To assess the use of scrubbers, two different perspectives were analysed with respect to costs and environmental damage:
4.1. Calculating payback time after scrubber installation
The Ship Traffic Emissions Assessment Model STEAM [28 and references therein], version 4.3.0, was used to estimate ship specific annual energy and main engine load, fuel consumption, amount of scrubber discharge water, amount of energy consumed for scrubber usage and kilometres travelled in different sea areas. The data was provided for each individual ship using Automatic Identification System (AIS), mandatory for ships > 300GT [39], between 2014–2022. These data were provided by Orbcomm Ltd and included position reports from both terrestrial and satellite AIS networks. Technical description of the global fleet, which enables STEAM modelling at vessel level, were obtained from SP Global. From all data, those ships that had registered a certificate of approval of scrubber installation within the timeframe (2014–2022) were selected for further analysis (maximum of 3922 ships in 2022). STEAM identifies ships based on IMO numbers, registry numbers that remain with the vessel from construction to scrapping, and MMSI codes, which is the Maritime Mobile Service Identity number of the ship's radio system, but the output data was anonymized by creating an artificial but unique Id-number for each ship.
Annual balance was calculated for each unique ship by accounting for investment cost, as starting conditions, and annual operational costs and monetary savings on fuels from using HFO instead of MGO or VLSFO (Fig. 6). Each ship was modelled from the date of installation until the end of 2022 (see example of ship with open loop scrubber in Fig. 6). The date of installation was given as year and months in STEAM based on the ship specific class certificate letter stating the date of approval to operate the scrubber.
The investment cost per kilowatt (€2019/kW, Fig. 4A) for scrubber systems was collected from literature (e.g. [40–46] and see detailed description in Supplementary Information B Table S.1) where the median (50th ), 5th and 95th percentiles were used in the different scenarios (Table 1 and Supplementary Information B Table S.1). Due to limited data availability, the hybrid systems were assigned the same investment cost as closed loop systems. Due to the variability in price connected to installed engine power, the ships and the cost were divided into three size categories based on total installed main engine power (Fig. 6). The total installed main engine power of the specific ships in the scrubber fleet were determined from SP Global ship database where power-regression equations based on a selection of 110000 ships (65000 excl. fishing vessels, tugs and service vessels) in different ship categories were used to calculate the engine power from the ship category and gross tonnage (derivations found in Supplementary Information C). Due to poor data fit, statistical data binning was used instead of power-regression for container ships and RoRo vessels. The total investment cost per ship is summarised in Supplementary Information B Fig. S.2 and Table S.3.
The operational costs were estimated from literature (e.g. [46–50] and see detailed description in Supplementary Information B Table S.1) and calculated for each ship based on annual main engine power output associated to scrubber-use from STEAM. For the hybrid systems, the fraction of power used in open (fracOL) versus closed (fracCL=1- fracOL) loop mode was calculated from the annual discharges of open and closed loop water according to Eq. (1).
$$\begin{array}{c}fra{\text{c}}_{\text{O}\text{L}}=\frac{\frac{{\text{V}}_{\text{O}\text{L}}}{{\text{Q}}_{\text{O}\text{L}}}}{\left(\frac{{\text{V}}_{\text{O}\text{L}}}{{\text{Q}}_{\text{O}\text{L}}}\right)+\left(\frac{{\text{V}}_{\text{C}\text{L}}}{{\text{Q}}_{\text{C}\text{L}}}\right)}\#\left(1\right)\end{array}$$
where QOL/CL is the discharge flow rate of open (90 m3/MWh) and closed (0.45 m3/MWh) loop systems [9] and VOL/CL are the annual volumes (m3) of open and closed loop water discharged from the specific ships. The annual operational cost of the different scrubber systems was then calculated from the annual engine power usage (MW/yr) during the time when the scrubber was operated (Pscrubber on ) and the power based operational costs (€2019/MW) for open and closed loop scrubbers (costoperation OL/CL) (Eq. (2)).
$$\begin{array}{c}cos{\text{t}}_{\text{o}\text{p}\text{e}\text{r}\text{a}\text{t}\text{i}\text{o}\text{n}}={\text{P}}_{\text{s}\text{c}\text{r}\text{u}\text{b}\text{b}\text{e}\text{r} \text{o}\text{n}}\left(\text{f}\text{r}\text{a}{\text{c}}_{\text{O}\text{L}}\times \text{c}\text{o}\text{s}{\text{t}}_{\text{o}\text{p}\text{e}\text{r}\text{a}\text{t}\text{i}\text{o}\text{n},\text{O}\text{L}}+\text{f}\text{r}\text{a}{\text{c}}_{\text{C}\text{L}}\times \text{c}\text{o}\text{s}{\text{t}}_{\text{o}\text{p}\text{e}\text{r}\text{a}\text{t}\text{i}\text{o}\text{n},\text{C}\text{L}}\right)\#\left(2\right)\end{array}$$
For the open loop scrubbers, fracOL=1 and for the closed loop scrubbers, fracCL=1.
The daily resolution of fuel price (from 2014–2022) of HFO, MGO and VLSFO (from 2019) was received from Ship & Bunker (Supplementary Information B Fig. S.1). The Global 20 Ports Average bunker prices were used which cover the 20 major global bunker ports and represent approximately 60–65% of absolute global bunker volumes. In the different scenarios (Table 1), the annual median (50th ) and the 5th and 95th percentiles of the fuel price difference between HFO/MGO and HFO/VLSFO was used when calculating annual balance (Fig. 4B). VLSFO was introduced to the market in late 2019 and from 2020, it was assumed that the alternative fuel to HFO and scrubbers are MGO in SECA and VLSFO outside SECA (Eq. (3)). Prior to the introduction of VLSFO, it is assumed that distillates were the only alternative to the use of scrubbers and the fuel price difference between MGO and HFO is applied. The annual monetary gain (Δcostfuel,yr in €2019/year) attributed to the use of HFO instead of low sulphur fuels are calculated from the fuel consumption (cons.HFO,yr in tonnes fuel/yr) and fuel price difference (Δprice in €2019/tonnes fuel) for the individual years (Eq. (3)).
$$\begin{array}{c}\varDelta cos{\text{t}}_{\text{f}\text{u}\text{e}\text{l},\text{y}\text{r}}=cons{.}_{\text{H}\text{F}\text{O},\text{y}\text{r}}\left({\Delta }\text{p}\text{r}\text{i}\text{c}{\text{e}}_{\text{V}\text{L}\text{S}\text{F}\text{O}-\text{H}\text{F}\text{O},\text{y}\text{r}}\times \frac{{\text{D}}_{\text{n}\text{o}\text{n}\text{S}\text{E}\text{C}\text{A},\text{y}\text{r}}}{{\text{D}}_{\text{t}\text{o}\text{t},\text{y}\text{r}}}\times 0.94+{\Delta }\text{p}\text{r}\text{i}\text{c}{\text{e}}_{\text{M}\text{G}\text{O}-\text{H}\text{F}\text{O},\text{y}\text{r}}\times \frac{{\text{D}}_{\text{S}\text{E}\text{C}\text{A},\text{y}\text{r}}}{{\text{D}}_{\text{t}\text{o}\text{t},\text{y}\text{r}}}\times 0.92\right)(\#3)\end{array}$$
Where DnonSECA/SECA,yr represent the distance travelled in SECA/non-SECA areas and Dtot,yr is the total annual distance sailed according to STEAM data output for each vessel and year (Supplementary Information B Table S.2). Fuel penalties of 2–3% from scrubber operations are the most common estimates [46, 51] and an additional factor of 0.94 (VLSFO) and 0.92 (MGO) is applied due to the fuel penalty of using a scrubber (2%) and the higher energy content, i.e. lower fuel consumption, of the low sulphur fuels [52].
The annual balance for each ship was calculated by summarising the costs (negative signs) and the monetary gain from using HFO instead of low sulphur fuels (positive sign) (equations (4) and (5)). For the first year, i.e. same year as installation, the balance was calculated from the investment cost (costinv.) and the cost of operation (costoperation,yr) and fuel consumption (i.e. monetary gain from using HFO instead of low sulphur fuels (Δcostfuel,yr)) where the two latter were adjusted to the number of months when the scrubber had been in service (Eq. (4)). For the remaining years, until 2022, the annual balance was calculated by summarising the balance from the previous year with the operational cost and the monetary gain on fuel by not switching to low sulphur fuels from the current year (equation ( 5)).
$$\begin{array}{c}balanc{\text{e}}_{\text{y}\text{r}=\text{i}\text{n}\text{s}\text{t}\text{a}\text{l}\text{l}\text{a}\text{t}\text{i}\text{o}\text{n} \text{y}\text{e}\text{a}\text{r}}=cos{\text{t}}_{\text{i}\text{n}\text{v}.}+\left(\text{c}\text{o}\text{s}{\text{t}}_{\text{o}\text{p}\text{e}\text{r}\text{a}\text{t}\text{i}\text{o}\text{n},\text{y}\text{r}}+{\Delta }\text{c}\text{o}\text{s}{\text{t}}_{\text{f}\text{u}\text{e}\text{l},\text{y}\text{r}}\right)\times \frac{12-\text{m}\text{o}\text{n}\text{t}{\text{h}}_{\text{i}\text{n}\text{s}\text{t}.}}{12}\#\left(4\right)\end{array}$$
$$\begin{array}{c}balanc{\text{e}}_{\text{i}\text{n}\text{s}\text{t}\text{a}\text{l}\text{l}\text{a}\text{t}\text{i}\text{o}\text{n} \text{y}\text{e}\text{a}\text{r}<\text{y}\text{r}\le 2022}=balanc{\text{e}}_{\text{y}\text{r}-1}+ cos{\text{t}}_{\text{o}\text{p}\text{e}\text{r}\text{a}\text{t}\text{i}\text{o}\text{n},\text{y}\text{r}}+\varDelta cos{\text{t}}_{\text{f}\text{u}\text{e}\text{l},\text{y}\text{r}}\#\left(5\right)\end{array}$$
To assess the variability of market fluctuations, the balance was estimated from three different calculation scenarios:
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Median balance scenario: Using the median for all costs, i.e. fuel price difference, investment cost and operational cost;
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Min balance scenario: using the 5th percentile in fuel price difference and the 95th percentile of investment and operational cost;
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Max balance scenario: using the 95th percentile in fuel price difference and the 5th percentile of investment and operational cost.
The net surplus of the global fleet was calculated by summarising the balance for every vessel at the end of 2022 (Eq. (6)).
$$\begin{array}{c}net surplu{\text{s}}_{\text{g}\text{l}\text{o}\text{b}\text{a}\text{l} \text{f}\text{l}\text{e}\text{e}\text{t}} = \sum _{\text{s}\text{h}\text{i}\text{p}}\text{b}\text{a}\text{l}\text{a}\text{n}\text{c}{\text{e}}_{2022,\text{s}\text{h}\text{i}\text{p}}\#\left(6\right)\end{array}$$
4.2. Cost of not restricting as damage cost on marine environment
To assess the societal cost of not restricting scrubber water discharge, the dataset was limited to a Baltic Sea case study. The selection of ships were based on their operating area, i.e. distance sailed in the Baltic Sea, the Gulf of Bothnia, Gulf of Finland, Gulf of Riga, Kattegat and Skagerrak (Supplementary Information Table S.2), since installing a scrubber. The HFO consumption and the volumes of scrubber water dishcarged within the Baltic Sea area is estimated from the total annual HFO consumption and the fraction sailed within Baltic Sea, calculated from the distance sailed in the Baltic Sea area divided by the total distance sailed for any given year.
The damage cost calculations were limited to marine ecotoxicity from the discharge of scrubber water, i.e. metals and PAHs in the scrubber water. The calculations were based on previous studies [29, 30] that valuated ecotoxicological impacts from the organotin compound tributyltin (TBT) in Sweden. The damage cost of marine ecotoxicity (in €2019 /kg 1,4-DCB eq, i.e. the toxicity potential expressed as 1,4-dichlorobenzene equivalents [31]) based on Willingness to pay estimates of Swedish households’ amounted to 1.07 €2019 /kg 1,4-DCB Eq. (0.73–1.29 €2019 /kg 1,4-DCB eq). The cumulative toxicity potential (i.e. kg 1,4-DCB eq/m3) of open and closed loop scrubber water was calculated from the concentrations (average and 95 confidence interval) of 9 metals and 10 PAHs and their respective characterization factors from ReCiPe (Supplementary Information B Table S.4). ReCIPe offers a harmonised indicator approach where characterisation factors for organic substances and metals for different envrionmental compartments, including marine waters, have been produced [31].
The annual damage cost for marine ecotoxicity resulting from scrubber discharge water in the Baltic Sea Area (including Skagerrak) was calculated by multiplying the total volume scrubber water discharged in the area with the damage cost of marine ecotoxicity (1.07 €2019 /kg 1,4-DCB Eq. [29]) and the marine toxicity potential (kg 1,4 DCB eq. /m3) of open and closed loop scrubber water (Supplementary Information B Table S.6 and S.7).