Introduction of method

The calculation of emissions for the SEA mode has been hampered by lack of data on consensus on methods. However, the IMO has presented data that connects installed engine power of a ship, of a specific type, to its size. These methods are also e used by the Clean Shipping index. Under assumptions on what fraction of the installed power that is normally used, the IMO data gives a way to calculate the CO2 emission per tonne-km for different ship types as a function of the ships’ deadweight (dwt). This method calculates the emissions with the inherent assumption that the dwt represents the cargo. However, in reality the maximum cargo capacity, the payload, is lower than the dwt. Further, the load capacity utilization (LCU) should be considered. For containers and trailers one should also consider the fill-factor for these. Finally, the fuel consumption of a specific ship (in mass of fuel per distance) will depend on the load since a more heavily loaded vessel will lie deeper and thus experience higher resistance by the water. Below are the different steps in the calculations described.

IMO data

The IMO data are presented as regression of data for installed engine power for a large number of ships to get specific functions with dwt as the parameter. This can be expressed as CO2 emissions per dwt-tonne and nautical mile as:

3.114 (0.190 SPME + 0.210 SPAE)/(dwt vref)

Where the factor 3.114 comes from the mass CO2 per mass of fuel used, the factors 0.190 and 0.210 comes from the specific fuel consumption (in kg/kWh) assumed for main engines and auxiliary engines, respectively. PME and PAE are the used power for the main and auxiliary engines. For the main engines this is calculated as 75% of the installed power (MCR). PAE is calculated as 2.5% of the main engine MCR+ 250 kW if MCR > 10 MW, or as 5% of main engine MCR if MCR < 10 MW. vref is the ships’ speed at full load (at dwt) and an engine power of 75% of MCR. vref is based on each ship´s design speed. As we since 2008 have seen a development towards slower speed (slow steam and super slow steam), the model is likely to exaggerate fuel consumption where the ship sail at lower speed than the design speed. NTM currently investigates how this correlation can match reality better.

The equation gives the value of emitted CO2 per dwt-tonne and nautical mile. To convert from nautical mile to km the value should be divided by 1.852.

The regression of the data for a number of ship types to the following type of function:

a * dwt-c

The values of the parameters a and c for the different ship types considered are given in table below (where a is recalculated to kg CO2/dwt -km).

Function parameter a and c for different ship types (MEPC 62/24/Add1, MEPC 59/2/23)

The values in above table are expected to be updated in the future. The values for tanker will be given corrections to reflect different densities of the load. The values for Ro-Ro and Vehicle carriers have not been adopted by IMO but are from a working paper. The fleet of Ro-Ro ships show a large spread in data reflecting a relatively large difference in fuel consumption between different Ro-Ro ships as related to transport work. Values for Ro-Ro ships should therefore be used with care. Data for passenger ships are missing.

The function above can be used to calculate the CO2 emissions from the ship per km (ED) as

ED(kg CO2/km) = a * dwt1-c, where ED is a function of dwt and ship type and applies for a fully loaded ship.

The ED can be expressed as mass of fuel rather than CO2 by dividing by 3.114. To get ED in heat content, multiply the latter by 0.041 GJ/kg fuel. Thus

ED(GJ/km) = 0.041/3.114 a dwt1-c.

Payload

The correlation between payload (in tonnes) and dwt, PDR, has been investigated for a number of ship types within the Clean Shipping Index. The resulting ratios are given in table below for a number of ship types.

The relationship between payload and deadweight for different ship types (Clean Shipping Index)

Gas carriers are assumed to have the same payload to dwt ratio as tankers.

Load capacity utilization

General load factors for ships are hard to find. Here we rely on load factors published by IMO. Although the origin of these values is unclear we choose to use them until more reliable figures are available. Table 3 gives the average load capacity utilization, LCU, for different types of ships.

Load capacity utilisation (IMO 2nd GHG study

In order to calculate mass the IMO study uses 7 tonnes per container, 2 tonnes per lanemetre and 1.5 tonnes per car equivalent unit (CEU). Note that for containers one should also consider the utilisation of the actual container (not included in the value in above table).

Fuel consumption as a function of load

There is not much data available on this issue. In principle the fuel consumption as a function of load for a specific ship is governed by the Admiralty coefficient

Const. = v3 D2/3 /P,

where v is the speed, D the displacement and P the engine power. D can be written as a linear function of the load and thus of LCU. Since ED is assumed to be proportional to P, it will be proportional to the displacement to the power of 2/3 when the speed is constant.

We here define the function F which is ratio the fuel consumption (per km) at a given load and the fuel consumption at full load which can be written as

F (LCU) = (d + e*LCU)2/3.

The IMO GHG study gives emissions in g/tonne-km for the ship types in table on capacity utilization in wiki load capacity utilization both at the utilization factor given in the table and at full load. From these two points d and e can be obtained giving a function that represent the relative decrease in fuel consumption per km with the load lower than full down to the utilisation factor.

The function F will be unity for a full ship. The parameters d and e for different ship types can be found in Table below. For ships where the cargo is not primarily characterised by mass no functions were obtained. For these cases F should be set to 1. The latter applies to Reefers, Container ships, Ro-Ro ships and Vehicle carriers.

Parameters giving fuel consumption per km as a function of load (adapted from IMO 2nd GHG study).

Calculation of fuel CO2 emissions

To get the CO2 emissions for a ship with a given dwt and capacity utilization, LCU, in kg CO2/tonne-km, the following equation should be used:

EI ship (dwt, LCU) = (a*dwt-c F (LCU) / (PDR ship LCU).

To get the emission of CO2, ECO2, for a specific transport with cargo mass M and travelled distance d, the following equation should be used:

E CO2  = EI M d

To get the fuel consumption the value for CO2 should be divided by 3.114 kgCO2/kg fuel and to get the heat conversion the fuel value should be multiplied by 0.041 GJ/kg fuel.

Emission factors

Emission factors will here be given in mass of emission per mass of consumed fuel. The basic emission factors used (from Cooper and Gustafsson) are given per type of engine (slow speed, medium speed, high speed, gas turbine, steam turbine) and fuel (residual oil, marine distillates). Here we need emission factors per ship type and fuel type. The distribution on engine types for different ship types are therefore obtained from Entec 2002 and emission factors per ship type and fuel type are calculated.

Engine type distribution for different ship types.

Sea cargo baselines

These baselines are based on data available by NTM, literature reviews and expert interviews. The settings aim to reflect on general sea transport. In general we recommend use of real data if available at sufficient quality. Proxio default data are conservatively assessed and should if used thereby not enable low emission calculations. Furthermore they should promote transport suppliers to present their real data in relation to Proxio default data, hence being used in the transport service procurement process. Providing default data for sea cargo transport will by definition entail elements of incorrect emissions and energy use assessment. The main reason is that ships more or less are individual units with its specific performance regarding emission and energy use. The below data is based on Proxio Emissions and includes the original design based data where modifications regarding speed reductions carried out are not taken into consideration. In order to compensate for this fact the IMO assessment on these actions has been added to the data as fuel consumption reductions. In this respect the user has a choice of calculating conservatively (using design data) or using adjusted numbers suggested in the table. For a more specific calculation it should be carried out based on real data from supplier or actual settings of ships used in Proxio Emissions.

References

Clean Shipping Index, Guidance Document, 2010

Cooper and Gustafsson, D. Cooper and T. Gustafsson. Methodology for calculating emissions from ships: 1. Update of emission factors. SMED 2004.

Entec, C Whall et al., Quantification of emissions from ships associated with ship movements between ports in the European Community, Entec, European Commission, 2002

IMO MEPC 62/24/Add1,

IMO MEPC 59/2/23

IMO GHG-WG 2/2/7, 2009

IMO Second GHG study, MEPC 59/INF.10

REDUCTION OF GHG EMISSIONS FROM SHIPS, IMO GHG Study 2020

Clean Cargo working group assessments