MnrAq

Inventories of CO2 emissions from international shipping 2007–2012 53
Figure 40: Variability within ship size categories in the tanker fleet (2012). Size category 1
is the smallest oil tanker (0–9,999 dwt) and size category 8 is the largest (200,000+ dwt)
1.3.4 Variability between ships of a similar type and size and the impact of slow steaming
The bottom-up method calculates ship type totals by summing the calculations for each individual ship
identified as in service in the IHSF database. This study therefore supersedes the Second IMO GHG Study
2009 in providing insight into individual ships within fleets of similar ships. To illustrate this, Figures 38–40
display the statistics for the bulk carrier, container ship and tanker fleets. The plots represent each ship type’s
population by ship size category (on the x-axis). The box plots convey the average ship (red line in the middle
of the box), the interquartile range (between the 25th and 75th percentile of the population) and the 2nd
to 98th percentile range (the extremes of the “whiskers”). Tabular data characterizing each ship type and
size category studied are included in Annex 2.The average sailing speed in 2012 of container ships in size
categories 4–7 (3,000 TEU to 14,500 TEU capacity) is between 16 knots and 16.3 knots (Figure 39). The
interquartile range of sailing speed is approximately 1 knot to 2 knots, depending on the size. This shows little
variability in operating speed across the sector (nearly 2,000 ships). The average speed of ships in those four
size categories varies between 24 knots and 29 knots. Therefore the sailing speed plot also shows the extent
to which ships are slow steaming in 2012. The ratio of operating speed to design speed (here approximated as
the IHSF reference speed) can be seen in the bottom left-hand plot (Figure 39), showing that larger ships (bin
8 in Figure 39) are on average operating at between 55% and 65% of their design speed. Although they have
lower design speeds than the larger ships, in ratio terms the smaller container ships (sizes 1 and 2) are slow
steaming less than the larger ships.
The top left of the plots portrays the estimated total annual main engine fuel consumption. In this instance
there is a comparatively higher variability within the population than observed for sailing speed. Some of this
is due to the variability in ship technical specifications (hull form, installed power and design speed). There
is also variability in the total fuel consumption because of variability in the number of sailing days in a year
(bottom right-hand plot). Holding all else equal, an increase in days at sea will increase total annual mainengine
fuel consumption by the same percentage.