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AGMA Technical Paper
Combined Marine Propulsion Systems: Optimization and Validation by Simulation By B. Pinnekamp, F. Hoppe and M. Heger, RENK AG
Combined Combined Marine Propulsio Propulsion n Systems: Systems: Optimizati Optimization on and Validation V alidation by Simulation Burkhard Pinnekamp, Franz Hoppe and Moritz Heger, RENK AG
[The statements and opinions contained herein are those of the author and should not be construed as an official action or opinion of the American Gear Manufacturers Association.]
Abstract ModernNavy and Coast ModernNavy Coast Guard Guard Vessels essels usuallyhave usuallyhave combine combined d propuls propulsion ion system systems s using using gas turbin turbines,diesel es,diesel engine eng ines s and electric electric mot motors ors as main main propul propulsor sors. s. Desire Desired d ope operat rating ing profil profiles es dem demand and for individu individual al optimization of the gear propulsion system with respect to efficiency, noise, operational flexibility and capital cost. Combined Combined systems are complex and therefore sensitive sensitive to dynamic dynamic excitation and resonance. resonance. To avoid unfavo unf avorab rable le dynami dynamic c effec effects, ts, it is necess necessary ary to valida validate te candid candidate ate arrang arrangeme ements nts using using mod modern ern too tools ls like like multi multi body simulation. The paper describes describes the evaluation evaluation process process for optimized optimized combined combined marine marine propulsion propulsion systems systems and system system validation by dynamic simulation. Copyright 2012 American Gear Manufacturers Association 1001 N. Fairfax Street, Suite 500 Alexandria, Virginia 22314 October 2012
ISBN: 978 978---1-1 --61481-614 81--039 -039---1
Combined Marine Propulsion Systems: Optimization and Validation by Simulation Dr. Burkhard Pinnekamp, Dr. Franz Hoppe and Moritz Heger, RENK AG
Introduction The maritime world experiences significant changes concerning tasks andmissions; globalprotectionof trade sea ways plays an increasing role. Considering constraints with defense budgets, future Naval concepts demand a reduction in the number of vessels and, at the same time, more efficiency and flexibility of propulsion systems. Various alternatives of mechanical and electric drive propulsion systems may be considered for future Naval ship building programs. The selection of the most appropriate propulsion system depends on the vessel platform and the intended mission profile. The most appropriate alternative enables optimum power management and minimum fuel oil consumption. A combined system using diesel engines for loitering, cruising, and --depending on vessel size and desired speed-- gas turbines for sprint speed is the optimum mechanical drive system. For modern hybrid marine propulsionsystems, electric motors more and more take over therole of a diesel engine for cruise speed in a combined plant, adding operational flexibility and other advantages. The heart of a combined propulsion system is a reduction gear which enables the flexible arrangement of prime movers and can thereby support various vessel missions. The following list shows examples for combined marine propulsion systems: S
CODAD (COmbined D iesel A nd D iesel);
S
CODELOD (COmbined D iesel-- ELectric O r D iesel);
S
CODOG (COmbined D iesel O r G as turbine);
S
CODAG (COmbined D iesel A nd G as turbine);
S
CODELAG (COmbined D iesel-- ELectric A nd G as turbine).
This paper provides an overview on the engineering background froma gear manufacturer’s perspective, and proposes considerations for the optimized propulsion solution. The theoretical validationby advanced calculation methods is also described.
Combined marine propulsion systems Overview
Table 1 shows an overview of different propulsion system installations comparing hybrid mechanical and hybridelectric propulsion. Mechanical systems were continuouslydeveloped until approximately one decade ago from simple CODAD solutions to highly complex CODAG arrangements with a cross connect gear. In parallel, electric systems were increasingly accepted throughout the maritime world. Any of the listed configurations have their specific advantages and should be considered reflecting the vessel’s needs. Driving factor for the selection of a certain configuration is not only low investment cost but also life cycle cost considering specific fuel consumption, maintenance intensity and overhaul periods as well as reliability and redundancy. The missions determine the platform, the platform determines the propulsion system and the propulsion system determinesthe reductiongear system. The potential prime movers in a propulsion system are given – their weights, dimensions and performance are individually selected. The prime movers can be considered black boxes. A flexibly designed reduction gear facilitates S
the matching and combining, if applicable, of the optimum prime movers with the propulsors,
S
the location of the prime movers in the machinery space, and
S
the optimum operation and maximum fuel efficiency of the prime movers.
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Table 1. Propulsion system variants and existing applications for surface combatants Propulsion type
Description
Installed power range, MW
CODELOD
Combined electric motor or diesel DE engine EM
5 ... 10 0.3 ... 0.6
CODELAD
Combined electric motor anddiesel DE engine EM
5 ... 10 0.7 ... 1.5
CODOG
Combined diesel engine or gas DE turbine GT
2.5 ... 9 15 ... 22
Typical application S S
Netherlands Navy OPV Korean Coast Guard OPV
No reference available yet S S
German Navy F123 ROKN FFX
Combined diesel and gas turbine DE (with or without cross connect GT gear)
5 ... 9 20 ... 36
With cross connect gear: German Navy f124 U.S. Coast Guard NSC Without cross connect gear: U.S. Navy -- Lockheed Martin LCS -- Austal LCS
CODELOG
Combined electric motor or gas EM turbine GT
2 ... 3 20 ... 32
S
Italian Navy FREMM
CODELAG
Combined electric motor and gas EM turbine GT
4 ... 6 20 ... 25
S
German Navy F125
S
CODAG
S
S
The reduction gear is a key determinant in developing a propulsion system that will meet the vessel’s operating envelope and will fit the machinery space physical envelope. The overview presented herein provides an overview and trade--off analysis of principal mechanical drive systems and can serve as a reference and sample to identify the optimum propulsion system for a vessel. At least ten major parameters need to be considered in determining the optimum propulsion system: S
S
S
S
Missions Operating profile Power Efficiency and fuel consumption
S
Weight
S
Dimensions
S
Maintenance and repair cost
S
Location flexibility
S
Survivability and redundancy
S
Signature
Once the propulsion system has been selected, the final arrangement can be further adjusted and optimized to fit within the design parameters. Operating speed profile
For decades, different prime movers have been combined to allow for flexible operation of propulsion plants. As a basis for the layout, the assumed speed profile of the vessel needs to be known. In this context, the comparison between former speed distribution assumptions and today’s approach is interesting. The two graphs in Figure 1 show typical operating profiles of frigates – 20 years ago and today. Thechange of operational demands is obvious: With a modern frigate concept, operation in slow speed or loiter mode has almost doubled, whereas sprint speed of 30+ knotsis rarely considered, theUS Navy LCS is the major exception fora surface combatant with versatile mission deployments.
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a) Frigate type CODOG F123 5400 t (1990)
b) Frigate type CODELAG F125 6800 t (2010)
Figure 1. Design operating profile (Source: German Naval Headquarters) Why combined propulsion plants?
A gas turbine is compact, light--weight and high--powered. It requires reasonable maintenance. However, when it operates at less than full or near--full load, its specific fuel consumption increases significantly. A high speed diesel engine has a high thermal efficiency over a broad range of loads and accordingly maintains economic and level specific fuel consumption over those loads. Combined systems take consideration of the above mentioned aspects for an optimized combined system. Combined propulsion system examples
CODAG is the result of a further development in propulsion systems, mainly derived from CODOG. A perfect CODAG system integration was performed in Germany in the late 1990s. The first German Navy F124 Frigate with CODAG propulsion (Figure 2) was put in operation in 2001. The main factors in the successful development of the F124 CODAG plant are as follows: S
S
S
Experience derived from CODOG and CODAD applications, e.g., self synchronizing overrunning clutch; light weight fabricated casings and carburized double helical gears; Optimized multi--disk clutch arrangement with hydraulic controls, lubrication and assembly on one common shaft; Ship control system and gear sub--controls following latest electronic standards are perfectly integrated using a programmable logic control unit (PLC) for operation, guarding and BUS data exchange. Local monitoring and operation is facilitated comfortably via the PLC touch screen.
The Lockheed Martin LCS monohull type (Freedom class) of the US Navy features a completely different propulsion train technology. Four water jets are driven by two symmetrically arranged CODAG systems, where the high speed combining gear and the low speed splitter gear are separately installed, linked by long intermediate shafts each, seeFigure 3. Gas turbines and dieselengines canbe engaged separately in cruise or higher speed propulsion modes, or jointly drive the water jets achieving a top speed in excess of 40 knotsin this CODAG mode. To match with different required water jet speeds, the diesel engine inputs are equipped with two gear stages where the adequate gear ratio is selected via a multi disc clutch engagement.
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Figure 2. F124 CODAG gear plant schematic
Figure 3. U.S. Navy Lockheed Martin LCS CODAG gear plant
More examples for combined propulsion are given with the variants for the propulsion system of a sample frigate in the following paragraphs.
Sample for propulsion system variants To demonstratevariants andevaluation of suitable combined propulsion systems, a 5200 ton frigate is chosen as an example. Mission profile and power requirement
The total required prime mover power is derived from theaverage power per ton of comparable Naval vessels and experience. Therequired installed prime mover power to achieve 29 kts fora 5200 tons frigate is approximately 40 MW. Figure 4 shows a simplified power vs. speed curve for a cubic law correlation. The frigate is assumed to operate underway approximately 150 days, i.e., 3600 hours per year.
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Figure 4. Power requirement for a 5200 ton frigate vs. speed
Based on current ship programs and experience, the speed profile as shown in Table 2 is applied for the evaluation. Prime movers and basic propulsion system arrangement
The propulsion system is based on diesel engines of approximately 1000 to 1300 rpm and gas turbines of 3300 to 3600 rpm. Diesel engines provide better fuel efficiency across the load and speed spectrum than other fossil fueled alternatives like gas turbines. Diesel engines maintain reasonable efficiency down to low load levelsof approximately 15%. Gasturbines feature lowdead weight andvolumeat rated power andthereforeprovide higher power densitythan dieselengines. A combined propulsion system,combiningtwo or more prime movers to one or more propulsors, further optimizes speed and endurance. Diesel engines and gas turbines of different manufacturers are considered in this paper. However, for any propulsion system configuration, alternative manufacturers can be adopted without major changes to the basic features of the propulsion system performance. The frigate is assumed to have two propeller shafts with CPPs and will be operated using both propellers except in case of emergency. Requirements summary
Table 3 shows an overview of the assumed basic specs for thefrigate. Any changes to these specs may have an impact on the final selection of prime movers and referring gear specs but will basically not affect the comparative aspects of the different propulsion system arrangements.
Propulsion system arrangements Overview
Five different propulsion system arrangements are investigated as potential solutions for the frigate’s power plant. The features and performance of each arrangement are evaluated against pertinent parameters listed earlier. Table 4 shows an overview of the subject arrangements. Table 2. Frigate mission profile Mode
Idle Loitering Low patrol High patrol Transit Sprint
Speed, kts 0 6 13 18 24 29
% of time 5 5 10 60 15 5
7
Propeller shaft power, MW 0 0.35 3.5 9.3 22.1 39.0
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Table 3. Frigate basic specs
Displacement Sprint speed Operating hours Propulsion power Propulsors Prime movers
5200 metric tons 28+ kts 3600 per year approx. 40 MW 2 CPP High speed diesel engines Gas turbines Electric motors Table 4. Propulsion system variants A: CODAD
B: CODELOD
C: CODOG
MW
@ rpm
Fuel1), g/kWh
No
DE 20V
9.1
1150
195
4
DE 12V
5.4
1000
195
2
(10.8)
GT1
21
3600
225
2
42
GT2
32
3300
225
Gen-- set DE 12V
5.2
1000
210 2)
1
(5.2)
2
(4.8)
Electric motor 1
2.4
1800
228 3)
Electric motor 2
4.7
150
228 3)
Power, MW
No
36.4
4
Power, MW
No
Power, MW
D: CODELAG with CC No
36.4
Power, MW
E: CODAG with CC No
Power, MW
0
2
18.2
0
1
21
1
32
2
(10.4)
2
9.4
Total input shaft power
36.4
36.4
42
41.4
39.2
Max speed, kts
28.0
28.0
29.4
29.2
29.0
NOTES: 1) At nominal load. 2) Generator efficiency considered. 3) Generator, converter, and electric motor efficiency considered.
A final solution is not restricted to one of these proposed arrangements but could also be any combination of individual prime movers, gear components, andpropulsors. However, the basic performance features canbe clearly seen from the comparison and a selection is hereby facilitated. Variant A: CODAD
The CODAD system features more than one diesel engine per propulsor and offers significant flexibility in running either of the engines individuallyor both(or more) simultaneously. The proposed system comprises 4 equal diesel engines, rated 9.1 MW, where two of which are connected to each, port and starboardcombining gear. Figure 5 shows one of many possible propulsion systems, Figure 6 the principal gear arrangement of a different configuration.
Figure 5. CODAD propulsion system
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Figure 6. CODAD gear arrangement variant
CODAD enables the propulsion system to run up to 80% of sprint speed (i.e., 22 kts) with only two of four diesel engines in operation, saving operating time and providing the opportunity for maintenance on non--operating engines while underway. In case of damage to one of the diesel engines, up to three engines can still be used retaining a max speed of 25 kts and full maneuverability. Variant B: CODELOD
A fully integrated sole electric propulsion system (IEP) is not considered for the following reasons: S
S
S
S
A combined propulsion system brings prime movers on line as required and canminimize operating hours of individual units thereby reducing maintenance and repair. With a well designed system, a mechanical propulsion has a full load efficiency in the range of 98% whereas an electric drive has an efficiency of about 90--92%. This discrepancy would be even more significant considering operation at low loads. Electric propulsion requires extensive electric and electronic equipment which easily compensates for any benefit with respect to required deck area, weight, survivability and reliability. The overall investment cost for electric propulsion is higher than for a mechanical solution.
A completely different approach is a combined mechanical and diesel electric system, often called a hybrid propulsion system. Such systems feature a main mechanical propulsion with diesel engines and a power-take--in (PTI) operated with an electric motor for loitering at low power with very low noise generation and still acceptable fuel efficiency, as the nominal load of the motor is moderate. Power generation for the electric motor can be intelligently combined with the integrated ship power system and temporarily be used for hotel and electrical weapon systems. Figure 7 shows the principal CODELOD propulsion system, Figure 8 the gear arrangement. There are multiple ways of connecting the electric motor to the gear, where two different solutions are shown in the principal and the gear arrangement.
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Figure 7. CODELOD propulsion system
Figure 8. CODELOD gear arrangement
CODELODsystemsfeaturea favorablecombination of the characteristics of a mechanical system withdiesel engine propulsion and an electric drive. For higher maximum propulsion power requirements, the diesel engines may be substituted by gas turbines, building a CODELOG ( COmbined Diesel-- ELectric Or Gas turbine) system. The newly launched USS Makin Island utilizes such a CODELOG approach. It has two GE LM 2500+ gas turbines as prime movers and two 3.5 MW electric motors for auxiliary propulsion. Variant C: CODOG
The CODOG configuration without a cross connect gear, Figure 9, has been applied on more than 50 frigates by many Navies worldwide. It includes two individual propulsion systems for the two propellers. A simultaneous use of diesel engines and gas turbines is not possible – they can only be used either/or. Power is automatically switched from one to the other prime mover by means of automatic, self synchronizingoverrunning clutches. CODOG is the predecessor of CODAG and is easier to control. It is particularly applicable where the power of the diesel engines is low compared to the gas turbines, so that adding the diesel engine power to the gas turbine power would only result in a very low incremental speed. Figure 10 shows a typical CODOG gear arrangement.
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Variant D: CODELAG
Figure 11 shows an advanced CODELAG propulsion system configuration. The center arranged gas turbine is only used just for high transient and sprint speed (> 17 kts), the majority of the operational time is covered with the two electric motors providing a power of 4.7 MW each. Theelectric motors are located in line with the propeller shaft in front of the main gears. By means of specifically developed propeller shaft clutches (APC) integral with the main reduction gear, the shaft lines are disengaged from the gears in electric mode. Hence, the reduction gear set is designed only for the gas turbine mode with the gears being at standstill in electric mode. In gas turbine mode, the electric modes can be disengaged with separate multi disk clutches. In CODELAG mode, all prime movers add their power for sprint speed propulsion.
Figure 9. CODOG propulsion system
Figure 10. CODOG gear arrangement
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Figure 11. CODELAG propulsion system Variant E: CODAG with cross connect gear
A CODAG system with a cross connect gear was first introduced for the GermanNavy type 124 frigates. The first propulsion system was delivered in 1999. The basic principle is the same for the propulsion system variants shown in Figure 12 and Figure 13. The center arranged gas turbine is only used for high transient and sprint speed (> 18 kts), the majority of the operational time is covered with the two diesel engines featuring a power of 9.1 MW each. The system can be operated fully flexible with one diesel engine driving both propellers, two diesels separatelydriving the referring propellers, the gas turbinedriving both propellers or, for sprint speed, with all prime movers commonly used for maximum propulsion power.
Propulsion system evaluation Fuel economy
Considering the specific fuel consumption of the prime movers for the four propulsion configurations, the fuel consumption per nautical mile vs. vessel speed is shown in Figure 14. Gear efficiency is considered for all fuel consumption figures, electrical conversion losses are taken account for with the CODELOD and CODELAG solutions. In a patrol of 5000 nm accordingto the mission profilelisted in Table 2, the frigate would consume fuel (propulsion only) as listed in Table 5 for the different propulsion systems. The overall consumption is very close for all configurations.
Figure 12. CODAG propulsion system schematic
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Figure 13. CODAG propulsion plant for the U.S. Coast Guard national security cutter
Figure 14. Specific fuel consumption
Table 5. Total fuel consumption for a 5000 nm patrol in metric tons
5000 nm patrol (tons)
CODAD
CODELOD
CODAG
680
663
703
13
CODELAG w/CC
717
CODAG w/CC
722
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Maintenance and repair
Maintenance and repair costs are determined by the hours of operation and loads on the prime movers. Combined systems operate only the prime movers which are needed to provide the actually required propulsion power. Thereby, fuel is saved by operating at a more favorable specific fuel consumption and, more importantly, operating hours accumulate on reduced basis per individual prime mover thus enabling longer intervals between maintenance and lowering repair cost. Figure 15 shows the cubic propeller curve, the mission profile as per Table 2, and the required prime movers to achieve the referring vessel speed. It is obvious, that combined systems and multiple prime movers can save operating time for the individual prime mover and thereby significantly extend the time interval between maintenance periods. Accumulated operating hours for dieselengines andelectric motors aresummarized in Figure 16. Location flexibility
Leading gear makers have demonstrated their ability to design, manufacture, and install reduction gears in the new forms of mono-- and multi--hulls with very confined engine room space. Additionally, combined propulsion systems are designed on an open architecture basis in respect to the ability to operate with the popular platform management systems and with any appropriate prime mover or propulsor.
Figure 15. Prime movers in operation
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Figure 16. Accumulated operating hours Survivability
Three of the reduction gear alternatives: S
CODAD
S
CODELOD
S
CODOG
involve individual gear units which solely serve prime movers dedicated to an individual propeller shaft. Therefore, the engine compartments can be separated by a longitudinal, water tight bulkhead. If one engine compartment sustains damage, the other can survive and provide propulsion power. CODELAG and CODAG with a cross--connect gear have a common engine compartment for the reduction gear system as the main gears are mechanically linked by the cross--connect gear. The gear compartment is between the aft engine compartment and the ahead diesel engine or the gas turbine compartment and separated in each case by a transverse, water tight bulkhead. The individual gears can be built water tight and operate under a water depth of up to six meters over the propeller shafts. The gears can be engaged and disengaged on a remote basis even if under water. If all automatic control systems fail, the gears can be manually engaged and disengaged. The CODELAG and CODAG plant with a cross--connect gear can sustain various types of damage and still enable ship propulsion as listed in Table 6. Considering all criteria, CODELAG or CODAG with CC turn out to be the arrangements with the highest redundancy in case of partial damage. Table 6. Damage and remaining operation modes for CODELAG or CODAG with CC Type of damage
Resulting operating mode
One EM/DE inoperable
GT or other EM/DE operating on both propellers
Two EM/DE inoperable
GT operates one or two propeller shafts
Cross connect gear inoperable
Each EM/DE drives its own propeller shaft
One main gear and cross connect gear EM/DE on other main gear and referring shaft inoperable Gear compartment flooded
All modes possible
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Signature
The key enabler in a combined propulsion system is the reduction gear which transmits the power from any or all of the prime movers to the propulsors as speed and operational conditions require. The gear system permitsthe prime movers to be located at favorable positions within theengine compartment(s). This requires complex gear transmission systemswhere the power transmittinggears as well as the idlinggear meshesare a potential source of air borne and structure borne noise. Optimum gear design considering basic parameters such as module, helix angle, pressure angle and face width matched with grinding tolerances to plus or minus several microns and furthermore high quality manufacturing can produce a gear system that confines structure borne noise to a level which is below customer specification. This is even possible without elastic mountings or dampening fluid couplings. There are only few gear manufacturers worldwide who succeeded in producing gears meeting the required noise levels for frigates like the example discussed in this paper. Further improvement of noise development can be made with CODELOD in EM drive mode. The electric power can be generated within an encapsulated area of a gen--set and little torsional vibration excitation is transmitted to the drive train. Ultimate low noise drive is possible with CODELAG, where the complete gear train can be disconnected in electric drive mode with the gears remaining at standstill. Hybrid drive evaluation
The hybrid variants CODELOD and CODELAG have the following advantages over CODAD, CODOG and CODAG: S
S
S
S
S
Maintenance and overhaul of main DE reduced in proportion with electric motor use. Electric motor maintenance is insignificant. Low power operation of GT (especially with CODOG) with extremely high fuel consumption can be avoided. In low load operation, deposits may form in DE combustion chambers. DE operation at less than 10% load limited to between 40 and 100 hours before relievinghigher load operation of several hours required. Fuel consumption and emissions (soot and other harmful atmospheric emissions) reduced as low load main diesel shifted to high load generator set. Loitering operation with extremely low noise emission, especially if gears can be disengaged.
System validation by advanced calculation Combined marine propulsion systems are complex mechanical systems with multiple potential failure modes regarding individual components and system behavior. Traditional design calculations, according to a classification society, only provide a certain level of risk reduction, especially for individual components. Cost and tight schedule do not allow for extended harbor and sea trials and rectification of unexpectedly upcoming failures and system performance deficiencies. The small numbers of produced gear units for a specific application make it impractical on a cost and time basis to test and optimize new designs of propulsion systems with prototypes. Prototype testing needs to be replaced by a conservative design approach, primarily based on experience over several decades and common calculation methods. To further reduce the risk of system based problems, the traditional methods are increasingly supported with the use of numeric simulation calculations. Well established methods such as torsional or lateralvibration calculations only provide partial informationon increasingly complex systems and transient mode dynamic behavior. The shortcomings can be reduced or even overcome with a modern dynamic calculation tool such as Multi Body Simulation (MBS). With the help of MBS, the complete propulsion system is transferred to a computer model using standard elements such as masses, spring stiffnesses and damping parameters. With this numericmodel, the dynamic behavior of the complete system canbe investigated withrespect to eigenmodes and referring critical excitations. Moreover, within the so--called timedomain simulation, MBS caninvestigate
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the response of the system on normal or unusual operating modes. The mode in question is applied to the model by a force or torque over time signal applied to thereferring place in thesystem. Thedynamic response at any other place of the model reveals the dynamic loads on the referring components. Multi body simulation (MBS) -- E xample
As an example, the application of time domain simulation within MBS is demonstrated for an electric propulsion system with reduction gear as shown in Figure 17. In case of a short circuit in the electric system, the motor can generate a large alternating torque. The simulation is performed to confirm the load factors (application factor, dynamic factor) in the gear calculations. The MBS model, Figure 18, consists of separate rigid bodies (shown in black) with specific mass and geometry. These bodies are connectedto theinertial system, which in this caseis theship’s hull, or with other bodies by joints, shown in blue. Joints describe the degree of freedom of movement between the connected bodies or relative to the inertial system. In case of this model, all bodies have one degree of freedom in rotation around the fore and aft ship direction, limiting this model to a torsional vibrations model as a special case of an MBS model. Interaction between the bodies is modeled by force elements. In this case force elements represent torsional stiffness of coupling and shafts as well as tooth contact forces between gear wheels, forces and damping at the propeller, and a time dependent exciting short circuit torque at the E--motor output shaft. A problem commonly experienced in model building for MBS is the availability of system parameters, either due to the components not yet being finally designed, or due to a generally poor knowledge of component parameters. Mass and stiffness data can be calculated to a relatively high accuracy from the parts’ geometry, or are provided by component suppliers, e.g., for elastic couplings. Mass and damping data, including the dynamic impact of thesurrounding data, canbe estimated if exact dataare notavailable. The E--motor torque curve for this model is based on data as typically given by the motor supplier based on test bench measurements.
Figure 17. Electric propulsion system
Figure 18. MBS model of ship propulsion system shown in Figure 17: black -- bodies; blue -- joints; red -- forces
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The damping parameters are generally not very well known for most components and usually have to be estimated based on experience or reference testing. Damping parameters in propulsion systems have no impact on the determination of the eigenmodes; main focus of dynamic calculations is to avoid such modes. Therefore, the lack of reliable damping parameters does not limit this part of the MBS calculations. However, limitation for not calibrated models (i.e., models where the damping parameters are not derived from experiment) lies in limitedaccuracy when determiningpeak torques/forces in the time domainsimulationfor specific torque or force excitation as described in this example or for operation conditions in resonance modes which may be transient or cannot be avoided for specific reasons. The model presented uses damping parameters at the propeller based on approximation algorithms and in the gear mesh based on default values from other models. The simplified MBS 3D model is shown in Figure 19 for the same system as shown in Figure 18. Gear wheels and tooth contact are modeled in detail to simulate variable stiffness for different positions on the path of contact and potential negative tooth load, meaning alternating torque and gear hammering. Gear hammering leads to increased tooth contact forces upon re--contact of the tooth flanks and unwanted gear noise. With respect to system dynamics, hammering adds a variable stiffness to the model, which may cause chaotic system behavior. This condition can only be handled by numeric simulation such as MBS rather than by analytic methods. The model calculation is started and transferred to a steady state operation point in the first 10 seconds of simulation time. After these 10 seconds, the torque excitation as shown in Figure 20 is simulated at the electric motor shaft, starting with time “0” in the diagrams.
Figure 19. Simplified 3D model -- joints and force elements not shown
Figure 20. Simulated electric short circuit dynamic torque excitation at the electric motor shaft
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The resulting input torque at the gear box is shown in Figure 21. It can be derived from this graph that the dynamic peak torque as compared to nominal torque reaches a factor of approximately 1.7 for this propulsion system with the defined short circuit dynamic torque excitation. The resulting tooth contact force in the gear mesh is shown in Figure 22. The maximum dynamic tooth load reaches a factorof approximately 1.8 as compared to nominal tooth load. This is to be considered to cover both, application factor, K A, and dynamic factor, K v. Negative torque figures and consequential tooth hammering do not occur.
Figure 21. Resulting torque at the gear box input flange
Figure 22. Resulting tooth contact force in the gear mesh
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General considerations on multi body simulation
Dynamic load factor values as determined by MBS within the time domain simulation can help to validate the traditional calculations performed during design. However, a direct comparison with classic design load factors may be difficult, because these are based on experiment and experience in combination with specific analytic calculation methods. Theanalytic designmethodsare coherent within themselves, but not necessarily with the numeric calculations, therefore caution is necessary. The use of MBS to validate a design with respect to classification rules is limited by specific requirements from the classification societies. Prior individual clarification is necessary. The main advantage of MBS becomes apparent when models become more complex than torsional vibration models. Such calculations may include the behavior of gear boxes elastically mounted to the foundation in combination withattached shafts and couplings, including longitudinal and lateral mode shapes of the shafts. Due to the complexity of the MBS calculation as compared to analytic torsional or lateral vibration calculation methods, MBS may not in any case be the best choice for early development. This is also due to the lack of reliable data formodeling at an earlydesignstage. Therefore, theuse of MBSat themoment is limitedto more complicated problems, for which no other tools exist. After final designof complex marine propulsion systems such as combined systems as described earlier in this paper, final validation with MBS is strongly recommended. It is a challenge to perform such MBS early enough to be able to reasonably react on undesirable system behavior before construction has progressed too far. MBS therefore needs to be an integrated element of the program schedule.
Conclusions S
S
S
S
S
Propulsion systems of surface naval vessels are to be specifically adapted ona case by case basis. Main drivers are efficiency, long term durability for extended operation without coastal reach, and minimized maintenance requirements. Many propulsion system variants are available supported by enhanced prime mover technology, including electric motors. Most sophisticated main reduction gear trains enable the various modes of operation. Due to the increasing complexity and number of variants of combined propulsion systems, the selection process for a distinct drive train is becoming more and more difficult. Together with all propulsion experts involved, it should be defined already in an early project phase what technical solutions are feasible for a specific vessel type; the selection should be based on the various parameters mentioned in this paper. Multi body simulation (MBS) is a powerful simulation tool for design evaluation and validation. It is still a challenge to integrate MBS already with the design of the propulsion system.
References [1]
Weiss, Toni; Hoppe, Franz: High Speed Gears for Extreme Applications in Industrial and Marine Fields. Proceeding Conference Verein Deutscher Ingenieure (VDI), September 2005
[2]
Hoppe, Franz: Naval Gear systems and their future demands. Proceeding Conference INEC 2008, Hamburg, June 2008.
[3]
Hoppe, Franz; Pinnekamp, Burkhard: Gear Noise – Challenge and Success based on Optimized Gear Geometries. AGMA Fall Technical Meeting, Milwaukee, October 2004
[4]
Pinnekamp, Burkhard: Optimization of Marine Propulsion Systems with Modern Tools. Proceedings RENK Symposium, Augsburg, Germany, June 8, 2011.
[5]
Hoppe, Franz: Marine Gears in the Environment of Naval Propulsion. Proceedings RENK Symposium, Augsburg, Germany, June 8, 2011.
[6]
Hoppe, Franz: Hybrid propulsion combining gears and systems for naval vessels. Proceeding Conference INEC 2012, Edinburgh, UK, May 15--17, 2012.
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