In-depth | EEDI
FEATURE THEME
Design constraints limit options for EEDI compliance
Te debate at IMO concerning the Energy Efficiency Design Index (EEDI) for tankers and bulkers continues to foment. Over the last two months the Danish position has been defined by the Danish Technical University (DTU); this month Michael Osborne, a RINA member, offers an opposing point of view.
T
he articles “Back to basics” by Hans Kristensen in the March and April editions of The
Naval Architect contain an interesting analysis of the trends in the design of tankers and bulk carriers over the last 20 years. The contention in the articles is that the hydrodynamic design of large tankers and bulk carriers, as measured by EEDI developed within the IMO, has become less efficient over this period. Kristensen further maintains that large tankers and bulk carriers could improve their hydrodynamic design, and EEDI, simply by increasing the ratio Length/3√(Volume of displacement) (slenderness ratio). The first point to make is that the
EEDI was not developed as a measure of hydrodynamic efficiency, which it certainly is not. Naval architects think of hydrodynamic efficiency in terms of maximum speed through the water for
minimum shaft power, or minimum fuel consumption and thereby minimum cost. The relationship between speed through the water and shaft power is a complex one. The required shaft power can be a function of between approximately the second and fourth power of the speed through the water. The EEDI, on the other hand, is a measure of CO2
emitted per tonne
mile of cargo carried. Its formulation includes engine power divided by speed, both to the power of one, so it cannot possibly represent a measure of hydrodynamic efficiency. Kristensen has carried out
a
retrospective analysis of EEDI values for tankers and bulk carriers over the last 20 years and is surprised to find that they have increased. He should not be surprised. The simple reason is that service speeds have increased over this period and since the required shaft
Figure 1.VLCC Spot Rates: source Richardson Lawrie.
Michael Osborne, is a retired naval architect and was Chief Naval Architect for a major oil company.
power increases at a much greater rate than speed, and the formulation for the EEDI contains power/speed, the trend in EEDI values is obviously upwards. But why have service speeds
increased? Basic economics dictate that the greater the value of the cargo, and the higher the freight rate, the higher is the most economical ship speed. Figure 1 shows the enormous variation in freight rates for VLCCs over the last 20 years and the most economical VLCC service speed would probably show a similar variation.
18 The Naval Architect May 2012
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