Conference Paper

NREL/CP-500-46725

February 2010

Model Development and Loads 

Analysis of a Wind Turbine on a 

Floating Offshore Tension Leg 

Platform

Denis Matha, Tim Fischer, Martin Kuhn

University of Stuttgart, Germany

Jason Jonkman

National Renewable Energy Laboratory

Presented at the 2009 European Offshore Wind Conference 

and Exhibition

Stockholm, Sweden

September 14-16, 2009
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iii

List of Acronyms

BEM blade-element / momentum

DEL damage-equivalent load

DLC design load cases

DOF degree of freedom

IEC International Electrotechnical Commision

LSS low-speed shaft

MIT Massachusetts Institute of Technology

PDF probability density function

RAO response amplitude operator

RMS root mean square

TLP tension leg platform
iv

Executive Summary

This report presents results of the analysis of a 5-MW wind turbine located on a floating offshore 

tension leg platform (TLP) that was conducted using the fully coupled time-domain aero-hydroservo-elastic design code FAST with AeroDyn and HydroDyn. Models in this code are of greater

fidelity than most of the models that have been used to analyze floating turbines in the past—

which have neglected important hydrodynamic and mooring system effects. The report provides 

a description of the development process of a TLP model, which is a modified version of a

Massachusetts Institute of Technology design derived from a parametric linear frequencydomain optimization process. The model has been verified using comparisons to frequencydomain calculations in terms of response amplitude operators and probability density functions. 

Important differences have been identified between the frequency-domain and time-domain 

simulations, and have generated implications for the conceptual design process. An extensive 

loads and stability analysis for ultimate and fatigue loads according to the procedure of the 

International Electrotechnical Commision 61400-3 offshore wind turbine design standard was 

performed with the verified TLP model. Response statistics, extreme event tables, fatigue 

lifetimes, and selected time histories of design-driving extreme events are analyzed and 

presented. Loads for the wind turbine on the TLP are compared to those of an equivalent landbased turbine in terms of load ratios. Major instabilities for the TLP are identified and described. 

The stability analysis identified an aerodynamic instability in the platform pitch, platform roll, 

and tower- and blade-bending modes and a platform-yaw instability.
v

Table of Contents

List of Acronyms..........................................................................................................................iii

Executive Summary..................................................................................................................... iv

Table of Contents.......................................................................................................................... v

1 Introduction....................................................................................................................... 1

2 Methods.............................................................................................................................. 1

2.1 Analysis Process............................................................................................................. 1

2.1.1 Model Development................................................................................................ 1

2.1.2 Loads and Stability Analysis .................................................................................. 2

2.2 Simulation Tools............................................................................................................. 2

2.3 Tension Leg Platform Design ......................................................................................... 3

3 Results................................................................................................................................ 4

3.1 Time-Domain and Frequency-Domain Comparison ...................................................... 4

3.2 Design Load Cases.......................................................................................................... 6

3.2.1 Ultimate Loads........................................................................................................ 6

3.2.2 Fatigue Loads.......................................................................................................... 7

3.2.3 Stability Analysis.................................................................................................... 8

4 Conclusions........................................................................................................................ 9

5 References........................................................................................................................ 10
1

1 Introduction

The present study aims to provide new insight in the modeling, loads, and dynamics of wind 

turbines on floating offshore tension leg platforms (TLPs). In the United States, China, Japan, 

Spain, Norway, and many other countries, shallow-water areas are scarce. Additionally, onshore 

space requirements and the public pressure to place wind turbines out of visual range further 

increase the demand for offshore wind turbines in deeper coastal waters. Huge potential sources 

for supplying coastal areas with offshore wind energy therefore are wind turbines located on 

floating support platforms in deep offshore waters (in depths of approximately 60 m to 900 m). 

This provides a huge future market for floating offshore wind turbines. Even at transitional 

depths of 30 m to 60 m, floating structures could be a viable alternative to conventional 

monopile, tripod, and jacket structures.

A requirement for deep-water floating offshore wind turbines is the development of reliable, 

viable floating-platform support structures. Developing cost-effective, high-performance floating 

wind turbines with structural and dynamic integrity and reliability requires integrated loads 

analysis according to the International Electrotechnical Commision (IEC) 61400-3 design 

standard for offshore wind turbines. Such analysis also is crucial for conceptual design and 

preliminary analysis. The work presented here describes the model development and loads and 

stability analysis of one primary floating concept, the TLP.

2 Methods

2.1 Analysis Process

The overall process for the model development and the loads and stability analysis performed in 

the present study can be summarized concisely as follows.

2.1.1 Model Development

• Define the wind turbine and blades: The NREL 5-MW baseline turbine was selected for 

this study [1].

• Define the geometry and structural properties of the TLP platform and mooring system: 

A Massachusetts Institute of Technology (MIT) design [2] was chosen and modified for 

this study [3] (referred to as MIT/NREL TLP herein).

• Define the wind turbine control system: Modification of the baseline control system was 

not required for the TLP (See Reference 1 for a definition and details of the control 

system; see Reference 4 for potential modifications required in some floating wind 

turbines).

• Define the installation site-specific statistical meteorological (wind) and oceanographic 

(wave) data: A location in the northern North Sea is selected representing severe 

environmental conditions (c.f. Reference 5).

• Compute the hydrodynamic frequency-domain properties (hydrodynamic-added-mass 

and hydrodynamic-damping matrices and wave-excitation force vector) of the TLP with 

the potential-flow based wave-interaction analysis tool, WAMIT [6].
2

• Compute the tower mode shapes of the combined turbine/platform system with the 

multibody simulation code MSC.ADAMS (TLP design-specific mode shapes are 

required in the FAST model to obtain viable simulation results).

• Create the FAST model of the MIT/NREL TLP floating platform and the NREL 5-MW 

baseline turbine.

• Verify the time-domain FAST model with frequency-domain results in terms of response 

amplitude operators (RAOs) and probability density functions (PDFs) for MIT’s extreme 

sea state reference-load case [2].

2.1.2 Loads and Stability Analysis

• Simulate selected design load cases (DLCs) with the verified TLP model according to the 

IEC 61400-3 design standard.

• Analyze the DLC statistics, time series, extreme event tables, and fatigue loads.

• Identify the critical DLCs: Investigate causes for extreme loads by study of critical DLC 

time series.

• Identify and analyze the instabilities.

o Identify instabilities in time series from DLC simulations.

o Isolate instability-driving wind and wave conditions.

o Linearize model at identified instability conditions.

o Perform linear stability analysis in terms of damping ratios.

o Analyze the causes for instabilities and identify possible solutions.

• Compare the results to an equivalent land-based system in terms of load ratios.

The model design and analysis process described here is not limited to the TLP, but rather can be 

applied to almost any floating-platform design.

2.2 Simulation Tools

This work uses the design tool FAST with AeroDyn and HydroDyn—developed and verified by 

Jonkman and Buhl [7,5] at NREL—to create the model of the MIT/NREL TLP floating wind 

turbine design. FAST accounts for the gravitational loads, the behavior of the control and 

protection systems, and the structural dynamics of the wind turbine. The latter contribution 

includes the elasticity of the rotor and tower, along with the elastic coupling between their 

motions and the motions of the support platform. The aerodynamics are calculated in the FAST 

module AeroDyn, which uses a state-of-the art blade-element / momentum approach (BEM) with 

empirical corrections to calculate the rotor aerodynamics.

The FAST module HydroDyn adds the capability of simulating time-domain hydrodynamic 

effects from linear hydrostatic restoring; added-mass and damping contributions from linear 

radiation, including free-surface memory effects; incident-wave excitation from linear 

diffraction; and nonlinear viscous drag, including sea-current loading. The code also includes a 

nonlinear quasi-static mooring line module.
3

These models are of higher fidelity than most models used in the past to analyze floating 

turbines, which neglected important hydrodynamic and mooring system effects. Additionally, 

FAST can produce linearized representations of the nonlinear aero-elastic wind turbine model to 

help identify natural frequencies, damping ratios, and instabilities, and to use for developing 

control systems. Figure 1 illustrates the various FAST modules and their interfaces. A close link 

between MATLAB (The Mathworks) and FAST exists, enabling the use of MATLAB’s 

comprehensive embedded analysis functions in user-programmed MATLAB routines as a preprocessor and postprocessor or for control-design purposes. To evaluate raw data from the loads 

analysis with FAST, the software utility Crunch/MCrunch—developed by Marshall Buhl at 

NREL [8]—is used.

FAST or

MSC.ADAMS

HydroDyn

AeroDyn

External 

Conditions

Applied 

Loads

Wind Turbine

TurbSim

Hydrodynamics

Aerodynamics

Waves & 

Currents

Wind-Inflow Power 

Generation

Rotor 

Dynamics

Platform Dynamics

Mooring Dynamics

Drivetrain 

Dynamics

Control System

Nacelle Dynamics

Tower Dynamics

Figure 1. MIT/NREL TLP with FAST, AeroDyn, and HydroDyn modules

2.3 Tension Leg Platform Design

As a basis for the development of the FAST TLP model, a TLP design from Tracy’s study 

“Parametric Design of Floating Wind Turbines” [2] (conducted at the MIT Mechanical 

Engineering Department) was selected. Tracy’s thesis contains a parametric optimization study 

conducted for several different floating-platform concepts for NREL’s 5-MW baseline wind 

turbine. The study resulted in a number of designs that show Pareto fronts for mean-square 

acceleration of the turbine versus multiple cost drivers, including platform displacement and total 

mooring-line tension.

Tracy’s study concludes that the TLP represents the most attractive choice because of its low 

root mean square (RMS) accelerations and negligible heave and pitch motions. For the present 

study, the TLP #1 for the 10-m reference sea state was chosen. Due to problems with the original 

design, however, the TLP was modified. The spokes—extending radially away from the TLP 

cylinder—were lengthened from 9 m to 18 m; the distance between the connection points for the 

mooring lines (fairleads) was increased from 36 m to 54 m. This design change primarily 

decreased the pitch and roll motions of the platform, thus ensuring the tautness of the mooring 

lines in the reference sea state. Note that concrete ballast is used to ensure that the combined 

turbine-platform system remains stable when towing the platform to its installation location in 
4

mild wind and wave conditions. The 

TLP design therefore could be much 

more shallow-drafted and could use

much less ballast if the installation 

process was solved alternatively. The 

MIT/NREL TLP is shown in Figure 1

and its basic properties are listed in 

Table 1. Detailed information 

regarding the modification and TLP 

design is provided in Matha [3].

3 Results

3.1 Time-Domain and Frequency-Domain Comparison

To verify the results from the FAST model, a comparison in terms of RAOs and PDFs between 

time- and frequency-domain simulations was conducted. An RAO represents the nondimensional response of a system to a unit-amplitude periodic incident wave at a given direction. 

It is the transfer function between the wave amplitude and the platform displacements, depending 

on the wave frequency and direction. Three different simulations (listed below) are compared to 

each other, and are presented in Figure 2 for the platform-pitch RAO.

1. Time-domain 22-degree of freedom (DOF) fully flexible model in FAST with AeroDyn 

and HydroDyn.

2. Time-domain 6-DOF rigid-turbine model in FAST with AeroDyn and HydroDyn, with 

only the 6 platform DOFs active.

3. Frequency-domain 6-DOF rigid-turbine model in WAMIT, with 6 platform DOFs active.

For the frequency-domain simulation the commercial hydrodynamics code WAMIT was used, 

but it only accounts for the 6 platform DOFs. WAMIT can directly compute RAOs if the nonhydrodynamic mass, damping, and stiffness matrices of the complete system (i.e., the turbine and

platform) about a linearization point are provided as inputs. (For the present study, these 

linearized matrices were derived from FAST and include the mass, damping, and stiffness 

matrices associated with turbine weight, rotor aerodynamics, and rotor gyroscopics.) In the 

chosen reference conditions, the rotor is spinning at rated 12.1 rpm at a constant hub-height wind 

speed of 11 m/s with no shear.

Table 1. Basic Properties of MIT/NREL TLP

Static Properties Value

Platform diameter 18 m

Platform draft 47.89 m

Total displacement 12,179 m3

Platform mass 8,600,000 kg

Distance between fairleads 54 m

Concrete (ballast) mass 8,216,000 kg

Concrete height (from bottom of platform) 12.6 m

Water depth 200 m
5

Figure 2. Response amplitude operator comparison for platform pitch

These frequency-domain results are compared with the RAOs and PDFs calculated using FAST 

in the time domain. In FAST, the RAOs are obtained simply by running numerous simulations 

with periodic 1-m amplitude waves at different wave periods while the control system is turned 

off. After the transient effects have disappeared, the amplitudes of the platform’s constantamplitude surge, sway, heave, roll, pitch, and yaw displacement oscillations are recorded for 

each selected wave period. The resulting amplitudes at different wave periods are equivalent to 

the RAOs for each platform DOF.

To imitate the frequency-domain approach in FAST, and thus validate the model by the 

comparison to the frequency-domain results, the turbine also is simulated as a completely rigid 

structure. This approach results in RAOs represented by the red curve shown in Figure 2. These 

FAST-generated RAOs match the WAMIT RAOs (shown in green) almost perfectly. The 

agreement of the RAOs thus constitutes a code-by-code verification of the FAST model with the 

commercial WAMIT code.

The most important differences between the RAOs obtained with the fully flexible FAST model 

and the WAMIT and rigid FAST models are the lower maxima and the shifting of the peaks 

towards smaller frequencies. The difference is caused by the missing turbine flexibility in the 

WAMIT and rigid turbine FAST models: The couplings between platform motions and the 

flexible tower and blades—which mostly are not captured in a typical frequency-domain 

approach—have a significant influence on the results. Particularly, the shift of the peaks to 

smaller frequencies in the time domain could cause problems in a design process that relies on 

frequency-domain simulations. A design which has no resonance problems in the frequency 

domain—because all RAO maxima are outside the critical resonance zone—could fail in the 

time domain. This is because downshifting to smaller frequencies could move these maxima into 

the critical resonance frequency ranges and cause severe resonance issues. In other simulations,

not included here, the tower-bending DOFs were found to have the most significant impact on 

the shift for this TLP design.

In addition to the deviations in the RAOs, the PDFs also show significant differences between 

time-domain and frequency-domain simulations (for charts of the PDF comparison, see Matha

Frequency- to 

time-domain shift

®
6

Figure 3. Ultimate load ratios for

DLCs 1.1, 1.3, 1.4, 1.5

[3]). In surge, heave, and pitch, the time-domain and frequency-domain results are similar. The 

PDFs have the same means and approximately the same standard deviation. In contrast, the PDFs 

for the platform’s sway, roll, and yaw motions differ considerably. The WAMIT solutions have a 

much smaller standard deviation and therefore have highly increased peak PDF values, but the 

means remain the same.

The frequency-domain model is based on a 6-DOF floating-turbine model linearized about a 

deflected position, based on the thrust and torque from the aerodynamic loads in the referenceload case. The incoming wind and waves—proceeding in downwind direction along the 

platform’s surge direction—primarily excite the pitch, surge, and heave modes and only 

modestly excite sway, roll, and yaw modes. In the frequency-domain model, the gyroscopic 

coupling between the rotor and support structure is captured, but the turbine flexibilities are 

neglected. The PDFs therefore show a very small standard deviation around the mean. The PDFs 

indicate that, in general, the standard deviations of frequency-domain simulations are much 

smaller (and the peaks higher) as compared to the time-domain simulations, because 

nonlinearities are neglected. In future frequency-domain design processes, these findings should 

be considered carefully. It is suggested that at least one tower fore-aft bending DOF should be 

added to the frequency-domain model to account for the neglections.

3.2 Design Load Cases

Using the FAST TLP model, multiple simulations of selected design load cases from the IEC

61400-3 standard [9] are defined and run. Only a subset of these DLCs is selected for the loads 

analysis of the TLP. First, DLCs 8.x which define transport, assembly, maintenance, and repair 

situations are discarded. The baseline control system routine does not include logic for start-up 

and shutdown events, therefore DLCs 3.x, 4.x, and 5.x—defining start-up, normal shutdown, and 

emergency shutdown situations—also are omitted. For fatigue analysis only DLC 1.2 is selected. 

The remaining DLCs simulated in the present study are:

• Power production: 1.1 (1.2), 1.3, 1.4, 1.5, 1.6a;

• Power production and fault: 2.1, 2.3;

• Parked: 6.1a, 6.2a, 6.3a; and

• Parked and fault: 7.1a.

3.2.1 Ultimate Loads

Figure 3 compares various output parameter 

extremes from the TLP and the land-based 

turbine ratios for DLCs 1.1, 1.3, 1.4, and 1.5. 

The bar chart presents the ratios of the TLP 

extreme loads divided by the land-based 

extreme loads. A ratio of 1, for example, 

implies that the loads are unchanged 

compared to the land-based system, and a 

ratio that is greater or less than unity indicates 

that the loads have increased or decreased.
7

Platform translations and rotations resulting in changed inflow increase the combined flap and 

edgewise blade-root bending moment by 13%. The total low-speed-shaft (LSS) bending moment 

is increased by 19%. These ultimate loads for blades and LSS occur in DLC 1.4, generated by 

the extreme coherent gust with direction changes. The total tower-top yaw bearing moment is 

14% greater than for the land-based system. The most significant increase of 25% occurs in the 

tower-base bending moment. The main causes of these increased loads are the comparatively 

large surge and pitch motions (displacements and accelerations) due to platform-wave interaction 

in DLC 1.1.

These load ratios provide a basis to estimate the impact on structural loads of placing a wind 

turbine on a floating TLP in an offshore environment. Based on this data, necessary changes to 

the land-based turbine design—such as strengthening of the tower base—can be estimated. 

These results make possible a detailed cost analysis, however it is out of the scope of this work. 

A more detailed analysis of the TLP’s ultimate loads, including time series and statistics 

analysis, is presented in Matha [3].

3.2.2 Fatigue Loads

The fatigue calculations in this analysis were performed according to the procedure described in 

Matha [3]. The damage-equivalent loads (DELs) were calculated according to equation 1, in

which RF Lk is the k-th load range transformed to a fixed mean (i.e., the load means are taken into 

account), is the number of lifetime cycles in the k-th load-range bin and j-th wind-speed bin, 

m represents the S/N slope, and Life,eq

j n in the denominator is the number of equivalent lifetime 

cycles in the j-th wind-speed bin. A detailed description of equation 1 and the fatigue-analysis 

process used in this study are provided in Matha [3].

( ) 1

Life RF m m

jk k Life j k

Life,eq

j j

n L

DEL

n

 

  =

 

∑ ∑

∑ 1

Figure 4 presents fatigue DEL ratios for DLC 1.2 for three different S/N slopes m. Ratios greater 

than one represent a higher DEL, meaning that the floating machine’s lifetime is less than that of 

the land-based unit. The blade-root bending 

moment ratio in-plane remains nearly

unchanged. The flap-wise moment ratios are 

increased because the greater pitch motion of 

the TLP influences the flap-wise blade-root 

moments and decreases the fatigue life of the 

blades. Ratios for the low-speed-shaft bending 

moments show only slight increases. This is 

due to the fact that both the TLP and the landbased turbine use the same control system and 

platform motions have negligible effects on 

the LSS. The tower-top yaw bearing moments 

show increased DELs, which are caused by 

wave-induced platform motions in pitch, 

surge, and roll. For the side-to-side and fore- Figure 4. Fatigue load ratios for DLC 1.2

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0

Ratio of Sea to Land

inplane

out-ofplane 0 90

sideside

foreaft sideside

foreaft

Blades:

Tower/LSS:
8

aft tower-base bending moment ratios, the greatest increase occurs for the fore-aft moment—

with ratios increased as much as 60%. The increased pitch motion is the driving mechanism here, 

because it has its greatest effect on the tower-base bending moment due to the large tower-top 

mass and long moment arm. This difference between the TLP and the land-based turbine—a

more than 60% increase in DELs—greatly influences the design of this particular tower. A 

significant increase in tower-base strength is necessary to equal the lifetime of the land-based 

system. Alternatively, advanced control strategies might achieve load reductions to increase the 

TLP tower’s lifetime.

3.2.3 Stability Analysis

Two major instabilities (such as negative damping in at least one mode) were identified. When 

analyzing DLC 2.1, a platform yaw instability was discovered—the same instability that 

Jonkman [5] discovered for the barge concept. The instability occurs when the rotor is idling and 

one blade is seized flat into the wind at a pitch angle of θ = 0° and the other two blades are fully 

feathered at θ = 90°. This fault event with one seized blade also occurs in DLC 7.1a and results 

in the same yaw instability as discovered in DLC 2.1. In DLC 7.1a, the TLP is operating in an 

extreme sea state with extreme winds, as compared to normal wave and winds in DLC 2.1, 

therefore the yaw instability is more severe in DLC 7.1a. In DLC 7.1a, extreme platform yaw 

displacements are reached—resulting in extreme loads on the platform and possible knotting of 

the mooring lines. For those extreme rotational displacements, the limits of the validity of 

FAST’s theory are exceeded significantly, therefore no quantitative values are presented here.

The yaw instability (which is oscillating at the platform yaw natural frequency of 0.0972 Hz) is 

caused by a coupling of the platform yaw with the azimuthal motion of the seized blade.

Compared with the barge, the instability is more severe for the TLP. This confirms Jonkman’s 

prediction that the instability would be more pronounced for a TLP design because it is more 

compliant to platform yaw motions and because smaller moment arms are available [5]. Several 

methods are suggested for preventing this yaw instability, including installing damping plates, 

applying the high-speed shaft brake, or reducing the pitch angle of the feathered blades to 

generate a low persistent torque producing a slow rotor rotation.

HydroDyn does not account for drag from vortex shedding, flow separation, skin friction, and 

drag from the spokes. The yaw instability therefore most likely will be less severe in reality,

because all of these effects can add to more damping of platform yaw. Note that there is the 

possibility that the vortices generated at the spokes might lead to regions of lower pressure which 

also could—contrary to the statement above—lead to less damping. Further research on the 

damping of the platforms therefore is needed, especially regarding the influence of the spokes.

When trying to identify the existence of the tower side-to-side instability in the TLP model in 

DLC 6.2a (which Jonkman [5] discovered for the land-based turbine), an expanded instability 

was identified. In design load case 6.2a, the rotor idles and all blades are fully feathered at a 

pitch angle of 90° and nacelle yaw misalignments range is –180° < Yaw < +180°. The turbine in 

this DLC floats when extreme 50-year wind and wave conditions are present.

It is difficult to analyze the roll/yaw instability in a severe sea state, therefore a simpler test case 

was created in which the instability could be investigated in isolation and without the disturbing 

influences of random waves and turbulent winds. The turbine floats in still water with a constant 
9

wind speed of 50 m/s, and the nacelle yaw misalignment range is –180° < Yaw < 180°. Figure 5

shows the damping ratios for the 6 platform DOFs at different yaw misalignments.

Figure 5. Tension leg platform damping ratios for 6 TLP DOFs

at different yaw misalignments

Negative damping in yaw occurs at yaw misalignments between 27° < Yaw < 46°, –68° < Yaw <

–29°, and 135° < Yaw < +145°. In roll, negative damping ratios are found at yaw misalignments 

of between –59° < Yaw < –47°, 22° < Yaw < 38°, and 166° < Yaw < 169°. For the remaining 

DOFs of the model not shown here (see Matha [3]), more negative damping ratios in flap-wise 

and edgewise blade modes and in tower side-to-side and fore-aft modes were identified.

Further investigation indicated that this instability is caused by a coupling of the blade 

aerodynamics and the turbine and platform. Interestingly, even simulations using only one DOF

(e.g., platform yaw) resulted in negative damping ratios. So far, no final conclusion has been 

reached regarding an exact explanation of the mechanism causing this instability. Although the 

exact cause has not been determined, what is known about the instability suggests three possible 

remedies. First, it might be possible to modify the shape of the airfoils in the blades to reduce the 

amount of energy absorption at the problematic angles of attack. A second remedy is to apply a 

fail-safe shaft brake to park the rotor in extreme winds to keep it from reaching the critical 

azimuth positions. A third option is to allow a slip in the nacelle-yaw drive to keep the rotor from 

reaching the critical yaw misalignments.

4 Conclusions

The study presented here analyzes a 5-MW wind turbine located on a floating offshore TLP

using the fully coupled time-domain aero-hydro-servo-elastic design code FAST with AeroDyn 

and HydroDyn. Significant differences between the higher-fidelity time-domain model and 

lower-fidelity frequency-domain model have been found and explained. These differences have 

important implications to the required design process for floating offshore-turbine systems.

An extensive loads and stability analysis for ultimate and fatigue loads was performed using the 

verified TLP model. Response statistics, ultimate loads, fatigue loads, and instabilities are 

analyzed and presented in this report. The ultimate loads for the TLP turbine are increased 25% 
10

for the tower and over 10% for the blades. Fatigue DELs are increased 60% for the tower-base

bending moment. The stability analysis identified a severe aerodynamic instability in the 

platform pitch, platform roll, and tower- and blade-bending modes for a TLP wind turbine that is 

idling in high winds at certain yaw misalignments with all blades fully feathered at 90o. It also 

identified a platform yaw instability that occurred when the TLP turbine was idling in high winds 

with one blade seized at 0o pitch (and the other two blades at 90o).

The model development and analysis process described here can serve as a blueprint for future 

analysis of new floating-platform concepts. Various designs can be compared to each other using 

the described analysis process and design tools to reach conclusions on the optimal floatingplatform design. The results of the loads and stability analysis can help resolve fundamental 

design challenges of the TLP floating system concepts. Yet, the results also indicate that further 

work on TLP design, controller improvements, and solving instabilities is necessary to develop 

economically feasible offshore floating wind turbines that can withstand the extreme conditions 

in the deep offshore environment. More details on this study are presented in the technical report 

by Matha [3]. Further information on the used hydrodynamic design tool HydroDyn is found in 

the report by Jonkman [5].

5 References

1. Jonkman, J.; Butterfield, S.; Musial, W.; Scott, G. Definition of a 5-MW Reference Wind 

Turbine for Offshore System Development. NREL/TP-500-38060. Golden, CO: National 

Renewable Energy Laboratory, February 2009.

2. Tracy, C. Parametric Design of Floating Wind Turbines. M.S. Thesis. Cambridge, MA: 

Massachusetts Institute of Technology, 2007.

3. Matha, D. Model Development and Loads Analysis of an Offshore Wind Turbine on a Tension 

Leg Platform, with a Comparison to Other Floating Turbine Concepts. NREL/TP-500-45891.

Golden, CO: National Renewable Energy Laboratory, 2009.

4. Larsen, T.J.; Hanson, T.D. “A Method to Avoid Negative Damped Low Frequent Tower 

Vibrations for a Floating, Pitch Controlled Wind Turbine.” Journal of Physics: Conference 

Series, The Second Conference on the Science of Making Torque from Wind, Copenhagen, 

Denmark, 28–31 August 2007 [online journal], 012073, Vol. 75, 2007, URL: 

http://www.iop.org/EJ/article/1742-6596/75/1/012073/jpconf7_75_ 012073.pdf?requestid=SpEuhRBu3BG0xV3r2wi7Kg, [cited 28 August 2007]; DOI: 10.1088/1742-

6596/75/1/012073.

5. Jonkman, J. Dynamics Modeling and Loads Analysis of an Offshore Floating Wind Turbine. 

NREL/TP-500-41958. Golden, CO: National Renewable Energy Laboratory, 2007.

6. Lee, C.H.; Newman, J.N. WAMIT User Manual, Versions 6.3, 6.3PC, 6.3S, 6.3S-PC. Chestnut 

Hill, MA: WAMIT Inc., 2006.

7. Jonkman, J.; Buhl, M. FAST User’s Guide. NREL/TP-500-38230. Golden, CO: National 

Renewable Energy Laboratory, 2005.
11

8. Buhl, M. MCrunch User’s Guide. NREL/TP-500-43139. Golden, CO: National Renewable 

Energy Laboratory, 2008.

9. International Electrotechnical Commission. IEC 61400-3. Wind Turbines—Part 3: Design 

Requirements for Offshore Wind Turbines. Geneva, Switzerland: International Electrotechnical 

Commission, 2009.
F1147-E(10/2008)

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February 2010

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Conference Paper

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4. TITLE AND SUBTITLE

Model Development and Loads Analysis of a Wind Turbine on a 

Floating Offshore Tension Leg Platform

5a. CONTRACT NUMBER

DE-AC36-08-GO28308

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6. AUTHOR(S)

Denis Matha, Tim Fisher, Martin Kuhn, and Jason Jonkman

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NREL/CP-500-46725

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WE101211

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National Renewable Energy Laboratory

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14. ABSTRACT (Maximum 200 Words)

This report presents results of the analysis of a 5-MW wind turbine located on a floating offshore tension leg platform 

(TLP) that was conducted using the fully coupled time-domain aero-hydro-servo-elastic design code FAST with 

AeroDyn and HydroDyn. Models in this code are of greater fidelity than most of the models that have been used to 

analyze floating turbines in the past—which have neglected important hydrodynamic and mooring system effects. 

The report provides a description of the development process of a TLP model, which is a modified version of a 

Massachusetts Institute of Technology design derived from a parametric linear frequency-domain optimization 

process. An extensive loads and stability analysis for ultimate and fatigue loads according to the procedure of the 

International Electrotechnical Commission offshore wind turbine design standard was performed with the verified TLP 

model. Response statistics, extreme event tables, fatigue lifetimes, and selected time histories of design-driving 

extreme events are analyzed and presented. Loads for the wind turbine on the TLP are compared to those of an 

equivalent land-based turbine in terms of load ratios. Major instabilities for the TLP are identified and described. 

15. SUBJECT TERMS

wind turbine floating platform; model development; loads analysis; tension leg platform

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