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Contract No. DE-AC36-08GO28308

Offshore Code Comparison 

Collaboration, Continuation: 

Phase II Results of a Floating 

Semisubmersible Wind System

Preprint

A. Robertson, J. Jonkman, and W. Musial

National Renewable Energy Laboratory

F. Vorpahl and W. Popko

Fraunhofer Institute for Wind Energy and Energy 

System Technology IWES

To be presented at EWEA Offshore 2013

Frankfurt, Germany

November 19–21 2013

Conference Paper

NREL/CP-5000-60600

November 2013
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“Offshore Code Comparison Collaboration, Continuation: Phase II Results of 

a Floating Semisubmersible Wind System”

Amy Robertson1, Jason Jonkman1, Walt Musial1, Fabian Vorpahl

2

, and Wojciech Popko2

1

National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, CO 80401, +1-

303-384-7157, Amy.Robertson@nrel.gov

2

Fraunhofer Institute for Wind Energy and Energy System Technology IWES, Am Seedeich 45,

27572 Bremerhaven, Germany

Summary

Offshore wind turbines are designed and analyzed using comprehensive simulation tools that 

account for the coupled dynamics of the wind inflow, aerodynamics, elasticity, and controls of the 

turbine, along with the incident waves, sea current, hydrodynamics, and foundation dynamics of the 

support structure. The Offshore Code Comparison Collaboration (OC3), which operated under the 

International Energy Agency (IEA) Wind Task 23, was established to verify the accuracy of these 

simulation tools [1]. This work was then extended under the Offshore Code Comparison 

Collaboration, Continuation (OC4) project under IEA Wind Task 30 [2]. Both of these projects 

sought to verify the accuracy of offshore wind turbine dynamics simulation tools (or codes) through 

code-to-code comparison of simulated responses of various offshore structures.

This paper describes the latest findings from Phase II of the OC4 project, which involved the 

analysis of a 5-MW turbine supported by a floating semisubmersible. Twenty-two different 

organizations from 11 different countries submitted results using 24 different simulation tools. The 

variety of organizations contributing to the project brought together expertise from both the offshore 

structure and wind energy communities.

Twenty-one different load cases were examined, encompassing varying levels of model complexity 

and a variety of metocean conditions. Differences in the results demonstrate the importance and 

accuracy of the various modeling approaches used. Significant findings include the importance of 

mooring dynamics to the mooring loads, the role nonlinear hydrodynamic terms play in calculating 

drift forces for the platform motions, and the difference between global (at the platform level) and 

local (at the member level) modeling of viscous drag. The results from this project will help guide 

development and improvement efforts for these tools to ensure that they are providing the accurate 

information needed to support the design and analysis needs of the offshore wind community.

Introduction

The vast offshore wind resource represents a potential to use wind turbines installed offshore to 

power much of the world. Design standardization is difficult, however, because offshore sites vary 

significantly through differences in water depth, soil type, and wind and wave severity. To ensure 

that offshore wind turbine (OWT) installations minimize cost, the application of a variety of support 

structure types is required. These types include fixed-bottom monopiles, gravity bases, spaceframes—such as tripods and lattice frames (e.g., “jackets”)—and floating structures. In this context, 

the offshore wind industry faces many new design challenges.
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Wind turbines are designed and analyzed using simulation tools (i.e., design computer codes) 

capable of predicting the coupled dynamic loads and responses of the system. The simulation tools 

that were developed to model land-based wind systems rely on the use of aero-servo-elastic codes, 

which incorporate wind-inflow, aerodynamic (aero), control system (servo), and structural-dynamic 

(elastic) models in the time domain in a coupled simulation environment. To accommodate the 

additional dynamics pertinent to offshore installations, these codes have been expanded to include 

the modeling of incident waves, sea current, hydrodynamics, and foundation dynamics of the 

support structure (see Figure 1). The high complexity and sophistication of these simulation tools 

underscores the need to verify and validate their accuracy. Two research tasks were developed 

under the International Energy Agency (IEA) to address this need: the Offshore Code Comparison 

Collaboration (OC3)1 and the Offshore Code Comparison Collaboration, Continuation (OC4) 2

projects.

Figure 1: Diagram of the components of offshore wind modeling tools

Overview of the OC3 and OC4 Projects

The OC3 project was operated under the IEA Wind Task 23, Subtask 2, and was the first 

international project to address the need to verify OWT modeling tools. The OC4 project was an 

extension of the original project and has operated under IEA Wind Task 30. The approach used in 

these projects was to verify offshore wind modeling tools by comparing simulated results of offshore 

wind systems from the various tools available (code-to-code comparisons). Accuracy was assessed 

through the consistency of the results reported. Validation was not addressed in these tasks due to 

 1 www.ieawind.org/task_23.html 2 www.ieawind.org/task_30/task30_Public.html
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the importance of verification before validation and due to the limited availability of measurement 

data from actual offshore wind systems.

To test the newly developed codes, the main activities of OC3 and OC4 were to discuss modeling 

strategies, develop a suite of benchmark models and simulations, run the simulations and process 

the simulation results, and compare and discuss the results. These activities fell under broader 

objectives including:

• Assessing the accuracy and reliability of simulations to establish confidence in the

predictive capabilities of the modeling tools

• Training new analysts on how to run and apply the tools correctly

• Investigating the capabilities and limitations of implemented theories

• Refining applied analysis methodologies

• Identifying further research and development needs.

Such verification work has led to dramatic improvements in model accuracy as the code-to-code 

comparisons and lessons learned have helped identify model deficiencies and needed 

improvements. These results are important because the advancement of the offshore wind industry 

is closely tied to the development and accuracy of dynamics models.

Project Approach and Phases

The theories used in offshore wind modeling tools are complex, and the diversity of the system 

designs means that a modeling theory that is applicable to one design may not be applicable to 

another. The approach of the OC3 and OC4 projects has therefore been to perform a very 

meticulous step-by-step methodology for examining the individual modeling components of the 

simulation tools, as well as their coupled behavior. The verification assessment has been performed 

for multiple architectures to ensure the applicability of the modeling approaches across the design 

space of offshore wind.

The code-to-code verification process is performed as follows. First, an offshore wind system 

design of interest is identified, and the information needed to model the system is developed and 

shared with the project partners. Second, a set of simulations (load cases) is defined to test the 

response behavior of the system. The simulations encompass system-identification tests and a 

stepped approach for examining the system response to wind, waves, and the combination of the 

two. Different components of the system are modeled as flexible or rigid within the load cases, to 

also examine the influence of system elasticity and its interaction with the offshore environment. 

Various environmental conditions are used to examine the response behavior in both benign and 

extreme conditions. Next, each of the participants builds a model of the given design in their 

respective modeling tools, and runs the prescribed load cases. The simulated response behavior 

(loads/motions) is then compared between the various codes at multiple points throughout the 

system, which results in the identification of mistakes in the modeling implementation or simulation 

settings, shows differences in the resulting loads/motions based on the modeling approach, and 

spurs discussion about the differences between and applicability of the various modeling theories. 

This procedure is repeated for multiple offshore wind system designs, and through this process an 

understanding of the applicability of modeling theories is developed, changes are made to the tools, 

and future tool improvement needs are identified.
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The OC3 and OC4 projects ran from 2005 to 2013 and examined five different offshore wind 

systems. The same wind turbine, the National Renewable Energy Laboratory (NREL) 5-MW 

Offshore Baseline Turbine [3], was used for all three systems. Only the offshore support structure 

was varied. An image of the five designs modeled is provided in Figure 2 and includes:

• OC3 Phase I: Monopile with a rigid foundation, 20-m water depth

• OC3 Phase II: Monopile with a flexible foundation to examine soil/pile interaction, 20-m 

water depth

• OC3 Phase III: Tripod, 45-m water depth

• OC3 Phase IV: Floating spar buoy, 320-m water depth

• OC4 Phase I: Jacket, 50-m water depth

• OC4 Phase II: Floating semisubmersible, 200-m water depth

Figure 2: OC3 and OC4 offshore wind system designs modeled

The systems analyzed span the design space of offshore wind, including shallow-water systems 

(monopile), transition water-depth systems (tripod and jacket), and deep-water floating systems 

(spar and semisubmersible). See references [1] and [2] for more information about the previous 

phases of the OC3 and OC4 projects.
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Participants and Codes

The OC3 and OC4 projects were performed through technical exchange among a group of 

international participants from universities, research institutions, and industry across the United 

States of America, Germany, Denmark, the United Kingdom, Spain, the Netherlands, Norway, 

Sweden, Korea, Japan, Portugal, Greece, and China. Each one of the participants has their own 

area of expertise, and therefore, their own unique contribution to the projects. Those who delivered 

simulation results for the most recent phase (Phase II of OC4) are summarized in Table 1.

Table 1: Active participants in OC4 Phase II

Country Institution

China Chinese General Certification Center (CGC), Goldwind

Denmark Danish Hydraulic Institute (DHI), Technical University of Denmark (DTU), 

PRINCIPIA

Germany Garad Hassan (GH), Endowed Chair of Wind Energy (SWE) at the University of 

Stuttgart

Greece National Technical University of Athens (NTUA)

Japan University of Tokyo

Korea Pohang University of Science and Technology (POSTECH), University of Ulsan 

(UOU)

Norway

4subsea, Centre for Ships and Ocean Structures (CeSOS) at the National 

Technical University of Norway (NTNU), Institute for Energy Technology (IFE), 

MARINTEK

Portugal Centre for Marine Technology and Engineering (CENTEC), Instituto Superior 

Técnico (IST), Wave Energy Center (WavEC)

Spain National Renewable Energy Centre (CENER)

USA American Bureau of Shipping (ABS), National Renewable Energy Laboratory 

(NREL)

Overview of OC4, Phase II

Phase II of the OC4 project involved the modeling of a semisubmersible floating offshore wind 

system developed for the DeepCwind project [4] as shown in Figure 3. This concept was chosen for 

its increased hydrodynamic complexity compared to the only other floating system analyzed in the 

OC3 and OC4 projects, the Hywind spar buoy [5]. Floating offshore wind designs are generally 

categorized into three groups: semis, spars, and tension leg platforms (TLPs). By analyzing the 

semi, the OC3 and OC4 projects have now examined two out of three of these categories.

DeepCwind is a U.S.-based project aimed at generating field-test data for use in validating floating 

OWT modeling tools. The semi and two other floating designs were tested by the DeepCwind 

project in a series of scaled tank tests at MARIN in 2011 [4]. The turbine modeled in this project is

the NREL 5-MW Offshore Baseline Turbine [3], which differs slightly from the scaled one tested by 

DeepCwind. This turbine was used in all phases of the OC3 and OC4 projects, but the control 

system properties changed to accommodate the differences in system dynamics.
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(a) Side View of System (b) Top View of Platform

Figure 3: OC4-DeepCwind floating wind system design

Twenty-one different organizations from 10 different countries submitted results using 24 different 

simulation tools for Phase II of the OC4 project. The variety of organizations contributing to the 

project brought together expertise from both the offshore structure and wind energy communities.

Some institutions provided multiple results, examining the influence of varying modeling 

approaches within their tool. Multiple institutions also used the same tool, which provided insight 

into the differences that could be obtained from a given simulation tool based on the user’s choice

of modeling parameters.

Results for OC4 Phase II

To compare the response behavior achieved by the different modeling approaches, twenty-one 

different load cases (simulations) were performed, encompassing varying levels of model 

complexity and a variety of metocean conditions. In addition to traditional wind/wave load cases, 

this phase included the computation of response amplitude operators (RAOs), which were shown to 

be a good way to examine offshore structure response characteristics across a range of wave 

conditions, an approach traditionally used in the offshore structural community, but new to the wind 

community. Damage cases were also modeled, which included the loss of a mooring line and the 

flooding of one column, to check the simulation tools’ capabilities in assessing system behavior in a 

variety of design conditions. The results obtained are the outcome of several revisions, which were 
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necessary due to the complexity of the model, code/user errors, the ongoing development of some 

of the codes, etc.

Some exemplary results for one of the load cases are shown in Figure 4. In this load case, the 

entire OWT is modeled as flexible and excited by steady wind and regular (periodic) waves that act 

on the system simultaneously. The outputs shown are the surge motion of the system, the mooring 

forces at the fairlead connection, and the resulting tower-base bending moment in the fore/aft 

direction. Differences in the results demonstrate the importance and accuracy of the various 

modeling approaches used. For the platform surge, a small difference is seen in the amount of 

mean displacement of the system, which can be attributed to differences in the aerodynamic rotor 

thrust calculations and the approach used for modeling the nonlinear hydrodynamics, which can 

create drift forces. The fairlead mooring loads show two distinct sets of results based on whether a 

dynamic or quasi-static mooring modeling approach is used. While the mooring loads may differ 

vastly between these two approaches, the mean values are similar and tend to have no significant 

effect on the overall dynamic response of the structure, but would be important in understanding the 

design loads for the mooring system. Finally, the tower bending moment shows the overall influence 

of the modeling approaches on the resulting loads in the system. The majority of simulation tools 

produce similar results, but overall there is an 18% difference in the spread, which is significant in 

the design of an offshore wind system.

(a) Platform surge response

0 10 20 30 40 50 60 3.5

4

4.5

5

5.5

6

6.5

7

Time (sec)

PtfmSurge

(m)

4Subsea_P

ABS

CENER

CENTEC

CGC

CSIC

CeSOS

DTU

GH

GH Adv

Goldwind

IFE

IST

IST2

MARINTEK

NREL

NTUA

NTUA_M

POSTECH

PRINCIPIA_M

PRINCIPIA_P

SWE

UOU
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(b) Fairlead tension of line 2, which is upwind of the turbine, and in-line with the wind direction

(c) Tower-base bending moment

Figure 4: Exemplary results from load case 3.1: 

Simulation of semi with deterministic wind (8 m/s) and waves (H = 6 m, T = 10 s)

0 10 20 30 40 50 60 1200

1250

1300

1350

1400

1450

1500

1550

Time (sec)

Fair2Ten

(kN)

4Subsea_P

ABS

CENER

CENTEC

CGC

CSIC

CeSOS

DTU

GH

GH Adv

Goldwind

IFE

IST

IST2

MARINTEK

NREL

NTUA

NTUA_M

POSTECH

PRINCIPIA_M

PRINCIPIA_P

SWE

UOU

0 10 20 30 40 50 60

2

3

4

5

6

7x 104

Time (sec)

TwrBsMyt

(kNm)

4Subsea_P

ABS

CENER

CENTEC

CGC

CSIC

CeSOS

DTU

GH

GH Adv

Goldwind

IFE

IST

IST2

MARINTEK

NREL

NTUA

NTUA_M

POSTECH

PRINCIPIA_M

PRINCIPIA_P

SWE

UOU
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This subset of results represents the type of comparisons performed during the OC4 Phase II 

project. Based on all the simulations run, a set of conclusions was drawn concerning the similarity in 

the predicted response of the semisubmersible based on the modeling approaches used. The 

following is a list of the main findings drawn from the project:

• There is not a clear need for the inclusion of radiation/diffraction loads from a potential-flow 

theory type solution for this type of system; strip-theory approaches are sufficient.

• Approximating the viscous-drag loads for the structure through a global drag matrix may not 

be sufficient as compared to calculating the member-level Morison drag terms.

• Varying levels of mean drift due to wave excitation are seen between the different models, 

based on the inclusion of nonlinear hydrodynamics modeling theory. The modeling 

approaches that create a drift force include wave stretching, applying loads at the 

instantaneous position of the structure, and including second-order terms in the potentialflow solution either directly or through Newman’s approximation. The drift offset is masked 

by wind loads when the turbine is operating.

• Those codes using a Morison-only approach for modeling the hydrodynamic loads need to 

be augmented with calculations of the dynamic pressure on the heave plates of this semi to 

get accurate heave excitation in the system from waves. The need is significant for this 

structure due to its shallow draft.

• Mooring loads differ significantly between codes using a quasi-static model versus those 

using a dynamic model. These loads have not been seen to have a significant impact on 

the system dynamics, but they are important in assessing ultimate and fatigue loads in the 

mooring lines.

• The predicted out-of-plane motion of the blades is slightly smaller for codes using a 

dynamic wake approach rather than the blade element momentum theory for the wind 

inflow model, especially in the higher frequency range.

• RAOs are a good way of concisely examining the response characteristics of a floating 

wind system across a range of wave conditions and comparing the response characteristics 

between codes.

• The sudden loss of a mooring line for a semisubmersible system does not appear to result 

in significant loading to the system during the event.

• The partial flooding of one column was not seen to be very significant in the overall 

response of the system, but the level of flooding examined may have been too minimal.

Conclusions

The comparisons performed in Phase II of OC4 and throughout the OC3 and OC4 projects have 

resulted in a greater understanding of offshore floating wind turbine dynamics and modeling 

techniques, and better knowledge of the validity of various modeling approaches. The results from 

this project will help guide development and improvement efforts for these tools to ensure that they 

are providing accurate information to support the design and analysis needs of the offshore wind 

community.

This project is now planned to be extended for an additional four years, with an expanded focus on 

validating the tools through the comparison to data from actual physical systems. While the OC3

and OC4 tasks have shown the differences between the modeling approaches, ambiguity still 
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remains as to which approach more accurately represents the true physical response; this new 

extension will begin to address this uncertainty.

Acknowledgments

The authors would like to acknowledge the International Energy Agency and its participating 

countries for their support of the OC3 and OC4 projects. We would also like to acknowledge all of 

the participants in the OC3 and OC4 projects for sharing their expertise and tremendous work. 

Without their active participation and engagement, these projects would not have succeeded.

References

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Offshore Wind Technology and Deployment. NREL/TP-500-48191; National Renewable Energy 

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Saetertro K, Okstad K, Nichols J, Nygaard T, Gao Z, Manolas D, Kim K, Yu Q, Shi W, Park H,

Vasques-Rojas A, Dubois J, Kaufer D, Thomassen P, de Ruiter M, Peeringa J, Zhiwen H, von 

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of the 22nd International Ocean and Polar Engineering Conference (ISOPE); June 2012,

Rhodes, Greece; 1:337–346.

3. Jonkman J, Butterfield S, Musial W, Scott G. Definition of a 5-MW Reference Wind Turbine for 

Offshore System Development. NREL/TP-500-38060; National Renewable Energy Laboratory:

Golden, CO, 2009.

4. Goupee AJ, Koo BJ, Lamrakos KF, Kimball R. Offshore Wind Energy: Model Tests for Three 

Floating Wind Turbine Concepts. Proceedings of the Offshore Technical Conference; April 30–

May 3, 2012, Houston, TX.

5. Jonkman J. Definition of the Floating System for Phase IV of OC3. NREL/TP-500-47535;

National Renewable Energy Laboratory: Golden, CO, 2010.