Textos de Apoio (pdf)

Case 1: Suezmax tanker & Capesize bulk carrier
In this case, a typical Suezmax tanker might be
specified with a six-cylinder Wärtsilä RT-flex68-D
main engine. However, if a seven-cylinder engine is
employed instead, the daily fuel consumption can be
reduced by some 3.4 per cent.
In the engine/propeller layout for this ship as
shown in figure 6, the CMCR points for the two
alternative engines are on the same rating line
(α = 0.3) through a common design point for the
same ship service speed (knots).
The calculation of annual fuel costs given in table
2 is based on 6000 hours running with heavy fuel oil
costing US$ 500 per tonne.
The resulting payback time for the extra cost
associated with the additional engine cylinder is
estimated to be between 3.5 and six years depending
on the bunker price of US$ 600–400 per tonne
respectively (Fig. 7). The calculations of the payback
are based on an interest rate of eight per cent.
A similar case may be made for a Capesize bulk
carrier as it would be similar in size and speed to a
Suezmax tanker and would thus require a similar
engine.
Table 1: Typical ship parameters for a Suezmax tanker
Length overall: about 274 m
Beam: 46–50 m
Design draught: 16 m
Scantling draught: 17 m
Sea margin: 15 %
Engine service load: 90 %
Table 2: Main engine options
Alternative engines: 6RT-flex68-D 7RT-flex68-D
Cylinder bore, mm: 680 680
Piston stroke, mm: 2720 2720
Stroke/bore ratio: 4:1 4:1
MCR, kW / rpm: 18,780/95 21,910/95
CMCR, kW / rpm: 18,780/95 18,460/89.7
BMEP at CMCR, bar: 20.0 17.9
CSR at 90% CMCR, kW/rpm: 16,902/91.7 16,614/86.6
BSFC at CMCR, g/kWh:
– 100% load: 169.0 164.8
– 90% load: 165.6 162.6
Daily fuel consumption, tonnes/day:
– ISO fuel, LCV 42.7 MJ/kg: 67.2 64.8
– LCV 40.5 MJ/kg: 70.8 68.4
– As percentage, %: 100 96.6 –3.4%
Annual fuel costs, US$: 8,853,000 8,544,000
Fuel saving, US$: 0 – 309,000
Engine length, mm: 8690 9870
Engine mass, tonnes: 472 533
— 4 — © Wärtsilä Corporation, June 2008