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MAN B&W Diesel A/S, Copenhagen, Denmark
Contents:
Shaft Generators for the MC and ME Engines
Preface .............................................................................. 3
1. Introduction ........................................................................ 3
2. Definitions and Designations .............................................. 4
3. Categories of Shaft Generators .......................................... 5
3.1 PTO/GCR (Power Take Off / Gear Constant Ratio) ................... 5
3.2 PTO/RCF (Power Take Off / Renk Constant Frequency)............ 5
3.3 PTO/CFE (Power Take Off / Constant Frequency Electrical) ........ 6
4. Physical Configuration ........................................................ 7
4.1 Front end installation (BW I) ............................................. 7
4.2 Front end installation (BW II) ............................................ 9
4.3 Side mounted installation (BW III) ..................................... 11
4.4 Aft end installation (BW IV) ............................................... 13
4.5 Front end installation (DMG/CFE) ..................................... 14
4.6 Aft end installation (SMG/CFE) ......................................... 15
5. Layout of Engine with Shaft Generator ............................... 16
6. Torsional Vibration .............................................................. 18
6.1 Engines with small shaft generators ................................. 18
6.2 Engines with large shaft generators .................................. 18
7. Engine Governing System ............................................... 19
8. Pros and Cons (shaft generators versus gensets) ............. 19
8.1 Advantages – shaft generators ......................................... 19
8.2 Disadvantages – shaft generators .................................... 19
9. Economic Comparison ....................................................... 20
10. Typical Applications ............................................................ 24
11. Special Applications ........................................................... 24
11.1 Shuttle tankers ................................................................ 24
11.2 Auxiliary propulsion system .............................................. 26
12. Summary ........................................................................... 27
3
Shaft Generators for the MC and ME Engines
Preface
The purpose of this paper is to providedetailed information about the various as-pects related to the application of shaftgenerators on the MAN B&W MC andME series of engines.
Shortly after introducing the first MC en-gines in 1982, MAN B&W Diesel startedinvestigating the possibilities of using shaftgenerators on the engines, and severalstandardised shaft generator types weredeveloped.
In this paper the individual types of shaftgenerators are defined and their physicalconfiguration is described together withthe interface with the diesel engine.
Given the nature of this paper, it can beused either to obtain a complete descrip-tion of the relevant aspects or it can beused for reference.
1. Introduction
During the 1980s the use of shaft gen-erators in conjunction with two-strokediesel engines rapidly became a popularmethod of producing electric power forthe various electricity consumers onboard ships.
At that time most gensets were unable tooperate on heavy fuel oil and, even if theywere, the fuel oil prices were muchhigher than today, so because of the dif-ferences in efficiency of the two-strokeengine and the medium-speed gensetengine, the use of a shaft generator oftenresulted in significant fuel cost savings.
Nowadays, a wide range of gensets, ableto operate efficiently on heavy fuel oil and,at the same time, offering a high degreeof reliability, are available, and it is tempt-ing to ask whether the installation of ashaft generator is really economical?
This paper highlights a number of param-eters which may influence the final selec-tion of the electricity production plant. Be-yond the fuel oil costs, it is important toconsider a number of other parameters,some of them impossible to quantify, if weare to obtain a clear picture of the prosand cons of the shaft generator concept.
References show that a number of ship-owners still consider a shaft generator tobe an attractive investment, e.g. on con-tainer vessels, product tankers, andshuttle tankers, and a number of differenttypes and various physical configurationsof shaft generators are available. The pa-per describes these types together withtheir interface with the two-stroke dieselengine, and examples are given of typicalas well as special applications.
As an alternative to producing electricity,a Power Take Off can be used, for ex-ample, to drive a hydraulic pump directly.However, only a marginal number of PTOsystems are made as direct drive sys-
tems, and they will not be discussed inthis paper.
In the following, the term “shaft genera-tor” is employed for any arrangementwhere a Power Take Off from the mainengine or the shaftline is used to drive analternator, i.e for the purpose of produc-ing electricity using the main engine asthe prime mover.
4
2. Definitions and Designations
Basically, MAN B&W Diesel distinguishbetween three main categories of shaftgenerators:
1 PTO/GCR (Power Take Off/GearConstant Ratio):Consists of flexible coupling, step-upgear, torsionally rigid coupling, and al-ternator.
2 PTO/RCF (Power Take Off/RenkConstant Frequency):Consists of flexible coupling, step-upgear, torsionally rigid couplings, RCF-gear, and alternator.
3 PTO/CFE (Power Take Off/Con- stantFrequency Electrical):Consists of flexible coupling, step-upgear, torsionally rigid coupling, al-ternator, and electrical control equip-ment or, alternatively, a slow-runningalternator with electrical control equip-ment.
The PTO/RCF and the PTO/CFE incorpo-rate different kinds of frequency controlsystems which make it possible to pro-duce electric power with constant electri-cal frequency at varying engine speed.The PTO/GCR has no frequency controlsystem.
Shaft generators of all three categoriescan be installed either at the front end ofthe engine, at the side of the engine, oraft of the engine. Figure 1 shows, in prin-ciple, the various possibilities for the instal-lation of a shaft generator.
The designations BW I ... BW IV are usedto distinguish between the various physi-cal configurations of the shaft generatorsystem.
In MAN B&W Diesel terms, a 700 kW (60Hz) shaft generator of the GCR type in-tended for installation at the side of anS50MC-C engine is thus designated:
BW III S50-C / GCR 700 - 60
PTO
/RC
FP
TO/C
FEP
TO/G
CR
5a
7a
6a
8a
9a
10a
12a
11a
13a
14a
5b
7b
6b
8b
9b
10b
Alternative types and layoutsof shaft generator
Design Seating Total efficiency (%)
BW IV/GCR
BW III/GCR
BW II/GCR
BW I/GCROn engine
(vertical generator)
On engine(vertical generator)
On tanktop
On tanktop
On tanktop
On tanktop
On tanktop
On tanktop(vertical generator)
On tanktop
On engine
On engine(vertical generator)
On engine
On engine
On engine
BW I/RCF
BW I/CFE
BW II/RCF
BW II/CFE
BW III/RCF
BW III/CFE
DMG/CFE
BW IV/RCF
BW IV/CFE
SMG/CFE
88!91
88!91
88!91
88!91
81!85
81!85
84!88
81!85
81!85
84!88
92
92
92
92
11b
12b
13b
14b
4
3
2
1
In the following chapters, the threetypes of PTO (PTO/GCR, PTO/RCF,PTO/CFE) and the various possibilitiesfor the configuration of the PTO (BW I... BW IV) are described.
Fig. 1: Types of PTO
5
3. Categories of ShaftGenerators
3.1 PTO/GCR (Power Take Off /Gear Constant Ratio)
The PTO/GCR is the simplest shaft genera-tor, as no speed control or frequency controlsystems are incorporated. In the vast ma-jority of cases, the PTO/GCR is used to pro-duce electric power with a constant electri-cal frequency during the voyage. Since thefrequency produced by the alternator is pro-portional to the speed of the engine, the en-gine must be operated at constant speed.This is only possible if a controllable pitchpropeller is installed. When a fixed pitch pro-peller is used, the speed of the propeller,and thus of the engine, varies with the re-quired speed of the ship and the resistanceacting on the ship.
Alternatively, the PTO/GCR can be used forpower production with floating frequency,e.g. between 50 and 60 Hz, which meansthat the speed of the main engine is allowedto vary between 83% and 100% of thespeed at specified MCR (Maximum Con-tinuous Rating). This also means that certainpower consumers sensitive to frequencyvariations must be provided with power sup-ply via a frequency converter or from agenset. The concept of long-term runningwith floating electrical frequency is only usedin rare cases.
The PTO/GCR is unable to run in parallelwith the auxiliary engines for long periods,because of the small engine speed varia-tions of the main engine, which occur evenin the constant speed mode of the control-lable pitch propeller plant. Consequently, thePTO/GCR is often used to supply electricpower to all power consumers during thevoyage, with the gensets out of operation.During manoeuvring, which involves reduc-ing the main engine speed, the PTO/GCRcan be used as a separate power sourcefor the bow thruster, which can often be runwith floating frequency, with the gensetssupplying electric power for all other powerconsumers.
The total efficiency of the PTO/GCR isaround 92%.
Except for the PTO BW I/GCR and thePTO BW III/GCR, which are built directlyonto the main engine, and is only producedby Renk in Germany, several manufacturersare able to supply the PTO/GCR system.Prices vary greatly with the physical configu-ration of the system and the different suppli-ers. The investment cost of a PTO/GCR ismuch smaller than the cost of a PTO/RCFor a PTO/CFE. On the other hand, the in-vestment cost of the controllable pitch pro-peller required in combination with a PTO/GCR is higher than the cost of a fixed pitchpropeller.
Finally, the operation of the engine at con-stant speed means reduced propeller effi-ciency at reduced propulsion load com-pared with a controllable pitch propeller run-ning in combinator mode (reduced speed atreduced propulsion load) or a fixed pitch pro-peller. The thermal efficiency of the main en-gine is also slightly lower in constant speedmode than in combinator mode.
3.2 PTO/RCF (Power Take Off /Renk Constant Frequency)
The PTO/RCF includes an RCF speedcontrolled planetary gearbox developedby Renk, and the system is only availablefrom Renk. Figure 2 illustrates the designprinciples of the RCF gearbox.
The RCF gearbox is an epicyclic gearwith a hydrostatic superposition drive.The superposition drive comprises a hy-drostatic motor controlled by an elec-tronic control unit and driven by a built-onpump.
The hydrostatic system drives the annu-lus of the epicyclic gear in either directionof rotation, based on the detected outputspeed, and thus continuously varies thegear ratio over an engine speed variationof 30%. In the standard layout the con-stant output speed range of the gearboxis set between 100% and 70% of the en-gine speed at specified MCR.
To panel
Hydrostaticmotor
Hydrostaticcontrol
Operator control panel(in switchboard)
Hydrostatic pump
Multi!disc clutch
Input shaft
Output shaft
ControllerTerminal
Fig. 2: PTO/RCF, developed by Renk
6
An electronic control system ensures thatthe control signals to the main electricalswitchboard are identical to those of thegensets, and the PTO/RCF is able to op-erate alone or in parallel with the gensetsthroughout the complete constant outputspeed range of the gearbox. The PTO/RCF is therefore suitable for installation onships with a fixed pitch propeller.
Internal control circuits and interlockingfunctions between the epicyclic gear andthe electronic control box provide auto-matic control of the functions necessaryfor the satisfactory operation and protec-tion of the RCF gear.
If any monitored value exceeds the nor-mal operation limits, a warning or analarm is given, depending on the origin,severity, and the extent of deviation fromthe permissible values. The cause of awarning or an alarm is shown on a digitaldisplay.
A multi-disc clutch, integrated into theRCF-gearbox input shaft, permits the en-gaging and disengaging of the epicyclicgear, and thus the alternator, from themain engine during operation.
Depending on the actual engine speedrelative to the speed at specified MCR,the total efficiency varies between 88%and 91%.
3.3 PTO/CFE (Power Take Off /Constant Frequency Electrical)
The PTO/CFE is, like the PTO/RCF, ableto produce electricity with constant elec-trical frequency over a wide engine speedrange. In the case of the PTO/CFE usinga step-up gear, the alternator may havea built-in electronic converter, which en-sures that corrections are made for thevarying engine speed, and hence thevarying alternator speed. Alternatively,and more usually, the electric power isproduced with varying frequency and isafterwards converted by thyristor controlto electric power with a constant fre-quency.
Over the years, different types of PTO/CFE based on step-up gears have beenintroduced, and a limited number of unitshave been built. However, to our knowl-edge, this type of shaft generator is notused for newbuildings at the moment.We therefore disregard the PTO/CFEbased on a step-up gear in this paper.
The PTO/CFE can, alternatively, be madeas a slow-running alternator coupled di-rectly to the front end of the engine(DMG/CFE, Direct Mounted Generator)or installed with the rotor integrated intothe intermediate shaft (SMG/CFE, Shaft
Mounted Generator). The slow-runninggenerators are much larger than the gen-erators running at high speed, but in re-turn the flexible coupling and the step-upgear are omitted.
Again, additional electrical control equip-ment must be installed to provide thyristorcontrol of the frequency. The DMG/CFEand the SMG/CFE are normally able tooperate in parallel with the gensets at thefull rated electric power, when the speedof the main engine is between 75% and100% of the engine speed at specifiedMCR. Between 40% and 75% of theSMCR speed, the electric output of thePTO/CFE is reduced proportionately tothe engine speed. Figure 3 shows thecontrol principle for the DMG/CFE.
The SMG/CFE (Shaft Mounted Genera-tor) is much more frequently used thanthe DMG/CFE (Direct Mounted Genera-tor). One reason may be that the SMG/CFE is not subject to any limitations fromthe installation on the main engine.
The total efficiency of the slow runningPTO/CFE types varies between 84% and88%.
G
Mains, constant frequency
SynchronouscondenserExcitation
converter
Diesel engineDMG
Staticconverter
Smoothingreactor
Fig. 3: PTO DMG/CFE
7
4. Physical Configuration
4.1 Front end installation (BW I)
In the BW I system, the step-up gear unit isbolted directly to the engine front end faceand comprises a bevel gear, which allowsthe alternator to be placed vertically on topof the gear unit. See Figure 4. This com-pact design gear unit is only available fromRenk.
A Geislinger elastic damping coupling is in-cluded in the delivery. The elastic elementsin the Geislinger coupling are packages ofsteel springs, and the damping is providedby oil, which is squeezed from one oilchamber to another by the steel springs,when they deflect during operation. The oilalso provides the necessary lubrication andcooling of the coupling. The Geislinger cou-pling is bolted to the front end of the crank-shaft, and the input shaft of the gear unit isconnected to the Geislinger coupling by atoothed coupling made as an integral partof the Geislinger coupling. Thanks to thisdesign, the gear unit is isolated from axialmovements of the crankshaft and pro-tected against torsional excitations.
Additionally, the toothed coupling allowssimple separation of the complete gear unitfrom the Geislinger coupling, and thus thecrankshaft, by shifting the shaft which car-ries the main gear wheel forward in theaxial direction. This procedure for discon-nection is only to be used in the specialevent that it is required to completely dis-connect the gear unit from the main en-gine.
Around the Geislinger coupling, betweenthe crankcase space and the gear unit,space is available for a tuning wheel,a moment compensator or a torsional vi-bration damper, if required.
The gear unit, often referred to as a “crank-shaft gear”, is bolted to the bedplate andframebox of the engine, and the weight ofthe gear wheels is, via bearings installed inthe gearbox, transferred to the enginestructure. Consequently, the crankshaft is
only loaded by the weight of the Geislingercoupling.
A multi-disc clutch is built into the RCFgearbox input shaft and ensures that theepicyclic RCF gear (if included) and the al-ternator, can be engaged or disengagedduring operation of the main engine.
An electrically driven built-on lubricating oilstand-by pump supplements a gear drivenpump when the engine is started and in theevent of malfunctioning of the gear drivenpump.
The PTO/RCF can be operated on thesame type of oil as is used for the main en-gine lubricating oil system if the lube oil meetsa minimum load level of 8 according to thestandardised FZG Gear Test (DIN 51354).
In the case of the PTO BW I/RCF the RCFgear is integrated into the step-up / bevelgear unit and because the hydrostatic driveof the RCF gear requires a 5 µm filtration ofthe oil, the complete gear unit has a sepa-rate lubricating oil system including the oilsupply to the Geislinger coupling.
Fig. 4: PTO BW I S70/RCF 850-60 (Renk)
8
For the PTO BW I/RCF, the shipyardmust provide cooling water for the built-on lubricating oil cooler, electric powersupply to the built-on lubricating oil stand-by pump, and cabling between the alter-nator and the switchboard. We furtherrecommend that the shipyard installs anexternal lubricating oil filling system, in-cluding a dosage tank made in accor-dance with the specified volume of oilused for one gearbox oil change. SeeFigure 5.
The PTO BW I/GCR is lubricated andcooled by using the main engine lubri-cating oil system and it has to be ensuredthat the lube oil has a minimum FZG loadlevel of 8. This means that the coolingwater supply and the external lubricatingoil filling system can be dispensed with. Inthis case, the capacities of the mainengine’s lubricating oil system with relatedcoolers must be increased in accordancewith the data given by the supplier of theshaft generator. The shipyard has to ar-range the external wiring of the controlsystem. The PTO BW I/GCR has not yetbeen produced.
The following preparations for the installa-tion of the PTO system must be made onthe engine. See also Figure 6.
• three machined blocks welded on tothe front end face for alignment of thegear unit
• machined washers to be placed be-tween the gearbox and the frameboxto compensate for the small differencein length between the bedplate andthe framebox
• rubber gasket to be placed betweenthe gearbox and the framebox
• electronic axial vibration measuringsystem
• free flange end on lubricating oil inletpipe (only for PTO BW I/GCR)
• oil return flange welded on to bed-plate (only for PTO BW I/GCR).
Cooling water supply to lub. oil x x x x x xcooler
PTO
BW
I/R
CF
PTO
BW
II/R
CF
PTO
BW
III/R
CF
PTO
BW
IV/R
CF
PTO
DM
G/C
FE
PTO
SM
G/C
FE
PTO
BW
I/G
CR
PTO
BW
II/G
CR
PTO
BW
III/G
CR
PTO
BW
IV/G
CR
Cooling water supply to alternator (x) (x)- if water cooled type is applied
Lub. oil dosage tank (option) (x) (x) (x) (x)
Electric power supply to lub. oil x x x x x x x xstand-by pump
Cabling between alternator and x x x x x x x xswitchboard
Seating for stator housing x
Seating for static converter cubicles x x
Seating for synchronous condenser x xunit
Seating for alternator x x
Seating for support bearing (x) (x)
Seating for gearbox x x x x
Electric cabling x x
External wiring of control system x x x x
Fig. 5: Shipyard installations for PTO
9
4.2 Front end installation (BW II)
The PTO BW II is placed in front of theengine but, in contrast to the PTO BW I,the gear unit is seated on a separatefoundation on the tanktop.
Various gearbox manufacturers are ableto supply the PTO BW II/GCR, whereasonly Renk is able to supply the PTOBW II/RCF.
The installation length in front of the en-gine, and thus the engine room length re-quirement, naturally exceeds the lengthof the other shaft generator arrange-ments. However, there is some scope forlimiting the space requirement, dependingon the configuration chosen.
Machined blocks welded on to engine x x x x xfront end face
Machined washers to be placed x x x xbetween gearbox and frame box
Rubber gasket to be placed between x x x xgearbox and frame box
Steel shim to be placed between xgearbox and frame box
Electronic axial vibration measuring x x x x x x xsystem
Free flange end on lubricating oil inlet x x xpipe
Oil return flange welded to bedplate x x x
Front end cover in two halves, with oil x xsealing arrangement
Intermediate shaft between crankshaft x xfront end and flexible coupling
Brackets mounted on side of x xbedplate
PTO
BW
I/R
CF
PTO
BW
II/R
CF
PTO
BW
III/R
CF
PTO
BW
IV/R
CF
PTO
DM
G/C
FE
PTO
SM
G/C
FE
PTO
BW
I/G
CR
PTO
BW
II/G
CR
PTO
BW
III/G
CR
PTO
BW
IV/G
CR
Fig. 6: Engine preparations for PTO
10
Figure 7 shows a space optimised con-cept where the alternator is placed ho-rizontally between the step-up gearboxand the front end of the engine and thusutilises the space which is anyway takenup by the flexible coupling.
A further reduction of the building-inlength can be obtained by the use of abevel gear in the step-up gearbox and avertically placed alternator, see Figure 8.
A rubber type elastic damping coupling isinstalled at the gearbox input shaft out-side the engine. The engine drives theshaft generator via an intermediate shaft,which is bolted to the front end of thecrankshaft and passes through the en-gine front end cover, which is made intwo halves with an oil sealing arrange-ment.
Alternator Torsionallyrigidcoupling
Step!upgear
Supportbearing
Flexible couplingMain engineside
Flexiblecoupling
Alternator
Step!upgear
Fig. 7: PTO BW II/GCR (A. Friedr. Flender AG)
Fig. 8: PTO BW II/GCR (Renk)
11
Often a small support bearing has to beinstalled between the front end of the en-gine and the flexible coupling. Whether asupport bearing is required can be deter-mined from MAN B&W Diesel’s specifica-tion of the permissible shear force andbending moment on the front end of thecrankshaft.
The PTO BW II has its own lubricating oilsystem, and an electrically driven built-onlubricating oil stand-by pump supple-ments a gear driven pump at start of theengine and in the event of malfunctioningof the gear driven pump.
The gearbox and the support bearing areseated on the tanktop, and the shipyardhas to make suitable foundations. Theshipyard must also provide cooling waterfor the built-on lubricating oil cooler, elec-tric power supply to the built-on lubricat-ing oil stand-by pump and cabling be-tween the alternator and the switch-board, as well as external wiring of thecontrol system (except for the PTO BWII/RCF).
For the PTO BW II/RCF we recommendthat the shipyard installs an external lubri-cating oil filling system, including a dosagetank made in accordance with the speci-fied volume of oil used for one gearbox oilchange.
The following preparations for the installa-tion of the PTO system must be made onthe engine:
• intermediate shaft between the en-gine and the flexible coupling
• front end cover in two halves with oilsealing arrangement
• electronic axial vibration measuringsystem.
4.3 Side mounted installation (BW III)
The investment cost of the PTO BW III istypically higher than for the other gearbased shaft generators. However, wehave adopted it as our standard, as it isthe most compact system available,which means more space for cargotransportation, and a further advantage isthat it is easy to install at the shipyard.
The gearbox is available as standard forthe 42 MC engines and upwards includ-ing the ME engines. Standard sizes of al-ternators are 700, 1200, 1800 and 2600kW, but others are available on request.
In the BW III system, the step-up gearunit is bolted directly to the engine frontend face, and is designed to allow the al-ternator to be placed horizontally at theside of the engine. See Figure 9, whichshows the PTO BW III/GCR. This com-pact design gear unit is only availablefrom Renk.
A Geislinger elastic damping coupling isincluded in the delivery. The Geislingercoupling is described in Section 4.1.
The gear unit, often referred to as a“crankshaft gear”, is bolted to the bed-plate and framebox of the engine, andthe weight of the gear wheels is, viabearings, installed in the gearbox, trans-ferred to the engine structure. Conse-quently, the crankshaft is only loaded bythe weight of the Geislinger coupling.
The step-up gear of the PTO BW III/GCR is lubricated and cooled using themain engine lubricating oil system, so itrequires no cooling water supply. The ca-pacities of the main engine’s lubricating oilsystem with related coolers are to be in-creased accordingly. A minimum FZGload level of 8 has to be observed for thelube oil.
A multi-disc clutch can be built into thegearbox output shaft ensuring that the al-ternator can be engaged or disengagedduring operation of the main engine. Therequired operating oil pressure for themulti-disc clutch is supplied by a geardriven pump. On engine start-up and inthe event of malfunctioning of the geardriven pump, an electrically driven built-onlubricating oil stand-by pump supple-ments the gear driven pump.
CombinedGeislingerand toothedcoupling
Crankshaftgear
Brackets
Toothed coupling
Alternator
Bedframe
Multi!disc clutch
Fig. 9: PTO BW III/GCR (Renk)
12
The step-up gear (crankshaft gear) ofthe PTO BW III/RCF is, similar to thecrankshaft gear of the PTO BW III/GCR,lubricated and cooled by using the mainengine lubricating oil system. Figure 10shows the PTO BW III/RCF.
The RCF gear, placed at the side of theengine, can be operated on the sametype of oil as the main engine lubricatingoil system. However, the hydrostaticdrive of the RCF gear requires a 5 µmfiltration of the oil, and the RCF gear,consequently, has its own lubricating oilsystem.
A multi-disc clutch is built into the RCFgearbox, and an electrically drivenlubricating oil stand-by pump, built on tothe RCF gear, supplements a geardriven pump on engine start-up and inthe event of malfunctioning of the geardriven pump.
For the PTO BW III/RCF the shipyardmust provide cooling water for thelubricating oil cooler built on to the RCFgear, electric power supply to thelubricating oil stand-by pump built on tothe RCF gear, and cabling between thealternator and the switchboard. Wefurther recommend that the shipyardinstalls an external lubricating oil fillingsystem, including a dosage tank made inaccordance with the specified volume ofoil used for one RCF-gear oil change.
The PTO BW III/GCR only requireselectric power supply to the built-onlubricating oil stand-by pump andcabling between the alternator and theswitchboard as well as external wiringof the control system.
The following preparations for theinstallation of the PTO BW III/GCR
system or the PTO BW III/RCF systemmust be made on the engine:
• three machined blocks welded on tothe front end face for alignment of thegear unit
• machined washers to be placedbetween the gearbox and the frame-box to compensate for the small dif-ference in length between the bed-plate and the framebox
• rubber gasket to be placed betweenthe gearbox and the framebox
• electronic axial vibration measuringsystem
• free flange end on lubricating oil inletpipe
• oil return flange welded on to bed-plate
• brackets mounted on starboard sideof bedplate to support the RCF gear(if installed) and the alternator.
Operatorcontrolpanel
Alternator
Controller
Brackets
Toothed couplingRCF unitincl. multi!disc clutch
Bedframe
CombinedGeislingerand toothedcoupling
Crankshaft gear
Fig. 10: PTO BW III/RCF (Renk)
13
4.4 Aft end installation (BW IV)
The PTO BW IV is placed aft of the en-gine and is made as a tunnel gear with ahollow shaft, which allows the intermedi-ate shaft, including the flange, to passthrough, see Figure 11.
A number of gearbox manufacturers areable to supply the PTO BW IV/GCR,whereas only Renk is able to supply thePTO BW IV/RCF, although it has not yetbeen produced.
The PTO BW IV can often be installedwithin the space already available aroundthe shaftline aft of the engine, without in-creasing the total building-in length.
A hollow, segmented elastic dampingcoupling based on rubber elements isbuilt around the shaft between the inter-
mediate shaft flange at the engine aft endand the hollow shaft of the tunnel gear-box. Some of the steel flanges used forthe coupling are made in halves to allowthem to be assembled around the shaft.
The flexible coupling only transfers thetorque corresponding to the power of theshaft generator, since the intermediateshaft for the propeller is bolted directly tothe thrust shaft of the engine.
The PTO BW IV has a separate lubricat-ing oil system, and an electrically drivenbuilt-on lubricating oil stand-by pumpsupplements a gear driven pump on en-gine start-up and in the event of any mal-functioning of the gear driven pump.
The gearbox is seated on the tanktop,and the shipyard has to make suitablefoundations, both for the gearbox and the
alternator. The shipyard must also pro-vide cooling water for the built-on lubri-cating oil cooler, electric power supply forthe built-on lubricating oil stand-by pump,and cabling between the alternator andthe switchboard, as well as external wir-ing of the control system (except for thePTO BW IV/RCF).
No preparations for the installation of thePTO system are needed on the engine,but the intermediate shaft flange must beprovided with additional bolt holes for theflexible coupling.
Vulkan Ratoflexible coupling
Alternator
Toothedcoupling
Tunnelgear
Fig. 11: PTO BW IV/GCR, Tunnel gear
14
4.5 Front end installation (DMG/CFE)
The PTO DMG/CFE is a large slow-running alternator with its rotor mounteddirectly on the crankshaft and its statorhousing bolted to the front end face ofthe engine. The PTO DMG/CFE does notinclude a gearbox, and no flexible cou-pling is required. See Figure 12. The alter-nator is separated from the crankcase bya plate and a labyrinth seal.
If the torsional characteristics of the shaftsystem require the application of an addi-tional inertia mass on the crankshaft foreend, a tuning wheel can be installed, asillustrated in Figure 13. A front endmounted moment compensator or a tor-sional vibration damper may be installedin a similar way.
If the limits for shear force and bendingmoment acting on the fore end flange ofthe crankshaft are exceeded, the statorhousing must be made with a front end
support bearing to reduce the load on thecrankshaft.
The electrical frequency generated de-pends on the speed of the main engineand the number of poles. Since the sizeof the alternator, and thus the number ofpoles, is limited by the ship’s hull, it is notpossible, with the low speed of the two-stroke engine, for the alternator itself toproduce electricity with a frequency of50 Hz or 60 Hz. It is therefore necessaryto use a static frequency converter sys-tem between the alternator and the mainswitchboard.
The static frequency converter system,see Figure 12, consists of a static part,i.e. thyristors and control equipment, anda rotary electric machine (synchronouscondenser).
The three-phase alternating current isrectified and conducted to a thyristor in-verter producing a three-phase alternat-ing current with constant frequency.
Since the frequency converter systemuses a DC intermediate link, it can supplyno reactive power to the main switch-board. A synchronous condenser is usedto supply this reactive power. The syn-chronous condenser consists of an ordi-nary synchronous alternator.
The DMG/CFE is normally able to oper-ate in parallel with the gensets at the fullrated electric power, when the speed ofthe main engine is between 75% and100% of the engine speed at specifiedMCR. Between 40% and 75% of theSMCR speed, the electric output of theDMG/CFE is reduced proportionately tothe engine speed.
The shipyard must provide seating for thesynchronous condenser unit and thestatic converter cubicles, as well as cool-ing water, if a water cooled alternator isused, and electric cabling.
Distribution cubicleSynchronouscondenser
Excitation cubicleConverter cubicle
Control cubicle
Static frequency converter systemTo
switchboard
Supportbearing
Cooler
Oil seal cover
Rotor
Stator housing
Fig. 12: PTO DMG/CFE
15
The following preparations for the installa-tion of the PTO system must be made onthe engine:
• three machined blocks welded on tothe front end face for alignment of thestator housing
• steel shim to be placed between thestator housing and the framebox tocompensate for the small differencein length between the bedplate andthe framebox
• electronic axial vibration measuringsystem.
References show that at present thePTO DMG/CFE is very rarely used.
4.6 Aft end installation (SMG/CFE)
The PTO SMG/CFE has the same work-ing principle as the PTO DMG/CFE, butinstead of being located on the front endof the engine, the alternator is installed aftof the engine, with the rotor integratedon the intermediate shaft. See Figure 14.This concept is much more frequentlyused than the PTO DMG/CFE, and hasthe advantages of a somewhat lower
price and a more straightforward designwith no physical interface with the mainengine.
In addition to the shipyard installationsmentioned in Section 4.5 for the PTODMG/CFE, the shipyard must provide thefoundation for the stator housing in thecase of the PTO SMG/CFE.
The engine needs no preparation for theinstallation of this PTO system.
Today, shaft alternators of type PTOSMG/CFE which are using PWM tech-nology allowing the inverter to produceboth the active and the reactive power,thus eliminating the need for the synchro-nous condenser, are on the market.Thereby, a simplification of the shaft gen-erator system with respect to installation,operation, and maintenance is obtained.
Stator housing Stator housingStuffing box Stuffing box
Crankshaft Crankshaft
Aircooler
Aircooler
Pole wheel
Standard engine, with directmounted generator (DMG/CFE)
Standard engine, with directmounted generator and tuning wheel
Mainbearing no. 1
Mainbearing no. 1
Supportbearing
Polewheel
Tuning wheel
Fig. 13: PTO DMG/CFE
Fig. 14: PTO SMG/CFE 1300-60 (AEG)
16
5. Layout of Engine with ShaftGenerator
Beyond the physical preparations of theengine for the installation of a shaft gen-erator, the layout of the engine in termsof power and speed is also affected bythe decision to install a shaft generator.
As an example the following describeshow the installation of a shaft generatoron an engine intended to drive a fixedpitch propeller influences the layout of theengine:
The specified maximum continuous rating(SMCR) of an engine without a shaft gen-
erator can be found on the basis of thepropeller design point (PD) by incorporat-ing the following factors, please refer toFigure 15:
• light running factor (normally 3-7% ofthe engine speed at PD is deducted)
• sea margin (traditionally 15% of thepower at PD is added – following theheavy running propeller curve and giv-ing the service propulsion point SP =S)
• engine margin (typically 10% of thepower at MP = M = SMCR is addedto the power at SP = S – followingthe heavy running propeller curve).
In most cases, the SMCR of an enginewith a shaft generator is found by addingthe maximum power consumed by theshaft generator to the specified propulsionMCR point (MP). See Figure 16.
Consequently, the engine with a shaftgenerator is specified with the maximumcontinuous rating (M), and the optimisingpoint (O), located on a propeller curveplaced to the left of the propeller curve(through MP) for heavy running propulsionwithout a shaft generator.
LR(5%)
Engine speed
Power
MP
Sea margin(15% of PD)
2 6
SP
2 Heavy propeller curve –hull and heavy weather
6 Light propeller curve –hull and calm weather
MP: Specified propulsion MCR pointSP: Service propulsion pointPD: Propeller design point
: Alternative propeller design pointLR: Light running factorHR: Heavy running
fouled
clean
PD´
HR
PD´
PD
Engine margin(10% of MP)
Fig.15: Ship propulsion running points and engine layout
17
The above means that the installation ofa shaft generator may involve that an en-gine with one more cylinder must be se-lected to ensure that the SMCR point isplaced inside the top of the layout dia-gram. However, this, of course, entailsextra costs and additional space require-ments for the main engine.
To avoid selecting a main engine with onemore cylinder, another solution is to re-strict the load on the shaft generatorwhen the engine is operated close to theSMCR.
More information about the layout of en-gines with and without shaft generators,as well as various examples, can befound in our paper P.254-01.04: BasicPrinciples of Ship Propulsion.
M: Specified MCR of engine
S: Continuous service rating of engine
O: Optimising point of engine
A: Reference point of load diagram
MP: Specified propulsion MCR point
SP: Service propulsion point
SG: Shaft generator power consumption
Definition of point A of load diagram:
Line 1: Propeller curve through optimising point (O)
Line 7: Constant power line through specified MCR (M)
Point A: Intersection between lines 1 and 7
Shaft generator
Propulsion curvefor heavy running
O7
1 2
Engine service curvefor heavy running
MP
SG
6
S
SG
SP
Engine speed
PowerA=M
Light propeller curve forclean hull and calm weather
Fig.16: Engine layout with shaft generator (normal case)
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6. Torsional Vibration
6.1 Engines with small shaftgenerators
The gear based shaft generator systems(PTO BW I ... BW IV) all incorporate aflexible coupling to protect the gearsagainst hammering caused by torsionalexcitation from the engine.
Normally, when the power output of theshaft generator is less than 10 per cent ofthe main engine power, the vibrationmodes of the shaft generator system willnot influence the vibration modes of thepropulsion shaft system. This means thatthe main propulsion shaft system can bedesigned and determined regardless ofwhether a shaft generator is to be in-stalled later on.
The PTO/GCR is normally designed tooperate at 100% of the speed at spe-cified MCR, and is therefore tuned sothat the critical speed of significant T/V-orders is placed outside the range 80 -120% of the speed at specified MCR.
Normally, the flexible couplings for thePTO/GCR types are selected on the ba-sis of the misfiring conditions, and thenormal service conditions will, conse-quently, usually be harmless to the flexiblecoupling and the gear.
In cases of misfiring, the 1st order excita-tion, which normally has a frequencyclose to the natural frequency of the 1-node vibration mode (the lowest naturalfrequency) occurring in the shaft genera-tor branch, increases substantially, irre-spective of the number of cylinders of theengine. This explains why it is essential totune the natural frequency of the 1-nodevibration mode in accordance with theengine speed.
The position of the natural frequency forthe 1-node vibration mode in the shaftgenerator branch mainly depends on theflexible coupling’s torsional flexibility andthe inertia of the alternator.
As a rule of thumb, the lowest natural fre-quency of the shaft generator branchshould not be less than 120%, or morethan 80%, of the frequency correspond-ing to the main engine speed at specifiedMCR. This means that either undercriticalor overcritical vibration conditions for 1storder excitation are obtained, with a sat-isfactory safety margin.
If a clutch for the alternator is incorpo-rated in the PTO/GCR, the tuning of thePTO system is normally made as follows:
• Alternator engaged:Overcritical running (1st order criticalspeed at 55 - 80% x SMCR speed)
• Alternator disengagedUndercritical running (1st order criticalspeed above 120% x SMCR speed,higher orders to be considered).
The ideal way of tuning the PTO/RCF,which is normally designed to operate be-tween 70% and 105% of the enginespeed at specified MCR, is to have a flex-ible coupling which leads to a natural fre-quency, around 50 - 60% of the fre-quency which corresponds to the speedat specified MCR.
Should the natural frequency be higher,because of a more rigid coupling, the al-ternator must be declutched in the eventof misfiring. The PTO/RCF is alwaysmade with a clutch built into the RCFgear, and operation during misfiring is of-ten prohibited. In this case, the magni-tude of the alternator’s inertia must,when the alternator is declutched, permitthe natural frequency of the shaft genera-tor branch which remains coupled to theengine, to ‘jump’ to a sensibly higher fre-quency than the frequency which corre-sponds to 105% of the speed at specifiedMCR. In order to obtain this, it may benecessary to tune the inertia of the alter-nator by fitting an additional mass (tuningwheel) at the alternator side of the clutch.
The DMG/CFE and the SMG/CFE do notincorporate a gear or a flexible coupling
but, because of the inertia of the rotor,they may naturally influence the torsionallayout of the shafting.
6.2 Engines with large shaftgenerators
Certain types of ships, such as shuttletankers with high electricity consumption,may use one or more large shaft genera-tors for the electricity production. Thepropulsion arrangement of a shuttletanker with a large shaft generator nor-mally comprises a controllable pitch pro-peller and a shaft clutch. See Section11.1.
The torsional vibrations of such an instal-lation are very complex, and require care-ful investigation of all possible operationmodes during the design stage.
In general, the elastic coupling, or coup-lings, should be sufficiently flexible to en-sure a natural frequency in the shaft gen-erator system below 75% of the fre-quency which corresponds to the mainengine speed to be used for operation ofthe shaft generator.
Alternatively, the shaft generator systemcan be designed to have a natural fre-quency of approximately 150% of the fre-quency which corresponds to the mainengine operating speed.
The above will give main critical reso-nances in the shaft generator system(4th, 5th and 6th order) at very low speedor even below the minimum speed. Fur-thermore, the 1st and 2nd order excita-tion, which becomes dominant in case ofmisfiring, will have resonance outside theshaft generator operating speed. Suchtuning of the natural frequencies will nor-mally require very elastic couplings.
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7. Engine Governing System
Basically, the following three types of gov-ernors can be used to control moderntwo-stroke camshaft diesel engines:
• conventional electronic governor withone speed pick-up and standard soft-ware
• advanced electronic governor withtwo or more speed pick-ups and spe-cial software
• mechanical-hydraulic governor (forsmall bore engines only).
The mechanical-hydraulic governor canbe used for simple installations involvingthe 26-46MC engines, where it may pro-vide sufficient governing abilities at an at-tractive cost. However, an electronicgovernor is also often used for the 26-46MC engines. All 50-98MC engines re-quire an electronic governor.
The installation of a shaft generator withan electric output power of less than15% of the main engine’s power atspecified MCR does not normally requirespecial considerations in the selection ofgovernor. However, if a PTO/GCR is in-stalled together with a controllable pitchpropeller, it may prove advantageous touse an electronic governor even on thesmallest engine types to obtain the moststable engine speed conditions and,hence, the most stable frequency in thegenerated electricity.
For large shaft generators, the combina-tion of a low natural frequency and a highmoment of inertia in the alternator mayrequire special facilities in the engine gov-ernor (i.e. the advanced governor) if insta-bilities in the system are to be avoided.For plants where the output power of theshaft generator exceeds 15% of the mainengine’s power, or a clutch or a flexiblecoupling is installed in the shaft line, wetherefore recommend investigatingwhether an advanced electronic gover-nor is needed.
For the electronically controlled ME en-gine, the governor functions are includedin the Engine Control System and thecontrol of an ME engine with shaft gen-erator is comparable to the control of acamshaft controlled diesel engine withshaft generator and electronic governor.
8. Pros and Cons (shaftgenerators versus gensets)
In the following, a number of advantagesand disadvantages related to the use ofshaft generators are discussed:
8.1 Advantages – shaft generators
• Small space requirementThe shaft generator is installed closeto the engine or the shaft line, and of-ten takes up no further space than isalready set aside for the engine instal-lation. In particular, the PTO BW III(engine side mounted) and the PTOBW IV (tunnel gear) require littlespace for the installation. The SMG/CFE and the DMG/CFE need extraspace elsewhere in the engine roomfor the synchronous condenser andthe control cubicles
• Low investment cost (PTO/GCR)The investment cost depends on thetype and make of the shaft generator.Depending on the origin of the shaftgenerator, the PTO/GCR can be pur-chased at a relatively low price,whereas the frequency controlledtypes (PTO/RCF and PTO/CFE) arerelatively expensive
• Low installation costThe shaft generator requires no sepa-rate (or only a simple) foundation, noexhaust gas system and only a fewconnections to the auxiliary equip-ment. Furthermore, the time spent in-stalling a shaft generator is normallyshort
• ReliabilityShaft generators are normally consid-ered highly reliable, as is the main en-gine which drives the shaft generator
• Low man-hour cost for maintenanceThe planned maintenance of a shaftgenerator during the first years of op-eration only involves regular checks ofproper functioning and regular re-placement of the lubricating oil andthe oil filter, if the shaft generator hasa separate lubricating oil system
• Low spare parts costThe high reliability of shaft generators,together with the low spare partsconsumption for planned mainte-nance, result in low spare parts costs
• Long lifetimeA shaft generator is generally not ex-posed to much wear, but of coursecomponents such as bearings, me-chanically driven oil pumps, frictionclutches, etc. need to be replaced orreconditioned after many years in op-eration
• Low noiseThe noise level of a shaft generator isconsiderably lower than the noiselevel of a genset.
8.2 Disadvantages – shaftgenerators
• No power production in harbourWhether a shaft generator is installedor not, the electric power consump-tion in harbour will generally have tobe covered by a genset. However, inspecial cases where a clutch is in-stalled in the shaft line, as on shuttletankers with high electricity consump-tion for cargo pumping, it is possibleto use the main engine and the shaftgenerator for electric power produc-tion in harbour
20
• Higher load on main engineThe load on the main engine, andthus the specific fuel oil consumptionand the cylinder oil consumption, in-crease when a shaft generator isused
• Reduced propeller and engine effi-ciency at low propulsion power forPTO/GCRIf the PTO/GCR is used for powerproduction with a fixed frequency,which is usually the case, the enginewith a controllable pitch propellermust be operated at constant speedeven at reduced load. The efficiencyof the controllable pitch propeller andthe engine would be higher if the en-gine was operated according to acombinator curve, where the speedwas reduced at reduced load
• No long-time parallel running abilityfor PTO/GCRThe PTO/GCR cannot run in parallelwith the gensets except during loadtake-over. This means that the powerdistribution between the electricpower producers is not as flexible aswith a pure genset installation
• More complex shaft arrangementThe installation of a shaft generatorcomplicates the shaft arrangement.Gears and flexible couplings need notbe installed for a two-stroke dieselengine used for propulsion if a shaftgenerator is not installed.
9. Economic Comparison
On the basis of the following data we have compared the operating costs, with andwithout a shaft generator, of a typical feeder container vessel.
The following alternatives have been compared:
Alternative 1: 1 x 7S50MC-C + PTO BW IV GCR/1200 + 2 x 7L16/24H
Alternative 2: 1 x 7S50MC-C + 3 x 7L16/24H
We have used the following ratings of the main engines:
Alternative 1: SMCR = 11,060 kW at 127 rpmCP-Propeller running at constant speed
Alternative 2: SMCR = 9,760 kW at 127 rpmCP-Propeller running at combinator curve
Propulsion load profile, Alternative 2:
• 90% load for 15% of time at sea• 80% load for 40% of time at sea• 70% load for 35% of time at sea• 50% load for 10% of time at sea
In order to compare two vessels operating at exactly the same speed, the propulsionloads for Alternative 1 have been increased to compensate for the reduced propellerefficiency at part load, caused by the propeller operating at constant speed.
Time at sea: 250 days/year
Time in harbour: 115 days/year
Electric load at sea: 900 kW
Electric load in harbour: 500 kW
We have compared the two alternatives with respect to fuel oil costs and lubricating oilcosts and found the following result:
Annual additional fuel and lube oil costs forAlternative 1= 9,500 USD/year
Details of the calculations are shown in Figures 17 and 18.
21
Additionally, we have analysed the maintenance costs for the two alternatives and estimatethat the average annual maintenance costs for a long operating period are as follows:
Alternative 1:
Main engine, 7S50MC-C: 98,200 USD/year
Shaft generator, PTO BW IV S50-C/GCR 1200 500 USD/year
Gensets, 2 x 7L16/24H: 4,700 USD/year
Total annual maintenance costs: 103,400 USD/year
Alternative 2:
Main engine, 7S50MC-C: 98,200 USD/year
Gensets, 3 x 7L16/24H: 25,200 USD/year
Total annual maintenance costs: 123,400 USD/year
Annual saving in maintenance costs for Alternative 1: 20,000 USD/year
The maintenance costs include man-hours for overhaul with an hourly wage of30 USD, and spare part costs in accor-dance with normal overhaul intervals. Themaintenance costs also allow for 30%extra man-hours for unscheduled over-hauls.
When the additional fuel and lube oilcosts and the expected savings in main-tenance costs are compared, Alternative1 turns out to be 10,500 USD cheaper inoperation per year than Alternative 2.However, the saving only representsaround 1.0% of the total annual operatingcosts.
Assuming a 30,000 USD extra invest-ment cost for the shaft generator com-pared with one extra 7L16/24H genset,the calculation shows a payback time forAlternative 1 (with shaft generator) ofthree years, using the Net Present Valuemethod with a 3% rate of inflation and a6% rate of interest.
However, the investment costs for theshaft generator and the genset may differsignificantly depending on the origin of theshaft generator and the genset.
The installation costs may be expected tofavour the shaft generator.
Other aspects also need to be consid-ered, e.g. in some cases a shaft genera-tor cannot be installed unless the engineis specified with one additional cylinder.The extra costs for the engine and itsauxiliary equipment related to an addi-tional cylinder, together with the eco-nomic impact of an increased engineroom length, have not been evaluated.
To conclude, many factors influence thefinal economic result, and the final con-clusion as to whether the installation of ashaft generator is attractive or not canonly be made by the owner.
22
Machinery arrangement
7S50MC-C with PTO
SMCR (kW) 11,060
Efficiency PTO (%) 92
2 x 7L16/24H gensets
Basic data
Total days at sea 250
Total days in harbour 115
HFO price (USD/ton) 140
System oil price (USD/ton) 800
Cylinder oil price (USD/ton) 900
Load pattern
Load case 1 2 3 4 5
Hours 900 2,400 2,100 600 2,760
Propulsion power (kW) 8,780 7,833 6,916 5,142 0
PTO mech. power (kW) 975 975 975 975 0
Main engine power (kW) 9,755 8,808 7,891 6,117 0
MEP (bar) 16.8 15.1 13.6 10.5 0
SFOC (g/kWh) 168.4 168.0 168.2 170.4 0
Genset power (kWel) 0 0 0 0 500
Fuel oil consumption
Load case 1 2 3 4 5 Total Cost (USD)
Hours 900 2,400 2,100 600 2,760
Main engine power (kW) 9,755 8,808 7,891 6,117 0
SFOC (g/kWh) 168.4 168.0 168.2 170.4 0
Genset power (kWel) 0 0 0 0 500
SFOC (g/kWhel) 0 0 0 0 202
HFO (tons/year) 1,571 3,773 2,961 664 296 9,265 1,297,100
(SFOC: ref. LCV = 42,700 kJ/kg)
(HFO: ref. LCV = 40,200 kJ/kg)
System oil consumption
Load case 1 2 3 4 5 Total Cost (USD)
Hours 900 2,400 2,100 600 2,760
Main engine (kg/24h) 31 31 31 31 0
Genset (kg/24h) 0 0 0 0 12
System oil (tons/year) 1.2 3.1 2.7 0.8 1.4 9.2 7,360
Cylinder oil consumption (based on 1.02 g/kWh and Alpha ACC with 3% sulphur)
Load case 1 2 3 4 5 Total Cost (USD)
Hours 900 2,400 2,100 600 2,760
Cylinder oil (tons/year) 9.0 21.6 16.9 3.7 0.0 51.2 46,080
Total cost per year (USD) excl. maintenance cost 1,350,540
Fig. 17: 7S50MC-C with PTO
23
Machinery arrangement
7S50MC-C without PTO
SMCR (kW) 9,760
Efficiency PTO (%) –
3 x 7L16/24H gensets
Basic data
Total days at sea 250
Total days in harbour 115
HFO price (USD/ton) 140
System oil price (USD/ton) 800
Cylinder oil price (USD/ton) 900
Load pattern
Load case 1 2 3 4 5
Hours 900 2,400 2,100 600 2,760
Propulsion power (kW) 8,784 7,808 6,832 4,880 0
PTO mech. power (kW) 0 0 0 0 0
Main engine power (kW) 8,784 7,808 6,832 4,880 0
MEP (bar) 16.2 14.8 13.2 9.9 0
SFOC (g/kWh) 166.6 165.5 164.9 168.6 0
Genset power (kWel) 900 900 900 900 500
Fuel oil consumption
Load case 1 2 3 4 5 Total Cost (USD)
Hours 900 2,400 2,100 600 2,760
Main engine power (kW) 8,784 7,808 6,832 4,880 0
SFOC (g/kWh) 166.6 165.5 164.9 168.6 0
Genset power (kWel) 900 900 900 900 500
SFOC (g/kWhel) 205 205 205 205 202
HFO (tons/year) 1,576 3,765 2,925 642 296 9,204 1,288,560
(SFOC: ref. LCV = 42,700 kJ/kg)
(HFO: ref. LCV = 40,200 kJ/kg)
System oil consumption
Load case 1 2 3 4 5 Total Cost (USD)
Hours 900 2,400 2,100 600 2,760
Main engine (kg/24h) 31 31 31 31 0
Genset (kg/24h) 24 24 24 24 12
System oil (tons/year) 2.1 5.5 4.8 1.4 1.4 15.2 12,160
Cylinder oil consumption (based on 1.49 g/kWh at nominal MCR and
reduced proportional to MEP at part load)
Load case 1 2 3 4 5 Total Cost (USD)
Hours 900 2,400 2,100 600 2,760
Main engine power (kW) 8,784 7,808 6,832 4,880 0
Cylinder oil (tons/year) 8.1 19.1 14.6 3.0 0.0 44.8 40,320
Total cost per year (USD) excl. maintenance cost 1,341,040
Fig. 18: 7S50MC-C without PTO
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10.Typical Applications
At present, the following shaft generatorsare most frequently installed:
PTO BW II/GCR
PTO BW III/GCR
PTO BW IV/GCR
PTO SMG/CFE
The PTO BW II/RCF and the PTO BW III/RCF are also installed from time to time.
Typically, the PTO BW II/GCR and thePTO BW IV/GCR are used on containervessels or chemical tankers with 26-50MC main engines (and CPP). The elec-trical output power is normally between500 and 1200 kW. The PTO BW III/GCRis mostly used for the same ship typeswith 50-60MC main engines (and CPP),and the shaft generator normally has anelectrical output power between 700 and1800 kW.
The PTO SMG/CFE is often installed onlarge container vessels with 70-98MCmain engines (and FPP) and a large num-ber of reefer plugs. For large containervessels, the shaft generator will often bespecified with an electric capacity ofaround 2,000-3,500 kW or even higher.
The PTO BW III/RCF has been used withall engines in the range from 42 to 90MCand can also be installed together withthe K98MC/ME-C and K108ME-C en-gines. For small ships with the smallestMC engines (26-35MC) with FPP, thePTO BW II/RCF with a power of around250-700 kW is occasionally used.
11.Special Applications
11.1 Shuttle tankers
Shuttle tankers, which load their cargofrom storage facilities at the field or di-rectly from the production platform, arewidely used to serve offshore oil fieldsfrom which pipeline connections are notpracticable.
High performance manoeuvring equip-ment is made necessary by the operatingprofile which, during loading of the ship,includes long periods of accurate dy-namic positioning at the field, by usingbow and stern thrusters as well as themain propeller. The time required for load-ing the oil depends on the loading facilitiesand may vary from one to ten days ineach round trip.
Shuttle tankers with 3 x 1750 kW bowthrusters and 2 x 1750 kW stern thrust-ers to match the above requirements arein operation.
The large side thruster power installed onthe vessels calls for equipment that canprovide sufficient electricity production.This means that large diesel generatorsor large shaft generators need to be in-stalled on all shuttle tankers intended fordynamic positioning operation. In order tominimise the total installed power of theauxiliary power producers, cargo pumpsdriven by electric motors are normally in-stalled. The cargo pumps are primarilyused to unload the cargo in port but mayalso be used in the field to distribute oilamong the segregated cargo tanks. Themaximum power consumption of thecargo pumps is typically around 4-5,000kW.
One propulsion system which meets theabove-mentioned requirements is thediesel-mechanical system with two lowspeed main engines and shaft genera-tors. Figure 19 shows an example ofsuch an arrangement, where the shaftgenerators are placed in the shaftlines.
Disconnectablethrust bearing
DGME
ME
DG
ME: Main engineSG: Shaft generatorDG: Diesel generator
Electricmotors
MESG
Cargopumps
Fig. 19: Shuttle tanker engine room arrangement with shaft generators
25
Clutches are installed in the shaftlines aftof the shaft generators, and are used todisconnect the propeller from the mainengine while in port, so that the main en-gine can be used for electric power pro-duction without turning the propeller.
When a clutch is positioned in theshaftline, an external thrust bearing is re-quired, so that both forward and aftwardthrust are transmitted to the tanktop, aftof the clutch. The clutch and the thrustbearing are normally made as a unit.
When a clutch is to be placed in theshaftline, a study of the engine accelera-tion behaviour must be performed, illus-trating the outcome of an immediate lossof electrical load on the shaft generatorwith the propeller disconnected.
Such a study normally results in the set-ting of minimum requirements for the in-
ertia of the alternator as well as require-ments for the control of the main engine,such as an advanced electronic governorand an additional overspeed shut-downsystem controlling a fuel cut-off device.As an additional safety feature it is rec-ommended that the flexible couplings aremade with a torsional limit device, so thatin the event of breakage of the flexible el-ements, steel parts will transmit thetorque until the safety system has shutdown the engine.
In harbour, the speed, and thus the effi-ciency, of the cargo pumps can be con-trolled by varying the speed of the enginethat drives the shaft generator and thusvarying the electrical frequency (in a pro-pulsion plant with two main engines, theother engine is at standstill and accessiblefor overhaul).
A frequency converter laid out for theship’s power consumption is requiredwhere the shaft generators are to beused for power supply for the generalelectrical consumption of the ship in allload conditions (harbour, steaming, anddynamic positioning) and where, in someof these load conditions, the enginespeed is not kept at a constant level.
Alternatively, one diesel generator can beused to supply the electric power for thegeneral electricity consumption when theengine speed is reduced.
In all cases, transformers are needed toprovide voltage regulation between thealternators and the ship service switch-board.
Fig. 20: An 8,600 kW PTO arrangement for a shuttle tanker, being tested at the Renk works
26
11.2 Auxiliary propulsion system
From time to time, an auxiliary propulsionsystem (Power Take Off / Power Take In)is requested, especially in connectionwith projects involving gas and chemicaltankers with main engines in the range ofS35-S42MC and equipped with a CP-propeller.
The auxiliary propulsion system must becapable of driving the CP-propeller by us-ing the shaft generator as an electric mo-tor while the main engine is stopped anddisengaged. The electric power is pro-duced by a number of gensets.
MAN B&W Diesel can offer a solutionwhere the CP-propeller is driven by thealternator via a two-speed tunnel gearbox. The main engine is disengaged by aclutch (Alpha Clutcher) made as an inte-gral part of the shafting. The clutch is in-stalled between the tunnel gear boxand the main engine, and conical boltsare used to connect and disconnectthe main engine and the shafting. SeeFigure 21.
The Alpha Clutcher is operated by hy-draulic oil pressure which is supplied bythe power pack used to control the CP-propeller.
A thrust bearing, which transfers the aux-iliary propulsion propeller thrust to the en-gine thrust bearing when the clutch is dis-engaged, is built into the Alpha Clutcher.When the clutch is engaged, the thrust istransferred statically to the engine thrustbearing through the thrust bearing builtinto the clutch.
To obtain high propeller efficiency in theauxiliary propulsion mode, and thus alsoto minimise the auxiliary power required,a two-speed tunnel gear, which provideslower propeller speed in the auxiliary pro-pulsion mode, is used.
The two-speed tunnel gear box is madewith a friction clutch which allows thepropeller to be clutched in at full alterna-tor/motor speed where the full torque isavailable. The alternator/motor is startedin the de-clutched condition with a starttransformer.
The requirements of some classificationsocieties differ depending on whether theauxiliary propulsion system is to be usedas a take home system (in the event offailure of the main engine at sea) or as analternative propulsion system (take awayfrom quay or alternative propulsion at lowvessel speed).
The auxiliary propulsion system offered byMAN B&W Diesel fulfils the requirementsof both alternatives, provided that suffi-cient electrical power for auxiliary propul-sion of the vessel can be produced bythe gensets.
The system can quickly establish auxiliarypropulsion from the engine control roomand/or bridge, even with unmanned en-gine room.
Re-establishing of normal operation re-quires attendance in the engine roomand can be done within a few minutes.
Oil distributionring
Generator/motorHydrauliccoupling
Two!speedtunnel gearbox
Alpha Clutcher Main engine
Intermediatebearing
Flexiblecoupling
Hydrauliccoupling
Fig. 21: Auxiliary propulsion system
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12.Summary
A wide range of shaft generators areavailable for installation in combinationwith the two-stroke engines in MANB&W Diesel’s comprehensive engineprogramme.
The shaft generators are available with orwithout frequency control systems. Someof them use step-up gears, some ofthem do not.
The installation of a shaft generator af-fects the layout of the engine relative tothe propeller, and the shaft generatormust be included in the torsional vibrationcalculations. Shaft generators with nor-mal electric capacity (less than 15% ofthe SMCR power) typically do not influ-ence the requirements of the engine gov-erning system. However, especially withshaft generators with no frequency con-trol system, the stability of the enginespeed needs to be considered.
Shaft generators can be used in specialapplications, including shuttle tanker pro-pulsion arrangements where the enginecan be disconnected from the propellerand used to drive a large alternator supply-ing electric power for the cargo pumps.
In rare cases, an auxiliary propulsion sys-tem is requested, in which the shaft gen-erator is used as an electric motor todrive the propeller, with the main enginedisconnected (the electric power is pro-duced by a number of gensets), andMAN B&W Diesel is able to offer a tailor-made concept, including the CP-propel-ler.
References show that most MAN B&Wtwo-stroke engines are at present in-stalled without a shaft generator. This re-flects the fact that many shipowners andshipyards rather than using a shaft gen-erator, prefer the simple engine room ar-rangement with a directly coupled two-stroke engine for propulsion, and a num-ber of gensets for electricity production
PTO/GCR Engine typeEngine typeEngine typeEngine typeEngine type UnitsUnitsUnitsUnitsUnits(Renk, A. Friedr. Flender AG, (BW IV/GCR)Newbrook) 35MC 2
42MC 1050MC 1960MC 1780MC 2 50(BW III/GCR)42MC 250MC 2160MC 2870MC 4 55(BW II/GCR)26MC 1635MC 2542MC 446 MC 450MC 3 52Total 157
PTO/RCF Engine type Units(Renk) (BW III/RCF)
42MC 550MC 2560MC 4470MC 2480MC 18 116(BW II/RCF)26MC 535MC 942MC 180MC 2 17(BW I/RCF)70MC 2 2Total 135
PTO/CFE Engine type Units(Renk) (BW III/CFE)
60MC 670MC 9 15Total 15
Reference List (Renk, A. Friedr. Flender AG, Newbrook), as at 2004.01.21
purposes. This may be because, in re-cent years, gensets have improved theircost effectiveness thanks to low prices,operation on heavy fuel oil, improved reli-ability and prolonged mean time betweenoverhauls.
On the other hand, when surplus capac-ity is available from the main engine, ashaft generator is still a viable solution.
References covering the supply of gearbased PTO systems from three majormakers are listed below.
