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

Offshore Code Comparison 

Collaboration Continuation (OC4), 

Phase I – Results of Coupled 

Simulations of an Offshore Wind 

Turbine with Jacket Support 

Structure

Preprint

Jason Jonkman and Amy Robertson (National Renewable Energy Laboratory)

Wojciech Popko, Fabian Vorpahl, Adam Zuga, and Martin Kohlmeier

(Fraunhofer Institute for Wind Energy and Energy System Technology IWES)

Torben J. Larsen and Anders Yde (Technical University of Denmark)

Kristian Saetertro and Knut M. Okstad (Fedem Technology AS)

James Nichols (Garrad Hassan & Partners Ltd.)

Tor A. Nygaard (Institute for Energy Technology)

Zhen Gao (Norwegian University of Science and Technology)

Dimitris Manolas (National Technical University of Athens)

Kunho Kim and Qing Yu (American Bureau of Shipping)

Wei Shi and Hyunchul Park (Pohang University of Science and Technology)

Andres Vasquez-Rojas and Jan Dubois (Leibniz Universitat Hannover)

Daniel Kaufer (Universitat Stuttgart)

Paul Thomassen (Norwegian University of Science and Technology)

Marten J. de Ruiter (Knowledge Centre WMC)

Johan M. Peeringa (Energy Research Centre of the Netherlands)

Huang Zhiwen (China General Certification)

Heike von Waaden (REpower Systems SE)

To be presented at the 22nd International Society of Offshore and Polar 

Engineers Conference

Rhodes, Greece

June 17 – 22, 2012

Conference Paper

NREL/CP-5000-54124

March 2012
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Offshore Code Comparison Collaboration Continuation (OC4), Phase I – Results of Coupled Simulations

of an Offshore Wind Turbine with Jacket Support Structure

Wojciech Popko 1, Fabian Vorpahl 1, Adam Zuga 1, Martin Kohlmeier 1, Jason Jonkman 2, Amy Robertson 2, Torben J. Larsen 3, Anders
 

Yde 3, Kristian Sætertrø 4, Knut M. Okstad 4, James Nichols 5, Tor A. Nygaard 6, Zhen Gao 7, Dimitris Manolas 8, Kunho Kim 9, Qing
 

Yu 9, Wei Shi 10, Hyunchul Park 10, Andrés Vásquez-Rojas 11, Jan Dubois 11, Daniel Kaufer 12, Paul Thomassen 13, Marten J. de
 

Ruiter 14, Johan M. Peeringa 15, Huang Zhiwen 16, Heike von Waaden 17
 

1. Fraunhofer Institute for Wind Energy and Energy System Technology IWES, Bremerhaven, Germany; 2. National Renewable Energy Laboratory, 

Golden, USA; 3. Technical University of Denmark, Department of Wind Energy, campus Risø, Roskilde, Denmark; 4. Fedem Technology AS, 

Trondheim, Norway; 5. Garrad Hassan & Partners Ltd., Bristol, UK; 6. Institute for Energy Technology, Kjeller, Norway; 7. Centre for Ships and 

Ocean Structures at the Norwegian University of Science and Technology, Trondheim, Norway; 8. National Technical University of Athens, Zografou, 

Greece; 9. American Bureau of Shipping, Houston, USA; 10. Pohang University of Science and Technology, Pohang, Korea; 11. Institute of Steel 

Construction at the Leibniz Universität Hannover, Hannover, Germany; 12. Endowed Chair of Wind Energy at the Institute of Aircraft Design at 

Universität Stuttgart, Stuttgart, Germany; 13. Norwegian University of Science and Technology, Trondheim, Norway; 14. Knowledge Centre WMC, 

Wieringerwerf, The Netherlands; 15. Energy Research Centre of the Netherlands, Petten, The Netherlands; 16. China General Certification, Beijing, 

China; 17. REpower Systems SE, Osnabrück, Germany 

ABSTRACT 

In this paper, the exemplary results of the IEA Wind Task 30 “Offshore 

Code Comparison Collaboration Continuation” (OC4) Project – Phase I, 

focused on the coupled simulation of an offshore wind turbine (OWT) 

with a jacket support structure, are presented. The focus of this task 

has been the verification of OWT modeling codes through code-to-code 

comparisons. The discrepancies between the results are shown and the 

sources of the differences are discussed. The importance of the local dynamics of the structure is depicted in the simulation results. Furthermore, 

attention is given to aspects such as the buoyancy calculation and methods of accounting for additional masses (such as hydrodynamic added 

mass). Finally, recommendations concerning the modeling of the jacket 

are given. 

KEYWORDS 

Offshore wind turbine; coupled simulation; aero-hydro-servo-elastic 

codes; jacket support structure; code verification; code-to-code comparison; OC4 

INTRODUCTION 

The analysis of offshore wind turbines relies on aero-hydro-servo-elastic 

simulation codes. These coupled time-domain-based tools take into account an interaction of various environmental conditions and the entire 

structural assembly of the turbine, including its control system. Due to 

the complexity of the models, verification and validation of the codes is 

required. Limited availability of measurement data impedes the validation of these simulation tools. Therefore, there is a need to perform codeto-code comparisons (verification) instead. The first international project 

dedicated to verification of simulation tools for wind turbines, including 

hydrodynamic loads, was undertaken within the “Offshore Code Comparison Collaboration” (OC3) Project (Jonkman and Musial, 2010). The 

cooperation was focused on coupled simulations of an offshore wind turbine supported by a variety of support structures. Further research needs 

triggered a follow-on project, the “Offshore Code Comparison Collaboration Continuation” (OC4) Project. The OC4 project was formed under 

the International Energy Agency (IEA) Wind Task 30 in 2010 to investigate wind turbine coupled simulations with a jacket support structure and 

a semisubmersible platform. Complex hydrodynamics of the latter and 

local vibration phenomena of the former have not been broadly studied 

yet, and therefore, their analysis is of interest. 

A number of academic and industrial project partners from 10 countries 

participate in the task. Those actively involved in Phase I are: Fraunhofer Institute for Wind Energy and Energy System Technology IWES 

(Germany), the National Renewable Energy Laboratory (NREL) (USA), 

Technical University of Denmark, Department of Wind Energy, campus 

Risø, Roskilde, Denmark (Risø DTU) (Denmark), Fedem Technology 

AS (Norway), Garrad Hassan & Partners Ltd. (UK), Institute for Energy Technology (IFE) (Norway), Pohang University of Science and 

Technology (POSTECH) (Korea), Centre for Ships and Ocean Structures (CeSOS) at the Norwegian University of Science and Technology 

(NTNU) (Norway), National Technical University of Athens (NTUA) 

(Greece), Institute of Steel Construction at the Leibniz Universität Hannover (LUH) (Germany), the Endowed Chair of Wind Energy at the 

Institute of Aircraft Design at Universität Stuttgart (SWE) (Germany), 

1
Norwegian University of Science and Technology (NTNU) (Norway), 

Knowledge Centre WMC (The Netherlands), Energy Research Centre of 

the Netherlands (ECN) (The Netherlands), American Bureau of Shipping 

(ABS) (USA), REpower Systems SE (Germany) and China General Certification (CGC) (China). Each one of the participants has their own area 

of expertise, and therefore, their own unique contribution to the project. 

A set of the state-of-the-art simulation codes for OWT modeling are represented in the OC4 project. Table 1 shows these codes and briefly summarizes some of their simulation capabilities that are important in Phase I 

of the project. 

Table 1: Overview of simulation capabilities of aero-hydro-servo-elastic 

codes used within the OC4 project, Phase I. 

Code Aerodynamics (aero) Hydrodynamics (hydro) Control (servo) Structural (elastic)

3DFloat BEM or GDW Airystr or UD Stream + 

ME 

UD FEM 

ADAMS + AeroDyn BEM or GDW + DS Airystr or UD or Stream + 

ME 

DLL or UD MBS 

ADCoS-Offshore BEM + DS Airystr or UD or Stream + 

ME 

DLL or UD FEM 

ASHES BEM + DS Airystr + ME Internal control 

system 

FEM 

Bladed V3.8X BEM or GDW + DS Airystr or UD or Stream + 

ME 

DLL or UD FEMp + Modal/MBS 

Bladed V4 Multibody BEM or GDW + DS Airystr or UD or Stream + 

ME 

DLL or UD MBS 

FAST-ANSYS BEM or GDW + DS 

(AeroDyn) 

Airystr or UD + ME DLL or UD or SM Support structure: FEM, 

Turbine: FEMp + 

Modal/MBS 

FEDEM WindPower BEM or GDW + DS 

(AeroDyn) 

Airy, Airystr, Stream + 

ME 

DLL or UD or 

Internal control 

system 

MBS/FEM Modal 

(CMS) 

Flex-ASAS BEM or DS Airystr or UD + ME DLL Modal, FEM 

Flex5-Poseidon BEM or GDW + DS Airystr or UD or Stream + 

ME, Interface to 

WaveLoads 

DLL or UD FEM + Modal 

GAST BEM or 3DFW + DS Airystr + PF or Stream + 

ME 

DLL or UD MBS/FEM 

HAWC2 BEM or GDW + DS Airystr or Stream or UD + 

ME 

DLL or UD or SM MBS/FEM 

OneWind BEM or GDW + DS Airystr or UD + ME DLL or UD MBS/FEM 

Phatas-WMCfem BEM or GDW + DS Airystr or Stream + ME DLL or Internal 

modeling 

Rotor-FD, Tower: FEM 

+ Craig Bampton 

USFOS-vpOne BEM + DS Airystr or Stokes’ 5th 

order or Stream + ME 

DLL or UD FEM 

3DFW – Free Wake Vortex particle method GDW – Generalized Dynamic Wake Theory, it is not distinguished between 

Airy – Airy theory GDW theory, which is simplified potential flow theory and GDW as BEM 

Airystr – Airy theory with stretching method modeled using different approaches to account for dynamic inflow. 

BEM – Blade Element Momentum Theory MBS – Multibody-dynamics formulation 

CMS – Component Mode Synthesis ME – Morison’s Formula 

DLL – External dynamic link library Modal – Modal reduced system 

DS – Dynamic Stall Implementation Rotor-FD – Nonlinear partial differential equations of the rotating and 

FEM – Finite-element method elastically deforming rotor (slender beams) solved by finite difference 

FEMp – Finite-element method for mode method and cubic spline for deformation field. 

pre-processing only SM – interface to Simulink with Matlab 

PF – Linear potential flow with radiation and Stream – Dean’s stream function 

diffraction UD – User-defined subroutine 

DEFINITION OF OFFSHORE WIND TURBINE 

The term OWT refers to the entire assembly of a wind turbine. In this 

case, this includes a rotor nacelle assembly (RNA) and a jacket support 

structure. In the OC4 project, the ”NREL 5-MW Offshore Baseline Turbine” defined by Jonkman et al. (2009) is supported by the UpWind reference jacket model developed by Vemula et al. (2010) and further adopted 

by Vorpahl et al. (2011) for the needs of this benchmark exercise. The 

definition of the jacket support structure, used within the OC4 project, 

consists of a jacket substructure, a transition piece and a tower. Four legs 

of the jacket are supported by piles, which are modeled as being clamped 

at the seabed. The legs are inclined from the vertical position and stiffened by four levels of X-braces. Additionally, mudbraces are placed just 

above the mudline to minimize the bending moment at the foundation 

piles. The jacket and the tower are connected through a rigid transition 

piece. The elevation of the entire support structure is 88.15 m, whereas 

the hub height is 90.55 m. The OWT is analyzed for a site of 50 m water 

depth. 

The definition of the OWT should be as simple as possible to minimize 

the effort and modeling errors in its implementation in various codes. On 

the other hand, its complexity should allow it to mimic the structural behaviour of a real OWT and to depict differences in results between the 

simulation codes. For simplification reasons, it is decided not to include 

appurtenances on the jacket structure such as boat landings, J-tubes, anodes, cables, ladders etc. Also, joint cans are not taken into account in 

the setup of the model. It was shown in Cordle et al. (2011) that the 

modeling of joint cans does not to lead to significant changes in the simulated loads. At joints, the connecting nodes of elements are defined at 

the intersection points of the members’ centerlines. This leads to overlap 

of elements in the analyzed jacket. Due to the overlapping members, the 

mass of the jacket is overestimated by about 9.7 %, though there is only 

a marginal influence coming from overlapping parts on eigenfrequencies 

and simulated loading as proved in Kaufer et al. (2010). The additional 

masses such as: hydrodynamic added mass, water in flooded legs and 

marine growth, have a strong influence on the dynamic response of the 

structure, and therefore, are included in the model description. Marine 

growth mass and hydrodynamic added mass are overestimated by about 

9.2 % and 4.6 %, respectively, due to the presence of overlapping members. 

DEFINITION OF LOAD CASES 

A set of 17 load cases of increasing complexity was defined to allow 

for a stepwise comparison of results and to enable the OC4 participants 

to trace back possible errors coming from different models and methods 

used among the codes. This is the same approach as already realized in 

the OC3 project. The load-case sets are specified in Table 2 and briefly 

described within this section. A detailed description of these load cases, 

including a precise definition of environmental conditions, simulation 

setup and output sensors, has been prepared within the project and can 

be found in Vorpahl and Popko (2011). 

The load-case set 1.0x is meant for the examination of modal properties 

of the structure, where eigenfrequencies should be calculated either with 

or without structural damping and gravity terms. This analysis is performed for a fully flexible OWT and for a flexible jacket support structure with a rigid RNA atop, respectively. The load-case set 2.X refers to a 

rigid OWT excited by diverse, non-combined wind and wave loads. The 

load sources are applied individually on the structure. In the next loadcase set 3.X, a flexible onshore wind turbine is simulated. The jacket 

support structure is replaced with a tubular tower described in Jonkman 

et al. (2009). This set is dedicated to verify basic aerodynamics and 

control responses of the turbine, including aero-elastic effects. In the 

load-case set 4.X, a flexible support structure with a rigid RNA (but no 

aerodynamics), is examined under different wave loadings. The most 

complicated load-case set 5.X accounts for a fully flexible OWT under 

the combined action of wind and wave loads. 

Turbulent wind fields are generated at Risø DTU (Larsen, 2011) according to the specification given in Vorpahl and Popko (2011). The stochastic wind data could also be generated individually by each project partner, 

2
Table 2: Overview of load cases. 

Load Case Enabled DOF Wind Conditions Wave Conditions

1.0x1 Support structure or all No air No water 

2.1 None No air Still water 

2.2 None, Rotor speed and blade Steady, uniform, no shear: No water 

pitch via controller Vhub = 8 m/s 

2.3a None No air Regular Airy: H = 6 m, T = 10s 

2.3b None No air Stream function: 

H = 8 m, T = 10s 

2.4a None: Rotor speed and blade NTM (Kaimal): No water 

pitch via controller Vhub = 11.4 m/s 

2.4b None: Rotor speed and blade 

pitch via controller 

NTM (Kaimal): Vhub = 18m/s No water 

2.5 None No air Irregular Airy with PM: 

Hs = 6m, Tp = 10s 

3.2 All, Rotor speed and blade pitch Steady, uniform, no shear: No water 

via controller Vhub = 8 m/s 

3.4a All, Rotor speed and blade pitch NTM (Kaimal): No water 

via controller Vhub = 11.4 m/s 

4.3b Support structure No air Stream function: 

H = 8 m, T = 10s 

4.5 Support structure No air Irregular Airy with PM: 

Hs = 6m, Tp = 10s 

5.6 All, Rotor speed and blade pitch Steady, uniform, no shear: Stream function: 

via controller Vhub = 8 m/s H = 8 m, T = 10s 

5.7 All: Rotor speed and blade pitch 

via controller 

NTM (Kaimal): Vhub = 18m/s Irregular Airy with PM: 

Hs = 6m, Tp = 10s 

NTM – Normal turbulence model T p – peak-spectral wave period 

PM – Pierson-Moskowitz wave spectrum V hub – average wind speed at the hub height 

H – wave height x1 – a and b cases without gravity and damping, c and d cases with Hs – significant wave height gravity and damping, a and c cases only support structure DOF T – wave period enabled, b and d cases all DOF enabled 

based on the above document. The alternative is given for those participants, whose codes are not able to utilize the provided wind fields due 

to a different grid format (i.e. Phatas and Flex5 utilize polar grid) or 

limitations imposed on the grid resolution (i.e. ADCoS-Offshore). The 

clear advantage of having these two possibilities is that the codes can be 

broken down into two groups for further comparison. The utilization of 

not the same time series of wind fields, even though they are based on 

the same definition of turbulence and stochastic properties, is an interesting source of discrepancies affecting the structural response. However, 

since only single cases are simulated, it is not possible to distinguish 

whether differences seen are caused by differences in turbulence modeling or stochastic uncertainty caused by the random seed selection. 

Deterministic and stochastic wave loads were generated individually by 

each project participant. Irregular waves may be generated based on the 

spectral input or the exact time history of the data could be reproduced. 

For the latter possibility, 50 wavelets with defined wave number, amplitude, phase angles and angular frequencies are specified in Vorpahl and 

Popko (2011). The simulation time for the deterministic load cases is 

30 s. The stochastic cases are run with one seed and the simulation time 

of 3600 s to get statistically comparable results. 

The load cases with deterministic inputs are compared in terms of timeseries outputs, whereas those with stochastic inputs are examined in 

terms of probability density functions (PDF), power spectral densities 

(PSD), and damage-equivalent loads (DEL). For each load case, the outputs are recorded at a number of nodal points (called sensors), located 

on the RNA and the jacket support structure as shown in Fig. 1. Sensor names result from leg numbering, side numbering, height level and 

joint type. TP stands for Transition Piece. The position of sensors is 

carefully chosen to provide maximum information regarding the OWT 

behaviour with the minimal effort necessary for data acquisition and 

post-processing. Those sensors located on the RNA are meant to capture 

aeroelastic response of a turbine and its power production. The support 

structure sensors are used for capturing global and local dynamics of the 

jacket. For example, local vibrations of the structure are studied at several places of the lowest X-braces, whereas global bending moments and 

forces are calculated at the very bottom, mudline points. There are also 

four additional sensors monitoring the environmental conditions such as 

the wind speed at the hub height and the sea elevation at the center of the 

structure. 

L3 L4

L2 L1

X4S3

mudbraceL4

X2S3

K1L4

mudbraceL2

X4S2

X2S2

K1L2

mudlineL2

mudlineL3

mudlineL4

mudlineL1

B59

B61

TP

X

Y

Blade 50% 

span

Tower top

Blade root

Generator

Low-speed shaft 

at main bearing

High-speed 

shaft

Blade tip

Fig. 1: Placement of sensors on jacket support structure (left) and wind 

turbine (modified sketch from GL (2005)) (right). 

COMPARISON OF THE RESULTS 

This section presents the exemplary results of the coupled simulations. 

The most interesting load cases and output sensors, according to the authors, are presented herein. Chosen results are meant to give a general 

overview of code-to-code differences and diverse approaches in modeling, that influence estimation of loads. The results shown herein are the 

effect of several revisions, which were necessary due to the complexity 

of models, user errors, the ongoing development of some of the codes 

etc. These account for corrections at all stages of OWT modeling, its 

simulation and post-processing of the data. Project participants put some 

effort into ensuring that their models: (1) are implemented according to 

the provided specification of the OWT, (2) initial conditions of the simulations are fulfilled and simulation start-up transients are eliminated, (3) 

use proper coordinate systems for the data outputs. However, some errors 

caused by a human factor may persist in the results. 

Comparison of Mass

Prior to the simulation of the prescribed load cases, a verification of the 

different implementations of the OWT is conducted in terms of structural 

and additional masses. The comparison of masses is important, as they 

are directly related to the structural dynamics. Structural masses encompass the jacket, transition piece, tower and RNA. Additional masses include marine growth, water in flooded legs and the hydrodynamic added 

mass imposed by water surrounding the structure. The results, obtained 

from 18 different models, are compared against each other in Fig. 2 and 

Fig. 3, respectively. 

3
349.0

349.5

350.0

350.5

351.0

Rotor Nacelle Assembly

Mass [t]

215.0

215.5

216.0

216.5

217.0

217.5

218.0

Tower

Mass [t]

666.0

668.0

670.0

672.0

674.0

676.0

678.0

Jacket

Mass [t]

Fraunhofer IWES/ADCoS-Offshore

FEDEM/WindPower

GLGH/Bladed V4.0

POSTECH/Bladed V3.85

SWE/Flex5-Poseidon

 CeSOS-NTNU/USFOS-vpOne

IFE/3DFloat

NTUA/GAST

LUH/Flex5-Poseidon

ECN-WMC/Phatas-WMCfem

Risø DTU/HAWC2

ABS/ANSYS-BModes

REpower/Flex-ASAS

Fraunhofer IWES/OneWind

CGC/Bladed V3.80

NTNU/ASHES

LUH/ABAQUS

LUH-IWES/ADAMS

Fig. 2: Structural masses. 

In general, a very good agreement between the implemented models is 

observed. The achievement of such results involved a widespread discussion about modeling strategies and implementation methods. The 

differences in RNA masses are mainly a result of discretization of the 

blade and its mass integration. Dissimilarities in the tower masses are 

attributed to whether stepped or conical elements are used for its implementation. Standard deviations of blade and tower masses coming 

from different codes are 0.2 t and 0.4 t, respectively. Discrepancies in the 

jacket mass are the result of slightly different modeling of the structure 

in diverse tools, though the standard deviation of 2.3 t is also very small. 

The transition piece is defined as a point mass of 666 t by most of the 

participants. Small discrepancies should not have a profound influence 

on the eigenfrequencies of the OWT. 

Hydrodynamic added mass and water in the flooded legs are defined from 

the mean sea level (MSL) to the seabed at -50 m. Marine growth is only 

implemented within the range of -40 m to -2 m. The discrepancies in 

masses mainly arise from the discretization of these threshold regions in 

the support structure. 

610.0

620.0

630.0

640.0

650.0

660.0

670.0

680.0

690.0

700.0

Hydrodynamic added mass to MSL

Mass [t]

197.0

198.0

199.0

200.0

201.0

202.0

203.0

204.0

205.0

Water mass in free flooded legs to MSL

Mass [t]

170.0

172.0

174.0

176.0

178.0

180.0

182.0

Growth mass

Mass [t]

Fraunhofer IWES/ADCoS-Offshore

FEDEM/WindPower

GLGH/Bladed V4.0

POSTECH/Bladed V3.85

SWE/Flex5-Poseidon

 CeSOS-NTNU/USFOS-vpOne

IFE/3DFloat

NTUA/GAST

LUH/Flex5-Poseidon

ECN-WMC/Phatas-WMCfem

Risø DTU/HAWC2

ABS/ANSYS-BModes

REpower/Flex-ASAS

Fraunhofer IWES/OneWind

CGC/Bladed V3.80

NTNU/ASHES

Fig. 3: Additional masses. 

Eigenanalysis

Four load cases are defined for eigenmodes analysis. The exemplary 

results from LC 1.0b are shown in Fig. 4. 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Frequency [Hz]

Fraunhofer IWES / ADCoS-Offshore

Risø DTU / HAWC2

GLGH / Bladed V4.0

POSTECH / Bladed V3.85

CGC / Bladed V3.80

IFE / 3Dfloat

Fedem Technology AS / FEDEM WindPower

CeSOS-NTNU / USFOS-vpOne

NTUA / GAST

SWE / Flex5-Poseidon

REpower /ANSYS

LUH / Flex5-Poseidon

ABS / ANSYS-BModes

NTNU / ASHES

ECN-WMC / Phatas-WMCfem

Fig. 4: Exemplary eigenfrequencies, LC 1.0b. 

Eigenmodes are identified based on their visualization. Additionally, the 

energy of the modal components could be used for this purpose. However, such information can be extracted only from a limited number of 

codes making the comparison rather difficult. 

Values of the very first eigenmodes are in a good agreement. Discrepancies increase for higher modes. This is expected, as different codes 

incorporate a different number of degrees of freedom (DOF) and somewhat dissimilar ways of structural modeling. Besides that, energy of 

higher modes usually originates from several different DOF, from which 

it is supplied in a different percentage. Consequently, it is difficult to 

clearly assess which coupled vibration mode, for the high frequency 

modes, is induced based on mode visualization. Furthermore, some of 

the codes use different reduction methods for solving the eigenvalue 

problem. Phatas-WMCfem and FEDEM WindPower utilize the CraigBampton method (with the Guyan Reduction in case of FEDEM), to reduce the size of their FEM models. Flex5-Poseidon calculates frequencies based on a “partly” reduced approach. Flex5 reduces the flexible 

beam elements of a tower and blades by Modal Decoupling, whereas 

no reduction of the substructure is performed by Poseidon, described in 

Böker (2010). Then, both parts are coupled on the synthesis of the equations of motion. In general, modal-based codes predict slightly higher 

frequencies compared to multibody- or FEM-based tools, which indicates a stiffer behaviour of a structure. Multibody and FEM codes accommodate more DOF and thus allow for more vibrational modes. This 

results in reduced stiffness of the structure, which should better mimic 

reality. This is well observed in the case of HAWC2, which predicts 

slightly higher frequencies than the modal-based Bladed V3.8X. The natural frequency for the fore-aft mode is slightly higher than for the sideto-side mode of the same order. The support structure is symmetric with 

respect to these modes. The RNA should be the only source of difference 

in the natural frequencies, as for instance, there are different moments of 

inertia of the nacelle, hub and rotor around two horizontal axes of symmetry. A number of codes have difficulty in detecting the 1st edgewise 

4
collective mode. Those that are able to capture it, show quite distinct 

frequencies due to the couplings with other modes. Bladed may overestimate the frequency, as the rotor inertia has to be artificially increased 

during the eigenanalysis to prevent the rotor from idling. This mode is 

close to the uncoupled edgewise vibration of the rotor. 3DFloat, Flex5 

and USFOS-vpOne show significant coupling of the collective edgewise 

blade motion with the 1st drivetrain mode, while in ADCoS-Offshore, 

the coupling with the 2nd side-to-side mode is present. 

The influence of gravity and damping on the eigenanalysis of the fully 

flexible OWT is analyzed in LC 1.0d. Gravity and damping terms 

are taken accounted for in Bladed, GAST, FEDEM WindPower and 

ANSYS-BModes. Whereas, HAWC2 and Phatas-WMCfem account for 

the damping term. The remaining codes neglect the influence of these 

two terms. A very low influence of gravity and damping on the eigenanvalues is observed; in general, much less than 1.5 %. In most of the 

cases, a slight decrease of frequency occurs. The highest, relative reduction is visible for the global modes such as side-to-side and the fore-aft. 

This is expected as gravity tends to reduce the bending stiffness of a vertical beam as shown in e.g. Jonkman (2003). A very small increase of 

frequencies for asymmetric flapwise and edgewise modes is observed. In 

general, the multibody code HAWC2 is expected to match GAST, Bladed 

V4.0 and FEDEM WindPower results. However, a much smaller change 

in the frequencies of global modes is observed, as only the damping term 

is used in the analysis (results not shown here). 

FEM- and multibody-based codes can be used to study higher local vibrational modes of the jacket. Mode-shape based tools might not accurately predict these vibrations due to the limited number of mode shapes 

used for the model. Local dynamics in the jacket are observed at higher 

frequencies, where diverse couplings with the RNA and the global structural modes are found by many multibody and FEM tools. Exemplary 

results are shown in Fig. 5. There is a distinct range of frequencies, 

where the lowest modes of the local jacket vibrations are detected by different tools. Therefore, more detailed analysis is necessary in the future. 

The presented modes do not account for hydrodynamic added mass. The 

presence of hydrodynamic added mass significantly reduces the frequencies of local modes as shown in i.e. Moll et al. (2010), which will bring 

the local brace bending frequencies closer to the low frequency modes 

(with higher energy content). 

(a) ADCoS­ (b) USFOS­ (c) 3DFloat (d) Fedem (e) GAST 5.59 Hz. 

Offshore VPOne 6.29 Hz. WindPower 

2.67 Hz. 5.40 Hz. 4.67 Hz. 

Fig. 5: Exemplary eigenmodes including local jacket vibrations and couplings detected by various codes. 

Rigid OWT

In the load-case set 2.X, special attention is given to proper calculation of 

buoyancy. There has been a wide discussion within the OC4 project concerning the modeling strategy for buoyancy and its physical correctness. 

Buoyancy can be accounted for based on the displaced volume method 

or the pressure integration method. The former estimates weight of water 

displaced by submerged elements of the structure. The latter integrates 

the external pressure acting on the surface of a structure accounting for 

all pressure forces imposed on individual members. The pressure integration method provides more accurate estimation of buoyancy and should 

be used in the analysis of jackets. This method accounts for hydrodynamic and hydrostatic pressures. The hydrostatic pressure is associated 

with pressure exerted by a column of water due to the gravity force. The 

hydrodynamic pressure refers to kinetic energy of water particles. 

The structure modeled within the OC4 project is cut and clamped fixed 

at the mudline (see Fig. 6a). In such a case, there is no upward buoyant 

force acting on the bottom, cross sectional area of a pile that is in contact 

with the seabed, as described in Clauss et al. (1992). As the piles are 

modeled as cut at the mudline (they do not penetrate the seabed), the 

pore water pressure in the seabed is also ignored in the modeling. 

(a) Pile modeled as cut and fixed at 

the mudline, arrows indicate the areas 

where pressure is integrated. 

-17200

-17000

-16800

-16600

-16400

-16200

-16000

-15800

-15600

-15400

MudFz

[kN]

(b) Vertical force at mudline for still water 

case, LC 2.1. 

-17400

-17200

-17000

-16800

-16600

-16400

-16200

-16000

-15800

-15600

-15400

 0 5 10 15 20 25 30

MudFz [kN]

Time [s]

Fraunhofer IWES/ADCoS-Offshore

FEDEM/WindPower

GLGH/Bladed V4.0

POSTECH/Bladed V3.85

SWE/Flex5-Poseidon

 CeSOS-NTNU/USFOS-vpOne

IFE/3DFloat

NTUA/GAST

LUH/Flex5-Poseidon

ABS/FAST-ANSYS

Risø DTU/HAWC2

REpower/Flex-ASAS

Fraunhofer IWES/OneWind

NTNU/ASHES

(c) Vertical force at mudline for Airy wave with 

Wheeler stretching case, LC 2.3a. 

Fig. 6: Resulting vertical force at the base of the structure and modeling 

strategy of a pile at the mudline. 

The resulting vertical forces at the base of the structure, for LC 2.1 and 

LC 2.3a, are shown in Fig. 6b and Fig. 6c, respectively. The shifts in 

the mean values of force are due to the diverse modeling approaches 

in accounting for buoyancy. Those project participants, that considered 

in their modeling strategies, that the structure is clamped fixed at the 

seabed, ended up with a mean force at about -16600 kN (a force directed 

upward) at the mudline. Other participants that did not adapt this approach, ended up with about -15800 kN. In those cases, the buoyancy 

force was exerted at the bottom surfaces of piles. In Bladed V4.0, there 

is an additional pressure force applied on the top of the grouted piles that 

leads to about -17150 kN. In Flex5-Poseidon utilized by SWE, buoyancy 

of legs was ignored, as the code could not handle hollow tubes at the 

5
 8.95

 9

 9.05

 9.1

 9.15

 9.2

 9.25

 0 5 10 15 20

RotSpeed [rpm]

Time [s]

 1700

 1720

 1740

 1760

 1780

 1800

 1820

 1840

 0 5 10 15 20

GenPwr [kW]

Time [s]

-1000

-800

-600

-400

-200

 0

 200

 400

 600

 800

 1000

 0 5 10 15 20 25 30

BSX [kN]

Time [s]

Fraunhofer IWES/ADCoS-Offshore

FEDEM/WindPower

GLGH/Bladed V4

POSTECH/Bladed V3.85

SWE/Flex5-Poseidon

 CeSOS-NTNU/USFOS-vpOne

IFE/3DFloat

NTUA/GAST

LUH/Flex5-Poseidon

ABS/FAST-ANSYS

Risø DTU/HAWC2

NTNU/ASHES

Fraunhofer IWES/OneWind

time this paper was written. Pressure integration was only applied on 

the surface of sealed braces, which led to the mudline vertical force of 

about -17100 kN. 

Differences in the peak-to-peak amplitude of the vertical forces at the 

mudline, shown in Fig. 6c, are mainly based on whether the displaced 

volume or the pressure integration method is used for calculation of 

buoyancy. The integration of the hydrodynamic pressure results in a reduced buoyancy effect seen at the mudline during the wave crest (higher 

upward buoyancy force at the wave crest), while it is increased during 

the wave through (lower upward buoyancy force at the wave through), 

compared to the displaced volume method. This is observed as smaller 

peak-to-peak amplitudes of the vertical force at the mudline. In reality, 

at the relatively deep position, the hydrostatic pressure does not change 

that much due to the changing water surface elevation. The influence of 

the hydrodynamic pressure is also small. Therefore, the variation in the 

vertical force at the mudline should also be small. This is observed in the 

results obtained with the pressure integration method. 

Summing up, the difference between the two furthest outliers is about 

9 %. This is acceptable, especially as different modeling assumptions for 

buoyancy were applied. Also, different masses of marine growth and the 

jacket substructure contributed to discrepancies. 

(a) Rotor speed. (b) Generator power. 

 1840

 1860

 1880

 1900

 1920

 1940

 1960

 1980

 2000

 2020

 2040

 2060

 0 5 10 15 20

RotTorq [kNm]

Time [s]

Fraunhofer IWES/ADCoS-Offshore

FEDEM/WindPower

GLGH/Bladed V4

POSTECH/Bladed V3.85

SWE/Flex5-Poseidon

 CeSOS-NTNU/USFOS-vpOne

IFE/3DFloat

NTUA/GAST

LUH/Flex5-Poseidon

ABS/FAST-ANSYS

Risø DTU/HAWC2

Fraunhofer IWES/OneWind

REpower/Flex-ASAS

CGC/Bladed V3.80

NTNU/ASHES

(c) Low-speed shaft torque. 

Fig. 7: Rotor speed, torque and generator power, LC 2.2. 

A very good match of the rotor speed and the generator power is achieved 

in LC 2.2 for the majority of codes as shown in Fig. 7a and Fig. 7b, respectively. The tower blockage effect is captured by most of the tools as 

small fluctuations of the generator power and the rotor speed. In ADCoSOffshore, power production under rated power is highly affected by the 

flow stagnation effect in front of the tower. A miscalculation of this 

effect under certain conditions was discovered during the OC4 project. 

Results presented herein include the improved tower blockage model, 

though some further refinements are still necessary. The tower blockage model was not implemented in 3DFloat and USFOS-vpOne tools at 

the time this paper was written. Larger downward peaks, corresponding to 3P frequency at about 0.46 Hz, are visible in the low-speed shaft 

torque for most of the codes (Fig. 7c). Upward and downward peaks in 

the low-speed shaft torque are present in the results of Flex5, utilized 

by LUH, SWE and REpower. Differences in the implementation of the 

tower blockage model are indicated as possible sources of those discrepancies. However, this issue has to be investigated further. The low-speed 

shaft torque is indirectly calculated in ADCoS-Offshore, through which 

a smoothening effect of the signal is introduced. High oscillations of the 

Flex5 (REpower) signals are due to the flexible model of the RNA used 

in the simulation, instead of the rigid defined in the load-case set 2.X. 

The model setup in ASHES still needs some refinements. The results for 

the remaining codes match very closely. They differ less than 1 % for the 

rotor speed, generator power and low-speed shaft. 

Fig. 8 shows the fore-aft force at the base of the structure for LC 2.3a. 

Some differences in the peak-to-peak amplitude are caused i.e. by dissimilarities in implementations of wave kinematics. The lowest amplitude comes from FAST-ANSYS simulations and needs to be investigated 

further. The simulation results are also affected by differences originating in the modeling of the support structure. Flex5-Poseidon, utilized 

by SWE, gives the largest amplitude of the fore-aft base shear force, 

whereas a lower amplitude is calculated by the same code used at LUH. 

To sum up, a good agreement is observed for most of the codes. The 

maximum difference of the peak-to-peak amplitudes is less than 11 % 

(for the majority of codes it is much less). 

Fig. 8: Fore-aft shear force at mudline, LC 2.3a. 

The PDF of the tower-top and base fore-aft shear forces for LC 2.4b, 

shown in Fig. 9a and Fig. 9b, are expected to be very similar. Results 

of FAST-ANSYS and GAST differ in the mean value, which is about 

10 % higher at the mudline. Such a shift is caused by the additional 

wind loads applied on the tower, which should not have been account for 

according to the definition of LC 2.4b. The mean fore-aft shear forces 

calculated in 3DFloat are higher than in other codes. 3DFloat has rigid 

blades, which are pitched according to the defined control system. One 

possible source of differences in the mean shear forces is the lack of the 

tower shadow model. The explanation of the higher mean of the Flex5­

Poseidon (LUH) signal is not that straightforward. The mean value of 

the generator power is higher than expected, but the mean pitch angle 

is lower. These facts may indicate wrong settings of the control system 

of the turbine. The utilization of the stochastic wind files with not the 

same time series is also a source of discrepancy, as already mentioned. 

HAWC2, FAST-ANSYS, FEDEM WindPower, 3DFloat and GAST utilize the same stochastic wind files as an input, whereas Bladed V3.85, 

ADCoS-offshore, OneWind and Flex5-Poseidon use their own wind files. 

For example, there is less fluctuation in the longitudinal component of the 

wind speed in ADCoS-Offshore, which is reflected in the narrower PDF 

of the shear force. In general, a very good agreement of presented PDF 

results is observed. 

DEL values of the mudline shear forces in LC 2.4b diverge a lot, as 

shown in Fig. 9c. The biggest outliers are GAST and ADCoS-Offshore. 

The energy content of the mudline signal of ADCoS-offshore is of an 

6
order less than other codes, and its distribution is narrower, resulting in 

the lowest DEL value. The opposite behaviour is observed for GAST (the 

PSD results are not shown herein). Apart from the outliers, the maximum 

difference between other tools is less than 20 %, which is a good result 

considering all the mentioned differences in the setup of LC 2.4b. 

 0

 0.001

 0.002

 0.003

 0.004

 0.005

 0.006

 0.007

 0.008

 0.009

 100 200 300 400 500 600

PDF [kN]-1

YawBrFxp [kN]

 0

 0.001

 0.002

 0.003

 0.004

 0.005

 0.006

 0.007

 0.008

 0.009

 100 200 300 400 500 600

PDF [kN]-1

BSX [kN]

(a) PDF – Tower-top fore-aft shear force. (b) PDF – Fore-aft shear force at mudline. 

 100

 120

 140

 160

 180

 200

 220

 240

 260

 280

 300

BSX

[kN]

Fraunhofer IWES/ADCoS-Offshore

FEDEM/WindPower

POSTECH/Bladed V3.85

IFE/3DFloat

NTUA/GAST

LUH/Flex5-Poseidon

ABS/FAST-ANSYS

Risø DTU/HAWC2

Fraunhofer IWES/OneWind

(c) DEL – Fore-aft shear force at mudline. 

Fig. 9: Fore-aft forces at tower-top and mudline of the structure, LC 2.4b. 

Flexible Onshore Turbine

In Fig. 10a, tip twist of Blade 1 for LC 3.2 is zero in the FAST, Flex5 and 

Bladed V3.85 outputs, as these codes do not consider torsion of the blade. 

USFOS/vpOne considers the torsional degree of freedom for the blade, 

however there is no output providing a time history of the tip angle. The 

blade torsion is included in the multibody release of Bladed V4.0 (no 

results presented herein) and should be soon implemented in FAST. The 

results of FEDEM WindPower and GAST match closely. The biggest 

outlier, with the highest peak-to-peak oscillation, is ADCOS-Offshore. 

This has to be investigated further. 

The fluctuations of the low-speed shaft torque in LC 3.2 are mainly 

caused by the tower blockage effect pronounced as 3P frequency at 

about 0.46 Hz and its higher harmonics as shown in Fig. 10b. Only the 

first 10 s of the time series are shown for better clarity. In ADCoSOffshore, the low-speed shaft torque is calculated indirectly through 

which a smoothening effect is introduced. The shift of HAWC2 signal 

corresponds to higher rotational speed of the rotor, achieved by this tool 

in LC 3.2. The low-speed shaft torque of Flex5, utilized by REpower, 

is not shown herein due to excessive oscillations of the signal. Further 

refinements of the model are necessary. In general, mean values of the 

low-speed shaft torque are close for all the tools. 

A very good agreement in the distribution of flapwise and edgewise shear 

forces at the blade root is observed among the codes in LC 3.4a, as shown 

in Fig. 11a and Fig. 11b, respectively. The greatest outlier is Flex5 (REpower) due to the incorrect specification of the stochastic wind. Minor 

shifts in the mean values of the flapwise force distributions are caused by 

differences in the thrust force. For the spinning-rotor cases, rigid blades 

are used in 3DFloat. Nevertheless, the distributions of the blade root 

forces closely match other codes where a flexible rotor is defined. 

-1

-0.8

-0.6

-0.4

-0.2

 0

 0.2

 0.4

 0 5 10 15 20 25 30

TwstDefl1 [deg]

Time [s]

 1940

 1950

 1960

 1970

 1980

 1990

 2000

 2010

 2020

 0 2 4 6 8 10

RotTorq [kNm]

Time [s]

(a) Blade 1 tip twist. (b) Low-speed shaft torque. 

Fraunhofer IWES/ADCoS-Offshore

FEDEM/WindPower

POSTECH/Bladed V3.85

 CeSOS-NTNU/USFOS-vpOne

NTUA/GAST

ABS/FAST-ANSYS

Risø DTU/HAWC2

REpower/Flex-ASAS

Fig. 10: Blade deflections and low-speed shaft torque, LC 3.2. 

 0

 0.005

 0.01

 0.015

 0.02

 0.025

 100 150 200 250 300 350 400

PDF [kN]-1

RootFxc1 [kN]

 0

 0.001

 0.002

 0.003

 0.004

 0.005

 0.006

 0.007

 0.008

 0.009

-250 -200 -150 -100 -50 0 50 100 150 200

PDF [kN]-1

RootFyc1 [kN]

(a) PDF – flapwise shear force at blade root. (b) PDF – edgewise shear force at blade root. 

 0

 0.002

 0.004

 0.006

 0.008

 0.01

 0.012

-200 -150 -100 -50 0 50 100 150

PDF [kNm]-1

RootMzc1 [kNm]

Fraunhofer IWES/ADCoS-Offshore

FEDEM/WindPower

POSTECH/Bladed V3.85

IFE/3DFloat

NTUA/GAST

ABS/FAST-ANSYS

Risø DTU/HAWC2

REpower/Flex-ASAS

(c) PDF – pitching moment at blade root. 

Fig. 11: PDF of blade root forces and pitching moment, LC 3.4a. 

The PDF of the pitching moment at the blade root is shown in Fig. 11c. 

The discrepancies originate from the rate and magnitude of the pitching action, dependent on whether or not a rated speed of the rotor is 

achieved. HAWC2, FAST, FEDEM WindPower, 3DFloat and GAST utilize the same stochastic wind files as input; only Bladed V3.85 uses its 

own wind files. Thus, the sources of discrepancies in the pitching moment at the blade root should mainly originate from the implementation 

of the blade model in various tools. For example, a shift of the blade 

mass center can affect the distribution of these values. Bladed V3.85 

is the biggest outlier. However, such a difference is not caused by the 

utilization of the wind files with not the same time series, as the PDF 

of the longitudinal wind component matches very well the same component in the wind files prepared by Risø DTU. The shift of the PDF of the 

pitching moment suggests incorrect setup of the model. For example the 

aerodynamic center, relative to the pitch axis, could be specified incorrectly. However, in other stochastic LCs 2.4a, 2.4b and 5.7, PDF of the 

pitching moment of Bladed V3.85 matches other tools. Summing up, a 

good agreement of the PDF is observed for the majority of codes. 

7
Flexible Support Structure Flexible OWT

The out-of-plane displacement in the fore-aft direction, of the central 

joint X2S2 of X-brace (see Fig. 1), is shown in Fig. 12 for LC 4.3b. 

The OWT is excited with a deterministic wave, described by the 9th order stream function wave. The agreement of signal trends is remarkably 

good, though the peak-to-peak amplitude is very small. A frequency shift 

of the output of Flex5-Poseidon, utilized by SWE, is caused by a bug in 

the stream function wave. LUH also uses Flex5-Poseidon. However in 

their case, the WaveLoads tool was used to compute the stream function 

wave and then linked to Flex5-Poseidon by a DLL library. The relatively 

highest oscillation amplitude of 3DFloat requires further attention. 

-0.01

-0.008

-0.006

-0.004

-0.002

 0

 0.002

 0.004

 0.006

 0.008

 0.01

 0 5 10 15 20 25 30

X2S2 [m]

Time [s]

Fraunhofer IWES/ADCoS-Offshore

FEDEM/WindPower

POSTECH/Bladed V3.85

SWE/Flex5-Poseidon

 CeSOS-NTNU/USFOS-vpOne

IFE/3DFloat

NTUA/GAST

LUH/Flex5-Poseidon-WaveLoads2

ABS/FAST-ANSYS

Risø DTU/HAWC2

REpower/Flex-ASAS

Fig. 12: Out-of-plane deflection at center of X-joint at level 2 on side 2, 

LC 4.3b. 

In LC 4.5, the OWT is excited with a stochastic irregular wave. The axial 

force of the member B59 at lowest X-brace (location presented in Fig. 1) 

is shown in Fig. 13. Frequency peaks at about 0.3 Hz, and in ranges of 1.1 

to 1.2 Hz and 2.6 to 2.8 Hz correspond to the 1st and 2nd global fore-aft 

modes and torsion, respectively, as detected in LC 1.0a and LC 1.0c. The 

results of the distribution of the axial member forces, shown in Fig. 13b, 

differ in the mean values mainly due to the differences in buoyancy modeling, discretization of the member, and therefore, slight offsets in placement of the output sensors. However, the widths and heights of most 

PDF are similar, indicating a good agreement in the fluctuation rate of 

the member axial force. 

1.0E-006

1.0E-004

1.0E-002

1.0E+000

1.0E+002

1.0E+004

1.0E+006

1.0E+008

 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

B59Ax [dB/Hz]

Frequency [Hz]

 0

 0.005

 0.01

 0.015

 0.02

 0.025

 0.03

-500 -450 -400 -350 -300 -250 -200 -150 -100 -50

PDF [kN]-1

B59Ax [kN]

(a) PSD – axial force in B59. (b) PDF– axial force in B59. 

FEDEM/WindPower

POSTECH/Bladed V3.85

SWE/Flex5-Poseidon

 CeSOS-NTNU/USFOS-vpOne

IFE/3DFloat

NTUA/GAST

LUH/Flex5-Poseidon

ABS/FAST-ANSYS

Risø DTU/HAWC2

Fig. 13: Axial force in center of brace 59, LC 4.5. 

The DEL of the axial forces, for LC 5.7, from sensors K1L2 and mudbraceL2 (see Fig. 1) from Leg 2 are shown in Fig. 14a and Fig. 14b, 

respectively. The biggest outlier in the DEL of the axial force at K1L2 

located in the splash zone is Flex5-Poseidon utilized by SWE. This is 

due to the incorrect implementation of the stochastic wave, which is observed in the case of the code used at SWE. The remaining codes ended 

up with relatively small differences. The discrepancies at mudbraceL2 

are relatively high, as many diverse modeling strategies influenced it. 

Shifts in the mean values of the PDF of the axial force at the mudbrace 

shown in Fig. 14c are caused by different approaches in modeling of 

buoyancy as previously explained. FAST-ANSYS is under further development concerning the modeling of irregular waves, and the results 

for LC 5.7 are not shown in Fig. 14. However, the preliminary result 

comparison has provided valuable input to support this development. 

 800

 850

 900

 950

 1000

 1050

 1100

 1150

 1200

 1250

 1300

 1350

K1L2

[kN]

 2000

 2200

 2400

 2600

 2800

 3000

 3200

MudbraceL2

[kN]

(a) DEL – axial force in Leg 2 at K1L2. (b) DEL – axial force in Leg 2 at mudbraceL2. 

 0

 0.0001

 0.0002

 0.0003

 0.0004

 0.0005

 0.0006

-6000 -4000 -2000 0 2000

PDF [kN]-1

MudbraceL2 [kN]

FEDEM/WindPower

POSTECH/Bladed V3.85

SWE/Flex5-Poseidon

IFE/3DFloat

NTUA/GAST

LUH/Flex5-Poseidon

Risø DTU/HAWC2

(c) PDF – axial force in Leg 2 at mudbrace 

level. 

Fig. 14: Damage equivalent loads at jacket leg, LC 5.7. 

CONCLUSION 

This paper presents the results of the OC4 project Phase I, focused on 

the coupled simulation of an offshore wind turbine with a jacket support 

structure. A detailed description of the support structure model is developed within the project. A set of deterministic and stochastic load cases 

of increasing complexity is discussed and simulated. The load cases with 

deterministic inputs are compared in terms of time-series output, and the 

stochastic cases are compared in terms of probability density functions, 

power spectral densities, and damage equivalent loads. Exemplary results of the simulations are presented in this paper. The exemplary discrepancies between the codes are shown and sources of differences are 

discussed. The presence of the local dynamic effects in the structure is 

depicted in the simulation results. Furthermore, attention is given to aspects like the buoyancy calculation and methods of accounting for additional inertia forces (such as from the hydrodynamic added mass), which 

were not analyzed extensively in the OC3 project. Finally, recommendations concerning the modeling of the jacket are given. 

The setup of coupled OWT simulations is an elaborated and difficult pro8
cess, involving multidisciplinary engineering knowledge within the fields 

of structural engineering, control, hydrodynamics, aerodynamics, aeroelasticity, data pre- and post-processing etc. Thus, some of the obtained 

results are not free from the human-inherited errors. Furthermore, differences in the implemented theories and diverse modeling strategies contributed to the discrepancies in the presented results. In the light of these 

facts, a very good agreement in the obtained results has been achieved. 

Through the participation in Phase I of the OC4 project, many of the 

participants have been able to verify their codes and methodologies developed for the dynamic analysis of a wind turbine supported with a 

jacket. Some inconsistencies and errors in models were detected in a 

direct code-to-code comparison of the results. Furthermore, the comparison with other codes has provided the first sanity checks for the newly 

developed tools, i.e. 3DFloat, OneWind and ASHES. Some examples of 

improvements, corrections and bugs, present in the tools and models, are 

briefly mentioned herein. 

Several improvements of the hydrodynamic load models were applied 

in FEDEM WindPower. For example, the mudline response from wave 

loads has been smoothed out due to improved interpolation of forces 

applied on the beam elements. Also, the integration of AeroDyn with 

FEDEM WindPower has been verified. The tower shadow effects in 

ADCoS-Offshore and FAST-ANSYS were improved. A bug in the implementation of the stream function in Poseidon has been discovered. 

Furthermore, an extended implementation of buoyancy, an interface to 

WaveLoads for computation of the stream function and the PiersonMoskowitz spectrum have been verified in Poseidon. The development 

of 3DFloat has been accelerated by the participation in the project. It 

has resulted in the implementation of i.e. models for marine growth, 

flooded members, irregular waves by the constant-energy method for discretization of the wave spectrum and stream function wave kinematics. 

In HAWC2, the position of the pitch axis in a blade model has been corrected and a bug in the Stream function wave theory implementation has 

been fixed. For calculation of the wave loads, the pressure integration 

method in USFOS-vpOne estimates the Froude-Krylov force by direct 

pressure integration using the incident wave velocity potential. During 

the modeling, it has been found out that this requires a correction on 

the mass coefficient so that the Froude-Krylov force is not accounted for 

twice. Then, it was checked that the wave loads obtained by the pressure integration method agrees well with those obtained by the Morison 

equation in the USFOS-vpOne tool. This was mathematically confirmed 

by Chung (1975). 

Overlapping members lead to increased material volume in the vicinity of joints. This leads to the overestimation of the structure weight, 

marine growth mass, buoyancy and hydrodynamic loads. The extent of 

the overestimation depends on a given jacket and varies due to the different topology, thicknesses and number of intersecting braces. Therefore, it is recommended to remove overlapping sections from models of 

jacket support structures, even though the OC4 project chose to use a 

simpler model, due to the setup simplicity, in which members overlap at 

the joints. 

Only some codes account for gravity and damping in the eigenanalysis. 

It was shown that these terms have a marginal influence on eigenfrequencies. 

Local out-of-plane vibrational modes of lower X-braces in the jacket are 

detected by FEM- and multibody-based codes. It would be of interest to 

further study this phenomena, as it might contribute to fatigue of some 

parts of the structure. 

Buoyancy should be calculated with the integrated pressure method, as 

this approach ensures correct results for multibraced structures like jackets. The jacket modeled within the OC4 project is cut and clamped fixed 

at the mudline, through which there is no upward buoyant force acting 

on the bottom, cross sectional area of a pile. This modeling approach 

is not physically correct for the jacket with piles penetrating the seabed, 

though it was used for the simplification of the model setup. The negligence of the upward buoyant force at the base of the structure, which is 

in contact with the mudline, is physically correct for structures with i.e. 

gravity-based foundations. 

ACKNOWLEDGEMENTS 

All the participants of Phase I of the OC4 project are acknowledged for 

their valuable comments, shared expertise and tremendous work. Without their active participation and true engagement, the success of the 

project would not be achieved. Walt Musial from NREL and the International Energy Agency (IEA) are acknowledged for setting up the 

project. Holger Schwarze and Mareike Strach from Fraunhofer IWES 

are acknowledged for fruitful discussions and plenty of ideas concerning 

the interpretation of the results. Alberto Grillone is acknowledged for his 

help in setting up the simulations in OneWind. 

REFERENCES 

Böker, C. (2010). Load simulation and local dynamics of support structures for offshore wind turbines. Leibniz Universität Hannover, Institute for Steel Construction, Germany. Dissertation Thesis. 

Chung, J. S. (1975). “A Note on the Two Force Equations for a Floating 

Platform”. Journal of Hydronautics, Vol. 9, No. 4, pp 170 – 172. 

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