Third IMO Greenhouse Gas Study 2014

166 Third IMO GHG Study 2014
Figure 7: Characterization of ship phases used in the extrapolation algorithm for an example
very large crude carrier (VLCC) in 2012. The top plot shows the phase labels for each data point
at given speeds (y-axis) and the lower plot classifies the data into high
and low standard deviation of speed within a six-hour window
The process of extrapolation follows the steps below:
• Speeds greater than 1.5 times the design speed of the ship are removed.
• Each hour where an activity report exists is classified as one of the phases indicated above.
• The activity data set is split by port activity, resulting in a sequence of discrete journeys.
• An acceptable missing period threshold is calculated as the median port-to-port time bounded by 6
and 72 hours.
• Where the contiguous missing periods are less than the missing period threshold, the intervening
hours are randomly sampled from the set of reported speeds for that phase.
• Where the missing periods are greater than the missing period threshold, the whole voyage to which
the contiguous missing periods belongs is stripped out and replaced with randomly sampled speeds
from the full set of reported speeds.
• A reliability indicator is applied to each data point. Data points that are based on actual reports and
those classified in step 4 are set as 1 and those sampled in step 5 are set as 0.
Naturally, the accuracy of the extrapolation would be improved by leveraging the ship location information.
However, as discussed in earlier sections, the location information was removed at the pre-processing phase.
An example of the extrapolation process is displayed in Figure 8. The first column displays a snapshot of
the speed time series for an example ship, followed by speed distribution for days at port and days at sea
respectively. The final column displays the histogram plot for the speed in each state. The first row displays
the raw data with the speed forward filled from the last activity report. The bar plots and the histogram are
based on the combined data set. The middle row displays only those data points for which there was an
activity report. The final row displays the extrapolated speed indicated by a reliability indicator. The plot
labels indicate the respective captured points (i.e. there are a total of 8,785 points in the year, of which 2,245
contained actual activity reports. Following application of the extrapolation algorithm, 7,170 were classified
as having a reliability of 1).
In the Figure 8 example, there were many activity reports missing in August; the contiguous missing period was
below the acceptable missing period threshold resulting in those missing data points being resampled from
in-phase activity reports. However, for the period from 17 July to 31 July, the missing data points were beyond
the acceptable missing period threshold and thus the speed was sampled from the full activity report data set.
This results in data points being selected from across all three phases and the resulting data points appearing