NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC.

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

Building and Calibration of a 

FAST Model of the SWAY 

Prototype Floating Wind Turbine

Preprint

J.H. Koh

Nanyang Technological University Singapore

A. Robertson, J. Jonkman, and F. Driscoll

National Renewable Energy Laboratory

E.Y.K. Ng

Nanyang Technological University Singapore

To be presented at the International Conference on Renewable 

Energy Research and Applications (ICRERA)

Madrid, Spain

October 20−23, 2013

Conference Paper

NREL/CP-5000-60096

September 2013
NOTICE

The submitted manuscript has been offered by an employee of the Alliance for Sustainable Energy, LLC 

(Alliance), a contractor of the US Government under Contract No. DE-AC36-08GO28308. Accordingly, the US 

Government and Alliance retain a nonexclusive royalty-free license to publish or reproduce the published form of 

this contribution, or allow others to do so, for US Government purposes.

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, 

express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of 

any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately 

owned rights. Reference herein to any specific commercial product, process, or service by trade name, 

trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, 

or favoring by the United States government or any agency thereof. The views and opinions of authors 

expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

This report is available at no cost from the National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy

and its contractors, in paper, from:

U.S. Department of Energy

Office of Scientific and Technical Information

P.O. Box 62

Oak Ridge, TN 37831-0062

phone: 865.576.8401

fax: 865.576.5728

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from:

U.S. Department of Commerce

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

phone: 800.553.6847

fax: 703.605.6900

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/help/ordermethods.aspx

Cover Photos: (left to right) photo by Pat Corkery, NREL 16416, photo from SunEdison, NREL 17423, photo by Pat Corkery, NREL 

16560, photo by Dennis Schroeder, NREL 17613, photo by Dean Armstrong, NREL 17436, photo by Pat Corkery, NREL 17721.

Printed on paper containing at least 50% wastepaper, including 10% post consumer waste.
1

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

BUILDING AND CALIBRATION OF A FAST 

MODEL OF THE SWAY PROTOTYPE FLOATING 

WIND TURBINE

Jian Hao Koh

Nanyang Technological University

Singapore

Amy N. Robertson

National Renewable Energy Laboratory

Golden, Colorado, USA

Jason Jonkman

National Renewable Energy Laboratory

Golden, Colorado, USA

Frederick Driscoll

National Renewable Energy Laboratory

Golden, Colorado, USA

Eddie Y. K. Ng

Nanyang Technological University

Singapore

Abstract—Present efforts to verify and validate aero-hydro-servoelastic numerical simulation tools that predict the dynamic response 

of a floating offshore wind turbine are primarily limited to code-tocode comparisons or code-to-data comparisons using data from 

wind-wave basin tests. In partnership with SWAY AS, the National 

Renewable Energy Laboratory (NREL) installed scientific wind, 

wave, and motion measurement equipment on the 1/6.5th-scale 

prototype SWAY floating wind system to collect data to validate a 

FAST model of the SWAY design in an open-water condition. 

Nanyang Technological University (NTU), through a collaboration 

with NREL, assisted in this validation.

This paper shows the use of the results of the SWAY open-water 

tests to calibrate the numerical FAST model, which will be used for 

future validation efforts. First, the modeling strategies and 

development of the FAST model for the SWAY prototype wind 

turbine are presented, including justification of the modeling 

assumptions. Next, the model calibrationbased on a subset of the 

free-decay test datais shown. This process involved tuning 

properties of the FAST model where uncertainties existed to better 

match the response of the prototype wind turbine. Finally, 

limitations of the FAST model and potential areas of improvement 

of the project are discussed.

Keywords-offshore wind; FAST; aero-hydro-servo-elastic; openwater testing

I. INTRODUCTION

In the design process, it is essential to use accurate numerical 

simulation tools to predict the complex aero-hydro-servo-elastic 

response of a floating wind turbine. Numerical simulation tools,

such as FAST, developed by the National Renewable Energy 

Laboratory (NREL) [1], [2] are capable of modeling floating 

wind turbines and predicting their dynamic response behavior.

Presently, efforts are primarily focused on verifying these 

tools through code-to-code comparisons (International Energy 

Agency (IEA) Wind Offshore Code Comparison Collaboration 

(OC3) [3] and OC3 Continuation (OC4) projects [4]) of 

simulated system behavior.

Few physical tests have been used to compare experimental 

results with simulation data for floating wind systems. Browning 

and Goupee [5], Stewart et al. [6], Prowell et al. [7], and Coulling 

et al. [8] worked on the calibration and validation of a FAST 

model with wind-wave basin tests conducted at the Maritime 

Research Institute Netherlands (MARIN) led by the DeepCwind 

consortium. In this work, the measurements from a 1/50th-scale 

spar, tension-leg platform, and semisubmersible floating wind 

turbine were scaled up and compared with results from full-scale 

FAST models of similar systems. In these three studies, the 

responses compared well between the experiment and the 

simulation in the wave-excitation frequency range after model 

calibration. These studies also indicated that the inclusion of 

second-order hydrodynamics and a dynamic mooring line model 

improve the accuracy of the simulations.

Almost all published studies to date on code-to-data 

validation work use data generated from a laboratory setting. 

Calibration and validation using response and performance data 

from real environmental conditions is beneficial to the 

development of aero-hydro-servo-elastic numerical tools as such 

tests are performed in realistic environmental conditions. In 

addition, larger-than-laboratory-scale testing will yield results 

with less scaling issues as it is able to show the overall system 

characteristic behavior more accurately.

This paper will focus on the effort to use the open-water test 

data of the SWAY prototype wind turbine to calibrate a FAST 

floating offshore wind turbine model for future validation efforts. 

This is the first time that the FAST tool has been compared to a 

real floating wind turbine system deployed in open water, which 

has a number of challenges compared to tank testing.

II. FAST WIND TURBINE SIMULATION TOOL

The FAST software tool developed by NREL is capable of 

modeling the coupled aero-hydro-servo-elastic response of a 

floating wind turbine subjected to combined wind and wave 

loading in the time domain. 

Aerodynamic loads are calculated in the AeroDyn module, 

which has the option of using blade element momentum (BEM) 

theory or a generalized dynamic wake theory with steady or 

unsteady airfoil aerodynamics, including dynamic stall. The user 

also has the option of including the effects of Prandtl’s tip and 

hub losses [9].
2

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

Structural components of the wind turbine are modeled by a 

combined nonlinear multibody dynamics and modal super 

position formulation. Structural components such as the blades, 

tower, and driveshaft can be modeled as flexible bodies, while 

the platform and nacelle are modeled as rigid bodies.

The HydroDyn module [2] computes the time-domain 

hydrodynamics loading based on nonlinear viscous drag from 

waves and current, the linear added mass and damping from wave 

radiation, and linear wave excitation including diffraction. A 

nonlinear, quasi-static mooring line model is also available for 

floating wind turbines to calculate loads from taut or catenary 

lines, including the effect of stretching, buoyancy, mass density, 

and seabed interactions.

The degrees of freedom (DOFs) of the floating wind turbine,

as modeled in FAST, include tower bending, blade bending, 

nacelle/rotor yaw, generator azimuth, driveshaft torsion, and 

platform motion. The platform DOFs are translated in the X, Y,

and Z directions and are called surge, sway, and heave; and 

rotations about the X, Y, and Z axes are called roll, pitch, and 

yaw. Each DOF can be turned on or off individually. Figure 1 

shows the coordinate systems and platform DOF terminology 

used in this paper; the origin is located at the mean sea level 

(MSL) and dimensions described in this paper will be referenced 

to this inertial frame coordinate system unless otherwise stated.

X

Surge

Z

Heave

Y

Sway

Roll

Pitch

Yaw

Figure 1. Coordinate systems and platform DOF.

III. SWAY SPAR-TYPE FLOATING WIND TURBINE

The SWAY spar-type floating wind turbine (Figure 2) has a 

three-bladed downwind configuration that uses a tension rod 

system for station-keeping and to maintain the hydrostatic 

stability of the wind turbine. The bottom of the spar structure is 

fixed to the tension rod with a universal joint. The other end of 

the tension rod is attached to a large steel mass (gravity anchor) 

on the seafloor with another universal joint. A downwind 

configuration allows the use of a passive yaw system that 

eliminates the cost that would be incurred by having an active 

yaw system in an upwind design. The wind turbine system, 

including the tower, yaws about the yaw mechanism located at 

the universal joint at the bottom of the tower. In addition, the 

blade clearance from the tower, which is an issue for larger 

turbines, is less of a concern for the downwind SWAY turbine.

The wind turbine has individual blade-pitch control, and the 

nacelle is fixed to the tower at an optimal angle such that the 

rotor axis is oriented horizontally (parallel to the wind) when the 

tower pitches during operation. Some of the key components in 

the system include unique spreader beams and tension cables, 

which help to stiffen the tower, reduce fatigue loads, and allow 

the tower to carry a larger turbine.

The full-scale SWAY wind turbine is designed to have a rated 

power of 2.5−10 MW, a rotor diameter up to 124 m, and a 

support structure up to 210 m in length.

Figure 2. SWAY floating wind turbine.

A 1/6.5th-scale prototype of the SWAY spar-type floating 

wind turbine was deployed in Hjeltefjorden, east of Øygarden in 

Hordaland, Norway, in May 2012 (www.sway.no). As part of a 

collaboration between NREL and SWAY AS, NREL installed 

scientific wind, wave, and motion measurement equipment on the 

SWAY system. The equipment enhances SWAY’s data 

collection and will allow SWAY to verify the concept and NREL 

to validate a model of the SWAY design in an open-water 

condition. NTU, in collaboration with NREL, is assisting with the

validation.

Table 1 provides the key specifications of the prototype wind 

turbine. Other detailed specifications, such as system dimensions 

and blade and mass properties, are not shown in the report 

because of proprietary protections.

TABLE I. SWAY 1:6.5 SCALE WIND TURBINE PROTOTYPE SPECIFICATIONS

Foundation Manufacturer SWAY

Make, Model of Turbine SWAY1/6.5th Scale Prototype

Production Year of Turbine 2011

Rotation Axis Horizontal

Orientation Downwind

Number of Blades 3

Rotor Diameter (m) 14.9

Hub Height (m) ~13

Control Individual Pitch Control

Tower Type Tubular

Floater Type Spar Buoy
3

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

The platform motions were measured with two motion 

reference units (MRU). A Teledyne DMS-05 MRU was located 

in the tower near the centerline just above the waterline and an 

Xsens MTi-G motion tracker unit was located out on an 

anemometer boom at nacelle height. A dual antenna Hemisphere 

VT101 GPS was used to provide position and heading. The wind 

speed and direction were measured with two three-axis ATI 

SATI/3K ultrasonic anemometers. One was mounted upwind on 

the same boom as the MTi-G and the other was mounted near the 

base of the nacelle. Profiles of the water velocity and directional 

wave spectra were measured by a Nortek 600 kHz acoustic wave 

and current profiler (AWAC) mounted in an OceanScience Sea 

Spider subsea instrumentation tripod approximately 12 m northnortheast of the turbine at a depth of about 20 m. 

Instruments aboard the turbine are sampled with a National 

Instruments PXI system located in the tower base. Data is stored 

both aboard the PXI system and transmitted to the shoreside 

computer. Data from the AWAC are internally recorded and 

loaded to a shoreside computer via an underwater communication 

cable. The PXI was chosen because of its ability to rapidly 

interrogate sensors that are widely distributed and its ability to 

use GPS time to tightly coordinate and synchronize 

measurements. This data acquisition system is monitored and 

controlled remotely at NREL in Golden, Colorado (USA). 

Figure 3 shows the profile of the tower, spreader beam, and 

tension rod. In FAST, the blue region is modeled as the wind 

turbine tower while the red region is modeled as the platform.

Yellow circles indicate the locations of the instruments.

Figure 3. Profile of the tower, spreader beam, and tension rod.

IV. FAST MODEL

A FAST model of the system was created using the 

specifications of the SWAY 1/6.5th-scale prototype spar-type 

floating wind turbine. As such, the model and test data obtained 

do not require any form of scaling when compared, eliminating 

potential scaling errors.

A. Assumptions

The SWAY wind turbine includes some innovative 

components that cannot be modeled directly (without 

customization) in the existing FAST tool.

The tension wires and spreader-beam system generates an 

asymmetric stiffening of the tower in the fore and aft directions. 

However, the current version of FAST is only able to model 

symmetric properties for the tower. For this work, the towerbending flexibility was considered to be negligible because the 

wind turbine is freely floating and no significant bending 

moments were applied at the bottom of the structure. As such, the 

tower was modeled as a rigid structure as a first step in getting a 

working model.

The yawing of the wind turbine occurs at the yaw mechanism,

which is located at the bottom of the spar, as opposed to a 

conventional yaw bearing located at the nacelle. There is a 

reasonable amount of yaw damping that arises from aerodynamic 

drag, hydrodynamic drag, and friction forces at the two universal 

joints. But, as yaw motion is not of specific interest in the freedecay work discussed in this paper, the nacelle and platform yaw 

DOF were switched off to restrict any yaw motion of the wind 

turbine.

The SWAY turbine uses a tension rod instead of typical slack 

or taut mooring lines commonly used for floating offshore 

platforms. As described earlier, FAST has a quasi-static mooring 

line model. To represent the tension rod in the FAST model, it is 

modeled as a taut mooring line. This assumption is valid because 

the SWAY wind turbine should only generate tension forces 

under the design conditions. Therefore, thickness, mass, and 

stiffness properties of the tension rod were used for the mooringline model.

V. MODEL CALIBRATION

For this study, most of the wind turbine dimensions and blade 

properties were provided by SWAY AS. However, there were 

some uncertainties in quantities, such as the mass moment of 

inertia and tower-mass distribution. Before the wind turbine was 

deployed, a center of gravity test was performed for the whole 

system. From the mass properties and center of gravity (CG) 

location data, the tower and platform properties were adjusted to 

match the CG location of the model with the real system. 

Because of the lack of measured data of the inertia values, they

were the primary values used to calibrate the model to the 

experimental data. An additional quadratic global platform 

damping matrix was used to calibrate the model.
4

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

A. Static Equilibrium Comparison and Calibration

A FAST linearization analysis of the model was conducted 

under static-equilibrium conditions to check the full system 

characteristics. Full-system inertia, damping, and stiffness values 

were obtained to get a general understanding of the system and to 

verify the system properties.

A static-equilibrium simulation was initially carried out on an

uncalibrated model to obtain the natural equilibrium position of 

the turbine. At equilibrium, the pitch offset (from upright 

position) was 1.8° and the surge offset was 0.514 m. The sway, 

heave, and roll offsets were negligible.

From experimental data, it was observed that the pitch offset 

is about 0.98° while the surge offset is about 0.34 m. To calibrate 

the offset, minor adjustments were made to the nacelle CG and 

the overhang values in FAST. This was a reasonable adjustment 

because of the inaccuracy in determining the mass distribution of 

the nacelle. After calibration, the pitch offset was 1° and the 

surge offset was 0.287 m.

B. Free-Decay Tests

Further calibration of the model was performed using freedecay test data. After turbine deployment and installation of the 

NREL instrumentation, five free-decay tests were conducted on 

the SWAY prototype by displacing the system and allowing it to 

return to equilibriumtwo in the roll direction, and one each for 

the pitch, surge, and sway directions. Ropes were attached to the 

nacelle and the base of the tower to perform the displacement. 

The ropes were held with a quick release system and pulled 

onshore (nearby to the prototype installation) to set the system to 

the desired initial displacement of the system. 

Testing was conducted during calm conditions with a mean 

wind speed of 1.7 m/s and minimum and maximum values of 

0.25 m/s and 5 m/s, respectively. The ocean was predominately 

flat with very small wind-generated capillary waves. The current

velocity ranged from 6 to 8 cm/s based on hourly averages. The 

turbine did not operate during the tests and it was expected that 

the calm conditions had minimal impact on the experimental 

data.

In an ideal condition, only one DOF would be excited at a 

time in a free-decay test to identify the frequency and damping of 

that DOF. However, during the actual test, the primary DOF of 

interest in each test was strongly coupled with other DOFs. For 

example, during a roll free-decay test, the sway DOF was heavily 

coupled while the pitch and surge of the system experienced 

small motions. Nonetheless, the results collected were sufficient 

to allow for manual tuning of the mass moment of inertia and 

drag coefficient (which results in hydrodynamic viscous 

damping) of the platform to calibrate the natural frequency and 

damping of the system. 

Out of five free-decay tests, only one roll free-decay test and 

one pitch free-decay test were needed for calibration while the 

remaining tests will be used for validation in the future.

1) Roll Free-Decay Test

The initial displacements (surge, sway, and heave) and 

rotations (roll and pitch) from the roll free-decay test were used 

as initial conditions in the FAST model. A free-decay simulation 

was then run, and the platform inertia and roll term (diagonalonly) in the damping matrix was tuned so that the roll period and 

magnitude matched the experimental data.

a) Roll DOF

The experiment exhibited an average platform roll period of 

45.6 s. Although, the FAST simulation had an average period of 

44.0 s after calibration. It is noted that both the experimental and 

simulated period varied slightly throughout the entire free-decay 

test (the first experimental oscillation period was 42.9 s). The roll 

motion of the system after tuning is shown in Figure 4. The 

simulated motion follows the experimental motion quite closely. 

Significant deviations of the period and amplitude occur after the 

fourth oscillation when the magnitude of oscillation decreases.

0 50 100 150 200 250 300 -15

-10

-5

0

5

10

Time (s)

Roll (degrees)

Experiment

FAST

Figure 4. Roll motion in roll free decay.

A fast Fourier transform (FFT) was carried out for the first 

280 s of experimental time and 5,000 s of simulation time to 

analyze the data in the frequency domain. Whereas the large 

amount of data for over 5,000 s of simulation time provided a 

decrease in power-spectral density across all frequencies, the 

peak frequencies became more pronounced, which is useful for 

improving the analysis and resolution of the FAST results. Figure 

5 shows the FFT results, while Figure 6 zooms in around the 

main peaks. The peak frequency of the experimental and 

simulated results occurs at values of about 0.02143 Hz and 0.023 

Hz, which correspond to a period of 46.7 s and 43.48 s,

respectively. This is the roll frequency of the whole system. 

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

10-4

10-2

100

102

104

Frequency (Hz)

Power Spectral Density (m2/Hz)

Experiment

FAST

Figure 5. Power spectral density (PSD) versus frequency of roll DOF.
5

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

0 0.02 0.04 0.06 0.08 0.1 0.12

10-1

100

101

102

103

X: 0.07143

Y: 19.31

Frequency (Hz)

PSD (m2/Hz)

X: 0.023

Y: 179.1

X: 0.02143

Y: 1205

X: 0.0724

Y: 10.28

Experiment

FAST

Figure 6. PSD versus frequency of roll DOF (zoomed in).

Because of the short duration of the roll test and sample rate,

the range and resolution of the frequency data is limited at 

0.00357 Hz, which is significant at lower frequencies. For 

example, the next frequency higher than 0.02143 Hz is 0.025 Hz, 

whereby the period would have decreased to 40.0 s. Nonetheless, 

the FFT analysis shows matching frequency trends between 

experimental and simulation results.

A second peak frequency was also observed for simulated and 

experimental results occurring at values of about 0.0724Hz and 

0.07143 Hz, respectively. The second peak frequency is caused 

by the flexibility of the universal joint between the spar structure 

and tension rod, which allows both to rotate about this joint. This 

is verified by analyzing the measured angle of the tension rod 

with a motion plot and FFT analysis.

Figure 7 shows that the primary frequency of motion of the 

tension rod is higher compared to the rolling motion of the 

system in Figure 4. Figure 8 and 9 shows the FFT results, 

indicating that the primary frequency is 0.0681 Hz and 0.07168 

Hz from the experimental data and FAST analysis respectively, 

which corresponds closely to the second peak shown in the FFT 

results in Figure 6.

0 50 100 150 200 250 300 -12

-10

-8

-6

-4

-2

0

2

4

6

8

Time (s)

Tension Rod Angle (degrees)

Experiment

FAST

Figure 7. Tension rod motion.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

10-4

10-2

100

102

104

Frequency (Hz)

Power Spectral Density (m2/Hz)

Experiment

FAST

Figure 8. PSD versus frequency of tension rod motion.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

100

101

102

103

X: 0.02151

Y: 380.8

Frequency (Hz)

Power Spectral Density (m2/Hz)

X: 0.02509

Y: 98.81

X: 0.07168

Y: 560.8

X: 0.0681

Y: 178.6

Experiment

FAST

Figure 9. PSD versus frequency of tension rod motion (zoomed in).

2) Pitch Free-Decay Test

Next, initial displacements and rotations were set in the FAST 

model to match the starting conditions for the pitch free-decay 

test. The simulated response in FAST was used to tune the 

platform inertia and damping matrix in the pitch direction 

(diagonal-term) so that the pitch period matched that of the 

experimental data.

a) Pitch DOF

The experiment had an average platform pitch period of 44.6 

s. After calibration, the FAST simulation had an average period 

of 42.9 s. Similarly, both the experimental and simulated period 

varied slightly throughout the entire free- decay test. For the first 

oscillation, the experimental period of the pitch motion was 42.5 

s while the simulation period was 43.7 s. The pitch offset was 

similar (at about 1°) for the experimental and simulation results. 

The motion plots are shown in Figure 10. The simulated motion 

follows the experimental motion quite closely. Noticeable

deviations for period and amplitude occur after the fifth 

oscillation when the magnitude of oscillation has decreased over

time.
6

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

0 50 100 150 200 250 300 -15

-10

-5

0

5

10

Time (s)

Pitch (degrees)

Experiment

FAST

Figure 10. Pitch motion in pitch free decay.

An FFT was carried out for the first 300 s of experimental

time and 5000 s of simulation time. Figure 11 shows the FFT 

results, while Figure 12 is zoomed in to show the peak values. 

The peak frequency of the measured and simulated pitch occurs 

at values of about 0.02333 Hz and 0.0226 Hz, which corresponds 

to a period of 42.86 s and 44.25 s, respectively. This frequency is 

the pitch frequency of the whole system. A second peak 

frequency is also observed for measured and simulated pitch

occurring at values of about 0.07333 Hz and 0.0714 Hz. Similar 

to the roll free-decay analysis, the second peak frequency is 

caused by the flexibility of the universal joint between the spar 

structure and tension rod.

Because of the duration of the pitch test and sample rate, the

range and resolution of the frequency data is limited at 0.00333 

Hz. This error is significant at lower frequencies. For example, at 

the next frequency higher than 0.02333 Hz is 0.02667 Hz, 

whereby the period would have decreased to 37.5 s. Nonetheless, 

the FFT analysis shows matching frequency trends between 

experimental and simulation results.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

10-4

10-2

100

102

104

Frequency (Hz)

Power Spectral Density (m2/Hz)

Experiment

FAST

Figure 11. PSD versus frequency of pitch DOF.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

10-1

100

101

102

103

104

Frequency (Hz)

PSD (m2/Hz)

X: 0.0226

Y: 191.1

X: 0.02333

Y: 625.7

X: 0.07333

Y: 20.11

X: 0.0714

Y: 12.87

Experiment

FAST

Figure 12. PSD versus frequency of pitch DOF (zoomed in).

The assumptions indicated earlier were likely to result in 

minor deviations between simulation and experimental results. In 

addition, there were two factors that might cause errors in period 

and amplitude as the magnitude of oscillation decreased over 

time.

The first factor is the inability to model frictional damping in 

the universal joints in the tension rod. During the initial few 

oscillations, the hydrodynamic viscous drag of the system is 

dominant. While the motion subsides overtime, the frictional 

damping in the universal joints may become significant in 

affecting the overall motion of the system.

The second factor was the inability to accurately simulate the 

drag coefficient of the cylindrical spar, which varies with the 

motion of the platform. This would likely result in minor errors 

for the simulated system.

In general, the calibrated FAST model was able to simulate 

the dynamics of the prototype SWAY wind turbine in free-decay 

motions with reasonable accuracy. However, to have better 

confidence in the model, more work needs to be done on the 

calibration process to reduce deviations in the simulated results 

and fine-tune the model over several periods. More data would 

help increase the confidence of the spectra.

VI. CONCLUSION

A FAST model of the SWAY prototype wind turbine was 

built and calibrated using two sets of free-decay tests. The 

comparison between the FAST simulations and experiment 

results matched with minor discrepancies because of the 

simplifying assumptions made in modeling the turbine. The 

inability to model frictional damping in the universal joints of the 

system and the inability to simulate the disturbed fluid field 

around the platform also contributed to discrepancies between 

measured and simulated results. Further work using more data 

sets and improved physics is needed to have a more accurate and 

reliable FAST model.

Future work may look at quantifying the assumptions and 

estimating the resulting errors. Also, the model fidelity may be 

increased to reduce assumptions. Some changes to the FAST tool 

might include altering the mooring-line model to better represent 

the tension-rod system and modifying the tower-stiffness model 
7

This report is available at no cost from the 

National Renewable Energy Laboratory (NREL) 

at www.nrel.gov/publications.

to account for the spreader beam system, which is unique for the 

SWAY system. In addition, simulations of different turbine 

operating and nonoperating conditions will be conducted and 

validated to further the accuracy of the model and FAST’s ability 

to model floating wind turbines.

ACKNOWLEDGMENT

The authors gratefully acknowledge the support provided by

the U.S. Department of Energy and the Energy Research Institute 

at Nanyang Technological University. In addition, the expertise 

and support of SWAY AS in conducting the model test and 

providing valuable test data is greatly appreciated.

REFERENCES

[1] J. Jonkman and M. L. B. Jr, “FAST User’s Guide,” 2005.

[2] J. Jonkman, “Dynamics Modeling and Loads Analysis of an Offshore 

Floating Wind Turbine,” Ph.D. dissertation, University of Colorado, United 

States, 2007.

[3] J. Jonkman and W. Musial, “Offshore Code Comparison Collaboration 

(OC3) for IEA Task 23 Offshore Wind Technology and Deployment,” 

United States, Rep, 2010.

[4] W. Popko, F. Vorpahl, and A. Zuga, “Offshore Code Comparison 

Collaboration Continuation (OC4), Phase I Results of Coupled Simulations 

of an Offshore Wind Turbine with Jacket Support Structure,” in 22nd 

International Society of Offshore and Polar Engineers Conference, Rhodes, 

Greece, 2012.

[5] J. R. Browning and A. J. Goupee, “Calibration and Validation of a SparType Floating Offshore Wind Turbine Model using the FAST Dynamic 

Simulation Tool Preprint,” in Science of Making Torque from Wind, 

Oldenburg, Germany, 2012.

[6] G. Stewart, M. Lackner, and A. Goupee, “Calibration and Validation of a 

FAST Floating Wind Turbine Model of the DeepCwind Scaled TensionLeg Platform Preprint,” in 22nd International Society of Offshore and Polar 

Engineers Conference, Rhodes, Greece, 2012.

[7] I. Prowell, G. M. Stewart, and A. J. Goupee, “Numerical Prediction of 

Experimentally Observed Behavior of a Scale Model of an Offshore Wind 

Turbine Supported by a Tension-Leg Platform Preprint,” in 2013 Offshore 

Technology Conference, Houston, Texas, 2013.

[8] A. J. Coulling, A. J. Goupee, A. N. Robertson, J. M. Jonkman, and H. J. 

Dagher, “Validation of a FAST semi-submersible floating wind turbine 

numerical model with DeepCwind test data,” Journal of Renewable and 

Sustainable Energy, vol. 5, no. 2, p. 023116, 2013.

[9] P. J. Moriarty and A. Hansen, “AeroDyn Theory Manual,” 2005.