State of the Art in Floating Wind

Turbine Design Tools

A. Cordle
GL Garrad Hassan and Partners Ltd.

J. Jonkman
National Renewable Energy Laboratory
Presented at the 21st International Offshore and Polar
Engineering Conference
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 State of the Art in Floating Wind Turbine Design Tools

Andrew Cordle
GL Garrad Hassan and Partners Ltd
Bristol, United Kingdom

Jason Jonkman
National Renewable Energy Laboratory
Golden, Colorado, USA

ABSTRACT that the tools continue to improve the accuracy of their loading and
response predictions, thus providing the confidence required for de-
This paper presents an overview of the simulation codes available to tailed floating platform design.
the offshore wind industry that are capable of performing integrated
dynamic calculations for floating offshore wind turbines. It provides a PREVIOUS RESEARCH
description of the modelling techniques employed by each of the differ-
ent codes, and analyzes the strengths and weaknesses of these methods. Frequency-domain methods commonly are used in the offshore oil and
A review of the testing and validation activities performed for a number gas industries to analyze and design floating structures. These methods
of the design tools is also included. Conclusions are presented about also have been employed in a number of instances for the preliminary
the development needs and future verification activities required for design of floating wind turbines. Bulder et al. (2002) used linear fre-
these codes. quency-domain hydrodynamic techniques to find response amplitude
operators (RAOs) to investigate a tri-floater concept. Lee (2005) used a
KEY WORDS: Offshore; floating wind turbine; integrated design similar process to analyze a tension-leg platform (TLP) design.
tools; state of the art; numerical simulation; testing; validation Vijfhuizen (2006) used frequency-domain analysis to design a barge for
a 5-MW turbine, including a wave energy device. Wayman (2006)
INTRODUCTION performed calculations in the frequency domain to model various TLP
and barge designs. Sclavounos et al. (2007) performed a parametric
The offshore wind industry has experienced significant growth in re- design study of floating wind turbine concepts and mooring systems
cent years, and continues to expand worldwide. Nearly all of the off- using a coupled linear dynamic analysis in the frequency domain.
shore wind turbines installed to date are located in North European
Seas and are mounted on fixed-bottom support structures in water There are numerous advantages to design calculations in the frequency
depths of 35 m or less. There are a limited number of suitable shallow domain. For example, the studies discussed above were useful in dem-
water sites available in offshore locations for countries currently active onstrating the initial technical feasibility of floating wind turbines.
in offshore wind. Much of the global offshore wind resource is in loca- They showed that turbines could be designed so that the natural fre-
tions where the water is much deeper than it is at the sites of current quencies are placed away from the wave-energy spectrum to minimize
installations. The offshore resources also exist where fixed-bottom dynamic response. Frequency-domain calculations, however, also have
support structures are not feasible, for instance off the coasts of the important limitations. They cannot capture nonlinear dynamic charac-
United States, China, Japan, Spain, Portugal, and Norway. The possi- teristics and cannot model transient loading events—both of which are
bility of mounting wind turbines on floating support structures opens important for wind turbines because the nonlinear dynamics introduced
up the potential to use such deepwater resources. The economic poten- through transient events and control system actions are significant for
tial of floating offshore wind turbines (FOWTs) is demonstrated in loads analysis. Matha (2009) performed a standard frequency-domain
Musial et al. (2004). Realization of this potential, however, requires analysis for a floating wind turbine and showed that some couplings
cost-effective floating wind turbine designs that can compete with other between the platform motion and the flexible tower and blades were not
energy sources. taken into account. This factor could lead to natural frequencies being
wrongly predicted and critical system resonances not being identified.
The design and manufacturing of optimized and cost-effective floating This result underscores the importance of performing calculations for
wind turbines requires reliable and sophisticated design tools that can floating wind turbines in the time domain. For the purposes of this
model the dynamics and response of floating wind turbine platforms in paper, therefore, frequency-domain design tools are not considered and
a comprehensive and fully integrated manner. Currently, several so- all the codes presented are based on a time-domain analysis.
phisticated simulation codes are capable of modelling floating offshore
wind turbines. This paper presents an overview of the current status of SUMMARY OF EXISTING DESIGN TOOLS
these codes, together with a description of the various modelling tech-
niques employed by the different codes, and an analysis of the strengths A number of design tools available to the offshore wind industry have
and weaknesses of these methods. The testing and validation of these the capability to model floating offshore wind turbines in a coupled
design tools is also reviewed, and conclusions are drawn about the time-domain dynamic analysis. This section presents the methods em-
development needs and future verification activities required to ensure ployed by those design tools known by the authors, and includes four

1
categories: structural dynamics, aerodynamics, hydrodynamics and tails of the quasi-static mooring line module are given in Jonkman
mooring lines. The summaries presented here apply to the design tool (2009).
capabilities available at the time of writing; future development is
planned for most codes to expand their capabilities.

The computational speeds of the various codes will depend on numer-

ous factors. These include the discretization chosen by the user, the
code features enabled, and the precise details of the coupling scheme
(in the case of coupled codes). Without a full knowledge of these vari-
ables a direct comparison of the computational speeds of the presented
codes is not possible. However it can be said in general that the compu-
tational speeds will be slower for codes with more complex coupling
schemes.

FAST with AeroDyn and HydroDyn by NREL The FAST code is a

publicly available simulation tool for horizontal-axis wind turbines that
was developed by the National Renewable Energy Laboratory (NREL), Figure 1: Interface between modules in the FAST code for FOWTs
largely by Jonkman (2007). The FAST code was developed for the (Jonkman, 2007)
dynamic analysis of conventional fixed-bottom wind turbines, but has
been extended with additional modules and to enable coupled dynamic The FAST with AeroDyn and HydroDyn code has been used in a num-
analysis of floating wind turbines. ber of research contexts to model coupled wind turbine and floating
platform dynamics. The configuration described above is that used by
Structural dynamics. The FAST code uses a combined modal and mul- Jonkman (2009). The various modules of the FAST code, however,
tibody system dynamics (MBS) representation. The wind turbine blades have also been coupled with a number of other dynamic analysis pro-
and tower are modelled using linear modal representation assuming grams to model the dynamics and response of floating wind turbines.
small deflections, with two flapwise bending modes and one edgewise Two examples of this are presented below.
bending mode per blade and two fore-aft and two side-to-side bending
modes for the tower. The finite element method (FEM) pre-processor FAST with Charm3D Coupling The FAST with AeroDyn code is
BModes (Bir, 2005) is used to calculate the mode shapes of the blades coupled with the floater-mooring dynamic analysis program Charm3D
and tower. The floating platform upon which the tower is cantilevered by Shim (2008). Charm3D is an FEM program for the dynamic analysis
has full six degree-of-freedom (DOF) rigid-body motion. The drivetrain of moored floating offshore structures. It was developed jointly by
is modelled using an equivalent linear spring and damper. Texas A&M University and Offshore Dynamics Inc., with partial fund-
ing from the Charm3D joint industry project. The coupling of this pro-
Aerodynamics. The aerodynamic subroutine package AeroDyn is used gram to FAST with AeroDyn enables the mooring line and rigid-body
to calculate aerodynamic forces in FAST. This model uses quasi-steady dynamics of a floating wind turbine system to be integrated with the
blade-element/momentum (BEM) theory or a generalized dynamic wind turbine dynamics in a coupled time-domain simulation.
inflow model. Both of these models include the effects of axial and
tangential induction. The BEM aerodynamic calculations include tip In Charm3D, the first- and second-order hydrodynamic coefficients of
and hub losses according to Prandtl and skewed-wake corrections. Dy- the floating platform are calculated in the frequency domain using a
namic stall is considered using the Beddoes-Leishman model. Further panel-based 3D diffraction and radiation program (in this case WA-
details can be found in Laino and Hansen (2002). MIT). In the time-domain analysis, various nonlinearities are taken into
account, including the drag force on the mooring lines, the large (trans-
Hydrodynamics. The hydrodynamic subroutine package HydroDyn is lational) motion of the platform, the free-surface effects, and the geo-
used to calculate applied hydrodynamic forces in FAST. Wave kine- metric nonlinearity of the mooring system. The mooring-line dynamics
matics is calculated using Airy wave theory with free-surface correc- are solved simultaneously at each time step by a coordinate-based FEM
tions. The hydrodynamic loading includes contributions from linear program. The coupling between FAST and Charm3D is available in
hydrostatic restoring, nonlinear viscous drag contributions from Mori- two forms. In the first method, the floating-body motions computed by
son’s equation, added mass and damping contributions from linear Charm3D are provided as inputs to FAST with AeroDyn, and the re-
wave radiation (including free-surface memory effects), and incident sulting dynamic loads from the wind turbine computed by FAST with
wave excitation from linear diffraction. Full details are given in Jonk- AeroDyn are returned to Charm3D as external forces. In the second
man (2009). The linearized radiation and diffraction problems are method, the motions of the tower base computed through the wind
solved in the frequency domain for a platform of arbitrary shape using turbine dynamics by FAST with AeroDyn are provided as input to
WAMIT (Wave Analysis at Massachusetts Institute of Technology), a Charm3D, which solves for the hydrodynamic and mooring-system
three-dimensional (3D) panel-based program for computing wave loads loads that are returned as external loads to FAST with AeroDyn.
and motions of offshore structures (Lee, 1995). The resulting hydrody-
namic coefficients are used in HydroDyn.

Mooring lines. The FAST code uses a quasi-static mooring system

module to represent the nonlinear mooring-line restoring forces. This
module accounts for the apparent weight of the mooring line in fluid,
the elastic stretching of the mooring line,and the seabed friction of each
line. For a given platform displacement, the module solves for the ten-
sions within each mooring line by assuming that each cable is in static
equilibrium at that instant, and uses the resulting tensions to solve the
dynamic equations of motion for the remainder of the system. Full de-

2
 ADAMS by MSC ADAMS (Automatic Dynamic Analysis of Me-
chanical Systems) is a commercially available general-purpose MBS
code developed by MSC Software Corporation. The code is not wind
turbine-specific and also is used by the automotive, aerospace, and
robotics industries. ADAMS models of wind turbines can be generated
using the FAST tool’s FAST-to-ADAMS pre-processor functionality.

Structural dynamics. The ADAMS code uses an MBS representation to

allow for numerous structural configurations and DOF. The wind tur-
bine blades and tower are modelled as flexible members consisting of a
series of rigid bodies with lumped mass and inertia connected by flexi-
ble joints with linear stiffness and damping. The drivetrain can either
be modelled similarly, either as a series of lumped masses or through a
simple hinge/spring/damper element. The ADAMS code also can be
used to model additional features, including torsional DOF in the
blades and tower, flap/twist coupling in the blades, mass offsets in the
blades and tower, and pitch actuator dynamics.

Aerodynamics. The AeroDyn aerodynamic subroutine package is used

to calculate aerodynamic forces in ADAMS, as described above in the
Figure 2: Model of TLP in Charm3D coupled code [13] section relating to the FAST code.

FAST with TimeFloat Coupling The TimeFloat software also has Hydrodynamics. The hydrodynamic forces can be calculated in AD-
been coupled to FAST with AeroDyn to model the dynamic response of AMS by interfacing with the hydrodynamic subroutine package Hy-
the WindFloat floating foundation concept for large offshore wind droDyn, as described above in the section relating to the FAST code.
turbines (Roddier et al., 2009). TimeFloat is a time-domain software Alternatively, an equivalent subroutine can be used for calculating
tool developed by Marine Innovation & Technology for the analysis of loads on the floating platform—see, for instance, Withee (2004).
floating structures. The coupling of TimeFloat to FAST with AeroDyn
enables the aerodynamic, hydrodynamic and mooring-system forces Mooring lines. The ADAMS code also can be extended in a similar
acting on the structure to be computed simultaneously, including the manner as the FAST code to enable the modelling of mooring lines.
nonlinear quasi-static mooring forces and the nonlinear viscous forces This can be done by solving the mooring line tensions quasi-statically
generated by the water-entrapment plates. As described above, the in a separate module and interfacing with the main code at each time
wave-interaction effects are processed in the frequency-domain soft- step. Alternatively, a look-up table specifying the relationship between
ware WAMIT and the resulting added-mass, damping, and mean-drift restoring force and platform displacement may be defined at the moor-
coefficients and wave-exciting forces are passed to the TimeFloat code. ing-line interface point.
The hydrodynamic forces then are calculated by TimeFloat and include
memory effects, wave-excitation forces (using force components com-
puted by WAMIT), viscous forces resulting from drag effects, drift
forces, and mooring-line forces. The hydrodynamic forces are provided
as an input to FAST with AeroDyn, which then solves the turbine and
tower equations of motion and passes the platform motion back to
TimeFloat.

Figure 4: FOWT simulation routine in MSC ADAMS by Withee

(2004)

Bladed by GL Garrad Hassan GH Bladed is an integrated software

tool for calculating wind turbine performance and dynamic response
(GL Garrad Hassan, 2010). It originally was developed by GL Garrad
Hassan for the modelling of onshore fixed-bottom wind turbines. It has
been extended, however, to include hydrodynamic loading for the mod-
elling of offshore wind turbines. In the last year, the core structural
dynamics of the code has been re-written to incorporate MBS.

Figure 3: WindFloat semi-submersible modelled in TimeFloat [14]

3
 particle kinematics is calculated using stream-function theory. The
order of the solution is chosen based on the input values of wave
height, wave period, and water depth.

Mooring lines. Mooring-line forces are applied in the Bladed code

using the foundation module, which includes the capability to model
non-linear relationships between the displacement of the platform and
the restoring force from the mooring line. These relationships are calcu-
lated separately by the user, and implemented via a stiffness matrix at
the fairlead position.

SIMO/RIFLEX by MARINTEK SIMO (Simulation of Marine Op-

erations) is a general-purpose time-domain program developed by
MARINTEK for the modelling and simulation of offshore structures.
The code has been extended by Fylling et al. (2009) to enable model-
ling of floating wind turbines. This is done by the addition of an exter-
nal module for the simulation of rotor aerodynamic forces. SIMO also
has been coupled with the nonlinear FEM code RIFLEX by Nielsen et
al. (2006). RIFLEX is a tailor-made code for the static and dynamic
analysis of slender marine bodies such as risers and mooring lines, also
Figure 5: Spar-buoy platform modelled in GH Bladed developed by MARINTEK.

Structural dynamics. The Bladed code uses a combined modal and

MBS representation to model the structural dynamics of a wind turbine.
The wind turbine structure can be composed of any number of separate
bodies. Flexible components such as the blades and tower are modelled
using a modal representation. Individual modal properties for each
component are computed independently using an FEM representation
of the body as a Timoshenko beam. The mode shapes and frequencies
are dependent on the mass and stiffness distribution and the position of
the neutral axis of the body, as well as other parameters specific to the
body in question. The modes are coupled using the appropriate equa-
tions of motion in the dynamic-response analysis. To model the tower,
a multi-member model consisting of an arbitrary space-frame structure
of interconnecting beam elements with user-specified mass and stiff-
ness properties can be used. Craig-Bampton (C-B) style modes are used
for the support structure. The resulting mode shapes are three-
dimensional with six DOF at each node. The use of MBS dynamics
enables more accurate modelling of floating structures. The turbine and
support structure are not constrained by a direct connection to the Figure 6: Rigid-body model of FOWT using 4-body configuration in
ground. Instead, the structure is connected to a reference frame by a SIMO (Fylling et al., 2009)
free joint and constrained by mooring-line forces. This enables all six
support-structure DOF to be modelled with large rotations and dis- Structural dynamics. The SIMO code uses interconnected MBS to
placements. model structural dynamics. To model a floating offshore wind turbine,
multiple bodies can be defined and coupled together. In Fylling et al.
Aerodynamics. The aerodynamic forces on the rotor are calculated in (2009), the turbine and support structure are defined using a few rigid
Bladed using BEM theory with corrections, including tip and hub loss bodies (two-body and four-body configurations are investigated). In
models based on Prandtl. A dynamic-wake model is included to ac- this case, the rotor loads are transferred to the support structure using
count for the effect of blade loading on wake vorticity. The model in- three flexible coupling elements consisting of two radial bearings and
cluded in Bladed is based on Pitt and Peters, and has received substan- one axial bearing. In Nielsen et al. (2006), the coupling with RIFLEX
tial validation from data on helicopters. Dynamic stall also is accounted enables a FEM formulation of the structure, allowing for unlimited
for using the Beddoes-Leishman model. displacements and rotations in 3D space. The rotor still is modelled as a
rigid body but the tower is made up of flexible beam elements, each
Hydrodynamics. The applied hydrodynamic forces on the wind turbine with 12 DOF, which means that the elastic behaviour of the tower can
support structure are calculated in Bladed using Morison’s equation. be investigated.
For linear sea states, the wave-particle kinematics is calculated using
Airy wave theory with free-surface corrections using Wheeler stretch- Aerodynamics. The aerodynamic forces are calculated in a separate
ing. If linear waves are used, then an irregular sea state can be defined module and implemented in SIMO as a user-specified external force.
using either a JONSWAP spectrum or a user-defined wave-energy BEM theory is used to calculate the forces on the rotor blades, with
spectrum. For linear irregular sea states, the effects of wave diffraction dynamic-inflow effects included. Individual blade-element forces then
can be accounted for by using a time-domain MacCamy-Fuchs ap- are summed and applied in SIMO as a six-component external load on
proximation. In this approach, the wave-energy spectrum is altered to a rotating body. The drag force on the tower and nacelle also is ac-
give the same resulting hydrodynamic load on the structure as the stan- counted for in the aerodynamic loading.
dard MacCamy-Fuchs method, in which the Cd and Cm coefficients are
modified in the frequency domain. For nonlinear waves, the wave-

4
Hydrodynamics. The hydrodynamic forces are modelled using the stan-
dard SIMO code. Linear Airy wave theory is assumed for calculating
wave kinematics. The calculation of hydrodynamic loads takes into
account linear and quadratic potential forces including frequency-
dependent excitation, added mass and damping contributions (calcu-
lated in the frequency domain using WAMIT), and slow drift. Viscous
drag forces from Morison’s equation, mooring-line forces, and body-to-
body hydrodynamic coupling force models are also included.

Mooring lines. The mooring lines are modelled using the RIFLEX
code. This enables the representation of mooring lines as finite ele-
ments, incorporating nonlinear material properties and dynamic proper-
ties. A separate mooring-system module is not required, as it is an inte-
grated part of the RIFLEX code.

SIMO/RIFLEX with HAWC2 Coupling The SIMO/RIFLEX code Figure 7: OC3-Hywind spar-buoy modelled in 3Dfloat (Nygaard et al.,
also has been coupled with the HAWC2 code in Skaare et al. (2007) 2009)
and Larsen and Hanson (2007). HAWC2 is an aeroelastic simulation
tool developed by Risø National Laboratory for the dynamic analysis of Structural dynamics. The 3Dfloat code uses FEM for modelling the
fixed-bottom wind turbines (Larsen and Hansen, 2007). The coupling structural dynamics of a floating wind turbine. Euler-Bernoulli beams
of these two codes enables detailed modelling of both the aerodynamic with 12 DOF are used, and geometric nonlinearities in the elements are
and hydrodynamic forces acting on a floating offshore wind turbine. taken into account by casting the model in a co-rotational framework.
The HAWC2 code also has been used to directly model a floating wind The rotor and drivetrain are modelled as rigid, with no interaction be-
turbine in Karimirad et al. (2009), with the mooring-line analysis per- tween the rotor and the tower. Flexibility is included in the tower. The
formed separately in SIMO/RIFLEX. global motion of the structure is taken into account by using structural
modes.
Structural dynamics. The HAWC2 code uses a combined linear FEM
and nonlinear MBS representation to calculate the structural dynamics Aerodynamics. Rotor aerodynamics is calculated in 3Dfloat using BEM
of a wind turbine. A number of separate bodies can be defined, consist- theory. Extensions for dynamic inflow and large yaw errors also are
ing of an assembly of linear Timoshenko beam finite elements. The included.
bodies are connected by algebraic constraint equations, which can take
the form of flexible joints, bearings, or rigid connections. Internal Hydrodynamics. The hydrodynamic forces are calculated in 3Dfloat
forces are calculated from these algebraic constraints. To couple the using Morison’s equation with wave particle kinematics derived using
two codes together, the position, velocity and acceleration vectors and linear Airy wave theory. The hydrodynamic loads include terms for
rotation matrix at the interface point are passed to HAWC2 by added mass of water from the acceleration of the structure, linear hy-
SIMO/RIFLEX. The reaction force at the interface point is returned to drostatic restoring, and nonlinear viscous drag.
SIMO/RIFLEX by HAWC2 at each time step.
Mooring lines. The mooring lines are modelled in 3Dfloat using beam
Aerodynamics. The aerodynamic forces on the rotor are calculated in finite elements with extensional stiffness included and bending stiffness
HAWC2 using BEM theory. The classic approach has been modified to neglected. The mooring lines also can be replaced by linear stiffness at
include the effects of dynamic inflow, dynamic stall, skewed inflow, the fairlead positions for the purposes of eigenfrequency analysis.
shear effects on induction, and effects from large deflections. The aero-
dynamic calculation points are positioned independently of the struc- SIMPACK by SIMPACK AG SIMPACK is a commercially available
tural nodes to provide an optimal distribution of these points. general-purpose MBS code developed by SIMPACK AG. The code is
used by the automotive, railway, aerospace, and robotics industries. A
Hydrodynamics and mooring lines. In Skaare et al. (2007) and Larsen version of SIMPACK—SIMPACK Wind—offers extensions to the
and Hanson (2007) the modelling of hydrodynamics and mooring lines original code that allow integrated wind turbine simulation. The SIM-
is performed in SIMO/RIFLEX, as described above. In Karimirad et al. PACK code has been used to model a floating wind turbine in Matha et
(2009) the hydrodynamic forces are calculated using Morison’s equa- al. (2011).
tion based on the instantaneous position of the platform. The mooring
lines are modelled in SIMO/RIFLEX using an FEM model and the
resulting force-displacement relationship applied as an external force at
the fairlead position.

3Dfloat by UMB The 3Dfloat code was developed by the Norwegian

University of Life Sciences (UMB) for the modelling of floating off-
shore wind turbines with full coupling between structural dynamics,
aerodynamics, hydrodynamics and control-system actions. The code
has been used to analyze floating offshore wind turbine models and to
compare conceptual designs by Nygaard et al. (2009).

Figure 8: Spar-buoy FOWT modelled in SIMPACK

5
Structural dynamics. The SIMPACK code uses an MBS representation A model of the floating wind turbine was built at 1:47 scale, with
to allow a large number of structural configurations and DOF. In SIM- Froude scaling applied. DC motors were used to control the rotational
PACK, the parts or bodies of the wind turbine structure are connected speed of the rotor and the blade pitch angle. A variety of sea states and
using complex joints with different types of force elements acting from wind velocities were tested, including the 100 year wave condition and
the inertial system on the bodies (e.g., aerodynamics on the rotor, hy- wind speeds above and below rated wind speed. The JONSWAP wave
drodynamics on the support structure) and between bodies (e.g., spring- spectrum was applied with turbulent wind for both the simulations and
damper elements). The parts of the wind turbine where the relative the model experiments. The measured hub wind speed from the model
deflection of the bodies is small in comparison to the rigid-body motion scale experiments was used as the basis for the turbulent wind field
are considered rigid. The SIMPACK code is able to include flexible used in the simulations, with corrections for Reynolds number effects.
FEM bodies of arbitrary geometry with the C-B method into the MBS The results of these tests showed very good agreement between the
model to account for larger deflections. This option is used for model- responses of the scale model and the predictions from the simulation
ling of the wind turbine blades and tower. An FEM blade model, con- code. The results also showed a significant increase in the damping of
sisting of Euler-Bernoulli or Timoshenko beam elements, is reduced by the tower motion when active blade-pitch damping was introduced.
the C-B method and is capable of considering bending in flap- and
edgewise direction, torsional and tensional rigidity, and the relevant Another floating wind turbine code which has been validated with the
coupling effects. The relevant geometric stiffening effects are included use of measurements is TimeFloat, a time-domain design tool for cou-
for the reduction, representing a nonlinear model for medium dis- pled analysis of floating structures (described above). The hydrody-
placements. The blade model also can be split into separate C-B re- namic calculations within this code were validated using wave-tank
duced flexible bodies that are connected with zero DOF, representing a tests performed at the University of California-Berkeley ship-model
nonlinear blade model for large displacements. The validation for the testing facility (Roddier et al., 2009). A 1:105 scale model of the float-
nonlinear behaviour is based on a comparison with the FEM code ing platform was fabricated. It included a foam disk at the tower top to
Abaqus, and is described in more detail in Matha et al. (2010) The represent wind forces and an electrical motor to model the gyroscopic
flexible tower is modelled with the same approach. Single- and multi- effect of the rotor. A three-hour realization of the 100-year sea state
torsional drivetrain models can be implemented to account for flexibil- was generated with and without steady wind, and the resulting platform
ity of the bedplate and other components. Drivetrain models for spe- motion measured using a digital video camera. The floating platform
cific analysis, mainly for frequency domain analysis, also can include also was modelled in the TimeFloat software using a simplified model
models for tooth contacts. for aerodynamic forces acting on the rotor. The results from these nu-
merical simulations then were compared with the measurements from
Aerodynamics. The AeroDyn aerodynamic subroutine package is used the tank tests. The comparison of model test results and numerical
to calculate aerodynamic forces in SIMPACK, as described above in simulations showed good agreement, with the TimeFloat software gen-
the section relating to the FAST code. erally underpredicting platform motion slightly. This is most likely due
to imperfections in the model and experiment data.
Hydrodynamics. The hydrodynamic forces in SIMPACK are calculated
by interfacing with the hydrodynamic subroutine package HydroDyn, Other measurement campaigns are being planned. The University of
as described above in the section relating to the FAST code. Maine DeepCwind Consortium in the U.S. has been awarded a $7.1-
million grant to develop floating offshore wind capacity. One of the
Mooring lines. SIMPACK can model mooring lines two ways. One stated aims of this project is to validate the coupled aero-hydro-elastic
method is to solve the mooring-line tensions quasi-statically in a sepa- models developed by NREL. The research program will include tank
rate module and interface with the main code at each time step. The testing, deployment of prototypes, and field validation.
other way is to use an integrated nonlinear MBS mooring-line model,
in which each line is discretized into separate rigid or flexible bodies The EOLIA project, led by Acciona, also has included some code-to-
connected by spring-damper elements. measurement tests. The objective of the project is to develop solutions
for the design and implementation of deepwater offshore wind farms.
TESTING AND VALIDATION OF DESIGN TOOLS As part of the project, the capabilities of FAST with AeroDyn and Hy-
droDyn have been extended and applied to the analysis of three floating
The development of design tools capable of modelling floating plat- concepts (spar buoy, TLP, and semi-submersible), alongside compari-
forms is an important step forward for the offshore wind turbine indus- sons with the SIMO/RIFLEX code. To verify the models, tank tests
try, but the results obtained from these codes must be shown to be ac- also have been performed at 1:40 scale for each of the concepts. No
curate and reliable. Comprehensive testing and validation therefore is public results from this project are currently known to the authors.
crucial for giving sufficient confidence to developers and investors.
The best way to achieve such confidence is to take measurements from The HiPRwind project is an EU research project awarded in the 2010
a real machine and compare the measured data with the results from Seventh Framework Programme. The project aims to develop and test
numerical simulations. The floating wind turbine industry is relatively new solutions for offshore wind farms at a large scale. One of the main
new, therefore very little measurement data is available to use to vali- aims of the HiPRwind project is to install a 1:10-scale model of a fu-
date the codes. Therefore, a second method also is employed—that of ture commercial 10-MW floating wind turbine installation. The model
comparing the results of different codes with each other. will be deployed in real sea conditions, and will be used to monitor and
assess the important operational parameters. The resulting measurement
Code-to-Measurement Comparisons A number of studies have been data will be an important contribution toward overcoming the gap be-
performed by Statoil for the development of the Hywind floating wind tween small-scale tank testing and full-scale offshore deployment.
turbine concept, see Nielsen et al. (2006). The Hywind floating plat-
form concept consists of a deep-water slender spar-buoy with three Other current and future FOWT research projects include the EU
catenary mooring lines. The integrated SIMO/RIFLEX/HAWC2 design DeepWind project, the Spanish Azimut project, the ZEFIR Test Station
tool was used in Skaare et al. (2007) to model the structure. As part of project off the coast of Spain, and the FOWT prototypes being jointly
the development of this concept, model-scale experiments were carried developed and tested by Sasebo Heavy Industries and Kyoto Univer-
out at the Ocean Basin Laboratory at MARINTEK in Trondheim to sity, Currently there is no detailed information about code-to-
validate the coupled wind and wave modelling of the Hywind concept. measurement campaigns planned for these projects.

6
Code-to-Code Comparisons In addition to validating codes using SESAM and DeepC. The SESAM and DeepC tools were not included
measurements, an important way to verify the predictive accuracy of in the discussion above because currently they cannot model the cou-
numerical simulation tools is through code-to-code comparisons. Most pled dynamics of the turbine with floating platform. A variety of differ-
of the codes used for the analysis of floating wind turbines have been ent load cases were performed. These included a full-system eigenana-
validated in this manner. One example is the FAST code, the aero- lysis; a static equilibrium test; free-decay tests for each of the six rigid-
elastic features of which have been verified through comparisons with body degrees of freedom of the platform; time series response tests with
ADAMS, described in Buhl (2001). Another example is the regular waves and irregular waves modelled with a rigid rotor and no
SIMO/RIFLEX code used to model the Hywind floating wind turbine wind; time-series response tests with regular waves and irregular waves
concept, which was validated in part through comparisons with Hy- modelled with a flexible rotor and steady and turbulent wind; and “ef-
windSim, a relatively simple MATLAB/Simulink code developed for fective RAOs” calculated with regular waves at varying frequencies.
the purposes of such comparison in Nielsen et al. (2006). Numerous Not all of the codes were able to contribute results to every test case
code-to-code comparison methods were used to verify the hydrody- performed due to various limitations on their modelling capabilities.
namic calculation module HydroDyn used in the FAST code. These The test cases provided a number of interesting results, some of which
methods included comparisons between the WAMIT frequency-to-time are outlined below.
conversion and HydroDyn calculations; comparisons between the
mooring-line force-displacement relationship calculated by the quasi- Structural dynamics. The participating codes all employ different
static method and that calculated by another code; and comparisons methods for modelling structural dynamics, which was illustrated in a
between time-domain results and frequency-domain results. The meth- number of differences in the results. The rotor-nacelle assembly was
ods are described in full in Jonkman (2009). The GH Bladed code has modelled rigidly in 3Dfloat and both the rotor-nacelle assembly and
recently undergone development from a pure modal representation of tower were modelled rigidly in SIMO, SESAM, and DeepC. This
structural dynamics to a MBS representation, as described above. The meant that these codes could not model structural deflections in these
new code structure is released in Bladed v4.0. Several levels of testing components. The FAST code predicted a higher natural frequency for
and validation were carried out for the new code structure, including the second blade asymmetric flapwise yaw frequency than that pro-
code-to-code comparisons and code-to-measurement campaigns. Full vided by the other codes. This is because FAST does not account for a
details are given in Witcher et al. (2010). torsional mode in the tower, whereas other codes that include tower
flexibility do account for this mode. The ADAMS code predicted less
The most extensive code-to-code comparison work in the offshore wind energy from the irregular wave simulations in the power spectra for
industry has been performed as part of the Offshore Code Comparison tower-top shear and rotor torque at the second tower and blade bending
Collaboration (OC3) project within the International Energy Agency natural frequencies than produced by FAST and Bladed. This might be
(IEA) Wind Task 23 (Jonkman et al., December 2010). In this project, because of an effect typical of ADAMS simulations, in which numeri-
a number of participants used different aero-elastic codes to model the cal damping increases with frequency.
coupled dynamic response of the same wind turbine and support struc-
ture, with the same environmental conditions. The results were com- Aerodynamics. Most of the participating codes use BEM theory for the
pared to verify the accuracy and correctness of the modelling capabili- calculation of aerodynamic loads with the exception of SESAM and
ties of the participant codes, and to improve the predictions. DeepC, which did not model aerodynamics for the purposes of this
project. The 3Dfloat, SIMO, SESAM, and DeepC codes modelled the
Offshore Code Comparison Collaboration Phase IV In Phase IV of rotor as rigid, which meant that the aero-elastic response was not rigor-
the OC3 project a floating offshore wind turbine was modelled (Jonk- ously modelled. One example of this was in the calculation of effective
man et al., April 2010). The turbine model used was the publicly avail- RAOs, for which the 3Dfloat code showed lower excitation in yaw,
able 5-MW baseline wind turbine developed by NREL, and the floating greater excitation in fairlead tensions, and greater excitation at the first
platform was a modification of the Hywind spar-buoy developed by tower bending frequency for all parameters. This was thought to be due
Statoil of Norway. The turbulent wind fields and irregular wave kine- to differences in aerodynamic damping due to the rigid rotor, although
matics were generated independently and were provided to all partici- it also could have been related to the modelling of the rigid spar with
pants to ensure tight control of all the inputs. A stepwise verification beam elements of artificially high stiffness. The 3Dfloat code also gave
procedure then was used, and the complexity of both the model and the a higher mean thrust in the simulations with regular wind and waves,
test cases was increased with each step. which corresponded with higher platform surge and pitch displace-
ments.

Hydrodynamics. The free-decay tests showed a few differences between

codes in their prediction of the amount of hydrodynamic damping pre-
sent in the various modes. The HAWC2 predicted too much heave and
pitch damping and ADAMS predicted too little pitch damping relative
to the other codes. The main difference in terms of hydrodynamic
analysis was between codes that used linear potential flow methods and
those that used the simple Morison’s equation. The most interesting
difference was found from the effective RAO calculations. The FAST
code used by POSTECH was missing one hydrodynamic damping
term, which led to the surge displacement and fairlead tension having a
negative effective RAO. The physical meaning of this is that there was
more system motion in still water than there was with waves. This oc-
curred because there was a controller-induced instability of the plat-
Figure 9: Illustration of NREL 5-MW wind turbine on OC3-Hywind form surge mode at the surge natural frequency, and there was negligi-
spar (Jonkman et al., April 2010) ble hydrodynamic damping in the model. With waves included, the
wave radiation damping at the wave excitation frequency damped out
A number of floating design tools were involved in Phase IV of the this instability, thus reducing platform motion considerably. This result
project, including FAST, ADAMS, Bladed, HAWC2, 3Dfloat, SIMO, indicates the importance of using potential flow-based solutions that

7
include wave radiation damping for the analysis of floating support ACKNOWLEDGEMENTS
structures.
The present work was funded by the Commission of the European
Mooring lines. Various methods are used in the different codes for Communities, Research Directorate-General within the scope of the
modelling mooring lines. These included both user-defined force- Integrated Project “UpWind – Integrated Wind Turbine Design” (Pro-
displacement relationships and full dynamic models. The SESAM and ject No. 019945 (SES6)).
DeepC codes used FEM for the mooring lines, and also predicted more
energy content above 0.1 Hz for fairlead tension in the power spectra REFERENCES
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