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54 Third IMO GHG Study 2014
The results for oil tankers show a similar level of variability within a given ship size group, a significant
(although not as significant as container ships) uptake of slow steaming and similarities between the larger ship
types in terms of sailing speeds and days spent at sea.
The bottom-up method also allows the influence of slow steaming to be quantified. Across all ship types and
sizes, the average ratio of operating speed to design speed was 0.85 in 2007 and 0.75 in 2012. This shows
that, in relative terms, ships have slowed down: the widely reported phenomenon of slow steaming that has
occurred since the financial crisis. The consequence of this observed slow steaming is a reduction in daily
fuel of approximately 27% expressed as an average across all ship types and sizes. However, that average
value belies the significant operational changes that have occurred in certain ship type and size categories.
Table 17 describes, for three of the ship types studied, the ratio between slow steaming percentage (average
at-sea operating speed expressed as a percentage of design speed), the average at-sea main engine load factor
(a percentage of the total installed power produced by the main engine) and average at-sea main engine
daily fuel consumption. Many of the larger ship sizes in all three ship type categories are estimated to have
experienced reductions in daily fuel consumption well in excess of the average value of 25%.
The ships with the highest design speeds have adopted the greatest levels of slow steaming (e.g. container
ships are operating at average speeds much lower than their design speeds); there is also widespread adoption
of significant levels of slow steaming in many of the oil tanker size categories. Concurrent with the observed
trend, technical specifications changed for ships. The largest bulk carriers (200,000+ dwt capacity) saw
increases in average size (dwt capacity), as well as increased installed power (from an average of 18.9 MW to
22.2 MW), as a result of a large number of new ships entering the fleet over the time period (the fleet grew
from 102 ships in 2007 to 294 ships in 2012).
A reduction in speed and the associated reduction in fuel consumption do not relate to an equivalent
percentage increase in efficiency, because a greater number of ships (or more days at sea) are required to do
the same amount of transport work. This relationship is discussed in greater detail in Section 3.