Third IMO Greenhouse Gas Study 2014

74 Third IMO GHG Study 2014
Both auxiliary engine comparisons (daily and total quarterly) imply that the bottom-up estimates of auxiliary
fuel consumption are of lower quality than those of the main engine. There are two possible explanations for
this: the low quality of noon report data for auxiliary fuel consumption, or the low quality of bottom-up method
estimates. Both are likely. Auxiliary fuel consumption in the noon report data set is commonly reported as
zero. This could be because:
1 a shaft generator is used;
2 the main and auxiliary power is derived from the same engine (in the case of LNG carriers);
3 the auxiliary fuel consumption is not monitored or reported.
Discussion with the operators from whom the data originated suggested that the second and third explanations
are the most likely.
As described in Section 1.2, the method for auxiliary engine fuel consumption estimation is derived from
samples taken from vessel boardings and averaged for ship type- and size-specific modes (at berth, at anchor
and at sea). This method is used because of the scarcity of data about the installed auxiliary engine in the
IHSF database and the shortage of other information in the public domain describing operational profiles of
auxiliary engines.
Figure 56 presents the comparison between the noon report and the bottom-up method in 2012, but with a
filter applied to include only data for which the AIS-derived activity was deemed reliable for more than 75%
of the time in the quarter. Otherwise, the data source is the same. The marked improvement of the agreement
is demonstration of the reliability of the bottom-up method in converting activity into fuel consumption and
shows that the largest source of uncertainty in the total fuel consumption is the estimate of activity, particularly
the estimate of days at sea.
Figure 57 and Figure 58 present the comparison between the noon report and the bottom-up method for 2007
and 2009 respectively. These quality assurance plots show that, consistent with the comparison of the activity
estimate data to noon report data, quality deteriorates between the earlier years (2007, 2008 and 2009) and
later years (2010, 2011 and 2012). The availability of noon report data in the earlier years is also limited, which
makes rigorous quality assurance difficult. However, even with the sample sizes available, the confidence
bounds clearly indicate that the quality deteriorates.
Table 25 summarizes the findings from the quality assurance analysis of the fuel consumption. Further data
from earlier years can be found in Annex 3.
Table 25 – Summary of the findings on the QA of the bottom-up method estimated fuel consumption
using noon report data
Consumer Quality, as assessed using noon report data Importance to the inventory of fuel
consumption and emissions
Main engine
Good: consistent agreement and close
High (71% of total fuel in 2012)
confidence bounds to the best fit
Auxiliary engine Poor: moderate, with some ships showing good Low (25% of total fuel in 2012)
agreement but many anomalies (very low values
in noon reports)
Boilers Unassessed Very low (3.7% of total fuel in 2012)