Ecological Transport Information Tool for Worldwide ... - Schenker

Page 94
IFEU Heidelberg, Öko-Institut, IVE, RMCON
6.3.1.2 The modelling of reduced vessel speed in EcoTransIT World
The default emission factors in EcoTransIT World are based on 4 % reduced vessel
speeds and 80 % main engine load. The design speed of a vessel, which is reached at
approximately 90 % main engine load, is considered 100 % or full speed. In the expert
mode, users may alter the service speed of the vessels up to 30 % below design
speed. This feature reflects the option of lowing greenhouse gas emissions effectively
by lowering the vessel speed (slow steaming). The effect is due to the potentiated decline
of required engine power in relation to reductions in vessel speed. Speeds below
30-40 % below design speed are not recommended by OEM for a longer period of time
without adjusting engine and lubrication parameters.
The propeller rotation of a vessel is not converted at as one to one ratio into forward
movement. Due to several factors of resistance, the propeller slips in the water, thus
increasing the power demand on the propeller shaft. Resisting forces that counter the
forward pulling force include wind, wave, friction and eddy resistance. All but the friction
resistance increase over-proportionally with increased vessel speed /MAN 2006/
The slip effect of a propeller in relation to the speed (or revolutions of the propeller) is
described in the propeller law. The propeller law states that the resistance with higher
ship speeds is proportional to the square of the vessel’s speed (R = c * V 2 with c = a
constant). The necessary power requirement is related to the ship resistance R and its
speed V. Thus the effect of vessel speed on required propulsion power is in proportional
to the power of 3.
P = R × V = c×
V
3
MAN /2006/ states that with high speed vessels such as container ships, empirical data
indicates a relationship to the power of 4.5 and for bulk carriers of 3.5. However, the
effects of slow steaming are countered by hull and propeller fouling as well as ocean
currents and heavy weather conditions. Thus, for EcoTransIT World the power relation
of 3 was chosen for all vessels as a more conservative approach.
Since the speed reduction only affects the main engine during the voyage at sea the
theoretical standard year is used as a baseline. The user may choose a reduction in
speed in percent of the average cruise speed of this vessel category. The engine load
and respectively the emissions are reduced by applying the propeller law. For bulk
vessels the formula is:
MEloi = ( Vi / Vn)
3
With MEloi as the main engine load on trip (i), Vi the reduced speed on trip (i) and Vn
the average cruise speed in this class.
At the same time the cargo carrying capacity is linearly reduced and the operating
hours of auxiliary engines at sea are linearly increased to the percent of the speed reduction
during the sea voyage. The time in port remains unchanged. Thus, starting
from the standard year, the theoretical time for carrying a given amount of cargo increases.
The linear reduction in vessel carrying capacity during the sea voyage is recognized
in the calculation. As a consequence, behind the aggregated emission factor
for a vessel, normalized to one tonne-kilometre are three separate emission factors
EcoTransIT World: Methodology and Data – July 15 th , 2010