Third IMO Greenhouse Gas Study 2014

240 Third IMO GHG Study 2014
b
from the extrapolation of observed activity to estimate the operating parameters when the ship
is not observed.
The first of these is a function of a relative speed between the water and the ground – e.g. tides and currents
– and is therefore a function of the metocean conditions in which the ship is sailing. These conditions cannot
be easily generalized; some ships may spend all their time operating in areas of high tidal flows and current
(typically coastal shipping) and others may spend little time operating in such areas (typically, although not
necessarily, when a ship is in the open ocean). To estimate the variability, we have used operator data supplied
for a fleet of twenty ships (a mixture of bulk carriers and tankers with a variety of ship sizes) for which
measurements of average speed through the water and average speed over ground were available, averaged
over 24 hours. The ships are owned by a variety of companies but managed by the same company and have
consistent data reporting mechanisms. In total they represent approximately 80 ship years of operation and
data. Figure 27 displays the estimate of the probability density function of the difference between speed over
ground and speed through the water. The average difference is -0.14 knots and the standard deviation is 0.95
knots. Implicit in this distribution is the measurement error associated with the speed logs used to obtain the
speed through the water and the GPS used to obtain the speed over ground, but these are assumed to be
negligible relative to the uncertainty in the difference between the two measurements.
Figure 27: Relationship between speed over ground and speed through the water
From the analysis described above in the section “Activity estimates temporal coverage QA/QC”, an estimate
was found for the standard deviation of the uncertainty of speeds during an hour of operation. These values
are:
• For an observed hour, 0.75 kt
• For an unobserved hour, 1.85 kt
Combining these sources of uncertainty, we can estimate the total uncertainty for the two types of observation
(see Table 18).
Draught uncertainty
Draught influences the underwater hull surface area and hull form. It varies during the course of a voyage and
from one voyage to another. The measurement of draught is obtained from the data reported in AIS messages
(see Section 1.2). On some ships, the value is entered manually (from draught mark readings or a loading
computer), and on others it is reported from sensors. As the value is entered manually and rarely audited
for quality, it is possible that spurious or null returns may be observed in the raw data. For the purposes of
estimating the uncertainty of this parameter, the comparison between the noon report and the reported AIS