Floating and fixed-bottom structures can benefit from further design optimization

Collage of two offshore wind turbines.

OC5's three phases used test data from a cylinder in a tank, a floating semisubmersible prototype turbine and substructure in a tank (left), and the open-ocean testing of a jacket foundation (right).

(Left) DeepCwind Research Program, NREL 19576; (Right) Gary Norton, NREL 27360

Researchers from the National Renewable Energy Laboratory (NREL) received a four-year extension of International Energy Agency (IEA) research that will advance the overall accuracy of offshore wind computer modeling tools.

The IEA Wind Technology Collaboration Program's Executive Committee awarded the extension of Wind Task 30 research and by doing so created the Offshore Code Comparison Collaboration, Continuation, with Correlation and unCertainty (OC6) project. This new project will dig deeper into differences between computer simulations and experimental measurements, building off of previous Task 30 research OC4 and OC5.

Offshore Code Comparison Projects by the Numbers


OC3 and OC4

•Created under IEA Wind Task 23 and Task 30, respectively
•Ran from 2005 to 2013
•Verified modeling tools by comparing simulation results from several different models
•Focused on verifying the coupled modeling tools through code-to-code comparisons of simulated responses for generic, representative offshore wind systems.



•Created under IEA Wind Task 30
•Ran from 2014 to 2018
•Validated modeling tools by comparing simulations to test data
•Focused on validating the tools by comparing the simulated responses to physical measurements of real systems.


OC3, OC4, and OC5 project objectives

•Assess simulation accuracy and reliability
•Train new analysts on how to run codes correctly
•Investigate capabilities of implemented theories
•Refine applied analysis methods
•Identify further R&D needs.

According to Amy Robertson, NREL project leader, the purpose of OC6 is to refine the accuracy of engineering tools used to design offshore wind turbines by improving their ability to estimate structural loads. The tools used to design offshore wind systems need to consider the coupling between the aerodynamic and hydrodynamic loading on the system, which is vital to optimization and stability.

Designing an offshore wind system involves running tens of thousands of simulations, which requires modeling tools accurate enough to provide realistic load results, but also fast enough to perform design iterations in a reasonable time frame.

"Coupled engineering-level tools (such as OpenFAST) that consider both the wind and wave loading simultaneously on the structure fit that need," Robertson said. "Improving these tools to better predict the actual behavior of offshore wind systems enables further design optimization and, therefore, cost reductions. Unvalidated models increase costs because the technology must be overdesigned to accommodate for large uncertainties in loading and operating conditions predicted by the numerical models."

The offshore code comparison projects are important for designers, certifiers, and research institutes that apply modeling tools for design, research, and instruction. In addition, many researchers have learned how to effectively model and simulate offshore wind turbines using information provided from these projects.

Including participants from the offshore wind industry as well as OC5 members (see Member chart), the OC6 project has the following objectives:

  • Perform more focused validation projects based on the issues identified in previous IEA Wind Task 23 and Task 30 projects (Offshore Code Comparison [OC3], OC4, and OC5)
  • Develop and employ more rigorous validation practices following American Society of Mechanical Engineers guidelines, with a strong emphasis on quantifying uncertainty in test campaigns used for validation
  • Include higher-fidelity modeling solutions in the validation process (when possible), performing a three-way validation among engineering-level tools and measured data.

The OC5 participants have already identified validation projects that are the most relevant to focus on within the extension and organized them into five projects, or "work packages" (see OC6 Validation Projects Identified). The objectives will be investigated through measurement data obtained across multiple test campaigns, including testing of several offshore turbine support structure types, including two floating semisubmersibles, a spar, and a monopile. When possible, multiple phenomena will be investigated using the data sets.

Improved models that better predict the actual behavior of offshore wind systems enable further design optimization and, therefore, cost reductions.

OC6 Validation Projects Identified

Researchers identified the following validation projects as the most relevant for OC6.

Work Package 1

Validation of nonlinear hydrodynamic loading on floating offshore wind support structures originating from the interaction of wave components, structure motion, and flow through a multibody structure.

Work Package 2

Incorporation and verification of advanced soil/structure interaction models to represent pile/foundation interaction.

Work Package 3

Validation of aerodynamic loading on a wind turbine experiencing wave motion caused by a floating support structure.

Work Package 4

Validation of the methodology for combining load models for floating offshore wind support structures.

Work Package 5

Optional fifth-year extension

Validation of the full-scale dynamic behavior of a floating wind turbine.

OC5 Active Membership

China General CertificationChina
Dalian University of TechnologyChina
Goldwind Science & Technology Co., Ltd.China
Envision Energy LimitedChina
Danish Hydraulic InstituteDenmark
Technical University of Denmark, Department of Wind Energy (DTU)Denmark
Electricité de France – Recherché et Développement (EDF)France
IFP Energies Nouvelles (IFPEN)France
National Institute of Applied Science of Rouen – Normandy Laboratory of MechanicsFrance
Fraunhofer Institute for Wind Energy and Energy System Technology IWESGermany
University of Stuttgart, Stuttgart Wind Energy (SWE)Germany
University Rostock, Chair of Wind Energy Technology (UR)Germany
Windrad Engineering GmbHGermany
Siemens Gamesa Renewable EnergyGermany
Politecnico di Milano, Department of Mechanical EngineeringItaly
Nippon Kaiji Kyokai, ClassNKJapan
Wind Energy Institute of Tokyo Inc.Japan
Institute for Energy TechnologyNorway
Norwegian Marine Technology Research InstituteNorway
Norwegian University of Science and Technology – Dept. of Marine TechnologyNorway
Norwegian University of Science and TechnologyNorway
Simis ASNorway
Centre for Marine Technology and Ocean EngineeringPortugal
WavEC – Ocean RenewablesPortugal
University of Ulsan, School of Naval Architecture and Ocean EngineeringRepublic of Korea
National Renewable Energy Centre (CENER)Spain
Universidad de Cantabria, Environmental Hydraulics InstituteSpain
Polytechnic University of Catalonia (UPC)Spain
Siemens Industry SoftwareSpain
Knowledge Centre WMCThe Netherlands
DNV GLUnited Kingdom
National Renewable Energy LaboratoryUnited States

OC6 will feature members of the offshore wind industry, as well as members of OC5.