In-depth | EEDI
But, naval architects cannot design for
the most economical service speed every day for the life of the ship – there has to be a compromise – so a variation of 1 or 2knots in service speed is not a surprise. Despite
this increase in service
speeds, there is evidence that hull form design has indeed become more efficient. A comprehensive regression analysis by a major ship model basin, in which all the hull parameters affecting the speed-power relationship were included, established that, surprisingly, the model number was a significant variable. Since model numbers are assigned in chronological order, this demonstrated that the more recent models are more efficient, i.e, they result in better speed-power relationships. Kristensen also suggests that owners
could design more efficient ships if they were to stick to sound design principles instead of being driven by economics. This strikes at the very heart of a naval architect’s purpose in life! Surely our aim is to design a ship that will maximise profits (or minimise losses) for our client, while meeting all recognised criteria for safety, structural strength and environmental protection. Taking the design of a typical modern
VLCC as an example: Lbp
Beam Depth
Draught
Cargo cubic Deadweight Service speed
322m 58m 31m 22m
343,000m3
298,000tonnes 16knots
The reasons why these dimensions are almost standardised are many:
• Crude oil is traded and transported in lots of 1 million, or half million,
barrels. A VLCC is designed to carry 2 million barrels (about 320,000m3
).
• Crude oil generally has a density of about 0.81 – 0.85tonnes/m3
This defines the required cargo capacity and hence depth.
. Required
• Many ports and berths have limits on length which are determined by berth
cargo deadweight is therefore about 272,000tonnes in order to carry the heavier crudes.
length, turning basins and other 20 The designer is, therefore, constrained
in the choice of dimensions and unless the VLCC is constructed entirely of very high tensile steel, the lightweight and displacement determine that the block coefficient is in the region of 0.82 - 0.84. Very high tensile steel could be used in order to reduce the lightweight and block coefficient, but it is not favoured because of its larger structural deflections and lower fatigue life than mild steel and moderately higher tensile steel. Given these dimensions and block
coefficient, the naval architect then has to decide how much propulsion power, or service speed, is needed. Nobody can see 20 years into the future to predict
Kristensen’s article in the April edition of The Naval Architect), we can see that the Froude number corresponding to a block coefficient of 0.84 is about 0.148 according to Watson and Gilfillan (the most recent of the three guidelines). For a VLCC with a length of 322m, a Froude number of 0.148 corresponds to a speed of 16.04knots. Far from being driven too fast, VLCC hulls with a service speed of 16knots meet almost exactly the most recent “guidelines”. Turning to ways of reducing the EEDI
of new VLCCs by 10%, Kristensen claims in his April article that a Panamax tanker can maintain its service speed and reduce its propulsion power by over 10% if the length is increased
The Naval Architect May 2012
• VLCC building docks are designed to construct 2 VLCCs side by side,
• The draught limit of 22m is imposed by various ports, channels and canals.
usually limiting the beam of each VLCC to about 58m.
constraints. Some offshore terminals have limits on displacement. An overall length of about 330m is usually the maximum. A tanker which is longer than this will be denied access to certain tanker terminals.
how freight rates will develop, nor predict how costs will change, so speed is usually chosen to be competitive with “the market”, resulting in a speed at the fully loaded draught of about 16knots. Kristensen maintains that common
naval architectural knowledge and guidelines have not been followed in the design of recent VLCCs, but if we look at the “guidelines*” for Froude number as a function of block coefficient (reproduced in Figure 2 of
Figure 2. Block coefficient for Panamax tankers delivered between 1971 and 2010.
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