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.

Contract No. DE-AC36-08GO28308

Calibration and Validation of a 

Spar-Type Floating Offshore 

Wind Turbine Model using the 

FAST Dynamic Simulation Tool 

Preprint

J.R. Browning

University of Colorado-Boulder

J. Jonkman and A. Robertson

National Renewable Energy Laboratory

A.J. Goupee

University of Maine

Presented at the Science of Making Torque from Wind

Oldenburg, Germany

October 9-11, 2012

Conference Paper

NREL/CP-5000-56138

November 2012
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1

Calibration and validation of a spar-type floating offshore 

wind turbine model using the FAST dynamic simulation tool

J.R. Browning,

1 J. Jonkman,2 A. Robertson,2 A.J. Goupee3

1 Department of Mechanical Engineering, University of Colorado, Boulder, Colorado, United 

States

2 National Renewable Energy Laboratory, Golden, Colorado, United States 3 Department of Mechanical Engineering, University of Maine, Orono, Maine, United States

E-mail: james.browning@colorado.edu

Abstract. High-quality computer simulations are required when designing floating wind 

turbines because of the complex dynamic responses that are inherent with a high number of 

degrees of freedom and variable metocean conditions. In 2007, the FAST wind turbine 

simulation tool, developed and maintained by the U.S. Department of Energy’s (DOE’s) 

National Renewable Energy Laboratory (NREL), was expanded to include capabilities that are 

suitable for modeling floating offshore wind turbines. In an effort to validate FAST and other 

offshore wind energy modeling tools, DOE funded the DeepCwind project that tested three 

prototype floating wind turbines at 1/50th scale in a wave basin, including a semisubmersible, a 

tension-leg platform, and a spar buoy. This paper describes the use of the results of the spar 

wave basin tests to calibrate and validate the FAST offshore floating simulation tool, and 

presents some initial results of simulated dynamic responses of the spar to several 

combinations of wind and sea states. Wave basin tests with the spar attached to a scale model 

of the NREL 5-megawatt reference wind turbine were performed at the Maritime Research 

Institute Netherlands under the DeepCwind project. This project included free-decay tests, 

tests with steady or turbulent wind and still water (both periodic and irregular waves with no 

wind), and combined wind/wave tests. The resulting data from the 1/50th model was scaled to 

full size and used to calibrate and validate a full-size simulated model in FAST. Results of the 

model calibration and validation include successes, subtleties, and limitations of both wave 

basin testing and FAST modeling capabilities.

1. Introduction

Simulation tools that are suitable for modeling offshore floating wind turbines are required for the 

design of utility-scale offshore systems because of both the increased loading expected in offshore 

systems from inertial effects and the economic impracticality of full-scale testing in the marine 

environment. One such tool, the FAST coupled aero-hydro-servo-elastic dynamic simulator, was 

developed for use with offshore floating wind turbines from the original FAST land-based simulation 

tool [1]. This paper focuses on an effort to use the DeepCwind 1/50th-scale test data to calibrate and 

validate a FAST turbine model of a spar buoy floating wind turbine.
2

1.1. The FAST wind turbine simulation tool

FAST is a nonlinear time-domain simulation tool that is capable of modeling the coupled aero-hydroservo-elastic response of floating offshore systems that are operating in an environment with 

combined wind and wave loading. Rotor aerodynamics are calculated using the AeroDyn software 

library—which relies on blade-element/momentum theory or generalized dynamic wake theory for the 

calculation of wake effects—and the Beddoes-Leishman model for calculation of dynamic stall, and 

provides the user with the option of incorporating the effects of tip losses and hub losses [2].

Structural components of the turbine are modeled as a combination of coupled rigid and flexible 

bodies. Flexible bodies include the blades, tower, and drive shaft [3, 4]. Time-domain hydrodynamics 

include the effects of hydrostatic restoring, viscous drag from waves and turbine motion, added mass 

and damping from wave radiation, and linear wave diffraction. Mooring lines are modeled as quasistatic taut or catenary lines and include the effects of stretching, mass density, buoyancy, geometric 

nonlinearity, and seabed interactions. Dynamic mooring line effects and mooring line drag are not 

included in the model [1].

The degrees of freedom (DOF) of the complete wind turbine and floating platform FAST model 

include edgewise and flapwise blade motions, rotor rotation, driveshaft torsion, nacelle/rotor yaw, first 

and second modal tower-bending motions (both side-to-side and fore-aft), as well as six degrees of 

platform motions, including surge, sway, heave, roll, pitch, and platform yaw. Surge, sway, and heave 

are translations in the X, Y, and Z directions, respectively; whereas roll, pitch, and yaw are rotations 

about the X, Y, and Z axes, respectively. Coordinate systems and platform DOF definitions used in 

this paper are illustrated in Figure 1.

1.2. The spar-type floating wind turbine

The Maritime Research Institute Netherlands (MARIN)/University of Maine (UMaine) scale test 

model and FAST full-scale, three-bladed horizontal-axis wind turbine model are based on the National 

Renewable Energy Laboratory’s (NREL’s) offshore 5-megawatt (MW) baseline wind turbine [5]. The 

turbine was attached to a spar buoy platform that was adapted from a spar design developed for Phase

IV of the Offshore Code Comparison Collaboration (OC3), which is based on Statoil’s Hywind spar 

[6]. The spar has three equally spaced mooring lines in a water depth of 200 meters (m). The UMaine 

model uses a bridle system for the attachment of the three mooring lines to the spar, providing 

additional yaw stiffness; whereas the FAST model has a direct attachment to the spar of each mooring 

line at a radius of 5.2 m and a draft of 70 m because of FAST mooring line geometrical definition 

limitations. Pertinent dimensions of the FAST model are given in Table 1.

Table 1. Dimensions of the spar type floating 

wind turbine model.

Hub Height [m] 90

Flexible Tower Length [m] 77.6

Blade Length [m] 61.5

Tower Top Mass [kg] 394,000

Tower Mass [kg] 303,145

Tower Base Above MSL [m] 10

Spar Length [m] 130

Spar Center of Mass Below MSL [m] 90

Spar Mass [kg] 7,280,000

Displacement Volume [m3] 7,948

Total System Mass [kg] 7,977,337

Figure 1. Coordinate system and definitions for 

platform DOF used in this paper. (Source: 

University of Maine)
3

1.3 MARIN wave tank testing (1/50thscale)

Tests were carried out in MARIN’s wind/wave basin on a 1/50th Froude-scaled model of the spar 

system built by UMaine and MARIN [7]. Researchers conducted static offset tests, six DOF decay 

tests, periodic wave tests with and without wind, and combinations of stochastic wind and wave 

conditions. In addition, hammer tests were performed on the system to obtain fundamental modal 

responses. Data recorded during the tests included six platform DOF positions and accelerations; rotor 

torque and position; accelerations at three locations spanning the tower, forces, and moments at the 

tower base and tower top; and mooring line tensions. The sampling frequency was 100 hertz (Hz),

corresponding to a Froude-scaled sampling frequency at full scale of roughly 14 Hz. All data from the 

MARIN tests were converted to full scale using Froude scaling prior to analysis [8]. All test data 

provided in this paper were presented at full scale, unless otherwise noted.

1.4 Introduction to calibration and validation

The FAST model was calibrated prior to validation to match the UMaine test model as closely as 

possible by using free-decay and periodic-wave tests (a small subset of the total experimental data 

available). Parameters in the FAST model were calibrated to match the test model when there was a 

known potential for discrepancy between the two. These discrepancies took the form of 

simplifications in the FAST model or simulation algorithms, or uncertainties in the characteristics of 

the scale test model. Once calibrated, the FAST model was validated by comparing the responses of 

the FAST model and test model for several tests, again including the free-decay and periodic-wave

tests, with the addition of tests with irregular waves and steady wind.

2. Calibration

The parameters of the FAST model, which were calibrated prior to validation, are included in Table 2,

along with a brief justification for calibration. Additionally, prior to this study, the aerodynamic 

coefficients of the blade were calibrated to match rotor thrust between FAST full-scale simulations 

and scaled-up test data because of poor aerodynamic performance of the UMaine test model resulting 

from Reynolds number (Re) dissimilitude [8].

Table 2. FAST model parameters calibrated prior to validation with a brief reason for calibration.

Calibrated Parameter Justification for Calibration

Mooring Line Mass, 

Stiffness, and Length

Matching the mooring system tensions in the FAST model caused by horizontal 

displacement to the UMaine test model; necessary because of a delta connection in the 

UMaine test model that was not directly modeled in FAST 

Tower Stiffness Matching of first tower vibrational mode in the FAST model to the UMaine test model

because of uncertainty in its stiffness, which was altered by sensors and sensor cables

Platform Displacement at 

Equilibrium

Matching zero heave at equilibrium of the FAST model and the UMaine test model because 

of uncertainties in mooring line fairlead angle and equilibrium displacement in the test 

model

Platform Yaw Stiffness Emulating the added yaw stiffness created by the UMaine mooring system’s delta 

connections in FAST

Heave and Yaw Damping Fixing discrepancies between the UMaine test model and the FAST model because of

FAST viscous drag simplifications

2.1 Mooring system calibration

The mooring system used in the MARIN tests consisted of three equally spaced primary mooring 

lines connected to the spar via delta connections that provided additional platform yaw stiffness than a 

single (direct) connection. Each of the three primary lines contained an inline linear spring intended to 

simulate the combined stiffness caused by mooring line axial stiffness and mass density of a full-scale

catenary mooring line. Because FAST was not able to simulate the more complex delta connection of 

the UMaine test model, and because it relied on a quasi-static catenary solution (rather than an inline

spring), the FAST mooring model was calibrated, as described in the following paragraph, to mimic 

the steady-state reaction of the MARIN model to X-direction displacements. Static offset tests in the 
4

X-direction were performed by MARIN for offset values ranging from 0 to 12.4 m; whereas heave

was kept constant at the equilibrium value. The length, axial stiffness, and mass density of the FAST 

mooring lines were tuned until lines 1 and 2, at 120 and 240 degrees from the X-axis, respectively, and 

line 3, at zero degrees from the X-axis, matched the MARIN results for line tension at offsets of 12.4 

m and at equilibrium (i.e., zero offset for all platform degrees of freedom). The anchor locations were 

kept fixed at a radius of 445 m for both models. The resulting line tensions at the spar connection for 

several offsets for the FAST model and UMaine test model are shown in Figure 2. The two models 

were in agreement for all offsets, with the largest discrepancy of 39 kN occurring for line 2 at an offset 

of 9.9 m.

2.2 Tower calibration

The as-tested UMaine test model included an instrument cable attached to the tower and a force and 

moment sensor at the base of the tower of unknown stiffness. To represent the interaction of the cable 

and the sensor with the structure, a single stiffness multiplier was used at all FAST tower nodes to 

decrease the stiffness from nominal UMaine test model design specifications. The multiplier was 

calibrated so that the tower’s first fore-aft frequency mode matched between MARIN and FAST, as 

measured by a fast Fourier transform (FFT) of tower-top acceleration with the turbine operating in 

periodic waves. The multiplier decreased the FAST model’s first tower-bending mode from 0.49 Hz to 

0.42 Hz.

2.3 Platform displacement, DOF stiffness, and damping calibration

Platform yaw stiffness was added to the FAST model until the yaw natural frequency matched that of

the MARIN tests. The first natural frequencies in yaw for both models were measured using an FFT of 

a time series from a yaw decay test.

The platform draft at zero heave was reduced from the design specification value of 8,029 m3 to 

7,948 m3 so that the model would float at zero heave in its equilibrium state with the calibrated 

mooring system.

FAST includes the capability of modeling nonlinear viscous drag on the platform in the X and Y

directions via a user-specified coefficient of drag and varying platform diameter. The coefficient of 

drag (Cd) for the FAST spar model was calculated as the coefficient of drag for an infinite cylinder, 

which was determined based on the oscillatory Re of the relative water flow [9]. Re, however, does 

not scale consistently with Froude scaling. Because the goal was to model the test model, the range of 

Re values used for calculating the appropriate Cd were based on the 1/50th-scale test data rather than 

the full-scale data. It was found that the likely range of Re for the scaled data corresponded to an area 

of low slope in a Cd versus Re curve, with a mean value of approximately 1.0. Therefore, a value of 

1.0 was used for Cd in the FAST model. It should be noted that the maximum Re expected from fullscale test data (using Froude scaling) was roughly 106

, corresponding to a Cd of approximately 0.6 for 

an infinite cylinder, thereby illustrating the importance of using Re from scale test data when 

approximating Cd for viscous drag for the purposes of simulating the behavior of a model-scale 

system.

Because viscous drag in FAST was calculated only in the X and Y directions, it had a damping

effect on the surge, sway, pitch, and roll motions of the spar platform. Additional linear damping was 

added to the heave and yaw DOFs in the FAST model to account for damping characteristics that are

currently not modeled in FAST—such as skin friction, drag on mooring lines, and the drag caused by

the abrupt edge at the bottom of the spar—that would be present in the MARIN tests. During 

calibration, 71.0×103 N/(m/s) and 10.1×106 N∙m/(rad/s) of damping were added to heave and yaw,

respectively, so that the average damping ratio over several peaks from the time series of heave and 

yaw decay tests were consistent between FAST simulations and MARIN tests.
5

Figure 2. Mooring line tensions at the spar connection for the FAST model and UMaine test model.

3. Validation

After calibrating the FAST model to account for any known discrepancies between it and the UMaine 

test model, the FAST model was validated by comparing the results between the simulation and test 

for a series of tests, including free-decay tests, periodic-wave tests with no wind, and irregular-wave 

tests [Joint North Sea Wave Project (JONSWAP) waves] with wind.

3.1. Free-decay tests

After calibration, damping properties and natural frequencies for the six platform DOFs were 

compared between the FAST model and the UMaine test model via decay tests. The tests were 

performed by translating or rotating the model in the direction of each of the platform DOFs and 

letting the model return to equilibrium. The tests were performed with no incident waves or wind and 

a stationary rotor. Natural frequencies were calculated by locating the dominant frequency in the FFT 

of the resulting free-decay time series. Figure 3 shows the resulting natural frequencies for the FAST 

model and UMaine test model. As described, yaw stiffness and damping, as well as heave damping, 

were added to the FAST model during calibration. All frequencies matched well between FAST and 

the UMaine test model with the exception of pitch and roll, for which FAST exhibited a noticeably 

lower frequency response than the MARIN test data (0.0290 Hz and 0.0315 Hz for FAST and 

MARIN, respectively). This lower frequency response may have been because of incorrect placement 

of mass along the tower during the tower mass calibration process, which would have resulted in an 

incorrect moment of inertia for the system and incorrect pitch and roll restoring associated with the 

system center of mass.

The damping ratio for the platform motions were calculated from the average ratio of successive 

peaks using peaks 2−9 and peaks 9−16 from the decay tests. Averages of two ranges of time series 

peaks were used because the FAST model included nonlinear viscous drag, which increased with the 

higher platform velocities that occurred with high-amplitude oscillations; whereas damping during 

lower amplitude oscillation was primarily because of radiation damping. The damping ratios are 

presented in Figure 4. Heave and yaw were consistent between FAST and the UMaine test model for 

peaks 9−16. The MARIN surge damping ratio increased for lower amplitude oscillations, which may 

have been caused by a problem with the test procedure or the average successive peak ratio analysis 

procedure. FAST showed self-consistency between surge and sway (small variations were caused by

different initial offset values to match MARIN tests as well as the greater influence of rotor drag in the 

surge direction) but did not match the MARIN test values. This inconsistency may be explained by 

large displacements of the mooring systems in surge and sway, leading to nonlinearities and differing

behavior of the mooring systems in the FAST model and UMaine test model. This explanation would 

also account for the lack of self-consistency in the MARIN test data for surge and sway because the 

initial offset for sway was significantly larger than for surge, at 10 m and 4 m, respectively. In general, 

the FAST model appeared to have greater damping in surge and sway and less damping in pitch and 
6

roll relative to the UMaine model. Drag on the mooring lines was not modeled in the current version 

of FAST, which may have accounted for some of the discrepancy in surge and sway.

Figure 3. Natural frequencies of platform motions 

for the UMaine test model and FAST model.

Figure 4. Average damping ratios from peaks 

2−9 and peaks 9−16 of platform DOF decay 

tests.

3.2. Periodic wave tests

The results of two periodic wave tests, with wave heights of 1.92 and 7.14 m and wave periods of 7.5

and 14.3 s, were compared between the FAST model and the UMaine test model to validate the 

system response to a relatively simple sea state. These tests were run with no wind, a stationary rotor,

and waves propagating along the positive X-axis (i.e., toward the rotor in the direction of platform 

surge).

Figure 5 shows the resulting power-spectral densities (PSDs) of the response to a periodic wave test 

with low-height (1.92 m) and low-period (7.9 s, 0.13 Hz) waves. Both heave and tower-top

acceleration response at the wave frequency was consistent between the MARIN experiment and 

FAST simulation. The response of the tower top at the pitch frequency as well as the heave response at 

the heave natural frequency was stronger for the simulation than the experiment. The difference in 

heave response at the natural heave frequency suggested insufficient heave damping in the FAST 

model. However, the damping ratios in Figure 4 indicated agreement for the heave DOF. Similarly, 

Figure 4 indicates that pitch had greater damping in FAST than the experiment for both of the large 

displacements. This fails to explain the greater response of the tower top in the FAST model at the 

natural pitch frequency.

The PSD of the tower top X-acceleration for the experimental data showed a strong response at 

0.26 Hz, which was twice the wave frequency. This could have been caused by second-order 

hydrodynamic excitation. The simulation did not show that this response was likely because it was not 

capable of modeling second-order hydrodynamic loading.

Figure 5. Tower-top X-direction acceleration response and platform-heave response of the UMaine

test model and FAST model in periodic waves with a height of 1.92 m and a wave period of 7.5 s.
7

PSDs from a periodic wave test with a wave height of 7.15 m and a period of 14.3 s (0.070 Hz 

frequency) are shown in Figure 6. Response of both platform-heave and tower-top acceleration at the 

wave frequency was fairly consistent between the simulation and the experiment. However, the 

simulation resulted in a significantly greater tower-top response at the platform-pitch frequency than 

the experiment, as well as greater heave response at the heave frequency than the experiment. Again, 

this outcome may point toward differences in damping between the two systems that are still occurring 

with larger waves and platform motions and may be partially explained by damping in the UMaine 

test model that was caused by nonlinear drag of the mooring lines (not modeled in FAST). The 

MARIN test data showed large responses at two and three times the wave frequency, 0.14 Hz and 0.21 

Hz, respectively; whereas the simulation showed only the 0.14 Hz response. This response was likely 

because of the quadratic viscous damping term in FAST, which caused a pronounced response at 

twice the regular wave frequency when subjected to higher waves and greater motions.

Figure 6. Tower-top X-direction acceleration response and platform heave response of the UMaine

test model and FAST models in periodic waves with a height of 7.14 m and a wave period of 14.3 s.

3.3. Irregular wave tests

The MARIN spar model was tested under a variety of metocean conditions with irregular waves and 

steady wind. Waves in both the experiment and simulation were based on JONSWAP spectra [1]. The 

recorded wave parameters in the experiment and wave spectra parameters in FAST were the same, and 

included significant wave height, peak-spectral period, peak shape factor, and propagation direction. 

The FAST simulation used these four parameters to produce a wave height time series that was based 

on the JONSWAP spectrum, which was then used in the simulation. This method resulted in sea 

surface elevations that were not identical in time, but had more consistent spectra than the MARIN 

tests.

The first irregular wave test used for validating the FAST model consisted of a significant wave 

height of 2 m, a peak-spectral period of 7.5 s (0.133 Hz), and a shape factor of 2.0. The steady 

horizontal wind speed was 11.23 meters per second (m/s) and the rotor was kept at a constant speed of 

7.8 rpm (0.13 Hz). The wave direction was zero degrees (i.e., propagation was toward the downwind 

side of the turbine). Figure 7 shows PSDs of the moment about the Y-axis at the base of the tower, and 

the tower-top acceleration in the X direction. Both plots show a similar response shape within the 

region of the JONSWAP wave frequencies (roughly 0.009 to 0.035 Hz) between both measurements

and between the experiment and simulation, as expected because of the influence of the tower-top

motions on the tower-base moments. The response of the tower-top acceleration at the platform-pitch

frequency was also similar between the experiment and simulation. It would be expected that similar 

tower-top motion spectra in the vicinity of the platform-pitch frequency would translate into similar 

tower-base moment spectra in that frequency range. However, it can be seen in Figure 7 that the 

magnitude of the tower-base moment response was somewhat larger for the FAST simulation than the 

experiment at the platform-pitch frequency (0.030 Hz). This outcome may have been caused by an 

incorrect tower/nacelle/rotor system rotational inertia, and thus a larger moment at the base for a given 
8

tower-top acceleration. This would also account for the lower pitch natural frequency for the FAST 

model than in the MARIN experimental model previously noted.

In both spectra, the experimental data showed prominent peaks at the blade-passing frequency (3P)

of 0.39 Hz, as well as the first two blade-passing frequency harmonics (6P and 9P). Although an effort 

was made to produce a low turbulence and constant wind field over the rotor area for the experiments, 

wind calibration results showed both vertical and horizontal variation in wind velocity, with a 

minimum wind speed measured across the rotor of roughly 70% of the maximum. The prominent 3P, 

6P, and 9P peaks may be because of these inconsistencies in wind velocity over the rotor. In 

comparison, the FAST simulations (for these tests) modeled wind velocity as a constant wind field 

over the rotor area; thus, for this experiment, the spectra did not exhibit pronounced peaks at the 

blade-passing frequency or its harmonics. FAST does include the capability for modeling wind fields 

with horizontal and vertical shear. These effects may be examined in the future.

Figure 7. Response in irregular waves of significant wave height, peak-spectral period, and shape 

factor of 2 m, 7.5 s, and 2.0, respectively. The horizontal wind speed was 11.23 m/s and the rotor 

speed was 7.8 rpm.

The next irregular wave test used for validation consisted of an increased wave height and peakspectral period of 7.1 m and 12.1 s (0.083 Hz), respectively. The shape factor increased to 2.2 and the 

wind velocity and rotor speed were kept the same as the previous test, at 11.23 m/s and 7.8 rpm (0.13 

Hz). As pitching motions increased, it was expected that yaw-pitch coupling would be present because 

of rotor gyroscopic forces. Figure 8 shows that the tower-top X-accelerations increased with the higher 

height and longer period waves relative to the previous test shown in Figure 7. As shown in the PSD 

of the yaw response, the simulation data showed the expected pronounced peak corresponding to the 

model’s natural platform-pitch frequency. However, rather than having a peak at the pitch frequency, 

the experimental data has a peak that corresponds with the natural heave frequency of 0.036 Hz. 

Another clear discrepancy was in the experimental peak yaw response at 0.08 Hz, or roughly the peak 

spectral wave frequency, which shifted to the right for the simulation response. The reason for these 

discrepancies is currently unknown. FAST currently utilizes the Massel wave cut-off frequency 

criterion, for which the JONSWAP wave spectra is truncated at three times the peak-spectral 

frequency [1].

An irregular wave test designed to emulate a survival condition was performed. The wave height 

and period were increased to 10.5 m and 14.3 s (0.070 Hz), respectively. Constant horizontal wind 

velocity was increased to 21.8 m/s and the rotor speed was increased to 12.7 rpm (0.21 Hz). The 

waves propagated at a 45-degree angle to the X-axis; the wind direction stayed at zero degrees.

The X and Y tower-top accelerations, shown on the left-hand side of Figure 9, showed agreement 

between them as well as between the experimental and simulation data. Unlike previous tests, a 

distinct peak was present at the 3P frequency for both the experiment and the simulation in both the Xand Y-acceleration plots. The presence of this 3P response in the simulation may have originated from 

the oblique 45-degree wave propagation direction. A noticeable discrepancy between the experiment 

and the simulation was the large 6P peaks in the X- and Y-acceleration PSDs that occurred only for 
9

the experimental data. Again, this discrepancy was likely caused by variations in wind speed over the 

rotor for the UMaine test model. Pure vertical shear produces 3P excitations because each blade passes

through a high and low wind speed region of the rotor plane once per revolution. Other 

nonuniformities in the wind field will result in 6P excitations. More complex variations, as were

observed during the wind calibration process at MARIN, could result in a noticeable response at 

higher order multiples of 3P. Although 3P response may occur in the simulation data because of 

platform pitching and the resulting misalignment between the rotor plane and the wind vectors, higher 

order responses will not be present because of the constant wind field modeled in the simulation.

As with the X and Y tower-top accelerations, surge and sway responses in Figure 9 show 

agreement between them as well as between the two experimental and simulation data in the range of 

wave frequencies. However, the simulation data has a distinct peak in surge response, at 0.023 Hz, and 

in sway, at 0.028 Hz, neither of which were apparent in the experimental data. The 0.023-Hz surge 

response was easily identified as the FAST model surge/sway natural frequency.

The UMaine test model demonstrated significantly greater response than the FAST model in the 

region of wave frequencies for heave; the opposite was true for yaw response.

Figure 8. Response in irregular waves of significant wave height, peak-spectral period, and shape 

factor of 7.1 m, 12.1 s, and 2.2, respectively. The horizontal wind speed was 11.23 m/s and the rotor 

speed was 7.8 rpm.

Figure 9. Response in irregular waves of significant wave height, peak-spectral period, and shape 

factor of 10.5 m, 14.3 s, and 3.0, respectively. The horizontal wind speed was 21.8 m/s and the rotor 

speed was 12.7 rpm.
10

4. Conclusions

FAST simulations of a 5-MW spar-type offshore floating wind turbine operating in various metocean 

conditions were compared to results of tank tests of a 1/50th Froude-scaled model of the same system

for the purposes of calibration and validation of the FAST model. The FAST model was calibrated to 

account for differences in mooring systems between the FAST model and UMaine test model and 

simplifications in the modeling of nonlinear viscous damping. Once calibrated, the natural frequencies 

of the platform DOFs, as well as the first tower mode, were mostly consistent between the experiment 

and the simulation, with a roughly 3% inconsistency in pitch and roll. Damping of the platform, as 

measured by the damping ratio from free-decay tests, was reasonably consistent between the 

simulation and experiment for heave and yaw decay (particularly for lower height motions), but was 

inconsistent for surge, sway, pitch, and roll. FAST surge and sway appeared to be less damped than 

the UMaine model; whereas pitch and roll appeared to have increased damping relative to the UMaine 

model.

The response of the two systems to periodic waves and zero wind compared well at the wave 

frequency and fundamental tower frequency, but the FAST model tended toward a greater response at 

the natural frequencies of platform DOFs. In addition, the experimental data showed greater responses 

at the first and second harmonics of the wave frequency than the simulation. A quadratic effect was 

noticeable at twice the wave frequency in the simulation data for higher waves, but was not present for 

lower waves.

Several irregular wave tests with wind were compared. The response of the two models was 

generally consistent at frequencies corresponding to the wave spectra. At lower wind velocities, the 

experimental data showed a 3P response that was not apparent in the FAST simulations until wind 

speeds were increased to 21.8 m/s, at which point the 3P FAST response exceeded that of the 

experiment, indicating an increased 3P simulation response with higher platform pitching and 

increased rotor loads. Responses of 6P and 9P were present in the test data but not in the simulation 

data. A yaw response at the heave natural frequency was present in the test data but not in the 

simulation. The response of the two systems in the pitch/roll and surge/sway frequency range was 

more consistent for simulations including wind, indicating that wind effects dominated in these lowerfrequency ranges.

In general, the responses compared well between the experiment and the simulation, particularly in 

the region of the wave-spectra frequencies. However, differences existed in the responses to periodic 

and irregular waves, which may be important for full-scale turbine design. More research is needed to 

understand the discrepancies between the simulation and experiment before an assessment of FAST’s 

ability to accurately model floating wind turbines can be made. In particular, there appears to be 

significant discrepancies in damping behavior between the experiment and the FAST simulation.

5. References

[1] Jonkman J 2007 Dynamics Modeling and Loads Analysis of an Offshore Floating Wind Turbine

NREL Technical Report NREL/TP-500-41958 National Renewable Laboratory Golden 

Colorado United States

[2] Hansen A C and Moriarty P J 2005 AeroDyn Theory Manual NREL Technical Report

NREL/TP-500-36881 National Renewable Laboratory Golden Colorado United States

[3] Jonkman J 2012 NWTC Design Codes – FAST http://wind.nrel.gov/designcodes/simulators/fast

last modified 08-March-2012 Accessed 08-March-2012

[4] Jonkman J M and Buhl M L 2005 FAST User’s Guide NREL Technical Report NREL/TP-500-

38230 National Renewable Laboratory Golden Colorado United States

[5] Butterfield W, Jonkman J, Musial W, and Scott G 2009 Definition of a 5-MW Reference Wind 

Turbine for Offshore System Development NREL Technical Report NREL/TP-500-38060 

National Renewable Laboratory Golden Colorado United States

[6] Jonkman J 2010 Definition of the Floating System for Phase IV of OC3 NREL Technical Report

NREL/TP-500-47535 National Renewable Laboratory Golden Colorado United States
11

[7] Martin H R, Kimball R W, Viselli A M, Goupee A J 2012 Methodology for Wind/Wave Basing 

Testing of Floating Offshore Wind Turbines Proceedings of the 31st International Conference on 

Ocean, Offshore and Arctic Engineering Rio de Janeiro, Brazil June 10-15 2012

[8] Martin H 2011 Development of a Scale Model Wind Turbine for Testing of Offshore Floating 

Wind Turbine Systems M.S. Thesis University of Maine Orono Maine United States

[9] Newman J N 1977 Marine Hydrodynamics MIT Press Cambridge Massachusetts