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

Investigation of a FASTOrcaFlex Coupling Module for 

Integrating Turbine and Mooring 

Dynamics of Offshore Floating 

Wind Turbines

Preprint

Marco Masciola, Amy Robertson, 

Jason Jonkman, and Frederick Driscoll

To be presented at the 2011 International Conference on Offshore 

Wind Energy and Ocean Energy

Beijing, China

October 31 – November 2, 2011

Conference Paper

NREL/CP-5000-52896

October 2011
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.

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) PIX 16416, PIX 17423, PIX 16560, PIX 17613, PIX 17436, PIX 17721

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

INVESTIGATION OF A FAST-ORCAFLEX COUPLING MODULE FOR INTEGRATING TURBINE AND 

MOORING DYNAMICS OF OFFSHORE FLOATING WIND TURBINES

MASCIOLA Marco, ROBERTSON Amy, JONKMAN Jason, DRISCOLL Frederick

National Renewable Energy Laboratory

National Wind Technology Center

Golden, Colorado, USA

Abstract

 To enable offshore floating wind 

turbine design, the following are 

required: accurate modeling of the 

coupled wind turbine structural 

dynamics, aerodynamics, platform 

hydrodynamics, a mooring system and 

control algorithms. Mooring and 

anchor design can appreciably affect 

the dynamic response of offshore 

wind platforms that are subject to

environmental loads. From an 

engineering perspective, system 

behavior and line loads must be 

studied well to ensure the overall 

design is fit for the intended 

purpose.

 FAST is a comprehensive simulation 

tool used for modeling land-based 

and offshore wind turbines. In the 

case of a floating turbine, quasistatic continuous cable theory is 

used to emulate mooring line 

behavior. Improved modeling fidelity 

can be achieved through the use of

finite element mooring theory. This 

can be accomplished by the new 

FASTlink coupling module, which 

couples FAST with OrcaFlex, a 

commercial simulation tool used for 

modeling mooring line dynamics. 

 In this coupling, FAST is 

responsible for capturing the 

aerodynamic loads, the turbine 

control system, the platform global 

motion and flexure of the wind 

turbine and its tower, and OrcaFlex 

models the mooring line and 

hydrodynamic effects below the water 

surface. The FASTlink coupling 

module was developed by Orcina, the 

creators of OrcaFlex, in 

collaboration with industrial 

partners. This paper can be 

considered as a third-party 

assessment of the coupling which

investigates the accuracy and 

stability of the FAST/OrcaFlex 

coupling operation.

Nomenclature

i Fap applied force for iteration i

Kz heave buoyant stiffness

m total mass specified in FAST

T0 z-axis tether tension from all 

fairleads

i T mean z-axis tether tension from 

all fairleads for iteration i

i z mean z-axis displacement for 

iteration i

AE mooring line axial stiffness

μ mooring line mass per length

L mooring line unstretched length

Introduction

 Although the conventions and 

practices used in simulating landbased wind turbines are well 

established, modeling tools 

specially tailored for floating wind 

turbines are still in development. 

To meet immediate design needs, 

tools can be assimilated from 

various disciplines to satisfy the

specialized needs of the offshore 

wind industry. 

 Conveniently, the resources needed 

to model, design and construct such

systems can be adapted from the 

offshore oil and gas industry. 

Perhaps most notable, collaboration 

between the wind and the offshore 

engineering industry is evident in 

Europe’s North Sea area, where wind 

turbines are attached atop monopiles

embedded in the seabed. Fixed towers 

are perhaps the simplest form of 

offshore wind installation

accessible today. The large moments 
2 

created from wind loads on the 

turbine, however, require upgraded 

foundation designs, likely making 

this concept less cost-competitive

as depths increase, effectively 

limiting the design to shallow 

waters.

 Floating offshore wind turbines

show promise for deep waters because 

they: 1) require less construction 

material, thereby reducing costs;

and 2) can withstand greater sea 

loads compared to their towered 

counterparts [1]. Floating offshore 

wind systems can be supported by a 

variety of platform designs, 

including a Tension Leg Platform 

(TLP), a spar buoy, or a simple 

barge (Figure 1). Offshore industry

innovations are malleable to meet 

immediate offshore wind development 

needs. 

 In most floating offshore systems, 

the mooring lines can alter the 

vessel response characteristics; 

therefore, the mooring lines should 

be included in the design process. 

The moorings are crucial additions 

to ensure safety and stability of 

the platform; however, if designed 

improperly, they can have 

catastrophic effects, especially if 

resonance matching between the 

platform and its tethers occurs. The 

likelihood of this happening 

increases as the floating system is 

installed in deeper waters [2,3]. 

For this reason, a rigorous analysis 

of the mooring line dynamics and its 

effects on the offshore wind turbine

response is important.

Offshore Wind Modeling Tools

 The purpose of this work is to 

assess the accuracy and stability of 

a coupling module used to combine 

the capabilities of two software 

tools to enable high-fidelity 

modeling of a floating offshore wind 

turbine. This coupling module is to 

be used exclusively with FAST [4] 

and OrcaFlex. FAST is a modeling 

tool used to compute the response of 

a land-based or offshore wind 

turbine. While FAST has many 

sophisticated features for modeling 

the aerodynamics, hydrodynamics, 

turbine structure and controller, it 

currently employs a simplistic

quasi-static cable model to 

approximate the mooring forces on an 

offshore floating wind turbine. 

Quasi-static models use continuous 

cable theory to estimate tether 

forces by assuming the cable is in 

static equilibrium at every time 

step; hence, this model accounts for 

cable weight, but not its inertia

[5,6]. Improvements can be made by 

replacing the quasi-static cable 

model with a dynamic cable model so 

that cable inertia, drag, and fluid

added mass can be modeled. This is 

done by linking FAST with OrcaFlex

using the appropriately named

FASTlink coupling module.

 FASTlink was assembled by Orcina, 

the originators of OrcaFlex, in 

collaboration with industrial 

partners. The National Renewable 

Energy Laboratory chose to perform 

an independent third-party 

evaluation to assess numerical 

stability, perform model-to-model 

comparisons, and verify the accuracy 

of the coupling module.

 This paper focuses on the 

usability and numerical stability

aspects of FASTlink by comparing 

rudimentary simulations in FAST with

results generated using the

FAST/OrcaFlex coupling tool, 

FASTlink. The analysis is performed 

using a 5-MW OC3-HyWind spar 

platform deployed in a water depth 

of 320 meters. The reader is 

directed to the study in Robertson 

and Jonkman [7] for a detailed 

summary of the OC3-HyWind spar, but 

the essential attributes of the 

system are listed in Table 1 below. 

Table 1: OC3-HyWind spar Properties

Spar + Turbine Mass 7.466x106 kg

Fluid Displacement 8,029 m3

Water Depth 320 m

Spar Draft 120 m

Spar Diameter 6.5 – 9.4 m

Mooring Mass per Length 77.7 kg/m

Mooring Axial Stiffness 3.842x108

N/m

Mooring Line Length 902.2 m
3 

Figure 1: Images of several concepts actively being pursued for offshore wind

interests. From left to right: a TLP, a spar and a floating barge.

FAST Capabilities

 FAST is an open source program 

developed by the National Renewable 

Energy Laboratory. Used by academics 

and in industry, FAST is a 

recognized tool for modeling landbased and offshore wind turbines. 

Among its many abilities, FAST can 

incorporate user defined controllers 

to adjust the nacelle heading and 

rotor blade pitch with changing wind 

speed and direction. 

 The current version of FAST 

features a fusion of several

programs to create one comprehensive 

and complete model. The main program, 

FAST, maintains the time-domain 

based equations of motion, system 

kinematics, kinetics and control

algorithms. FAST also models the 

natural flexure of the tower, the

rotor blades and drive train. A 

secondary program, AeroDyn [8], 

provides the aerodynamic wind 

resistance, rotor blade lift and 

drag forces using quasi-steady or 

dynamic blade-element/momentum 

theory and static or dynamic stall. 

Similarly, the HydroDyn module 

solves the hydrodynamic forces on 

the submerged portion of the 

offshore wind turbine support 

platform [9]. This module is used in 

conjunction with the WAMIT [10] 

preprocessor to resolve the 

radiation, diffraction and 

hydrostatic restoring force

coefficients. In parallel, Morison’s 

equation gives the fluid resistance

caused by viscous effects. 

OrcaFlex Capabilities

 OrcaFlex is a time-domain, finite

element commercial software used to 

model the response of cables or to 

couple behavior between a surface 

vessel and its moorings. Such models 

are often referred to as discretized 

cable models because the mooring 

line is idealized as a system of 

mass components (nodes) connected to 

visco-elastic elements. The inherent 

mass, stiffness, and damping

properties of the mooring ensure it 

responds to varying end forces and 

displacements. This results in 

excitation loads that cannot be 

anticipated by the quasi-static 

model.

 The basic underlying theory for 

such models has existed in some form

since the 1960s [11,12], and over 

the years, incremental refinements 

have led to the models being used 

today. OrcaFlex is commonly used in 

the offshore industry as a means to 

predict the mooring line snap loads, 

rapid re-tensioning, and transient 

OC3-

Hywind Spar

ITI Energy 

Barge

MIT/NREL

TLP
4 

motions. As an added benefit, 

OrcaFlex also provides a graphical 

user interface (GUI), whereas FAST 

does not; therefore, OrcaFlex is 

useful in the visualization process. 

OrcaFlex is designed to be robust 

for a variety of cable 

configurations, whether the cable is 

slack or taut. Similar to FAST’s

HydroDyn, WAMIT input files can be 

loaded into OrcaFlex to capture 

vessel reactions to waves.

FASTlink Capabilities

 FASTlink is a coupling module

being developed by Orcina in 

conjunctions with industrial 

partners to exploit the strengths of 

FAST and OrcaFlex to create a highfidelity modeling tool for offshore 

wind turbines. The wind turbine, its 

aerodynamic loads, control system,

tower and the six degree-of-freedom 

rigid-body platform motion is 

modeled in FAST. The subsea 

components, such as the mooring 

lines and support platform

hydrodynamics, are modeled in 

OrcaFlex. Therefore, when FASTlink 

is being utilized, HydroDyn is no 

longer performing calculations.

Naturally, as part of this coupling 

process, the quasi-static cable 

model in FAST is disabled. Mooring

forces would then be represented in 

OrcaFlex. 

 In the FASTlink coupling module, 

FAST passes the vessel position and 

velocity vectors into OrcaFlex; the 

line tension and hydrodynamic added 

mass matrix and non-acceleration 

dependent hydrodynamic forces are 

calculated by OrcaFlex; the

resulting added mass matrix and

total force and moment on the 

platform is passed back into FAST;

then the resulting platform motion 

is solved in FAST. There is an 

inconsistency between FAST and 

OrcaFlex regarding the way platform 

static equilibrium is calculated, 

and this will be discussed next.

Code Coupling Considerations

 One challenge with coupling

software packages together is 

ensuring program compatibility is 

maintained. The purpose of FASTlink 

is to ensure compatibility issues 

are settled between FAST and 

OrcaFlex. A unique feature of 

FASTlink is that it selects the 

units in OrcaFlex to match FAST’s 

units automatically. This feature 

prevents an obvious mismatch between 

the models, which often times is 

overlooked. There are, however, 

considerations the end user must 

account for when using FASTlink to 

avoid double counting of forces,

continuity between reference frames

and consistent time-stepping between 

programs. The following steps are 

taken to ensure uniformity between 

models: 

• The mooring lines should be 

arranged in OrcaFlex relative to 

the same reference frame origin

identified in FAST, which is 

level with the mean surface line.

• To avoid double counting of the 

gravitational forces, all mass

quantities (both submerged and 

wind turbine components) are 

specified in FAST, and the vessel 

mass is set to zero in OrcaFlex.

• In this work, the OrcaFlex 

integration time-step is equal to 

that of FAST’s to ensure

consistent resolution between the 

coupled and uncoupled data sets.

The last point identified above is 

not ideal if the goal is to run the 

simulation quickly. Instead, it is 

acceptable to increase OrcaFlex’s

time-step by an integer multiple of 

FAST’s. 

 Issues may arise in the coupling 

process because of how the platform 

weight and buoyancy in handled in 

the FASTlink exchange. This can lead 

to a system not beginning at static 

equilibrium. In the next section, 

corrective measures are prescribed

to account for this particular 
5 

disparity occurring between the two 

models. 

Resolving the Static Solution with FASTlink

 The platform static configuration 

must first be determined before the 

simulation is initiated. This is 

done to ensure the ensuing platform 

motions are not caused by an

imbalance in the initial conditions. 

Although there are several ways to 

force the platform into a static 

configuration, this correction is 

applicable to circumstances where

the total mass (platform and wind 

turbine combined) is specified in 

the FAST input file. To begin, an 

applied force equal to the 

undisplaced buoyancy is specified at 

the center of buoyancy in the 

OrcaFlex model.

0

1

ap += TmgF (1) 

The simulation is run for a 

sufficient amount of time to ensure 

the initial start-up transients are 

converging toward a steady-state

value. If transients still occur, 

the solution for Fi

ap at any 

simulation iteration i can be 

refined using:

( ) i 0

i zi

ap

1iap = − TT-zK-FF + (2) 

Equation (2) can be repeated as 

necessary until the static errors 

are sufficiently small. This process 

resolves the static equilibrium for 

the heave direction because the 

applied force is inserted in the z 

direction only.

Preliminary Results and Stability Assessment

 Several simulations with varying 

conditions were performed; however,

for brevity, only subsets of results 

are reported to illustrate accuracy, 

stability, and usability of 

FASTlink. All simulations are 

performed over an 800-second time 

interval with a constant time step 

of ∆t = 0.01 seconds in both 

OrcaFlex and FAST. The results being 

presented include:

• A single degree-of-freedom freedecay simulation to ensure

accuracy of the FAST/OrcaFlex 

coupling by comparing platform

motions to results generated with 

the standalone FAST program

• OC3HyWind spar response to 

regular waves for the purpose of 

illustrating multi degree-offreedom motion and to ensure 

solution stability to wave loads

• The spar platform in calm water 

with a below-cut-off wind speed 

of 8 m/s. This is done to ensure 

wind load calculations are being 

carried out correctly in the 

simulation process.

Each simulation is performed with 

the rotor in a ‘parked’ arrangement

and with turbine control turned off, 

even when wind velocity is 

prescribed. This ensures aerodynamic 

loads are computed, but control 

forces are not introduced into the 

system.

CASE I: Single Degree-of-Freedom 

Offset, z0 = 0.2 meters 

 A total of six free-decay 

simulations (one for each platform 

degree-of-freedom) are carried out 

to ensure consistency between the 

FAST/OrcaFlex coupling tool and the 

standalone FAST program. Figure 2

shows the free decay response for

the heave (z) direction with the 

platform initially displaced z0 = 

0.2 meters. The platform natural 

frequency is used to assess 

continuity between FAST (using the 

quasi-static cable model) and 

FASTlink (labeled as ‘FAST/OrcaFlex’ 

in the figures). This step serves as 

a check to ensure the time-step is 

properly selected, to ensure similar

transient characteristics are 

present and assess simulation 

stability. 

 Spectral analysis is an effective 

means to compare time series data 

because subtle phase shifts in the 
6 

data may skew results. Different 

mooring forces are calculated in 

FAST versus OrcaFlex, which may lead 

to different platform displacements. 

This, in turn, may lead to differing 

wave forces because the platform 

will be positioned differently in 

the wave field. Hence, there is no 

guarantee the time series will be 

identical between the two models.

 In the figure below, the time 

series z(t) displacement plots are 

nearly identical, except for the

exponential decay appearing in the 

FAST/OrcaFlex model. This is likely 

caused by tether damping, which is 

not present in FAST’s default quasistatic model. The frequency response 

plot, however, shows exceptional 

agreement, and it is concluded that 

FASTlink operates reasonably well

under the prescribed conditions. 

 Appearing to the right of the zaxis displacement plot is the tether 

tension time history and its 

associated spectral response. The 

OC3-HyWind spar has three mooring

lines, and the one being represented 

in the plot is aligned with the 

platform x-axis. Observing the 

mooring line tension is a concise 

and effective way to monitor the 

platform response, health, and 

condition, because a displacement in 

any degree-of-freedom will influence 

the leash tension. As expected, the 

mooring line tension is heavily 

influenced by the heave natural 

frequency. This is a characteristic 

shared by both models. In the 

FAST/OrcaFlex model, several high

frequency vibration components are 

present. This is because of the wind 

turbine and mooring line axial 

natural frequency. The mooring line 

natural frequency is easily 

identifiable from [13]: 

μ

AE

2L

n fn

u = (3) 

where n is the nth vibrations mode. 

The first three modes, which agree 

with the Power Spectral Density 

plot below, are fu

n = {1.23 ; 2.46 ; 

3.70} Hz.

Figure 2: Simulation results for Case I: free decay with z0 = 0.2 meters. 

CASE II: Regular Waves, 

H = 5 meters, T = 20 seconds

 A test case is performed with the 

OC3-HyWind platform in regular waves 

to evaluate FASTlink’s stability to 

environmental loads. In Figure 3, 

the time series for surge (x), heave 

(z), and pitch (θ) directions, as 

well as the mooring line tension at 

the platform, is given. Because the 

waves are progressing in a direction 

aligned with surge, there is no 

motion in sway (y) and roll (φ); 

hence, they are not shown alongside 

Figure 3. The Power Spectral Density

plot for each degree-of-freedom

complements the time-series plots. 

Once analyzed in the frequency 

domain, the data sets reveal strong 
7 

agreement between FAST and the 

FAST/OrcaFlex coupling tool.

 Although the surge time histories

look uncoordinated between FAST and

FAST/OrcaFlex, the surge Power 

Spectral Density plot, X(f),

illustrates that both data sets are 

in agreement. Equally satisfactory 

data are reported for the remaining 

degrees-of-freedom. In reference to 

the tension Power Spectral Density 

plot T(f), large differences between 

the two models occur at frequencies 

coinciding with the wave-band 

frequencies; however, it is stressed 

that the purpose of this exercise is 

not to investigate why difference 

occurs, but rather to show that the 

coupling tool reports comparable 

results to its FAST counterpart. 

Differences between the two models 

can be attributable to:

• Longitudinal and transverse cable 

excitations;

• Ringing or springing;

• Mathieu instabilities;

• Resonance matching between the 

wind turbine and tethers;

• Internal cable damping, crossflow cable drag and fluid added 

mass.

Naturally, differences in mooring 

line tension are expected because

the two cable models are

fundamentally different. This modelto-model comparison projects the 

appearance that FASTlink is 

operating as designed, the results 

given by its solution are

reasonable, and the coupling module 

has a high degree of usability. 

Figure 3: Simulation results for Case II: 

Regular Waves – H = 5 meters, T = 20 seconds.
8 

CASE III: Below Rated Wind, 8 m/s

 The final case is initiated to 

ensure the aerodynamic calculations 

between FAST and the FAST/OrcaFlex 

coupling modules are consistent

under wind excitation. The platform 

is initially at its equilibrium 

position with zero translational and 

rotational displacement. A uniform 

unsheared wind of 8 m/s is 

prescribed in a direction aligned 

with the platform x-axis, with no 

waves or currents acting on the 

platform. Similar to Case II, model 

symmetry and the wind direction 

confine the platform motion to 

primarily the surge (x), heave (z), 

and pitch (θ) directions. The 

results from this simulation are

given in Figure 4. Once again, the 

model-to-model comparison suggests 

each program is executing its 

assigned calculations correctly and 

the coupling process between FAST 

and OrcaFlex appears to be operating 

properly.

Figure 4: Case III: Uniform, unsheared wind. The turbine is operating at a 

below-rated capacity with the wind speed set to 8 m/s.

Other Simulations Considered

 Although the authors have 

presented a limited number of test 

cases in this manuscript, other 

starting conditions were examined to 

ensure consistency between models 

and ensure functionality of the 

FAST/OrcaFlex coupling tool. Similar

to Case I, a free-decay numerical 

experiment was performed on the 

remaining degrees-of-freedom. This 

test revealed that the FAST/OrcaFlex 

tool obtains platform natural

frequencies comparable to the 

standalone FAST program. This test 

was essential to make certain the

platform and/or wind turbine weight 

was not counted twice in the 

simulation. 
9 

 A test combining the attributes of 

Case II and Case III was also 

performed. In this case, a steady 

wind of 8 m/s and a regular sea

defined by H = 5 meters and T = 20 

seconds were introduced into the 

model concurrently. This simulation 

was performed with the seas 

propagating in the same direction as 

the platform x direction, as well as 

one with the sea offset by 20°

relative to the x direction. Three

simulations which integrated wind 

turbine control algorithms were also 

performed to further verify 

stability of the coupling module. 

These simulations introduced 

irregular sea conditions using the 

JONSWAP spectrum; the sea was 

defined by a significant wave height 

of Hs = 6 meters and an average 

period of Tavg = 20 seconds. The wind 

conditions were generated using a 

normal turbulence model, and the

three mean wind speeds considered 

were Um = 10, 12 and 18 m/s. An 

analysis of the simulation result 

supports the same conclusions 

presented in this paper.

 While only results for the OC3-

HyWind spar are presented in this 

manuscript, the authors have also 

performed limited analysis on the 

MIT/NREL TLP, and our preliminary 

findings reveal the MIT/NREL TLP 

tends to be less stable compared to 

the OC3-HyWind spar. The source of 

instability is likely attributable 

to the mooring implementation rather 

than the platform design itself. 

Instability issues are more likely 

to occur with taut leg systems (such 

as the MIT/NREL TLP) as opposed to 

systems using slack mooring lines 

(such as the OC3-HyWind spar) since 

the cable can alternate between 

slack and taut conditions. This 

swing in operation mode can 

introduce large jumps in cable 

tension, even if the platform 

displacement is small. These 

instability issues are often cured 

by simply decreasing the time step 

resolution. A further investigation 

is needed to identify failure modes, 

track the source of instability and

offer remedies to correct this 

issue. 

Remarks and Conclusions

 In this work, the stability, 

usability, and functionality of 

FASTlink are assessed. The results 

reported with the FAST/OrcaFlex 

coupling tool are consistent with 

those obtained while using FAST with

its default quasi-static cable 

model. The FAST/OrcaFlex coupling 

module reports promising results, 

which warrants a further study to

identify the mechanisms that cause 

the mooring lines to influence the

wind turbine response. 

 As expected, the simulation runtime is extended when using the

FASTlink coupling module; however, 

high-fidelity modeling almost always 

requires simulation speed to be 

sacrificed. Earlier, it was noted 

that that simulation speed can be 

increased by simply increasing the 

simulation time-step. In this work, 

the time-step used in FAST equals 

that of OrcaFlex’s. However, 

recalling that FAST calculates the 

tower and rotor blade aerodynamics

and deflection, the size of FAST’s 

time-step must be balanced to ensure 

the tower/blade dynamics resolution

is acceptable between integration 

steps. On the same note, OrcaFlex’s 

time step must be small to ensure 

the cable does not go unstable. 

Increasing the integration time step

may not be a viable option for some 

systems, such as a TLP.

 The method used to select timesteps between FAST and OrcaFlex is 

up to the user, and no clear rules 

are stipulated. One area needing

further investigation involves

defining a method to select timestep length. In terms of stability, 

the time-step has a profound effect 

on the solution, and in general, a 

smaller time-step implies greater 

simulation stability.

 In terms of usability, OrcaFlex’s

GUI allows easy management of

platform hydrodynamic properties and 

cable configuration. Once the 

simulation is run with FASTlink, the 
10 

simulation file can be opened in

OrcaFlex, allowing the user to gain 

an appreciation of the wind turbine 

motions through three-dimensional 

rendering, although the turbine 

dynamics are not animated in 

OrcaFlex. 

 In terms of applicability, the

FASTlink tool appears suitable for a 

multitude of offshore wind turbine 

designs, including semi-submersibles 

and TLPs, whether the mooring lines 

are taut or slack. Our analysis of 

FASTlink suggests that the results 

are reasonable, the solution method 

is stable, and the overall coupling 

is meaningful. By combining the best 

attributes of FAST and OrcaFlex, a 

high-fidelity offshore wind turbine 

model is garnered. 

Acknowledgements

This work was performed at NREL in 

support of the U.S. Department of 

Energy Wind and Water Power Program. 

The authors would like to extend 

gratitude to Peter Quiggin of 

Orcina, Antoine Peiffer of Principle 

Power, and Pierre Coulombeau of 

IDEOL Offshore for their assistance 

with FASTlink.

References

1. Wilson, J.F., 2003. Dynamics of 

Offshore Structures. John Wiley 

& Sons. Hoboken, New Jersey, USA.

2. Oran, C., 1983. “Overall Dynamic 

Characteristics of Tension Leg 

Platform.” 15th Annual Offshore 

Technology Conference, May 2-5. 

Houston, TX, pp. 507-513. 

3. Datta, T.K., Jain, A.K., 1988. 

“Nonlinear Surge Response of a 

Tension Leg Platform to Random 

Wave Forces” J. Engineering 

Structures, Vol. 10, pp. 204-210. 

4. Jonkman, J., and Buhl, M., 2005. 

FAST User’s Guide. NREL/EL-500-

38230. National Renewable Energy 

Laboratory. Golden, CO, USA. 

5. Wang, L., Guo, Z., and Yuan, F., 

2010. “Quasi-Static ThreeDimensional Analysis of Suction 

Anchor Mooring System.” Ocean 

Engineering, 37, pp. 1127-1138. 

6. Irvine, M., 1992. Cable 

Structures. Dover publications. 

Mineola, NY, USA. 

7. Robertson, A., and Jonkman, J., 

2011. “Loads Analysis of Several 

Offshore Floating Wind Turbine 

Concepts.” 2011 International 

Society of Offshore and Polar 

Engineers (ISOPE) Conference, 

Maui, HI, USA. 

8. Laino, D.J., and Hansen, A.C., 

2002. User’s Guide to the Wind 

Turbine Dynamics Aerodynamics 

Computer Software AeroDyn. 

Windward Engineering LLC., Salt 

Lake City, UT, USA. 

9. Jonkman, J.M., 2009. “Dynamics of 

Offshore Floating Wind Turbines 

– Model Development and 

Verification.” Wind Energy 12, 

pp. 459-492. 

10. Lee, C.H., and Newman, J.N., 

2006. WAMIT User’s Manual, 

Version 6.3, 6.3PC, 6.3S, 6.3SPC. WAMIT, Inc. Chestnut Hill, 

MA, USA. 

11. Walton, T., and Polacheck, 1960. 

“Calculation of Transient Motion 

of Submerged Cables.” 

Mathematics of Computation, Vol. 

14, No. 69. 

12. Merchant, H., and Kelf, M., 1973. 

“Non-linear Analysis of 

Submerged Buoy Systems.” 

Proceedings of the MTS/IEEE 

OCEANS. Vol. 1, pp. 390-395. 

13. Inman, D., 2007. Engineering 

Vibrations. Prentice Hall. Upper 

Saddle River, NJ, USA.