scholarly journals Bichromatic Wave Selection for Validation of the Difference-Frequency Transfer Function for the OC6 Validation Campaign

2019 ◽  
Author(s):  
Nathan Tom ◽  
Amy Robertson ◽  
Jason Jonkman ◽  
Fabian Wendt ◽  
Manuela Böhm
Keyword(s):  
Nonlinear Wave ◽  
International Energy Agency ◽  
Low Frequency ◽  
Wave Interaction ◽  
Offshore Structures ◽  
Difference Frequency ◽  
Offshore Wind ◽  
Energy Agency ◽  
Validation Data ◽  
Irregular Wave

Abstract The focus of the Offshore Code Comparison Collaboration, Continuation, with Correlation and unCertainity (OC6) project, which operates under the International Energy Agency Wind Task 30, is to refine the accuracy of engineering tools used to design offshore wind turbines. In support of this work, a new validation campaign is being developed that seeks to better understand the nonlinear wave loading that excites floating wind systems at their low-frequency, rigid-body modes in surge and pitch. The validation data will be employed in a three-way validation between simplified engineering tools and higher-fidelity tools, such as computational fluid dynamics (CFD). Irregular wave spectrums, which are traditionally used to examine the nonlinear wave interaction with offshore structures, are too computationally expensive to be simulated in CFD tools, and so we will employ bichromatic wave cases instead. This paper reviews the process used to choose the bichromatic wave pairs to be applied in the campaign to validate the second-order difference-frequency quadratic and potential loads at the surge and pitch natural frequencies of a floating semisubmersible.

Offshore wind energy generation has huge potential

2019 ◽  
Keyword(s):  
Wind Energy ◽  
International Energy Agency ◽  
Offshore Wind ◽  
Offshore Wind Energy ◽  
Chinese Firms ◽  
Energy Agency ◽  
Content Type ◽  
Wind Energy Generation ◽  
Cost Reductions ◽  
Order Books

Subject Offshore wind costs and potential. Significance The International Energy Agency (IEA) released a report on October 25 estimating that offshore wind capacity will rise 15-fold over the next two decades. Costs have been falling ahead of expectations and further cost reductions will help the sector to build more momentum. Impacts North European turbine producers and wind project developers see huge export potential, but Chinese firms will provide stiff competition. Offshore construction vessels will support rising offshore wind deployment and help to bolster currently weak shipyard order books. Governments and regulators will create supportive policies for offshore wind, but this will occur gradually and differently across regions.

scholarly journals Numerical Analysis of a Catenary Mooring System Attached by Clump Masses for Improving the Wave-Resistance Ability of a Spar Buoy-Type Floating Offshore Wind Turbine

2019 ◽  
Vol 9 (6) ◽  
pp. 1075 ◽  
Author(s):  
Zhenqing Liu ◽  
Yuangang Tu ◽  
Wei Wang ◽  
Guowei Qian
Keyword(s):  
Wind Turbine ◽  
International Energy Agency ◽  
Wave Resistance ◽  
Offshore Wind ◽  
Irregular Waves ◽  
Mooring Line ◽  
Offshore Wind Turbine ◽  
Induced Response ◽  
Mooring Lines ◽  
Energy Agency

The International Energy Agency (IEA), under the auspices of their Offshore Code Comparison Collaboration (OC3) initiative, has completed high-level design OC-3 Hywind system. In this system the wind turbine is supported by a spar buoy platform, showing good wave-resistance performance. However, there are still large values in the motion of surge degree of freedom (DOF). Addition of clump masses on the mooring lines is an effective way of reducing the surge motion. However, the optimization of the locations where the clump masses are added is still not clear. In this study, therefore, an in-house developed code is verified by comparing the results of the original OC3 model with those by FAST. The improvement of the performance of this modified platform as a function of the location of the clump masses has been examined under three regular waves and three irregular waves. In the findings of these examination, it was apparent that attaching clump masses with only one-tenth of the mass of the total mooring-line effectively reduces the wave-induced response. Moreover, there is an obvious improvement as the depth of the location where the clump masses mounted is increased.

scholarly journals Development and Verification of an Aero-Hydro-Servo-Elastic Coupled Model of Dynamics for FOWT, Based on the MoWiT Library

Energies ◽  
2020 ◽  
Vol 13 (8) ◽  
pp. 1974
Author(s):  
Mareike Leimeister ◽  
Athanasios Kolios ◽  
Maurizio Collu
Keyword(s):  
Wind Turbine ◽  
International Energy Agency ◽  
Coupled Model ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
System Behavior ◽  
Energy Agency ◽  
Realistic Representation ◽  
Non Linear System ◽  
Fully Coupled

The complexity of floating offshore wind turbine (FOWT) systems, with their coupled motions, aero-hydro-servo-elastic dynamics, as well as non-linear system behavior and components, makes modeling and simulation indispensable. To ensure the correct implementation of the multi-physics, the engineering models and codes have to be verified and, subsequently, validated for proving the realistic representation of the real system behavior. Within the IEA (International Energy Agency) Wind Task 23 Subtask 2 offshore code-to-code comparisons have been performed. Based on these studies, using the OC3 phase IV spar-buoy FOWT system, the Modelica for Wind Turbines (MoWiT) library, developed at Fraunhofer IWES, is verified. MoWiT is capable of fully-coupled aero-hydro-servo-elastic simulations of wind turbine systems, onshore, offshore bottom-fixed, or even offshore floating. The hierarchical programing and multibody approach in the object-oriented and equation-based modeling language Modelica have the advantage (over some other simulation tools) of component-based modeling and, hence, easily modifying the implemented system model. The code-to-code comparisons with the results from the OC3 studies show, apart from expected differences due to required assumptions in consequence of missing data and incomplete information, good agreement and, consequently, substantiate the capability of MoWiT for fully-coupled aero-hydro-servo-elastic simulations of FOWT systems.

scholarly journals Uncertainty Assessment of CFD Investigation of the Nonlinear Difference-Frequency Wave Loads on a Semisubmersible FOWT Platform

2020 ◽  
Vol 13 (1) ◽  
pp. 64
Author(s):  
Lu Wang ◽  
Amy Robertson ◽  
Jason Jonkman ◽  
Yi-Hsiang Yu
Keyword(s):  
Low Frequency ◽  
Uncertainty Assessment ◽  
Difference Frequency ◽  
Offshore Wind ◽  
Wave Loads ◽  
Wave Excitation ◽  
Wave Load ◽  
Frequency Wave ◽  
Floating Offshore Wind Turbines ◽  
Frequency Excitation

Current mid-fidelity modeling approaches for floating offshore wind turbines (FOWTs) have been found to underpredict the nonlinear, low-frequency wave excitation and the response of semisubmersible FOWTs. To examine the cause of this underprediction, the OC6 project is using computational fluid dynamics (CFD) tools to investigate the wave loads on the OC5-DeepCwind semisubmersible, with a focus on the nonlinear difference-frequency excitation. This paper focuses on assessing the uncertainty of the CFD predictions from simulations of the semisubmersible in a fixed condition under bichromatic wave loading and on establishing confidence in the results for use in improving mid-fidelity models. The uncertainty for the nonlinear wave excitation is found to be acceptable but larger than that for the wave-frequency excitation, with the spatial discretization error being the dominant contributor. Further, unwanted free waves at the difference frequency have been identified in the CFD solution. A wave-splitting and wave load-correction procedure are presented to remove the contamination from the free waves in the results. A preliminary comparison to second-order potential-flow theory shows that the CFD model predicted significantly higher difference-frequency wave excitations, especially in surge, suggesting that the CFD results can be used to better calibrate the mid-fidelity tools.

Investigation of Nonlinear Difference-Frequency Wave Excitation on a Semisubmersible Offshore-Wind Platform With Bichromatic-Wave CFD Simulations

2021 ◽  
Author(s):  
Lu Wang ◽  
Amy Robertson ◽  
Jason Jonkman ◽  
Yi-Hsiang Yu ◽  
Arjen Koop ◽  
...  
Keyword(s):  
Potential Flow ◽  
Low Frequency ◽  
Flow Theory ◽  
Second Order ◽  
Difference Frequency ◽  
Offshore Wind ◽  
Viscous Drag ◽  
Cfd Simulations ◽  
Frequency Wave ◽  
Potential Flow Theory

Abstract The natural surge and pitch frequencies of semisubmersible offshore wind platforms are typically designed to be below the wave frequencies to avoid direct excitation. However, surge or pitch resonance can be excited by the nonlinear low-frequency loads generated by irregular incident waves. Second-order potential-flow models with added Morison drag have been found to underpredict this low-frequency excitation and response. As part of the OC6 project1, the authors performed computational fluid dynamics (CFD) simulations to enable a better understanding of the low-frequency loads and the limitations of lower-fidelity models. The focus of this paper is to set up a computationally cost-effective CFD simulation of a fixed semisubmersible platform to investigate nonlinear difference-frequency loads and establish the corresponding uncertainty in the results. Because of the high computing cost, CFD simulations of irregular waves can be challenging. Instead, simulations were performed with bichromatic waves having a shorter repeat period. A preliminary comparison with quadratic transfer functions from second-order potential-flow theory shows that CFD models consistently predict higher nonlinear wave loads at the difference frequency, likely because of flow separation and viscous drag not accounted for in potential-flow theory.

Hydrodynamic Responses of a Spar-Type Floating Wind Turbine in High Waves

2014 ◽  
Vol 31 (1) ◽  
pp. 105-112 ◽  
Author(s):  
Y-J. Lee ◽  
C.-Y. Ho ◽  
Z.-Z. Huang
Keyword(s):  
International Energy Agency ◽  
Computer Code ◽  
Experimental Results ◽  
Offshore Wind ◽  
Scale Model ◽  
Tropical Storms ◽  
Energy Agency ◽  
Deep Waters ◽  
Floating Offshore Wind Turbines ◽  
The Time Domain

AbstractFloating offshore wind turbines (FOWTs) can be used to exploit the enormous wind energy present over deep waters. Numerous studies have examined the dynamics of FOWTs, but few have focused on validating numerical results with experimental results, particularly for a deep draught FOWT in regions with frequent tropical storms. For this study, we developed a computer code and conducted experiments with a scale model to validate the simulation results. The computer code was first verified by comparing the results with those of the International Energy Agency Wind Task 23. Numerical simulations were implemented in both the frequency domain and the time domain. A comparison of the numerical and experimental results of the scale model in high waves showed good agreement. The flexibility of blades and the tower did not observably affect the motion of the deep draft spar-type FOWT. Therefore, it can be ignored in the preliminary design. The pitch motion of the scale model was within 1°. Therefore, the spar-type FOWT may be an effective power source for regions with frequent tropical storms.

scholarly journals Response of the IEA Wind 15 MW – WindCrete and Activefloat floating wind turbines to wind and second-order waves

2021 ◽  
Author(s):  
Mohammad Youssef Mahfouz ◽  
Climent Molins ◽  
Pau Trubat ◽  
Sergio Hernández ◽  
Fernando Vigara ◽  
...  
Keyword(s):  
Wind Turbines ◽  
Reference Model ◽  
Numerical Models ◽  
International Energy Agency ◽  
Second Order ◽  
Offshore Wind ◽  
Wave Forces ◽  
Energy Agency ◽  
Floating Offshore Wind Turbines ◽  
Floating Platforms

Abstract. The EU Horizon 2020 project COREWIND has developed two floating platforms for the new International Energy Agency (IEA) Wind 15 MW reference model. One design – WindCrete – is a spar floater, and the other – Activefloat – is a semi-submersible floater. In this work the design of the floaters is introduced with their aero-hydro-servo-elastic numerical models, and the responses of both floaters in both static and dynamic simulations are verified against the operational and survival design limits. The static displacements and natural frequencies are simulated and discussed. Additionally, the effects of the mean wave drift forces, and difference second order wave forces on the systems' responses are presented. The increase in the turbine's power capacity to 15MW in IEA Wind model, leads to an increase in inertial forces and aerodynamic thrust force when compared to similar floating platforms coupled to the Danish Technical University (DTU) 10MW reference model. The goal of this work is to investigate the floaters responses at different load cases. The results in this paper suggest that at mild wave loads the motion responses of the 15MW Floating Offshore Wind Turbines (FOWT) are dominated by low frequency forces. Therefore, motions are dominated by the wind forces, and second order wave forces rather than the first order wave forces. After verifying and understanding the models' responses, the two 15MW FOWT reference numerical models are publicly available to be used in the research and development of floating wind energy.

scholarly journals OC6 Phase Ia: CFD Simulations of the Free-Decay Motion of the DeepCwind Semisubmersible

Energies ◽  
2022 ◽  
Vol 15 (1) ◽  
pp. 389
Author(s):  
Lu Wang ◽  
Amy Robertson ◽  
Jason Jonkman ◽  
Jang Kim ◽  
Zhi-Rong Shen ◽  
...  
Keyword(s):  
Damping Ratio ◽  
International Energy Agency ◽  
Offshore Wind ◽  
Cfd Simulations ◽  
Energy Agency ◽  
Equivalent Damping ◽  
Damping Coefficients ◽  
Hydrodynamic Damping ◽  
Free Decay ◽  
Quadratic Damping

Currently, the design of floating offshore wind systems is primarily based on mid-fidelity models with empirical drag forces. The tuning of the model coefficients requires data from either experiments or high-fidelity simulations. As part of the OC6 (Offshore Code Comparison Collaboration, Continued, with Correlation, and unCertainty (OC6) is a project under the International Energy Agency Wind Task 30 framework) project, the present investigation explores the latter option. A verification and validation study of computational fluid dynamics (CFD) models of the DeepCwind semisubmersible undergoing free-decay motion is performed. Several institutions provided CFD results for validation against the OC6 experimental campaign. The objective is to evaluate whether the CFD setups of the participants can provide valid estimates of the hydrodynamic damping coefficients needed by mid-fidelity models. The linear and quadratic damping coefficients and the equivalent damping ratio are chosen as metrics for validation. Large numerical uncertainties are estimated for the linear and quadratic damping coefficients; however, the equivalent damping ratios are more consistently predicted with lower uncertainty. Some difference is observed between the experimental and CFD surge-decay motion, which is caused by mechanical damping not considered in the simulations that likely originated from the mooring setup, including a Coulomb-friction-type force. Overall, the simulations and the experiment show reasonable agreement, thus demonstrating the feasibility of using CFD simulations to tune mid-fidelity models.

scholarly journals Offshore Code Comparison Collaboration Continuation Within IEA Wind Task 30: Phase II Results Regarding a Floating Semisubmersible Wind System

2014 ◽  
Author(s):  
Amy Robertson ◽  
Jason Jonkman ◽  
Fabian Vorpahl ◽  
Wojciech Popko ◽  
Jacob Qvist ◽  
...  
Keyword(s):  
Wind Turbine ◽  
International Energy Agency ◽  
Lessons Learned ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Energy Agency ◽  
Code Comparison ◽  
Mooring Dynamics ◽  
Offshore Floating Wind Turbine ◽  
Incident Waves

Offshore wind turbines are designed and analyzed using comprehensive simulation tools (or codes) 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, mooring dynamics, and foundation dynamics of the support structure. This paper describes the latest findings of the code-to-code verification activities of the Offshore Code Comparison Collaboration Continuation project, which operates under the International Energy Agency Wind Task 30. In the latest phase of the project, participants used an assortment of simulation codes to model the coupled dynamic response of a 5-MW wind turbine installed on a floating semisubmersible in 200 m of water. Code predictions were compared from load case simulations selected to test different model features. The comparisons have resulted in a greater understanding of offshore floating wind turbine dynamics and modeling techniques, and better knowledge of the validity of various approximations. The lessons learned from this exercise have improved the participants’ codes, thus improving the standard of offshore wind turbine modeling.

scholarly journals Multipoint high-fidelity CFD-based aerodynamic shape optimization of a 10 MW wind turbine

2019 ◽  
Vol 4 (2) ◽  
pp. 163-192 ◽  
Author(s):  
Mads H. Aa. Madsen ◽  
Frederik Zahle ◽  
Niels N. Sørensen ◽  
Joaquim R. R. A. Martins
Keyword(s):  
Shape Optimization ◽  
Wind Turbine ◽  
International Energy Agency ◽  
Operating Conditions ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
High Fidelity ◽  
Energy Agency ◽  
Cross Sectional

Abstract. The wind energy industry relies heavily on computational fluid dynamics (CFD) to analyze new turbine designs. To utilize CFD earlier in the design process, where lower-fidelity methods such as blade element momentum (BEM) are more common, requires the development of new tools. Tools that utilize numerical optimization are particularly valuable because they reduce the reliance on design by trial and error. We present the first comprehensive 3-D CFD adjoint-based shape optimization of a modern 10 MW offshore wind turbine. The optimization problem is aligned with a case study from International Energy Agency (IEA) Wind Task 37, making it possible to compare our findings with the BEM results from this case study and therefore allowing us to determine the value of design optimization based on high-fidelity models. The comparison shows that the overall design trends suggested by the two models do agree, and that it is particularly valuable to consult the high-fidelity model in areas such as root and tip where BEM is inaccurate. In addition, we compare two different CFD solvers to quantify the effect of modeling compressibility and to estimate the accuracy of the chosen grid resolution and order of convergence of the solver. Meshes up to 14×106 cells are used in the optimization whereby flow details are resolved. The present work shows that it is now possible to successfully optimize modern wind turbines aerodynamically under normal operating conditions using Reynolds-averaged Navier–Stokes (RANS) models. The key benefit of a 3-D RANS approach is that it is possible to optimize the blade planform and cross-sectional shape simultaneously, thus tailoring the shape to the actual 3-D flow over the rotor. This work does not address evaluation of extreme loads used for structural sizing, where BEM-based methods have proven very accurate, and therefore will likely remain the method of choice.

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