Showing papers by "Jason Jonkman published in 2013"

Validation of a FAST semi-submersible floating wind turbine numerical model with DeepCwind test data

Abstract: There are global efforts in the offshore wind community to develop reliable floating wind turbine technologies that are capable of exploiting the abundant deepwater wind resource. These efforts require validated numerical simulation tools to predict the coupled aero-hydro-servo-elastic behavior of such systems. To date, little has been done in the public domain to validate floating wind turbine simulation tools. This work begins to address this problem by presenting the validation of a model constructed in the National Renewable Energy Laboratory (NREL) floating wind turbine simulator FAST with 1/50th-scale model test data for a semi-submersible floating wind turbine system. The test was conducted by the University of Maine DeepCwind program at Maritime Research Institute Netherlands' offshore wind/wave basin, located in the Netherlands. The floating wind turbine used in the tests was a 1/50th-scale model of the NREL 5-MW horizontal-axis reference wind turbine with a 126 m rotor diameter. This turbine was mounted to the DeepCwind semi-submersible floating platform. This paper first outlines the details of the floating system studied, including the wind turbine, tower, platform, and mooring components. Subsequently, the calibration procedures used for tuning the FAST floating wind turbine model are discussed. Following this calibration, comparisons of FAST predictions and test data are presented that focus on system global and structural response resulting from aerodynamic and hydrodynamic loads. The results indicate that FAST captures many of the pertinent physics in the coupled floating wind turbine dynamics problem. In addition, the results highlight potential areas of improvement for both FAST and experimentation procedures to ensure accurate numerical modeling of floating wind turbine systems.

New Modularization Framework for the FAST Wind Turbine CAE Tool: Preprint

Abstract: NREL has recently put considerable effort into improving the overall modularity of its FAST wind turbine aero-hydro-servo-elastic tool to (1) improve the ability to read, implement, and maintain source code; (2) increase module sharing and shared code development across the wind community; (3) improve numerical performance and robustness; and (4) greatly enhance flexibility and expandability to enable further developments of functionality without the need to recode established modules. The new FAST modularization framework supports module-independent inputs, outputs, states, and parameters; states in continuous-time, discrete-time, and in constraint form; loose and tight coupling; independent time and spatial discretizations; time marching, operating-point determination, and linearization; data encapsulation; dynamic allocation; and save/retrieve capability. This paper explains the features of the new FAST modularization framework, as well as the concepts and mathematical background needed to understand and apply it correctly. It is envisioned that the new modularization framework will transform FAST into a powerful, robust, and flexible wind turbine modeling tool with a large number of developers and a range of modeling fidelities across the aerodynamic, hydrodynamic, servo-dynamic, and structural-dynamic components.

Summary of Conclusions and Recommendations Drawn From the DeepCwind Scaled Floating Offshore Wind System Test Campaign

Abstract: The DeepCwind consortium is a group of universities, national labs, and companies funded under a research initiative by the U.S. Department of Energy (DOE) to support the research and development of floating offshore wind power. The two main objectives of the project are to better understand the complex dynamic behavior of floating offshore wind systems and to create experimental data for use in validating the tools used in modeling these systems. In support of these objectives, the DeepCwind consortium conducted a model test campaign in 2011 of three generic floating wind systems: a tension-leg platform (TLP), a spar-buoy (spar), and a semi-submersible (semi). Each of the three platforms was designed to support a 1/50th-scale model of a 5-MW wind turbine and was tested under a variety of wind/wave conditions.The focus of this paper is to summarize the work done by consortium members in analyzing the data obtained from the test campaign and its use for validating the offshore wind modeling tool, FAST.© 2013 ASME

Importance of Second-Order Difference-Frequency Wave-Diffraction Forces in the Validation of a FAST Semi-Submersible Floating Wind Turbine Model

Abstract: To better access the abundant offshore wind resource, efforts are being made across the world to develop and improve floating offshore wind turbine technologies. A critical aspect of creating reliable, mature floating wind turbine technology is the development, verification, and validation of efficient computer-aided-engineering (CAE) tools. The National Renewable Energy Laboratory (NREL) has created FAST, a comprehensive, coupled analysis CAE tool for floating wind turbines, which has been verified and utilized in numerous floating wind turbine studies. Several efforts are underway to validate the floating platform functionality of FAST to complement its already validated aerodynamic and structural simulation capabilities. The research employs the 1/50th-scale DeepCwind wind/wave basin model test dataset, which was obtained at the Maritime Research Institute Netherlands (MARIN) in 2011. This paper describes further work being undertaken to continue this validation. These efforts focus on FAST’s ability to replicate global response behaviors associated with dynamic wind forces and second-order difference-frequency wave-diffraction forces separately and simultaneously.The first step is the construction of a FAST numerical model of the DeepCwind semi-submersible floating wind turbine that includes alterations for the addition of second-order difference-frequency wave-diffraction forces. The implementation of these second-order wave forces, which are not currently standard in FAST, are outlined and discussed. After construction of the FAST model, the calibration of the FAST model’s wind turbine aerodynamics, tower-bending dynamics, and platform hydrodynamic damping using select test data is discussed. Subsequently, select cases with coupled dynamic wind and irregular wave loading are simulated in FAST, and these results are compared to test data. Particular attention is paid to global motion and load responses associated with the interaction of the wind and wave environmental loads. These loads are most prevalent in the vicinity of the rigid-body motion natural frequencies for the DeepCwind semi-submersible, with dynamic wind forces and the second-order difference-frequency wave-diffraction forces driving the global system response at these low frequencies. Studies are also performed to investigate the impact of neglecting the second-order wave forces on the predictive capabilities of the FAST model. The comparisons of the simulation and test results highlight the ability of FAST to accurately capture many of the important coupled global response behaviors of the DeepCwind semi-submersible floating wind turbine.Copyright © 2013 by ASME

Modeling and Control to Mitigate Resonant Load in Variable-Speed Wind Turbine Drivetrain

Abstract: Failure of the drivetrain components is currently listed among the most problematic failures during the operational lifetime of a wind turbine. Guaranteeing robust and reliable drivetrain designs is important to minimize the wind turbine downtime as well as to meet demand in both power quantity and quality. While aeroelastic codes are often used in the design of wind turbine controllers, the drivetrain model in such codes is limited to a few (mostly two) degrees of freedom, resulting in a restricted detail in describing its dynamic behavior and assessing the effectiveness of controllers on attenuating the drivetrain load. In the previous work, the capability of the well-known FAST aeroelastic tool for wind turbine has been enhanced through integration of a dynamic model of a drivetrain. The drivetrain model, built using the Simscape in the MATLAB/Simulink environment, is applied in this paper. The model is used to develop a power-electronics-based controller to prevent excessive drivetrain load. The controller temporarily shifts the closed-loop eigenfrequency of the drivetrain through the addition of virtual inertia, thus avoiding the resonance. Simulation results demonstrating the fidelity of the expanded drivetrain model as well as the effectiveness of the virtual inertia controller are presented.

Assessment of the Importance of Mooring Dynamics on the Global Response of the DeepCwind Floating Semisubmersible Offshore Wind Turbine

Abstract: This paper studies the influence of mooring line dynamics on the response of a coupled floating offshore wind turbine against an equivalent uncoupled model. The semisubmersible modeled in this paper is based on a design developed by the DeepCwind program and uses the National Renewable Energy Laboratory’s (NREL’s) 5megawatt (MW) baseline wind turbine to represent the tower, nacelle, and blade properties. The uncoupled model was formed using FAST, an open-source program that models the wind turbine aerodynamics, control, motion, tower/blade flexure, and wave forces, but with the mooring line forces treated using a quasi-static approximation. In contrast, the coupled model was enabled by pairing FAST with OrcaFlex. OrcaFlex replaces FAST’s wave force and quasi-static cable model with an equivalent subsea fluid-structure representation and a lumped-mass cable system to capture the mooring line dynamics. This analysis revealed that an uncoupled model using the quasi-static mooring approximation can underestimate peak mooring line loads versus a coupled model using a dynamic mooring line.

Offshore Code Comparison Collaboration, Continuation: Phase II Results of a Floating Semisubmersible Wind System: Preprint

Abstract: 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.

Implementation of a Multisegmented, Quasi-Static Cable Model

Abstract: The Mooring Analysis Program (MAP) is a library designed to be used in parallel with other computer-aided engineering (CAE) tools to model the static and dynamic forces of mooring systems. In this paper, the implementation of a multisegmented, quasi-static (MSQS) mooring model in MAP is investigated. The MSQS model was developed based on an extension of conventional single-line static solutions. Conceptually, the MSQS program simultaneously solves the algebraic equations for all elements with the condition that the total force at connection points sum to zero. Seabed contact, seabed friction, and externally applied forces can be modeled with this tool, and it allows multielement mooring systems with arbitrary connection configurations to be analyzed. This paper provides an introduction to MAP’s MSQS model, its underlying theory, and a demonstration of its abilities.

Simulation-Length Requirements in the Loads Analysis of Offshore Floating Wind Turbines

Abstract: The design standard typically used for offshore wind system development, the International Electrotechnical Commission (IEC) 61400-3 fixed-bottom offshore design standard, explicitly states that “the design requirements specified in this standard are not necessarily sufficient to ensure the engineering integrity of floating offshore wind turbines” [1]. One major concern is the prescribed simulation length time of 10 minutes for a loads-analysis procedure, which is also typically used for land-based turbines. Because floating platforms have lower natural frequencies, which lead to fewer load cycles over a given period of time, and ocean waves have lower characteristic frequencies than wind turbulence, the 10-min simulation length recommended by the current standards for land-based and offshore turbines may be too short for combined wind and wave loading of floating offshore wind turbines (FOWTs). Therefore, the goal of this paper is to examine the appropriate length of a FOWT simulation — a fundamental question that needs to be answered to develop design requirements.To examine this issue, we performed a loads analysis of an example FOWT with varying simulation lengths, using FAST, the National Renewable Energy Laboratory’s (NREL’s) nonlinear aero-hydro-servo-elastic simulation tool. The offshore wind system used was the OC3-Hywind spar buoy, which was developed for use in the International Energy Agency (IEA) Offshore Code Comparison Collaborative (OC3) project, and supports NREL’s offshore 5-MW baseline turbine. Realistic metocean data from the National Oceanic and Atmospheric Administration (NOAA) and repeated periodic wind files were used to excite the structure. The results of the analysis clearly show that loads do not increase for longer simulations. In regard to fatigue, a sensitivity analysis shows that the procedure used for counting half cycles is more important than the simulation length itself. Based on these results, neither the simulation length nor the periodic wind files affect response statistics and loads for FOWTs (at least for the spar studied here); a result in contrast to the offshore oil and gas (O&G) industry, where running simulations of at least 3 hours in length is common practice.Copyright © 2013 by ASME

State-Space Realization of the Wave-Radiation Force within FAST

Abstract: Several methods have been proposed in the literature to find a state-space model for the wave-radiation forces. In this paper, we compared four methods, two in the frequency domain and two in the time domain. The frequency-response function and the impulse response of the resulting state-space models were compared against those derived from the numerical code WAMIT.A new state-space module was implemented within FAST, an offshore wind turbine computer-aided engineering tool, and we compared the results against the previously implemented numerical convolution method. The results agreed between the two methods, with a significant reduction in required computational time when using the new state-space module.Copyright © 2013 by ASME

A New Structural-Dynamics Module for Offshore Multimember Substructures Within the Wind Turbine Computer-Aided Engineering Tool FAST

Abstract: FAST, developed by the National Renewable Energy Laboratory (NREL), is a computer-aided engineering (CAE) tool for aero-hydro-servo-elastic analysis of land-based and offshore wind turbines. This paper discusses recent upgrades made to FAST to enable loads simulations of offshore wind turbines with fixed-bottom, multimember support structures (e.g., jackets and tripods, which are commonly used in transitional-depth waters). The main theory and strategies for the implementation of the multimember substructure dynamics module (SubDyn) within the new FAST modularization framework are introduced. SubDyn relies on two main engineering schematizations: 1) a linear frame finite-element beam (LFEB) model and 2) a dynamics system reduction via Craig-Bampton's method. A jacket support structure and an offshore system consisting of a turbine atop a jacket substructure were simulated to test the SubDyn module and to preliminarily assess results against results from a commercial finite-element code.

Assessing the Importance of Nonlinearities in the Development of a Substructure Model for the Wind Turbine CAE Tool FAST

Abstract: Design and analysis of wind turbines are performed using aero-servo-elastic tools that account for the nonlinear coupling between aerodynamics, controls, and structural response. The NREL-developed computer-aided engineering (CAE) tool FAST also resolves the hydrodynamics of fixed-bottom structures and floating platforms for offshore wind applications. This paper outlines the implementation of a structural-dynamics module (SubDyn) for offshore wind turbines with space-frame substructures into the current FAST framework, and focuses on the initial assessment of the importance of structural nonlinearities. Nonlinear effects include: large displacements, axial shortening due to bending, cross-sectional transverse shear effects, etc.

Investigation of Response Amplitude Operators for Floating Offshore Wind Turbines

Abstract: This paper examines the consistency between response amplitude operators (RAOs) computed from WAMIT, a linear frequency-domain tool, to RAOs derived from time-domain computations based on white-noise wave excitation using FAST, a nonlinear aero-hydro-servo-elastic tool. The RAO comparison is first made for a rigid floating wind turbine without wind excitation. The investigation is further extended to examine how these RAOs change for a flexible and operational wind turbine. The RAOs are computed for below-rated, rated, and above-rated wind conditions. The method is applied to a floating wind system composed of the OC3-Hywind spar buoy and NREL 5-MW wind turbine. The responses are compared between FAST and WAMIT to verify the FAST model and to understand the influence of structural flexibility, aerodynamic damping, control actions, and waves on the system responses. The results show that based on the RAO computation procedure implemented, the WAMIT- and FAST-computed RAOs are similar (as expected) for a rigid turbine subjected to waves only. However, WAMIT is unable to model the excitation from a flexible turbine. Further, the presence of aerodynamic damping decreased the platform surge and pitch responses, as computed by both WAMIT and FAST when wind was included. Additionally, the influence of gyroscopic excitation increased more » the yaw response, which was captured by both WAMIT and FAST. « less

Numerical Stability and Accuracy of Temporally Coupled Multi-Physics Modules in Wind-Turbine CAE Tools

Abstract: In this paper we examine the stability and accuracy of numerical algorithms for coupling time-dependent multi-physics modules relevant to computer-aided engineering (CAE) of wind turbines. This work is motivated by an in-progress major revision of FAST, the National Renewable Energy Laboratory's (NREL's) premier aero-elastic CAE simulation tool. We employ two simple examples as test systems, while algorithm descriptions are kept general. Coupled-system governing equations are framed in monolithic and partitioned representations as differential-algebraic equations. Explicit and implicit loose partition coupling is examined. In explicit coupling, partitions are advanced in time from known information. In implicit coupling, there is dependence on other-partition data at the next time step; coupling is accomplished through a predictor-corrector (PC) approach. Numerical time integration of coupled ordinary-differential equations (ODEs) is accomplished with one of three, fourth-order fixed-time-increment methods: Runge-Kutta (RK), Adams-Bashforth (AB), and Adams-Bashforth-Moulton (ABM). Through numerical experiments it is shown that explicit coupling can be dramatically less stable and less accurate than simulations performed with the monolithic system. However, PC implicit coupling restored stability and fourth-order accuracy for ABM; only second-order accuracy was achieved with RK integration. For systems without constraints, explicit time integration with AB and explicit loose coupling exhibited desired accuracy and stability.

Numerical Prediction of Experimentally Observed Behavior of a Scale-Model of an Offshore Wind Turbine Supported by a Tension-Leg Platform

Abstract: Realizing the critical importance the role physical experimental tests play in understanding the dynamics of floating offshore wind turbines, the DeepCwind consortium conducted a one-fiftieth-scale model test program where several floating wind platforms were subjected to a variety of wind and wave loading condition at the Maritime Research Institute Netherlands wave basin. This paper describes the observed behavior of a tension-leg platform, one of three platforms tested, and the systematic effort to predict the measured response with the FAST simulation tool using a model primarily based on consensus geometric and mass properties of the test specimen.

Simulation of Thunderstorm Downbursts and Associated Wind Turbine Loads

Abstract: This study is focused on simulation of thunderstorm downbursts and associated wind turbine loads. We first present a thunderstorm downburst model, in which the wind field is assumed to result from the summation of an analytical mean field and stochastic turbulence. The structure and evolution of the downburst wind field based on the analytical model are discussed. Loads are generated using stochastic simulation of the aeroelastic response for a model of a utility-scale 5-MW turbine. With the help of a few assumptions, particularly regarding control strategies, we address the chief influences of wind velocity fields associated with downbursts—namely, large wind speeds and large, rapid wind direction changes—by considering different storm scenarios and studying associated turbine loads. These scenarios include, first, an illustrative case to understand details related to the turbine response simulation; this is followed by a study involving a different storm touchdown location relative to the turbine as well as a critical case where a shutdown sequence is included. Results show that the availability of and assumptions in wind turbine control systems during a downburst clearly influence overall system response. Control system choices can significantly mitigate turbine loads during downbursts. Results also show that different storm touchdown locations result in distinct characteristics in inflow wind fields and, hence, in contrasting turbine response.

Building and calibration of a fast model of the SWAY prototype floating wind turbine

Abstract: Present efforts to verify and validate aero-hydro-servo-elastic numerical simulation tools that predict the dynamic response of a floating offshore wind turbine are primarily limited to code-to-code comparisons or code-to-data comparisons using data from wind-wave basin tests. In partnership with SWAY AS, the National Renewable Energy Laboratory (NREL) installed scientific wind, wave, and motion measurement equipment on the 1/6.5th-scale prototype SWAY floating wind system to collect data to validate a FAST model of the SWAY design in an open-water condition. Nanyang Technological University (NTU), through a collaboration with NREL, assisted in this validation. This paper shows the use of the results of the SWAY open-water tests to calibrate the numerical FAST model, which will be used for future validation efforts. First, the modeling strategies and development of the FAST model for the SWAY prototype wind turbine are presented, including justification of the modeling assumptions. Next, the model calibration-based on a subset of the free-decay test data-is shown. This process involved tuning properties of the FAST model where uncertainties existed to better match the response of the prototype wind turbine. Finally, limitations of the FAST model and potential areas of improvement of the project are discussed.

Assessing Fatigue and Ultimate Load Uncertainty in Floating Offshore Wind Turbines Due to Varying Simulation Length

Abstract: With the push towards siting wind turbines farther offshore due to higher wind quality and less visibility, floating offshore wind turbines, which can be located in deep water, are becoming an economically attractive option. The International Electrotechnical Commission's (IEC) 61400-3 design standard covers fixed-bottom offshore wind turbines, but there are a number of new research questions that need to be answered to modify these standards so that they are applicable to floating wind turbines. One issue is the appropriate simulation length needed for floating turbines. This paper will discuss the results from a study assessing the impact of simulation length on the ultimate and fatigue loads of the structure, and will address uncertainties associated with changing the simulation length for the analyzed floating platform. Recommendations of required simulation length based on load uncertainty will be made and compared to current simulation length requirements.

Hybrid Electro-mechanical Simulation Tool for Wind Turbine Generators

Abstract: Wind turbine generators (WTGs) consist of many different components to convert kinetic energy of the wind into electrical energy for end users. Wind energy is accessed to provide mechanical torque for driving the shaft of the electrical generator. The conversion from wind power to mechanical power is governed by the aerodynamic conversion. The aerodynamic-electrical-conversion efficiency of a WTG is influenced by the efficiency of the blades, the gearbox, the generator, and the power converter. This paper describes the use of MATLAB/Simulink to simulate the electrical and grid-related aspects of a WTG coupled with the FAST aero-elastic wind turbine computer-aided engineering tool to simulate the aerodynamic and mechanical aspects of a WTG. The combination of the two enables studies involving both electrical and mechanical aspects of a WTG. For example, mechanical engineers can formulate generator control that may preserve the life of the gearbox or mitigate the impact of transient events occurring on the transmission lines (faults, voltage and frequency dips, unbalanced voltages, etc.). Similarly, electrical engineers can study the impact of high-ramping wind speeds on power systems, as well as the impact of turbulence on the voltage and frequency of a small balancing area.

Assessing the importance of nonlinear structural characteristics in the development of a jacket model for the wind turbine cae tool fast

Abstract: Design and analysis of wind turbines are performed using aero-servo-elastic tools that account for the nonlinear coupling between aerodynamics, controls, and structural response. The NREL-developed computer-aided engineering (CAE) tool FAST also resolves the hydrodynamics of fixed-bottom structures and floating platforms for offshore wind applications. Primarily due to the required modal characteristics, monopiles become progressively less economical and more difficult (or impossible) to fabricate for multi-megawatt turbines and water depths of more than 25-30 m. Derived from the oil and gas industry experience, light and stiff space-frame alternatives have been proposed to alleviate this problem. Lattice structures (e.g., jackets) are more complex to analyze and design than cantilevered monopiles, especially in terms of structural dynamics of the coupled turbine-support structure system. This paper outlines the implementation of a structuraldynamics module (SubDyn) for offshore wind turbines with space-frame substructures into the current FAST framework, and in particular focuses on the initial assessment of the importance of structural nonlinearities. Nonlinear effects include: large displacements, axial shortening due to bending, cross-sectional transverse shear effects, etc. A nonlinear computational analysis is resource-intensive, thus it is important to assess the applicability of a linear approach to maintain high-fidelity results while still allowing for fast and efficient design simulations. Space-frame structural behavior can be controlled by a number of factors (e.g., member crosssectional properties, number of legs, batter angles). Additionally, nonlinearities may manifest only at certain load levels. Several finite-element analyses were carried out via commercial and open-source codes that can capture nonlinear effects in the structural behavior of turbine substructures under different load cases. Results were compared to the output of the new linear module SubDyn. The configurations considered in this study included 5-MW, 7-MW, and 10-MW platforms: OC3 1

State-Space Realization of the Wave-Radiation Force within FAST: Preprint

Abstract: Several methods have been proposed in the literature to find a state-space model for the wave-radiation forces. In this paper, four methods were compared, two in the frequency domain and two in the time domain. The frequency-response function and the impulse response of the resulting state-space models were compared against the ones derived by the numerical code WAMIT. The implementation of the state-space module within the FAST offshore wind turbine computer-aided engineering (CAE) tool was verified, comparing the results against the previously implemented numerical convolution method. The results agreed between the two methods, with a significant reduction in required computational time when using the state-space module.

Simulation tool to assess mechanical and electrical stresses on wind turbine generators

Abstract: Wind turbine generators (WTGs) consist of many different components to convert kinetic energy of the wind into electrical energy for end users. Wind energy is accessed to provide mechanical torque for driving the shaft of the electrical generator. The conversion from wind power to mechanical power is governed by the aerodynamic conversion. The aerodynamic-electrical-conversion efficiency of a WTG is influenced by the efficiency of the blades, the gearbox, the generator, and the power converter. This paper describes the use of MATLAB/Simulink to simulate the electrical and grid-related aspects of a WTG coupled with the FAST aeroelastic wind turbine computer-aided engineering tool to simulate the aerodynamic and mechanical aspects of a WTG. The combination of the two enables studies involving both electrical and mechanical aspects of a WTG. For example, mechanical engineers can formulate generator control that may preserve the life of the gearbox or mitigate the impact of transient events occurring on the transmission lines (faults, voltage and frequency dips, unbalanced voltages, etc.). Similarly, electrical engineers can study the impact of high-ramping wind speeds on power systems, as well as the impact of turbulence on the voltage and frequency of a small balancing authority area. This digest includes some examples of the capabilities of the FAST and MATLAB coupling, namely the effects of electrical faults on the blade moments.

Investigation of Response Amplitude Operators for Floating Offshore Wind Turbines: Preprint

Abstract: This paper examines the consistency between response amplitude operators (RAOs) computed from WAMIT, a linear frequency-domain tool, to RAOs derived from time-domain computations based on white-noise wave excitation using FAST, a nonlinear aero-hydro-servo-elastic tool. The RAO comparison is first made for a rigid floating wind turbine without wind excitation. The investigation is further extended to examine how these RAOs change for a flexible and operational wind turbine. The RAOs are computed for below-rated, rated, and above-rated wind conditions. The method is applied to a floating wind system composed of the OC3-Hywind spar buoy and NREL 5-MW wind turbine. The responses are compared between FAST and WAMIT to verify the FAST model and to understand the influence of structural flexibility, aerodynamic damping, control actions, and waves on the system responses. The results show that based on the RAO computation procedure implemented, the WAMIT- and FAST-computed RAOs are similar (as expected) for a rigid turbine subjected to waves only. However, WAMIT is unable to model the excitation from a flexible turbine. Further, the presence of aerodynamic damping decreased the platform surge and pitch responses, as computed by both WAMIT and FAST when wind was included. Additionally, the influence of gyroscopic excitation increased the yaw response, which was captured by both WAMIT and FAST.

Effect of Second-Order Hydrodynamics on Floating Offshore Wind Turbines: Preprint

Abstract: Offshore winds are generally stronger and more consistent than winds on land, making the offshore environment attractive for wind energy development. A large part of the offshore wind resource is however located in deep water, where floating turbines are the only economical way of harvesting the energy. The design of offshore floating wind turbines relies on the use of modeling tools that can simulate the entire coupled system behavior. At present, most of these tools include only first-order hydrodynamic theory. However, observations of supposed second-order hydrodynamic responses in wave-tank tests performed by the DeepCwind consortium suggest that second-order effects might be critical. In this paper, the methodology used by the oil and gas industry has been modified to apply to the analysis of floating wind turbines, and is used to assess the effect of second-order hydrodynamics on floating offshore wind turbines. The method relies on combined use of the frequency-domain tool WAMIT and the time-domain tool FAST. The proposed assessment method has been applied to two different floating wind concepts, a spar and a tension-leg-platform (TLP), both supporting the NREL 5-MW baseline wind turbine. Results showing the hydrodynamic forces and motion response for these systems are presented and analysed, andmore » compared to aerodynamic effects.« less

Numerical Prediction of Experimentally Observed Behavior of a Scale Model of an Offshore Wind Turbine Supported by a Tension-Leg Platform: Preprint

Abstract: Realizing the critical importance the role physical experimental tests play in understanding the dynamics of floating offshore wind turbines, the DeepCwind consortium conducted a one-fiftieth-scale model test program where several floating wind platforms were subjected to a variety of wind and wave loading condition at the Maritime Research Institute Netherlands wave basin. This paper describes the observed behavior of a tension-leg platform, one of three platforms tested, and the systematic effort to predict the measured response with the FAST simulation tool using a model primarily based on consensus geometric and mass properties of the test specimen.

Hybrid Electro-Mechanical Simulation Tool for Wind Turbine Generators: Preprint

Abstract: This paper describes the use of MATLAB/Simulink to simulate the electrical and grid-related aspects of a WTG and the FAST aero-elastic wind turbine code to simulate the aerodynamic and mechanical aspects of the WTG. The combination of the two enables studies involving both electrical and mechanical aspects of the WTG.