Third IMO Greenhouse Gas Study 2014

32 Third IMO GHG Study 2014
1.2 Bottom-up CO 2 inventory calculation method
The bottom-up method derives estimates of emissions from data sources describing shipping activity. The
primary source of vessel activity used is the AIS data, which describe, among other things, a ship’s identity,
position, speed and draught at a given time-stamp. The data are transmitted by the ship with a broadcast
frequency of one message every six seconds. The data are received by shore-based stations, satellites and
other ships and the consortium acquired access to a number of shore-based station and satellite receiver
archives. These were used to build time histories of shipping activity, which could be used, in conjunction
with ship specifications, to calculate the time histories of fuel consumption and emissions. Calculations were
carried out for every individual ship identified as in service in the IHSF database and for every hour of the year.
1.2.1 Overall bottom-up approach
The bottom-up method is split into two stages:
1 initial estimation of observed per-ship activity, energy consumption and emissions;
2 estimation of per-ship activity and associated energy consumption and emissions for ships not
observed in the AIS database.
The first stage is performed only on ships that appear coincidentally in both the IHSF and AIS databases. The
second stage is performed for all ships listed as “in service/commission” within the IHSF database and uses
estimated activity for similar ships in stage 1, in combination with IHSF technical specifications to estimate
power requirements, fuel consumption and emissions. The total energy consumption and emissions for a fleet
of similar ships is then found by summing the calculations for each ship, estimated either at stage 1 or stage 2.
The total shipping emissions are then found by summing across all ship type and size categories. International
shipping emissions are estimated by defining which ship type and size categories are involved in international
shipping.
Figure 23 is a diagram of the flow of data through the processes and calculation stages that make up the
bottom-up method.
Figure 23: Data assembly and method for Sections 1.2 and 2.2