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A December report by the International Council on Clean Transportation (ICCT) says global bunker consumption was 266.275 million metric tonnes in 2015 - lower than the 298 million metric tonnes documented in an ICCT study published in October.

According to the latest report, entitled 'Black carbon emissions and fuel use in global shipping 2015', the world's shipping fleet consumed 210 metric tonnes of residual fuel in 2015 and 50 million tonnes of distillates, whilst around 6 million tonnes of liquefied natural gas (LNG) is said to have been utilized by ships as fuel.

Residual fuel consumption represents 79 percent of fuel use by vessels; distillates represent around 19 percent; and LNG makes up the rest - around 2 percent.

In the previous October report, named 'Greenhouse gas emissions from global shipping, 2013-15', total shipping fuel consumption was calculated to have increased by 2.4 percent, from 291 million tonnes to 298 million tonnes, between 2013 and 2015.

Calculating consumption

In the December-published study, fuel consumption was estimated on a ship-by-ship basis based on the amount of carbon dioxide (CO2) that each ship emitted and its main fuel type.

Marine fuels emit varying amounts of CO2 when burned; this is called the CO2 intensity of the fuel and is reported in units of g CO2/g fuel. CO2 intensity was calculated to be 3.114 for residual fuel, 3.206 for distillates, and 2.75 for both LNG and gas boil-off.

Fuel carriage is calculated in the report using its main fuel type capacity (cubic metres) as derived from the IHS ShipData database and the assumed density of the fuel.

When estimating the amount of fuel on board each vessel, the study assumes that each ship's fuel tanks are 65 percent full at all times, consistent with Det Norske Veritas (Det Norske Veritas, 2013).

The study uses a density of 0.985 tonnes per cubic metre (t/cbm) for residual fuel, 0.860 for distillates, and 0.456 for LNG and gas boil-off.

In the case of LNG, it is assumed that gas boil-off is the same density as LNG because the fuel source for gas boil-off is LNG until it is converted to compressed natural gas.