Wartsila SP Tech 2008 Derating

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Derating: a solution forhigh fuel savings and lower emissions

Rudolf Wettstein1 & David Brown2-

Wrtsil Switzerland Ltd, Winterthur-

Summaryis paper sets out ways to achieve worthwhile reductions in the fuel consumption of Wrtsil low-speed engines

when designing newbuildings. e key approach is to use the flexibility offered by the full power/speed layout field toselect a better layout point at a derated power with a lower BSFC and also possibly a higher propeller efficiency.

Engine power, %R1

Engine speed, %R1

100

90

100

80

70

60908070

R1

R2

R3

R4

0-1-2-3

-4

-5

-6

-7

BSFC

g/kWh

Higher propulsive

efficiency

Lower

specific

fuel

consumption

Con

stantt

orqu

elin

e

Rx

Fig. 1: Typical layout field for RTA and RT-flex engines. econtracted maximum continuous rating (CMCR) can beselected at any point, such as Rx, within the layout field. e

BSFC is the reduction in full-load BSFC for any ratingpoint Rx relative to that at the R1 rating.[08#044]

IntroductionFuel efficiency and environmental friendliness arehigh on the list of requirements for ship propulsionengines from todays shipping- and shipbuildingindustries. us Wrtsil is committed to creatingbetter technology in these areas that will benefit boththe customers and the environment.

Yet it is often forgotten by many ship designers

and those specifying low-speed main engines thatadvantage can be taken of the power/speed layoutfield of Wrtsil low-speed engines to select an enginerating point with a still lower fuel consumption.

e concept of the power/speed layout field forlow-speed marine diesel engines originated in the1970s. e layout options were step-by-step wideneduntil, in 1984, our low-speed engines began to beoffered with a broad power/speed layout field. Anengines contracted maximum continuous rating(CMCR) can be selected at any point in the power/speed field defined by the four corner points: R1,R2, R3 and R4 (Fig. 1). e rating point R1 is themaximum continuous rating (MCR) of the engine.

Most recently, the layout fields for certainengines, the RT-flex82C, RTA82C, RT-flex82T andRTA82T, are extended to increased speeds for theR1+ and R2+ points (Fig. 2). e extended fieldsoffer widened flexibility to select the most efficientpropeller speed for lowest daily fuel consumption,and the most economic propulsion equipment,

namely the propeller, shafting, etc.One basic principle of the engine layout field is

that the same maximum cylinder pressure (Pmax)is employed at all CMCR points within the layoutfield. us the reduced brake mean effective pressure(BMEP) obtained at the reduced power outputs in

the field results in an increased ratio of Pmax/BMEPand thus lower brake specific fuel consumption(BSFC).

e other principle behind the layout field is

1- Rudolf Wettstein is Director, Marketing &

Application Development, Ship Power, WrtsilSwitzerland Ltd.

2- David Brown is Manager, Marketing Support,Wrtsil Switzerland Ltd.

1 Wrtsil Corporation, June 2008-


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Engine power, %R1

Engine speed, %R1

R1+

R2+

R3

R4

100

100

90

80

9080

R1

R2

Fig. 2: For the RT-flex82C, RTA82C, RT-flex82T andRTA82T engines the layout fields are extended to the ratingsR1+ and R2+ at the same powers as R1 and R2 respectivelybut with increased shaft speed.[08#049]

Engine power, %R1

Engine speed, %R1

100

90

100

80

70

60

908070

R1

R2

R3

R4

Rx2

Rx1

Rating lineslope =

Fig. 3: For a given ship, a rating line (slope) can be appliedto the layout field so that all rating points on that line would

give the same ship speed with a suitably optimized propeller.Rating points at lower speeds on the rating line requirea larger propeller diameter and give a greater propulsiveefficiency.

Fig. 4: Since the 1980s engine ratings have been selected overa steadily smaller area of the layout field.-[08#051]-

Engine power, %R1

Engine speed, %R1

100

90

100

80

70

60908070

R1

R2

R3

R4

Area of recent

CMCR selection

Area of CMCRselection in

the 1980s

that the lower CMCR speeds allow flexibility inselection of the optimum propeller with consequentbenefits in propulsion efficiency and thus lower fuelconsumption in terms of tonnes per day.

One feature to be borne in mind when selectingthe rating point for the derated engine is the rating

line (Fig. 3). is is the line through a CMCR ratingpoint such that any point on the line representsa new power/speed combination that will givethe same ship speed in knots. e points on therating line all require the same propeller type butwith different adaptations to suit the power/speedcombination. In general, lower speeds of rotationrequire larger propeller diameters and therebyincrease the total propulsive efficiency. Usually theselected propeller speed depends on the maximumpermissible propeller diameter. e maximumdiameter is often determined by operationalrequirements, such as design draught and ballastdraught limitations, as well as class recommendations

concerning propellerhull clearance (pressureimpulse induced by the propeller on the hull).

e slope of the rating line () depends broadlyupon the ship type. It can range from 0.15 fortankers, bulk carriers and general cargo ships up toabout 10,000 tdw to 0.22 for container ships largerthan 3000 TEU and 0.25 for tankers and bulkcarriers larger than 30,000 tdw.

Changing engine selection strategiesWhen the broad layout field was introduced in

RTA engines in 1984 it was widely welcomed byshipowners and shipbuilders. Afterwards RTAengines were frequently selected at ratings in thelower part of the layout field to gain the benefits of



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100

200

300

400

500

2004 2005 2006 2007 2008

Bunker price, US$/tonne380cSt HFO

Fig. 5: Bunker prices have considerably increased in recent times. e chart shows the average price of 380 cSt heavy fuel oil (HFO) from various ports around the world from 2004 to 2008. e green bars indicate the mean price for each year.-

-[08#045]

lower fuel consumption.However, under the pressure of first costs and

softening bunker prices the strategy was changed andthe selected power/speed combination has, duringthe past 15 years or so, been selected to be closer tothe R1 rating (Fig. 4).

Yet, more recently, bunker prices have steadilyclimbed, rising by some 85 per cent in the course of2007 from US$ 270 to US$ 500 per tonne (Fig. 5).e result is that bunkers are now the dominant partof ship operating costs.

Such drastic increases in bunker prices give astrong impetus to reduce fuel costs. ey can also

justify additional investment cost such as selectingan engine with an extra cylinder. e consequentfuel saving may make for an acceptable payback timeon the additional investment cost. It would justifyany efforts to increase the overall efficiency of thecomplete propulsion system.

Further impetus to implementing such changesin engine selection strategy will come from a futureneed to cut CO

2emissions. If a carbon trading

scheme is imposed on shipping it would give furthereconomic advantage to reducing fuel consumption

and further help to pay for any necessary extrainvestment costs.

In addition it is important to bear in mind thatthe fuel savings measures discussed here will alsoresult in lower NO

Xemissions in absolute terms.

Derating engines for greater fuel savingsIn the following pages are some case studies for shipinstallations for which an engine is selected with anextra cylinder without increasing the engines power.ese cases demonstrate that such engine deratingcan be an advantageous solution with remarkablesaving potential. Depending on bunker costs, such astrategy can have a very attractive pay-back time.

e four case studies are for a Suezmax tanker,a Capesize bulk carrier, a Panamax container shipand a Post-Panamax container ship. ey includeestimations of the respective pay-back times for theadditional engine costs.



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Case 1: Suezmax tanker & Capesize bulk carrier-

In this case, a typical Suezmax tanker might bespecified with a six-cylinder Wrtsil RT-flex68-D

main engine. However, if a seven-cylinder engine isemployed instead, the daily fuel consumption can bereduced by some 3.4 per cent.

In the engine/propeller layout for this ship asshown in figure 6, the CMCR points for the twoalternative engines are on the same rating line( = 0.3) through a common design point for thesame ship service speed (knots).

e calculation of annual fuel costs given in table2 is based on 6000 hours running with heavy fuel oil

costing US$ 500 per tonne.e resulting payback time for the extra cost

associated with the additional engine cylinder isestimated to be between 3.5 and six years dependingon the bunker price of US$ 600400 per tonnerespectively (Fig. 7). e calculations of the paybackare based on an interest rate of eight per cent.

A similar case may be made for a Capesize bulkcarrier as it would be similar in size and speed to aSuezmax tanker and would thus require a similarengine.

Table 1: Typical ship parameters for a Suezmax tanker

Length overall: about 274 m

Beam: 4650 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 680Piston 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



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7RT-flex68-D

= 0.3Constant ship speed CMCR

18,460 kW

Design pointCMCR = R1

18,780 kW, 95 rpm

6RT-flex68-D

89.7 rpm

CSR

CSR

16,902 kW

16,614 kW

86.6 rpm

91.7 rpm

Fig. 6: Engine/propeller layouts fora typical Suezmax tanker with a

derated seven-cylinder RT-flex68-Dengine compared with a six-cylinder

engine at the full MCR power andspeed.

[08#052]

Engine power, kW

22,000

20,000

18,000

16,000

75 80 85 90

Engine speed, rpm

95 100

Fig. 7: Variation of payback timesfrom fuel savings according to

bunker costs for the derated enginewith an extra cylinder for a typical

Suezmax tanker.[08#144]

3.0

2.0

1.0

0

Millions US$

Bunker price, HFO:$600/tonne

$500/tonne

$400/tonne

Investment approx. ($)

2 4 6 8 10 12 14

Years

Case 1: Suezmax tanker & Capesize bulk carrier-



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Case 2: Panamax container ship-

In this case, a typical Panamax container ship witha container capacity of up to 5000 TEU might be

specified with an eight-cylinder Wrtsil RT-flex82Cmain engine. However, if a nine-cylinder engine isemployed instead, the daily fuel consumption can bereduced by some two per cent.



costing US$ 500 per tonne.e resulting payback time for the extra costassociated with the additional engine cylinderis estimated to be between four and seven yearsdepending on the bunker price of US$ 600400 pertonne respectively (Fig. 9). e calculations of thepayback are based on an interest rate of eight percent.

Table 3: Typical ship parameters for a Panamax

container ship


Beam: 32.2 m


Scantling draught: 13.5 m

Sea margin: 15 %


Table 4: Main engine options

Alternative engines: 8RT-flex82C 9RT-flex82C


Stroke/bore ratio: 3.2:1 3.2:1

MCR, kW / rpm: 36,160/102 40,680/102

CMCR, kW / rpm: 36,160/102 35,480/97.5


CSR at 90% CMCR, kW / rpm: 32,544/98.5 32,250/94.3


100% load: 169.0 166.6

90% load: 166.5 164.6

Daily fuel consumption, tonnes/day: ISO fuel, LCV 42.7 MJ/kg: 130.0 127.4

LCV 40.5 MJ/kg: 137.1 134.3

As percentage, %: 100 98 2.0%



Engine length, mm: 14,055 16,500




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Fig. 8: Engine/propeller layouts for atypical Panamax container ship witha derated nine-cylinder RT-flex82C

engine compared with an eight-cylinder engine at the full MCR

power and speed.[08#062]

Engine power, kW

42,000

40,000

38,000

36,000

34,000

32,000

85

8RT-flex82C

9RT-flex82C

= 0.2Constant ship speed

90 95 100 105

Engine speed, rpm

CMCR35,850 kW

97.5 rpm

Design point

CMCR = R1+36,160 kW, 102 rpm

CSR32,544 kW

98.5 rpm

CSR

32,250 kW94.3 rpm


bunker costs for the derated enginewith an extra cylinder for a typical

Panamax container ship.[08#145]

Millions US$

4.0

3.0

2.0

1.0

0


$500/tonne

$400/tonne


2 4 6 8 10 12 14

Years

Case 2: Panamax container ship-



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Case 3: Post-Panamax container ship-

In this case, a typical Post-Panamax containership with a container capacity of around 7000

TEU might be specified with an eleven-cylinderWrtsil RT-flex96C main engine. However, if a12-cylinder engine is employed instead, the daily fuelconsumption can be reduced by some 2.4 per cent.



costing US$ 500 per tonne.e resulting payback time for the extra costassociated with the additional engine cylinder isestimated to be between two-and-a-half and fouryears depending on the bunker price of US$ 600400 per tonne respectively (Fig. 11). e calculationsof the payback are based on an interest rate of eightper cent.

Table 5: Typical ship parameters for a Post-Panamax

container ship


Beam: 42.8 m


Scantling draught: 14.5 m

Sea margin: 15 %


Table 6: Main engine options-

Alternative engines: 11RT-flex96C 12RT-flex96C


Stroke/bore ratio: 2.6:1 2.6:1

MCR, kW / rpm: 66,330/102 72,360/102

CMCR, kW / rpm: 66,330/102 65,919/98.9


CSR at 90% CMCR, kW / rpm: 59,697/98.5 59,327/95.5


100% load: 171.0 168.0

90% load: 166.8 163.8

Daily fuel consumption, tonnes/day: ISO fuel, LCV 42.7 MJ/kg: 239 233.2

LCV 40.5 MJ/kg: 252 245.9

As percentage, %: 100 97.6 2.4%



Engine length, mm: 21,550 23,230




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Fig. 10: Engine/propeller layouts fora typical Post-Panamax container

ship with a derated 12-cylinder RT-flex96C engine compared with an

11-cylinder engine at the full MCRpower and speed.

[08#127]

Engine power, kW

72,000

70,000

68,000

66,000

64,000

62,000

60,000

58,000

Engine speed, rpm90 95 100 105

11RT-flex96C

12RT-flex96C

= 0.2Constant ship speed

CSR

59,697 kW

98.5 rpm

CMCR65,919 kW

98.9 rpm

Design point

CMCR = R1

66,330 kW, 102 rpm

CSR

59,327 kW

95.5 rpm


bunker costs for the derated enginewith an extra cylinder for the typical

Post-Panamax container ship.[08#146]

Millions US$

8.0

6.0

4.0

2.0

0


$500/tonne

$400/tonne


2 4 6 8 10 12 14

Years

Case 3: Post-Panamax container ship-



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Case 4: Derating without adding anengine cylinderIt is also feasible to apply a derated engine to obtainfuel savings in such a way that an additional enginecylinder is not required.

An example of this can be seen with the WrtsilRT-flex50 engine. In October 2007, the D versionof this engine was announced, in which the enginepower was increased by 5.1 per cent and the BSFCat full-load was reduced by 2 g/kWh compared withthe B version.

us if a -D engine is derated to the samecylinder power output as the original version of theRT-flex50, then the BSFC at full load is reducedby 4.5 g/kWh, or 2.7 per cent (see Table 7). For atypical bulk carrier with a six-cylinder RT-flex50

engine this can translate into annual savings ofUS$ 124,000 when operating for 6000 runninghours a year with heavy fuel oil costing US$ 500per tonne. Even greater savings are possible if theengine is derated to a lower running speed (rpm)at the derated power to gain the benefits of a betterpropulsion efficiency.

ere are already a number of standard shipdesigns delivered and on order with RT-flex50-B oreven the original RT-flex50 engine. So it would beperfectly feasible to install a derated RT-flex50-Din further newbuildings to the same ship designs

and obtain the benefit of the substantial savings inoperating costs. e overall dimensions of the Dversion are identical to those of the B and originalversions of the RT-flex50. ere would, however, be

a modest increase in cost of the D version for thehigher-efficiency turbochargers used, but the extracost would soon be repaid by the fuel cost savings.

Derating with flexibility to full rating

Although derating offers attractive economics, itcan be frustrating to buy more engine than seemsnecessary. Yet there is an interesting option to retainan ability to utilise the full available installed enginepower, even up to the full R1 rating for future use toobtain higher ship service speeds.

e concept would be to set up the engine forthe derated output at the chosen reduced servicespeed. en for a later date, the engine could bere-adapted to the higher output. However, this needscorresponding provisions in the selection and design

of the propeller, shafting and ancillary equipment tomeet the requirements of the envisaged higher power.Furthermore the engine would need to be testedand approved by the Classification Society for bothratings with all the necessary emissions certification.

RT-flex technology as an importantcontribution to fuel savingWrtsil RT-flex technology plays an important rolein fuel saving. Wrtsil RT-flex low-speed enginesincorporate the latest electronically-controlled

common-rail technology for fuel injection and valveactuation. e result is great flexibility in enginesetting, bringing benefits in lower fuel consumption,lower minimum running speeds, smokeless operation

Table 7: Options for the Wrtsil RT-flex50 engine type

Alternative engines: 6RT-flex50 6RT-flex50-D

Cylinder bore, mm: 500 500

Piston stroke, mm: 2050 2050

S/B ratio: 4.1:1 4.1:1MCR, kW / rpm: 9720/124 10,470/124

CMCR, kW / rpm: 9720/124 9720/124


CSR at 90% CMCR, kW / rpm: 8748/119.7 8748/119.7


100% load: 171 165.7

90% load: 167.6 163.0

Daily fuel consumption, tonnes/day:

ISO fuel, LCV 42.7 MJ/kg: 35.2 34.2

LCV 40.5 MJ/kg: 37.1 36.2

As percentage, %: 100 97.3 2.7%Annual fuel costs, US$: 4,637,000 4,513,000




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at all running speeds, and better control of otherexhaust emissions.

Not only do RT-flex engines have a lower part-load fuel consumption than RTA engines but theycan be adapted through Delta Tuning so that theirpart-load fuel consumtion is even lower. [1]

Owing to the interaction between fuel economyand NO

Xemissions, there is always the possibility

that fuel saving measures will have an impact onNO

Xemissions. As with all new marine engines

nowadays, Wrtsil RTA and RT-flex engines are allfully compliant with the NO

Xemission regulation of

Annexe VI of the MARPOL 1973/78 convention.Moreover, the engines in the Wrtsil portfolio willbe adapted to meet the coming IMO NO

Xreduction

level Tier II.

Conclusione paper shows that there are techniques to achieve

worthwhile reductions in the fuel consumptionof Wrtsil low-speed engines when designingnewbuildings. e key approach is to use theflexibility offered by the full power/speed layout fieldto select a better layout point with a lower BSFC and

also possibly a higher propeller efficiency.It must also not be forgotten that any fuel savings

achieved at the ship design stage will have benefits inalso reducing exhaust emissions.

If you have a project for which you wish toexplore the fuel-saving possibilities through deratingas set out in this paper, then please contact yournearest Wrtsil office. Our experts will be delightedto calculate various alternatives for your evaluation.

References1. German Weisser, Fuel saving with RT-flex,

Wrtsil Switzerland Ltd, July 2004.


Published June 2008 by:Wrtsil Switzerland LtdPO Box 414

CH-8401 WinterthurTel: +41 52 262 49 22Fax: +41 52 262 07 18

www.wartsila.com

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Wartsila SP Tech 2008 Derating