casestudy

WIND ENERGY
Wind Energ. 0000; 00:1–12
DOI: 10.1002/we
RESEARCH ARTICLE
A Systems Engineering Analysis of three-point and
four-point Wind Turbine Drivetrain Configurations
T. Parsons1,2
, R. N. King1,3
, Y. Guo1
, K. Dykes1
1
National Renewable Energy Laboratory, 15013 Denver W Pkwy, Golden, CO 80401. 2
Colorado School of Mines, Department
of Mechanical Engineering, 1500 Illinois St, Golden, CO 80401. 3
University of Colorado at Boulder, Department of Mechanical
Engineering, 1111 Engineering Drive, UCB 427, Boulder, CO 80309.
ABSTRACT
A systems engineering analysis of the effects of drivetrain configuration on the mass and cost of three-point and four-point
wind turbine drivetrains is performed for 1.5 MW and 5.0 MW turbines. Our analysis is performed with the Wind-Plant
Integrated System Design & Engineering Model (WISDEM), which uses physics based relationships to size all major
drivetrain components to meet given rotor loads based on IEC Design Load Cases (DLC’s). We analyze the model’s
sensitivity to input loads which contain a high degree of variability. Using stochastic simulations which calculate forces and
moments on the drivetrain, we perform a series of case studies on the effects of three-point and four-point configurations
on onshore 1.5 MW reference, onshore 5 MW reference, and 5 MW offshore reference turbines. DLC’s which govern each
component’s mass are applied to the model, and we present results from our simulations that quantify the trade-offs in mass
and component cost when using three-point and four-point drivetrain configurations. We find on average a 16.7% decrease
in total nacelle mass when using a three-point drivetrain configuration which translates to a 3.5% reduction in turbine
capital cost. This analysis is based off of extreme loads and does not consider fatigue, so the effects of configuration choices
on reliability and serviceability are not captured. Important insights related to the design choices and cost implications are
gathered from a comparison of 1.5 MW and 5.0 MW turbines and in onshore and offshore operation. Copyright c 0000
John Wiley & Sons, Ltd.
KEYWORDS
Systems engineering; drivetrain design; sensitivity analysis; cost optimization
Correspondence
T. J. Parsons, National Renewable Energy Laboratory, 15013 Denver W Pkwy, Golden, CO 80401. E-mail: taylor.parsons@nrel.gov
Received . . .
1. INTRODUCTION
This paper presents an analysis of three and four point drivetrain configurations, referring to one or two main bearings
respectively, for two open source turbine designs in onshore and offshore loading conditions. Three-point and four-point
suspensions are the most common wind turbine drivetrain architectures, and the choice of one or two main bearings is a
primary driver of nacelle component sizes, masses, and costs. Nacelle mass directly affects the structural and dynamical
requirements of the turbine tower, foundation or offshore platform, and construction main crane. Consequently, main
bearing configuration is a critical design choice in systems engineering optimization of wind plants.
In the three-point suspension configuration, the rotor is rigidly connected to the main shaft, which is supported by a
single main bearing near the rotor. A shrink disk typically connects the downwind side of the shaft to the low-speed stage
of the gearbox. The gearbox is supported by two torque arms that are connected to the bedplate elastically. These two
torque arms, along with the single main bearing, provide a total of three-points of support. Turbines which utilize this
configuration include the General Electric GE 1.5 MW, Siemens SWT108 2.3 MW, Nordex N117 2.4 MW and Vestas
V112 3.0 MW[3].
Four-point suspension configurations, sometimes referred as two-main-bearing suspension, place an additional main
bearing near the down-wind side of the main shaft with the intent of isolating any non-torque rotor loads upwind of the
gearbox. The result is a design with two main bearings and two torque arms, creating an undetermined support system
Copyright c 0000 John Wiley & Sons, Ltd. 1
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A Systems Engineering Analysis of three-point and four-point Wind Turbine Drivetrain Configurations T. J. Parsons et al
Figure 1. Three-point (top) and four-point (bottom) architectures[11].
which can be sensitive to deflections and tolerances in the assembly[3]. Turbines which utilize this configuration include
the Gamesa G114 2.0 MW, Vestas V80 2.0 MW or General Electric GE120 2.5 MW turbines.
Publicly available, peer-reviewed literature comparing the design trade-offs associated with these two drivetrain
architectures is somewhat lacking. Industry reports have claimed that four-point suspensions are more costly due to
the additional main bearing, but their advantages lie in the isolation of non-torque loads from the gearbox[5, 10]. Test
results published through the Gearbox Reliability Collaborative (GRC), which uses a three-point suspension drivetrain in
their analysis[20], indicate that such non-torque loads should not affect tooth contact patterns in the low-speed stage in a
manner that would decrease gearbox life[18], but may significantly affect planet-ring gear mesh pattern and planet carrier
misalignment. The resultant edge loading, they concluded, may contribute to high contact stresses and shorter gear life[18].
The true effects of loading on the gearbox and how to prevent failures is an evolving discussion, and the need for such
knowledge is recognized in order to create an accurate model which accounts for known failure modes. The gearbox model
used in this study does not fully consider the effects of non-torque loads on gearbox design and reliability, and therefore
may underscore the gearbox cost benefits of using a four-point suspension drivetrain.
This drivetrain analysis also serves as a demonstration of the capabilities of a new systems engineering modeling tool.
Work in this field began with a University of Sunderland study in 1993, which defined a method for approximating the
location, and size, and cost of key components in horizontal axis wind turbines (Sunderland Model)[12]. The Sunderland
model was based on a set of semi-empirical models for each component which estimated design loads at the rotor,
propagated these loads through the entire system, and used the loads to estimate the size of each component calibrated to
data on actual turbines of the time[12, 8]. In 2005, following the Wind Partnerships for Advanced Component Technology
(WindPACT) work that occurred between 2002 to 2005[1], the National Renewable Energy Laboratory (NREL) released
their wind turbine design Cost and Scaling Model, which utilized the work from the University of Sunderland, and updated
the cost and size equations to better reflect modern turbine technology[8].
While the NREL Cost and Scaling Model improved the overall cost estimation for larger turbines, it abstracted away
from the engineering analysis foundations of the original Sunderland model. Seeing the need for physics-based models
which are sensitive to loading and physical design considerations, the systems engineering for wind energy group at NREL
released DriveSE, RotorSE, TowerSE, JacketSE and a large suite of other physical and cost models as a part of its 2015
release of the Wind-Plant Integrated System Design & Engineering Model[7]. WISDEM integrates wind turbine, wind
plant, and various costs models to allow for integrated wind plant optimization. The model is compatible with the standard
Framework for Unified System Engineering and Design of Wind Plants (FUSED-Wind) that has been jointly developed by
NREL and DTU. Both WISDEM and FUSED-Wind are built around NASA’s Open Multi-disciplinary Design Analysis
& Optimization (OpenMDAO) which is a modular optimization tool written in Python that can wrap around individual
simulation codes. For more information on the software and its functionalities, detailed documentation can be found
online[7].
In this study, we examine the trade-offs associated with three-point and four-point drivetrain designs using the WISDEM
model. Because the WISDEM drivetrain module DriveSE sizes drivetrain components using load information, model
outputs such as component geometry, mass and cost properties can be more closely linked to rotor loads and site selection.
Integrating these analysis capabilities brings new analytical tools to integrated turbine and wind farm design optimization.
This integrated analysis is crucial in evaluating design choices such as main bearing configuration that has a minimal impact
2 Wind Energ. 0000; 00:1–12 c 0000 John Wiley & Sons, Ltd.
DOI: 10.1002/we
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T. J. Parsons et al A Systems Engineering Analysis of three-point and four-point Wind Turbine Drivetrain Configurations
on annual energy production (AEP), but strongly affects turbine capital costs. Consequently, cost-driven optimization using
WISDEM provides valuable insights about fundamental turbine design decisions such as main bearing configuration.
2. WISDEM DRIVETRAIN MODEL
DriveSE consists of a series of interacting mathematical models of drivetrain subcomponents as shown in Figure 2. At
this time, DriveSE contains physical loads-based models for the low speed shaft, main bearings, gearbox, bedplate and yaw
system. The remainder of the components in the hub and nacelle systems are sized using HubSE and NacelleSE which
are primarily scaling relationships based on empirical data. Master routines in HubSE and NacelleSE interface with other
wind turbine components, namely, the rotor and tower. At this top level, design criteria on allowable stress, deflection,
and nacelle center of gravity are inherently included for drivetrain subcomponents. These design criteria, together with
the minimum weight objective for sub-optimizations, are used to determine the subcomponent dimensions. This is an
improvement over previous systems engineering drivetrain models which used empirically-based scaling relationships
developed for stall regulated turbines with fundamentally different drivetrain architectures.
Yaw System
Main Bearings
Design Inputs:
• Configuration Parameters
• Bearing types
• Gearbox Type and Location
• Up/Down Tower transformer
• Fatigue Parameters (optional)
Main Shaft Gearbox
Generator
Coupling
Generator
Transformer
(optional)
Bedplate
DriveSE
Turbine Inputs:
• Rotor Diameter
• Rotor mass properties
• Rotor forces and moments
• Overhang length
• Tower top diameter
Drivetrain Outputs:
• Mass Properties
• Dimensions
Figure 2. DriveSE calculation flow chart
Key model inputs include the extreme aerodynamic rotor loads (torque and non-torque), gravity loads, gearbox
configurations such as the number and type of gear stages, and design parameters such as rotor overhang and gearbox
location. The outputs of DriveSE, HubSE and NacelleSE fall into two categories: subcomponent outputs and system
outputs. Subcomponent outputs include the dimension and weight of individual subcomponents, and gearbox stage ratio
and stage volume, which are preliminary design parameters for these subcomponents. The mass outputs for all the
individual components are then used in a turbine cost model as part of overall system cost analysis. The system outputs are
the cumulative weight, moments of inertia, and center of gravity of the entire hub and nacelle assemblies, which are used as
inputs at the tower design level. The individual drivetrain component models in WISDEM were thoroughly vetted against
known component specifications and against higher order finite element analysis software for verification and validation
purposes. A forthcoming NREL Technical Report details the model equations and verification and validation results[11].
2.1. Drivetrain Modeling Assumptions
A number of general assumptions have been made to facilitate drivetrain modeling in DriveSE. The current modeling
effort focuses on component designs driven by maximum stress analyses and deflection criteria. By using industry standard
safety factors this approach sizes components to avoid catastrophic failure. The deflection constraints ensure components
are properly aligned and within geometrical limits for bearing alignment, gear tooth meshing, etc. These geometrical
Wind Energ. 0000; 00:1–12 c 0000 John Wiley & Sons, Ltd. 3
DOI: 10.1002/we

requirements are an important part of the integrated design work-flow connecting main bearings, lowspeed shaft, and
gearbox configurations. Other general assumptions include homogeneously distributed component masses for calculating
moment of inertia (MOI) and center of mass (CM). Where models rely on scaling arguments, such as for the hub or
transformer mass in sizing the bedplate, these departure from physics are noted in documentation. For more information
on the DriveSE model formulation, validation, and other documentation, refer to the DriveSE model report [11].
3. METHODS
3.1. Loads Sensitivity Analysis
Each component in DriveSE is sized based off of a multitude of input parameters which specify the desired drivetrain
configuration, the extreme rotor forces and moments in all coordinate directions, and the torque at rated power conditions.
Table I shows the necessary input variables for each component model. Due to the uncertainty involved in the aerodynamic
rotor loads, an understanding of individual component sensitivity to these loads is valuable for further comparison between
turbines.
Table I. External input variables used in DriveSE component sizing
Input Variable Main Shaft Main Bearing(s) Gearbox Bedplate Yaw
Aerodynamic Rotor Loads - -
Torque at Rated Power - -
Power Rating - -
Gearbox Ratio - - - -
Rotor Diameter - - - -
Rotor Weight - -
Overhang Distance - -
Tower Top Diameter - - -
Because we are predominantly interested in the subcomponent outputs and their sensitivity to each load, this section
investigates the impact a change in load magnitude has on the mass of each component. A base set of loads which included
forces and moments in each coordinate direction are taken from a 5 MW onshore reference turbine[13] which will be
discussed in further detail in the following section. A multiplier is added to each load individually, and the impacts on
component mass in both three-point and four-point configuration are plotted. The bedplate and low speed shaft (LSS) are
the only components analyzed in this study because the main bearing models are directly coupled to the shaft model, and
the gearbox size is determined from the rated torque, which is a significantly less uncertain input variable. This analysis
yields a qualitative representation of the effects of a wide range of load magnitudes on the DriveSE model results. The base
loads used in sensitivity analysis are included in Table II, and were approximated from simulated loads found in Appendix
A. As these values show, the mass of the rotor and wind shear significantly impact design loads, leading to significantly
larger downward rotor force and a similarly large moment in the negative y-direction.
Table II. Base loads used in sensitivity analysis
Fx (kN) Fy (kN) Fz (kN) Mx (kNm) My (kNm) Mz (kNm)
255 -180 -1300 5000 -14000 -5500
3.2. Drivetrain Case Study Methodology
After quantifying the sensitivity of drivetrain components to changes in input loads, an analysis of 3 point and 4 point
main bearing configurations was carried out for two open source wind turbine designs: the NREL 5 MW Reference Turbine
[13] and the WindPACT 1.5 MW Turbine [19]. Table III provides a summary of the different simulation cases considered
in this study. A unique set of loads was generated for each case by running a suite of simulations using NREL’s aero-
servo-hydro-elastic multibody dynamic simulator, FAST [15]. The design load cases and the result extreme loads for each
simulation case are presented in Appendix A.
DOI: 10.1002/we

Table III. Summary of simulation cases
Case Turbine Drivetrain Design Location
1 1.5 MW WindPACT 3 point Onshore
2 1.5 MW WindPACT 4 point Onshore
3 5.0 MW NREL Reference 3 point Onshore
4 5.0 MW NREL Reference 4 point Onshore
5 5.0 MW NREL Reference 3 point Offshore
6 5.0 MW NREL Reference 4 point Offshore
3.3. NREL 5 MW Reference Turbine
The NREL 5 MW Reference Turbine is a conventional utility scale turbine with a three-bladed upwind, variable
speed, variable pitch design. It is loosely based on the REpower 5M, RECOFF [9], and DOWEC [16] designs and is a
representative design for offshore turbines of a similar nameplate power rating. It is commonly used as a baseline design
for wind energy research on diverse topics such as hydrodynamics of floating turbines [14], blade design [22], large eddy
simulations [4], extreme and fatigue load studies [21, 17], and fluid-structure interactions [2]. Relevant geometrical and
mass properties for the NREL 5 MW Reference Turbine and its drivetrain are given in Table IV, all of which are taken
from Jonkman, 2009 [13].
3.4. WindPACT 1.5 MW Turbine
The WindPACT (Wind Partnership for Advanced Component Technologies) 1.5 MW Turbine is the result of an NREL-
funded study on how new technologies and larger rotors would affect the cost of energy. The WindPACT study examined
several nameplate sizes, however we focus only on the 1.5 MW nameplate baseline design as it is similar to the GE 1.5
MW turbine that is commonly installed in the U.S. Similar to the NREL 5.0 MW Reference Turbine, the WindPACT 1.5
MW turbine is a three bladed, upwind, variable speed, variable pitch conventional ”Danish” wind turbine design. Details
on the WindPACT 1.5 MW are also included in Table IV.
Table IV. Wind turbine specifications
Parameter 1.5 MW WindPACT 5.0 MW NREL Reference
Nameplate (kW) 1500 5000
Rotor Diameter (m) 70 126
Hub Height (m) 84 90
Cut-in, Rated, Cut-out Windspeed (m/s) 4, 16.18, 25 3, 11.4, 25
Cut-in, Rated Rotor Speed (rpm) 8.5, 20.5 6.9, 12.1
Gearbox Ratio 78:1 97:1
Overhang Distance (m) 3.3 5.0
Shaft Tilt (deg) 5.0 5.0
Hub center to main bearing along shaft (m) 1.535 1.912
Tower Top Diameter 2.30 3.78
Rotor Mass (kg) 32,016 110,000
Nacelle Mass (kg) 52,839 240,000
Hub Mass (kg) 15,104 56,780
Tower Mass (kg) 122,522 347,460
3.5. Mass Comparison Details
Because the DLC’s are extreme loads and not expected to occur simultaneously, the masses of each component were
calculated for individual DLC load cases given in Appendix A independently. The governing load case was taken to be
the one which produced the maximum component mass, and may be different for each part. In addition to rotor loads,
the bedplate model takes into account the individual component weights which the bedplate must support. Because of
this, a second iteration of the analysis was used in which DLC’s were applied to the bedplate with the maximum masses
from each other component as manual inputs. The final assembly mass, including all above-yaw components which do not
depend on turbine loading, is summed to arrive at a total nacelle mass figure.
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The current version of DriveSE seeks to minimize the forces on the main bearings and reduce non-torque moments on
the gearbox. This results in the main bearings for the four-point drivetrain being spaced as far apart as possible without
exceeding main shaft deflection constraints. However, the gearbox model does not currently account for the effects of
non-torque loads on the gearbox design. For this reason, the trade-off between a more massive main shaft resulting from
a longer shaft and less massive gearbox resulting from decreased loading are not captured. Because this design trade-off
is very important for a comparison between drivetrain configurations, the mass of the three-point gearboxes have been
increased manually to account for the bearing and housing mass increase which is normally accounted for in design. The
following process is followed in order to account for the mass difference in the gearbox:
Radial and axial loads on the gearbox are calculated in the shaft model, and it is assumed that two cylindrical roller
bearings equally carry the non-torque loads at the low-speed stage of the gearbox. Bearings whose load ratings can
accommodate half of the radial load each, and with a bore diameter equal to the shaft diameter at the gearbox connection,
are selected from SKF bearing catalogs[23]. The gearbox housing mass increase is taken to be directly proportional to the
incoming loads on the gearbox, and scaled to fit available industry data which is unfortunately proprietary in nature. The
increase in bearing mass and housing mass within the gearbox is then summed to account for the total mass increase.
3.6. Cost Comparison Details
The most important comparison is not the mass of the individual components and nacelle but how these masses translate
to differences in machine costs and overall wind plant costs. At a system level, the design of the drivetrain predominantly
affects cost of individual components and the overall turbine capital costs. In order to analyze the cost effects of drivetrain
configuration on the capital cost of each component, mass results from the previous section are connected to the inputs of
the WISDEM TurbineCostSE model, which approximates component costs from scaling relationships between mass and
cost[6]. This approach considers capital cost reduction to be the objective behind design, but does not consider the effects
of configuration on reliability, which would affect the operation and maintenance costs of the turbine.
4. RESULTS
4.1. Loads Sensitivity Results
The results of the loads sensitivity analysis on a three-point suspension drivetrain are shown in Figures 3 and 4. As the
former figure conveys, the three-point main shaft model is most highly sensitive to the non-torque moments My and Mz.
These loads contribute to stress concentrations at the location of the upwind main bearing, such that the shaft diameter
at this location must be increased to sustain these larger bending moments, and the mass of the resultant tapered shaft is
especially sensitive to these two moments. The moment in the y-direction has the largest impact on this model because its
already high magnitude is compounded by the rotor mass (shown in the large-magnitude Fz) and the moment arm from
the rotor to the main bearings. In much the same way, Mz and Fy will compound their effects, but due to their lower
magnitudes they are shown to have less of a total effect on the mass of the shaft. The main shaft mass is also dependent
on the torque (Mx), which contributes to a torsional shear stress throughout the shaft, but only significantly contributes
to the stress at the smaller-diameter, downwind end of the component. The forces in the x- and y-directions are shown to
have a nearly negligible effect on the mass of the main shaft because they do not contribute to the stress concentrations
and deflections that drive LSS sizing.
In Figure 4, the bedplate model is also shown to be highly sensitive to the moment in the y-direction for two distinct
reasons: firstly, the same compounding effects of moment and complimentary force, My and Fz impact the stress on the
bedplate’s upwind support structure. Secondly, the weight of the main shaft and bearings is an internal input to the bedplate
model, such that the more massive shaft/bearing assembly weighs down the bedplate more and necessitates a larger and
more massive support. In this way, the effects of this moment will always compound to impact the bedplate sizing in a large
way. The only other load which has a direct effect on the bedplate model is the force in the z-direction, which contributes
to the bending stress along the upwind portion of the support. All other loads, to which this model is expectedly shown to
have less sensitivity, only indirectly impact bedplate mass because of their effects on other component weights.
Figures 5 and 6 show the same analysis performed with a four-point suspension drivetrain model. The main difference
between these configurations is the existence of a downwind main bearing which helps to carry forces and moments from
the rotor. The main shaft results still show that the moments My and Mz have the largest effect on this component’s
mass. However, the model’s sensitivity to each moment is almost the same, as opposed to the previous configuration which
was much more sensitive to the moment in the y-direction. Under the three-point configuration it is assumed that the
gearbox trunnion force in the y-direction is negligible such that the moments are not carried completely through the shaft.
Because of this, the rotor moments are less directly carried in the shaft without a second main bearing. In the four-point
configuration, however, both moments are assumed to be carried by the main bearings such that the model’s sensitivity to
each load is nearly the same. The difference between the four-point sensitivity between these moments arises mainly from
DOI: 10.1002/we

Figure 3. LSS loads sensitivity under three-point configuration
Figure 4. Bedplate loads sensitivity under three-point configuration
the fact that component weights also effect the bending stress from My, so its effects normalized to the baseline loads are
smaller.
After normalizing each mass against the mass of the component under the base loads, the slopes of these lines indicate
that the four-point main shafts are less sensitive to a change in loading, while the four-point bedplate is more sensitive to
the two driving loads. The upwind and downwind diameters between the main shaft models are similar because the stress
concentrations at these locations are nearly the same. However, the shaft under a four-point configuration is significantly
longer than its three-point counterpart. This leads to a higher sensitivity of shaft mass to length, which is most often
determined from variables other than loading. Only when the deflection constraints are not met does the model shorten the
main shaft according to loading[11]. Consequently, in these four-point cases, loads have less of an effect on the mass of
the main shaft than the maximum shaft length which is derived from the rotor overhang distance.
It is important to note that the same trends in loads sensitivity are shown in the main bearing of the three-point suspension
machine, due to the fact that the bearing model is closely coupled to the shaft diameter. For this reason, we can see that
the DLC’s which produce the largest moments in the y-direction will often determine the main shaft diameter under these
stress and deflection criteria used in DriveSE.
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Figure 5. LSS loads sensitivity under four-point conﬁguration
Figure 6. Bedplate loads sensitivity under four-point conﬁguration
Despite the increased similarity between My and Mz sensitivities for the four-point shaft model, the bedplate model
is still more sensitive to the My input. This is because, although Mz effects the weight of the shaft and bearings to be
supported by the bedplate, the bedplate model does not directly consider stiffness in the y-direction to be a driver for the
bedplate design. Much like in the three-point design, Fz and My, in addition to the component weights, are the only loads
considered in the bedplate model. This is important to note because in several cases the DLC which produces the largest
moment in the y-direction will determine the size of the bedplate, while the DLC with the largest combined moment will
produce the largest main shaft. Indeed, from the 5 MW Onshore loads case found in the appendices, row 10, the load
instance with the highest moment in the y-direction determined the bedplate size while row 11, the instance with the
maximum combined moment at the main bearing location, determined the shaft sizing. This validates the methods in the
case study, which took the shaft mass from the rotor torque maximum instance and used it to size the bedplate with the
maximum moment in the y-direction.
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4.2. Case Study Mass Results
Table V shows the total nacelle mass from each of the cases and the corresponding masses of the affected components in
this study. We find good agreement with known masses for the 1.5 MW WindPACT turbine and 5.0 MW NREL Reference
turbine in onshore conditions under 4 point suspensions[13, 19]. Because the reference turbine configurations for these
machines include a second main bearing in actuality, these results verify the fact that the model and its methods are
accurate. Further model validation can be found in the NREL Technical Report, ”DriveSE: An Analytical Formulation for
Sizing and Estimating the Dimensions and Weight of Wind Turbine Hub and Drivetrain Components”[11].
The bedplate is the single heaviest component in both configurations, and its mass and dimensions are sensitive to the
location and external dimensions of the other nacelle components, resulting in a strong coupling between shaft and bearing
system mass and bedplate mass.
Table V. Total nacelle mass and the corresponding component masses for each case
Case Total Nacelle (kg) LSS (kg) Main Bearing (kg) Second Bearing (kg) Gearbox (kg) Bedplate (kg)
1 47200 5379 3339 - 19297 8646
2 56830 9532 2178 1742 18668 12825
3 216374 26073 12884 - 60379 69566
4 250568 33017 9543 4627 59076 92747
5 253001 38746 12882 - 60742 91121
6 314509 50879 16576 6665 59076 125562
Cases 5 and 6 are more massive than their onshore counterparts due to the higher loads found in the offshore load cases.
The loads experienced in an offshore setting are larger due to the coupled dynamics of wind and wave loading, and result
in heavier components when using loads-driven sizing as in DriveSE. Offshore loads are heavily dependent on local wave
spectra as well as the design and damping of the offshore platform. Consequently, we expect offshore component masses
to be sensitive to site-specific conditions and designs but generally to be heavier than the onshore models.
Cases 1, 3, and 5 in Table V show that the single-main-bearing designs results in a lower overall nacelle mass than
the two-main-bearing designs. The onshore four-point cases result in upwind main bearings which are less massive than
the single main bearings in the three-point cases, but the additional mass from the second bearing offsets this benefit. The
addition of a second bearing increases the dimensions of the LSS in each case. This is largely due to the extra length needed
for the bearings in a four-point main shaft and the difference in the way the model simulates the load path downwind of
the main bearing. The increase in main shaft and bearing mass is coupled with a decrease in gearbox mass, but because the
gearbox is generally located above the tower-top center, the resultant loads are transferred directly to the tower. In turn, the
bedplates from the four-point cases were designed to be significantly larger than the three-point ones in order to carry the
additional loads and longer moment arms from the shaft and bearings and transfer them to the tower. As a result, the four-
point drivetrain configurations are 13-20% heavier than three-point drivetrain configurations in an equivalent nameplate
and operating environment.
These results show that under the design assumptions within the DriveSE model, and with total mass reduction as the
only goal, a three-point suspension configuration would be selected regardless of loading scenario. The following section
discusses the impact of these weight savings on the turbine capital cost.
4.3. Case Study Cost Comparison
Table VI summarizes the component costs for each case study. In each case, the three-point configuration nacelles are
less expensive than the four-point counterparts. From a cost optimization standpoint, the difference between the two design
choices is less pronounced than in the mass comparison. On average, the nacelle mass decrease from four-point to three-
point suspension is 16.7%, while the cost savings between the two averages 3.5%. This is because the majority of the mass
savings comes from the bedplate and main shaft, which are less costly on a per-mass basis than the gearbox and bearings.
Still, a 3.5% decrease in drivetrain cost is a significant reduction when considering the cost effects on large-scale wind
project.
Table VII shows that the four-point gearbox cost savings are modeled to be substantial, but not enough to overcome the
increase in the cost of the other components in the nacelle. In the 1.5 MW case, the gearbox savings more than offsets
the increase in bedplate and bearing cost. However, as the loads on the bearing and shaft system increase to the 5.0 MW
offshore case, the bearings and shaft are significantly more expensive between the two configurations. This suggests it may
be best for larger machines to use a three-point configuration.
Despite the apparently lower capital cost for three-point suspension drivetrains, several common wind turbines up to the
5 MW scale have been shown to employ a four-point suspension drivetrain design, including the Gamesa G114 2.0 MW,
Vestas V80 2.0 MW, General Electric GE 120 2.5 MW turbine, and the original NREL 5 MW Reference Turbine. Because
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Table VI. Component costs and their effects on total nacelle capital cost
Case Nacelle LSS Bearings Gearbox Bedplate
1 $1,121,144 $49,918 $20,006 $413,493 $36,361
2 $1,147,795 $71,920 $23,487 $398,892 $41,895
3 $3,561,073 $159,551 $77,194 $1,367,085 $117,019
4 $3,662,826 $196,339 $84,899 $1,336,840 $147,728
5 $3,744,044 $226,690 $77,182 $1,375,511 $145,575
6 $3,961,672 $290,967 $139,248 $1,336,840 $191,182
Table VII. Cost difference between three- and four-point drivetrains
Turbine Total Savings LSS Bearings Gearbox Bedplate Other Components
1.5 MW Onshore $ 26,651 $ 22,002 $ 3,481 $ (14,600) $ 5,534 $ 10,234
5.0 MW Onshore $ 101,753 $ 36,788 $ 7,705 $ (30,245) $ 30,710 $ 56,796
5.0 MW Offshore $ 217,628 $ 64,278 $ 62,066 $ (38,671) $ 45,607 $ 84,348
wind turbine original equipment manufacturers (OEM’s) typically make design decisions based off of cost considerations,
the results of this study seem to conflict with the occurrence of four-point designs in the industry. The reason for this
discrepancy may very well be that the current models do not accurately account for the complex cost relationships of the
wide range of components in the drivetrains of these four-point turbines. The relationship between component mass and
cost is indeed simplified in the models, which do not consider the effects of bearing type, gearbox configuration and other
such design choices on the cost of individual components.
There is, however, a more compelling reason why designers would chose a four-point drivetrain when it may be more
costly on a turbine capital level. As the DriveSE model shows, the non-torque loads imparted on the gearbox of a four-
point suspension turbine are much lower than if the low speed shaft were supported by only a single bearing. Gearbox
reliability has been shown to be a key goal for wind turbine design due to the costly consequences and frequency of
gearbox failures[20, 3]. Because DriveSE and the WISDEM cost models do not currently account for the full effects of
non-torque loads on a gearbox, or the operational costs associated with gearbox failures, the effects of these considerations
are lost. This limitation prevents a quantitative analysis of the reliability benefits of a four-point drivetrain, other than to
say that OEM’s may consider them to outweigh the increase in capital cost.
5. CONCLUSIONS
We have found that the bedplate, main bearings, and main shaft are largely sensitive to the overhang moment My, and
the other non-torque moment Mz. Our mass analysis shows a 16.7% weight savings by using a three-point suspension,
due mainly to the lack of a second bearing, shorter drivetrain length, and lighter resultant bedplate. The additional bearing
causes shaft and bedplate masses to increase substantially, but cause a comparatively smaller decrease in gearbox mass. As
a result of the mass differences, significant cost savings are seen in the main shaft and bedplate of three-point suspension
drivetrains. Because gearboxes are more expensive than the main shaft and bedplate on a mass to cost basis, the percentage
cost savings is less than the percentage mass savings. Three-point suspension turbines cost an average of 3.5% less in terms
of capital cost than their corresponding four-point versions. This analysis does not fully consider the positive effects of a
second main bearing on the reduction of non-torque loads on the gearbox, which will improve gearbox reliability. Future
development and analysis will aim to improve the fidelity of modeling the effects of fatigue and O&M costs on different
drivetrain architectures.
6. ACKNOWLEDGMENTS
This work would not be possible without the funding and support of the Systems Engineering group at the National
Wind Technology Center and NREL. Thanks also to Paul Veers and Rick Damiani for providing their comprehensive
industry insight.
DOI: 10.1002/we

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DOI: 10.1002/we

APPENDIX A: DLC’S USED IN CASE STUDY ANALYSIS
WindPact 1.5 MW Extreme Loads Table
RotThrust LSShftFys LSShftFzs RotTorq LSSGagMys LSSGagMzs
Load Case Type (kN) (kN) (kN) (kNm) (kNm) (kNm)
DLC 2.3 Minimum -155.994 -3.400 -284.130 -231.330 -248.930 -334.620
DLC 1.1 Maximum 448.889 3.249 -407.028 1173.450 -706.950 661.500
DLC 1.3 Minimum 91.732 -77.703 -332.640 949.590 1336.223 -918.405
DLC 1.3 Maximum 130.275 68.330 -355.320 921.611 -1902.150 -597.645
DLC 1.1 Minimum 147.375 -13.292 -461.946 1091.194 119.260 1542.000
DLC 2.3 Maximum 46.617 -1.951 -204.366 -99.847 -165.000 -281.380
DLC 2.3 Minimum -56.232 -5.345 -244.200 -270.294 -148.339 -643.720
DLC 1.1 Maximum 185.250 -15.285 -405.956 1340.390 741.580 396.150
DLC 1.3 Minimum 262.043 28.026 -347.625 843.885 -2675.768 752.220
DLC 1.3 Maximum 153.090 -57.537 -353.025 1053.540 2129.116 -283.365
DLC 1.3 Minimum 306.990 -1.293 -332.100 1048.005 -241.380 -2273.131
DLC 1.3 Maximum 273.914 8.733 -403.110 894.240 556.740 2650.607
NREL 5 MW Reference Turbine Onshore
DLC 2.3 Minimum -894.190 1.906 -1166.000 -1423.400 -1675.300 -1384.900
DLC 1.3 Maximum 1506.600 -3.178 -1474.200 6115.500 -537.975 902.070
DLC 6.2 Minimum -5.222 -1093.950 -855.690 1124.200 -8072.900 6613.200
DLC 6.2 Maximum 78.848 1119.800 -864.160 -1035.980 1078.550 -3135.000
DLC 1.1 Minimum 581.400 -27.045 -1761.000 6177.000 -1794.000 4776.000
DLC 6.1 Maximum 31.887 -465.480 -313.335 90.760 -1786.050 6980.850
DLC 2.3 Minimum -580.800 -87.076 -1049.070 -2253.900 2063.600 -5431.800
DLC 1.1 Maximum 647.700 -8.625 -1454.400 7836.000 313.350 -9579.000
DLC 7.1 Minimum 599.610 186.780 -842.710 330.770 -16665.000 2896.300
DLC 1.3 Maximum 254.475 -179.145 -1364.850 4942.350 14053.500 -5404.050
DLC 1.3 Minimum 300.510 43.227 -1238.085 4326.750 -1055.160 -14607.000
DLC 1.3 Maximum 1362.150 40.257 -1513.350 6347.700 -3508.650 13132.800
NREL 5 MW Reference Turbine Offshore
DLC 1.1 Minimum -2780.000 1530.000 -938.000 3470.000 -16400.000 -3460.000
DLC 1.1 Maximum 4400.000 -424.000 -1100.000 8540.000 -2120.000 10700.000
DLC 1.1 Minimum -1180.000 -2070.000 265.000 4430.000 5500.000 6480.000
DLC 1.1 Maximum -2540.000 2150.000 -107.000 3160.000 -4650.000 -12500.000
DLC 1.1 Minimum -1540.00 143.000 -2080.000 3360.000 -10200.000 7760.000
DLC 1.1 Maximum -602.000 -169.000 2000.000 4390.000 -6490.000 -1090.000
DLC 1.1 Minimum -1920.000 -1350.000 1060.000 -1650.000 -16900.000 5550.000
DLC 1.1 Maximum 2490.000 366.000 -1310.000 10700.000 2140.000 -1250.000
DLC 1.3 Minimum 488.000 492.000 822.000 4500.000 -29100.000 15800.000
DLC 1.1 Maximum 43.300 426.000 -821.000 2070.000 32600.000 7210.000
DLC 1.3 Minimum 1610.000 -996.000 456.000 6510.000 -5110.000 -27700.000
DLC 1.1 Maximum -1500.000 -1020.000 -987.000 5100.000 1530.000 26200.000
DOI: 10.1002/we

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