Third IMO Greenhouse Gas Study 2014

176 Third IMO GHG Study 2014
The model can perform calculations for ships only for which there are both activity and IHSF technical data
available; these are referred to as “matched ships”. Procedures for estimating the fuel demands and emissions
of ships that are not matched are described in the section on fleet estimation.
Table 10 – Description of bottom-up model subroutines and calculation stages
Subroutine
Read_fleet
Read_status
Emissions_in
Type_size_match
EF_match
Active_calcs
Power_at_op
Emissions_at_op
Assemble
Output
Description
Reads in and formats data from the database structure containing ship technical characteristics
Reads in and formats data from the database structure containing ship quarterly status definition
Reads in the emissions factor data for all engine types, fuel types and emissions species
Reads in additional assumptions characterizing aggregate ship type and size fleets
For each matched ship, looks up the machinery specification to identify the appropriate emissions
factors from Emissions_in
For each matched ship, uses the data describing hourly observations of a ship’s activity in a series of
subroutines to estimate hourly power demands, fuel consumption and emissions
Calculates the power demanded from main engine, auxiliary engine and boiler for each hour of
observed and extrapolated activity
Calculates the fuel consumed and emissions (nine species) for each hour of observed and
extrapolated activity
Calculates a series of annual and quarterly statistics to characterize activity, power, fuel use and
emissions for each matched ship
Structures and writes the databases produced in Assemble for producing aggregate statistics,
performing QA/QC, and undertaking uncertainty analysis
Algorithms for reading in and formatting input databases do not manipulate the data and therefore are not
described in greater detail here. However, there are a number of subroutines that perform operations on the
activity data, technical data or both, and for transparency the method used in those steps is described in
greater detail below.
Powering subroutine: Power_at_op
This subroutine estimates the main, auxiliary and boiler power output in a given hour of operation. The main
engine’s power output is dominated by the propulsion requirements of the ship, which in turn is dominated
by the operation (speed, draught) and condition (hull condition, environmental conditions). The auxiliary and
boiler power demands are a function of service loads (including cargo operations), and vary depending on
the cargo carried, the operation of the main machinery and the mode of operation (e.g. whether the ship is at
berth, at anchor, at sea, etc.).
Key assumptions
Some ships have shaft generators, which produce electrical power for auxiliary systems from the propeller
shaft. This represents main engine power output that would be additional to the propulsion power demand
and would be expected to reduce the power output of the auxiliary machinery. There is no data in the IHSF
database that could be used to reliably determine whether a ship is equipped with a shaft generator, and so an
assumption was applied that for all ships, only the main engine produces propulsion power and only auxiliary
engines produce service power. This assumption should not significantly impact the total power produced,
but because main engines/shaft generators and auxiliary engines have different specific fuel consumptions
and emissions factors, there will be an effect on these calculations which is discussed in greater detail in
Sections 1.5 and 2.5.
A number of ships recover energy from waste heat (either exhaust, jacket waste heat or cooling water waste
heat). This recovered energy can be used to provide both propulsion and service power supply, which reduces
the power demands on the main engine, auxiliary engines and boiler to produce a given level of performance/
service. The assumption applied for these calculations is that the majority of these reductions occur in the
auxiliary and boiler systems, and that any reductions in their power demands are already factored in to the
empirically derived power outputs. For the small number of ships that use waste-heat recovered energy for
propulsion, this will be misrepresented by the model as written. The consequence can be observed in the
discussion on quality of the bottom-up model in Section 1.4.