions (kg/min) in maneuvering will be lower than that of cruise mode (Ammar and Seddiek 2017; EEA 2000; ICF 2009a). At the same time, NOx emissions are reduced by 90% for the cases where SCR is used for reducing NOx emissions. These rates are reduced from 14.0 and 13.15 to 1.4 kg/min and 1.31 kg/min for A7 class container ship operated with HFO and ULSHFO, respectively, during ship cruise. Therefore, NOx and SOx emission rates of A19 and A7 class container ships do not present challenge or cause any adverse effect on the maritime environment after applying the proposed emission control methods.

Fig. 5
figure 5

NOx and SOx emissions rates compared with IMO limits

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On the other hand, IMO approved Energy Efficiency Design Index and Energy Efficiency Operational Indicator (EEDI and EEOI) for reducing CO2 emissions. The maximum permissible values determined by IMO for the EEDI can be calculated using Eq. (2) based on the ship deadweight at summer load draft as shown in Fig. 6. These values are planned to show a reduction by 10%, 20%, and 30% according to three phases depending on the year of built for ships (GL 2013). The reference EEDI values set by IMO for A19 and A7 container ships are 14.99 and 17.78 gCO2/ton-NM, at deadweights of 199744 MT and 85384 MT, respectively. In 2025, these reference values will be reduced to 10.49 and 12.44 gCO2/ton-NM, respectively.

Fig. 6
figure 6

Reference values of the EEDI for container ships

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The calculated EEDI values of the case study should be compared with the reference EEDI set by IMO shown in Fig. 6. These values can be calculated using Eq. (3). The correction factors for capacity, cubic capacity, and general cargo ships (fc, fi, and f1) are equal to 1.0 for container ships (GL 2013). In addition, the correction factor due to weather and environmental conditions can be calculated using Eq. (6). The deadweight and the operational ship speed are used as the capacity and the reference speed in Eq. (3) (Perera and Mo 2016). The reference speeds are 21.21 knots and 24.0 knots for A19 and A7 class container ships, respectively. Figure 7 shows the relative calculated and required IMO-EEDI values for the case study ships. The calculated EEDI values are 4.65 and 18.28 gCO2/ton-NM for A19 and A7 class container ships, respectively. It is noticed that A19 complies with the required IMO-EEDI values while A7 is not compliant with all the required EEDI phases.

Fig. 7
figure 7

Reference values of the EEDI for container ships

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The second measure for the energy efficiency of the ships is the EEOI. It is developed by the IMO to enable ship operators to measure the fuel efficiency and carbon emissions during ship operations. The values of the EEOI change for each voyage based on the operating conditions of the ship. The mean EEOI values are calculated using Eq. (7) based on the transported TEU cargoes of 19,870 and 6921 for A19 and A7 container ships, respectively, over the distance of 1300 nautical miles. These values are 0.000058 tonCO2/TEU-NM and 0.00017 tonCO2/TEU-NM, respectively. During ship maneuvering and standby modes, the fuel consumption is calculated at engine load percentages of 20% and 5%, respectively. It can be noticed that the average EEOI values are improved when the ship speed is reduced from the design to the operational.

One of the short-term options to improve the energy efficiency through reducing EEDI value is to reduce the ship speed. In order to study the effect of ship speed reduction on the energy efficiency of the A7 class container ship, the specific fuel consumption of the main engine at different engine loads should be determined. Using WÄRTSILÄ RT-flex96C project guide (Wartsila 2008), the relation between engine load and fuel consumption can be determined using curve fitting method. As the ship speed is reduced, the main engine power will be reduced. This will consequently reduce the emitted CO2 emissions.

The effect of speed reduction on A7 class container ship performance can be evaluated from the operational ship level through EEOI and from the design level using EEDI. As the ship speed reduces, the EEOI value will be reduced as shown in Fig. 8. The main engine output power (PME) will be reduced with the decrease in ship speed. This reduction will affect the EEDI value as expressed in Eq. (3). The improvement in the EEDI should be compared with IMO limiting values in three phases as shown in Fig. 7. On one hand, the required IMO-EEDI values are 16.0, 14.22, and 12.44 gCO2/ton-NM during the first, second, and third phases. On the other hand, the calculated EEDI values are 15.39, 13.72, and 12.14 gCO2/ton-NM in accordance with speed reduction by 12.5%, 17.5%, and 22.5%, respectively. At ship speed reduction of 22.5%, the calculated EEDI value will meet the required EEDI values for the three phases. The relative attained to the required IMO values for the first, the second, and the third phases will be 76%, 85%, and 98%, respectively.

Fig. 8
figure 8

The improved EEDI values for A7 container ship after speed reduction

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Finally, the effect of different improvement percentages in EEDI values using various methods for A19 and A7 class container ships can be shown in Fig. 9. The improvements in the EEDI for A19 class container ship include the use of Shaft Generator/Motor (SGM), the waste heat recovery (WHR) using turbo generator, and the dual-fuel engine. The power taken off the shaft generator eliminates the need for the auxiliary power during cruise (PAE = 0.0) and consequently reduces the EEDI value. The output power of the installed turbo generator improves the EEDI value, expressed in Eq. (3), through the term PAEeff. The improvement in the calculated EEDI value due to the shaft generator is higher than that due to the waste heat recovery system. This is because of the higher output power of the shaft generator (5620 kVA) compared with the turbo generator output power (2650 kW). In addition, using dual fuel will improve the EEDI value by 10.13% through reducing CO2 emissions. On the other hand, reducing A7 class container ship speed by 22.5% will improve the EEDI value by 33.5% due to reducing both the main engine power and the exhaust gas emissions. Another improvement from the ship design point of view is that the lower the length-to-beam ratio for the case study (A19), the more improved EEDI value.

Fig. 9
figure 9

Improvement percentages in EEDI for A19 and A7 class container ships

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Economic results

In this section, the economic results for using emission reduction measures onboard A19 and A7 container ships are discussed. In order to estimate the financial advantages for the case studies, the yearly profit function should be calculated for each case of study using Eq. (9). This function includes the calculation of the annual cargo transporting, cost of installed emission reduction technologies, fixed fees for passing through canals, and fuel consumption costs for the case study. Table 5 shows the fuel cost split for the case studies. The calculations are based on natural gas and ULSHFO fuel prices of 0.1075 $/m3 and 772.6 $/m3, respectively (Bunkerworld 2019; EIA 2019). The bunkering prices for natural gas and diesel oil fuels are 0.009 $/m3 and 8 $/m3, respectively (Ammar 2019b; Ammar and Seddiek 2017). The highest annual cost of fuel consumption is 39.61 million US$ for the A7 class container ship operated with ULSHFO. This is due to the high fuel consumption and diesel engine output power compared with A19 class container ship.

Table 5 Annual cost of fuel consumption for the case studies
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The cost of installed emission reduction technologies includes the initial, operating, and maintenance costs of SCR and SWS systems. For A19 class container ship, operating and maintenance costs are only included because the system is already installed onboard the ship. The capital costs for installing SCR and SWS onboard A7 class container ship are 50 US$/kW and 160 US$/kW, respectively. The average operating and maintenance costs for SCR system are 3.75 $/MWh and 0.9 $/MWh, respectively. For SWS system, the average operating costs are 3% from the initial installation costs (Ammar 2019b; Ammar and Seddiek 2017; INTERTANKO 2007; Nikopoulou 2008; Wang and Corbett 2007). Figure 10 shows the annual costs for SCR and SWS emission reduction methods onboard A19 and A7 class container ships. The annual costs for SCR system are higher than SWS systems due to its higher operating and maintenance costs. In addition, the average costs per kW are high for A7 operated with HFO. This is because of its high carbon and sulfur contents compared with ULSHFO. It is noted that using ULSHFO instead of the currently used HFO onboard A7 class container ship can be considered one of the measures for reducing SOx emissions. The fuel costs will be increased by 11.5 million $/year when the ship operates with ULSHFO compared with HFO.

Fig. 10
figure 10

Total annual and average costs for using emission reduction methods

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In order to assess the financial advantages of the emission reduction method, the annual cost-effectiveness should be calculated. If the ship is operated with ULSHFO, SCR is the only used system to reduce NOx emissions. Figure 11 shows the annual cost-effectiveness for the two options proposed to decrease NOx and SOx emissions for A7 class container ship. The annual cost-effectiveness for a combined system of SCR and SWS installed onboard A7 class container ship for reducing NOx and SOx emissions are 748 $/ton and 1189 $/ton, respectively. Using SCR+ULSHFO system will reduce these emissions with cost-effectiveness of 3232 $/ton and 5035 $/ton, respectively. On the other hand, the cost-effectiveness for the installed SCR system onboard A19 class container ship reduces NOx emission with cost-effectiveness of 393 $/ton.

Fig. 11
figure 11

Annual cost-effectiveness for combined ECMs

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The cost of cargo transporting can be calculated using the ship route freight rate and the number of transported containers for each ship. The freight rate for transporting one container from Shanghai to Northern European is 876 $/TEU (Brent-Petersen and Rex 2018; Deloitte 2019; UNCTAD 2011). The average annual costs of cargo transporting should be subtracted from the total annual operating and maintenance costs for the ship in order to calculate the ship profit. The annual fees for passing through Suez canals are 14.6 and 3.98 million US$/year for A19 and A7 class container ships, respectively. The average specific profit per transported TEU is 313 and 286 US$/TEU for the currently operated A19 and A7 class container ships, respectively, according to the company that owns the ships.

On the other hand, the speed reduction of A7 class container ship for improving its energy efficiency will lead to loss in ship profit in addition to reducing CO2 emissions. This will also reduce the cost of fuel consumption particularly at increased fuel prices (Tezdogan et al. 2014; Tezdogan et al. 2016). The annual loss in ship profit due to speed reduction will considerably affect the cost-effectiveness for reducing CO2 emissions. The average annual loss percentage in the transported capacity for A7 class container ship is assumed 6.9% for each knot reduction in ship speed (Ammar 2019b; Eide et al. 2009). Figure 12 shows the cost-effectiveness for each CO2 emission reduction percent for A7 class container ship operated with HFO. The cost-effectiveness is calculated at three different loss percentages in the transported capacities of 4%, 7%, and 10%. At 22.5% of ship speed reduction (% of ship design speed) and 7% loss in transport capacity, the annual CO2 cost-effectiveness is 95.54 $/ton.

Fig. 12
figure 12

Cost-effectiveness and CO2 emission reduction percentages for speed reduction of A7

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In addition to the cost-effectiveness for reducing CO2 emissions, the annual fuel saving costs as a result of ship speed reduction will considerably influence the total ship profit. Figure 13 shows the variation of the fuel saving and CO2 emission reduction cost-effectiveness due to speed reduction for the remaining life cycle time of A7 class container ship. The remaining work period for the ship is 17 years based on 28 years of maximum ship operations (Ammar and Seddiek 2017; Mikelis 2008). The annual increment in fuel cost (PI) is assumed 10%. Ship speed reduction is restricted by the criteria of IMO for minimum power requirements (Faber et al. 2017; MEPC 2016). In addition, the loss in ship transport capacity is assumed 6.9% for each knot reduction (Ammar 2018; Eide et al. 2009). The higher the reduction in ship speed, the more CO2 emission reduction cost-effectiveness will be gained with higher fuel saving costs as shown in Fig. 13. The maximum value for the fuel saving costs will be gained at the end of the ship life cycle. After 17 years of the ship operations, with 22.5% speed reduction from the design speed, the fuel saving costs will reach 87.7 million dollars with CO2 cost-effectiveness of 52.54 $/ton CO2.

Fig. 13
figure 13

Fuel saving costs and CO2 cost-effectiveness map for the A13 container ship

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Moreover, using dual natural gas-diesel fuels onboard A19 class container ship would comply with NOx and SOx emissions without using SCR system as discussed in Fig. 5. The annual and maintenance operating saved costs of SCR system will be 1.33 million US$/year. Another benefit for using dual-fuel engine is the reduced dual fuel costs as presented in Table 5. Figure 14 presents the total annual saving costs after operating the dual fuel installed onboard the ship at different fuel price increments over the remaining ship life cycle. The remaining ship working age is 25 years, assuming the ship life cycle is 28 years. At the end of the ship life cycle, the total annual saving for using dual-fuel engines will be 41.11 million US$/year, assuming 2% fuel price increment.

Fig. 14
figure 14

Total saving costs after operating dual-fuel system for the A19 container ship

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From the previous results, it can be concluded that the contribution percentage of container ships to green gaseous as a function in million tons of CO2 emitted is more than the percentage of seaborne trade as a function of million cargo ton loaded per year. Therefore, as steps toward reducing the gap between both percentages, most companies start to depend on the medium and mega containerized size in order to reduce the ratio of fuel consumption per ton transported. The main driver for ordering bigger vessels is to reduce the energy needed to transport each individual container. More energy efficiency lowers costs and helps to minimize CO2 emissions, which improves profitability and reduces the environmental impact of global supply chains (Knowler 2019; Martin et al. 2015). The previous objective was achieved through the modification of container ship design to increase the loading area. The new-built container ship TEU capacity showed a continuous increase throughout the last decades, as illustrated in Fig. 15 (Dragan et al. 2014; Pani et al. 2015; UNCTAD 2011, 2019). It can be noticed that, in the last 10 years, there was a dramatic increase in TEU with 60%, which reached to 230,000 in last delivery order 2020, by optimization of ship length-to-beam ratio which could be reached to 6.514. This agrees with the results obtained from the current study. The improvement percent in EEDI and ship emissions can be achieved by increasing container ship breadth and capacity.

Fig. 15
figure 15

Development of container ships during 1980–2020

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Finally, there are pre-emptive steps which can be carried out by shipbuilders and owners to reach the best levels of environmental and economic operations for container ships. The results from the current study showed that there is a clear difference between the outcomes of the two selected container ship classes A7 and A19. On the other hand, there are general aspects which can be concluded for the other classes of containerships. One of the most important aspects is the relationship between the number of containers and the ship length-to-breadth ratio. The study indicated that the reduction in ship length-to-breadth ratio by 0.1 can increase the number of transported containers from 1000 to 1200. Moreover, all of containership operators are planning to comply with IMO emission regulations through reducing the operational ship speed. The latest generations of container ships are sailing at moderate speeds of 21 to 23 knots to be eco-friendly.

Conclusions

The environmental and economic analysis of the different emission control methods with emphasis on the improved Energy Efficiency Design Index (EEDI) for A19 and A7 class container ships was discussed. Three different options are considered including natural gas, treatment equipment, and ship speed reduction. The calculated EEDI value is compared with the mandatory IMO values and its phases. The main conclusions from the current study are:

  • From environmental point of view, the lowest annual SOx and CO2 emission rates per transported cargo are achieved by A19 container ship when operated with dual-fuel engine. These rates are 0.93 and 1.8 kg/TEU, respectively. The lowest NOx emission is 18.9 kg/TEU emitted from A19 class container ship operated by ULSHFO and after using SCR system. The calculated energy efficiency for A19 and A7 class container ships is 3.94 and 16.3 gCO2/ton-NM, respectively. In order to comply with the mandatory EEDI values set by IMO for A7 class container ship, its speed should be reduced by 22.5%. This will improve its EEDI value by 33.5%. The improvement percentages in the calculated EEDI values due to using shaft generator, waste heat recovery system, and dual-fuel engines are 25.5%, 7.5%, and 10.1%, respectively for A19 class container ship. In addition, the lower the values of the length-to-beam ratio and the operational index (EEOI) for the case study, the more improved the EEDI value.

  • From economic and cost-effectiveness points of view, the highest annual cost of fuel consumption is 39.61 million US$ for the A7 class container ship operated with ULSHFO. The average total annual costs for the installed SCR and the combined (SCR+SWS) system onboard A19 and A7 class container ships are 2.9 $/kW and 62.5 $/kW, respectively. Reducing A7 class container ship speed by 22.5% (from the design speed) will save fuel costs by 87.7 million dollars with CO2 cost-effectiveness of 52.54 $/ton CO2, at the end of ship life cycle. On the other hand, using the installed dual-fuel engine infrastructure onboard A19 container ship will save annual SCR operating and maintenance costs of 1.33 million US$ with total annual savings of 41.11 million US$, at the end of the ship life cycle and assuming 2% fuel price increment.

  • Finally, the main engine power, ship speed, and ship length-to-beam ratio, as main characteristics of container ship design, need to be optimized for new ships in order to improve the profitability and reduce the environmental impact of the global container supply chain.