Joshua A. Paquette

Structural Health Monitoring of Wind Turbine Blades

As electric utility wind turbines increase in size, and correspondingly, increase in initial capital investment cost, there is an increasing need to monitor the health of the structure. Acquiring an early indication of structural or mechanical problems allows operators to better plan for maintenance, possibly operate the machine in a de-rated condition rather than taking the unit off-line, or in the case of an emergency, shut the machine down to avoid further damage. This paper describes several promising structural health monitoring (SHM) techniques that were recently exercised during a fatigue test of a 9 meter glass-epoxy and carbon-epoxy wind turbine blade. The SHM systems were implemented by teams from NASA Kennedy Space Center, Purdue University and Virginia Tech. A commercial off-the-shelf acoustic emission (AE) NDT system gathered blade AE data throughout the test. At a fatigue load cycle rate around 1.2 Hertz, and after more than 4,000,000 fatigue cycles, the blade was diagnostically and visibly failing at the out-board blade spar-cap termination point at 4.5 meters. For safety reasons, the test was stopped just before the blade completely failed. This paper provides an overview of the SHM and NDT system setups and some current test results.

Modal Testing for Validation of Blade Models

The focus of this paper is a test program designed for wind turbine blades. Model validation is a comprehensive undertaking which requires carefully designing and executing experiments, proposing appropriate physics-based models, and applying correlation techniques to improve these models based on the test data. Structural models are useful for making decisions when designing a new blade or assessing blade performance, and the process of model validation is needed to ensure the quality of these models. Blade modal testing is essential for validation of blade structural models, and this report discusses modal test techniques required to achieve validation. Choices made in the design of a modal test can significantly affect the final test result. This study aims to demonstrate the importance of the proper pre-test design and test technique for validating blade structural models.

Uncertainties in Prediction of Wind Turbine Blade Flutter.

The blades of a modern wind turbine are critical components central to capturing and transmitting most of the loads experienced by the system. Blades are complex structural items composed of many layers of fiber and resin composite material and typically, one or more shear webs. Simplification of the blade structure into equivalent beams is an important step prior to aeroelastic simulation of the turbine structure. There are a variety of approaches that can be used to reduce the three-dimensional continuum blade structure to a simpler beam representation: two-dimensional cross section analysis, extraction of equivalent properties from three-dimensional blade finite element models and variational asymptotical beam sectional analysis. This investigation provides insight into discrepancies observed in outputs from these three approaches for a real blade geometry. Wind turbine blades of the future will be longer and more flexible as weight is optimized. Innovative large blade designs may present challenges with respect to aeroelastic flutter instabilities. Sensitivity of computed flutter speed with respect to variations in computed beam properties is demonstrated at the end of this paper.

Development of Validated Blade Structural Models

The focus of this paper is on the development of validated models for wind turbine blades. Validation of these models is a comprehensive undertaking which requires carefully designing and executing experiments, proposing appropriate physics-based models, and applying correlation techniques to improve these models based on the test data. This paper will cover each of these three aspects of model validation, although the focus is on the third – model calibration. The result of the validation process is an understanding of the credibility of the model when used to make analytical predictions. These general ideas will be applied to a wind turbine blade designed, tested, and modeled at Sandia National Laboratories. The key points of the paper include discussions of the tests which are needed, the required level of detail in these tests to validate models of varying detail, and mathematical techniques for improving blade models. Results from investigations into calibrating simplified blade models are presented. I. Introduction HERE are a number of reasons why one desires to develop models of wind turbine blades, and in each case one wants to ensure that these models are useful for the intended purpose. For example, correctly predicting failure in blades using a model can reduce the need for numerous costly tests including the fabrication of additional blades for a test-based failure prediction approach. An additional benefit of modeling and simulation is that the time required to complete the design and fabrication cycle can be reduced significantly when validated models are used to evaluate key aspects of the design that would otherwise require testing. Additionally, modern blades are large and costly – a validated predictive tool would be useful for assessing larger blades of the future. An important step in ensuring that a model is useful for the purpose of the analysis, that is ensuring that a model accurately predicts the behavior of interest, is a process called model validation. A validated model is one in which an analyst or designer can place a great deal of confidence – one can use this model to accurately predict performance. The validation process incorporates both testing and analysis. A set of calibration experiments are designed which provide enough data to improve the model so that the observations from the test and the corresponding predictions from the analysis are suitably correlated. In the next step, additional “validation experiments” are conducted in order to ensure that the model is predictive for the conditions of the validation experiments. If the validation experiments can be predicted, then the model is considered validated, otherwise additional experiments must be performed to provide data for further improvement of the model. It is important to note that a model which has been calibrated to match the test data is not necessarily a valid model. The process of calibrating a model is called model updating, while model validation includes the additional step of performing validation experiments. The main objective of the paper is to detail a general model validation process applied to wind turbine blades designed and tested at Sandia National Laboratories. The key points to be covered include those related to 1) testing (experiment design), 2) analysis (model development), and 3) comparison of test-analysis data for use in model calibration. Key points are covered in each of these areas; however, the focus of this paper is on model calibration. Optimization of the model parameters incorporating various types of blade test observations, including modal testing and static testing, is a novel development. As an example encompassing the key points, a program aimed at improving current modeling capabilities for a research-sized wind turbine blade is discussed in detail.

Modeling and Testing of 9m Research Blades

Wind turbines and their blades continue to grow in size. The resulting increase in blade mass and cost requires the implementation of new design concepts. Among these is the selective use of carbon fiber. In 2002, Sandia National Laboratories (SNL) initiated a research program to investigate the use of carbon fiber in 9m subscale blades. Two sets of blades were designed, one with a carbon spar-cap and the other with off-axis carbon in the skin which produces bend twist coupling. Blades of each design have recently undergone modal and structural testing. In addition, finite element analysis (FEA) of both blades has been performed. This paper describes the design, testing, and analysis work that have been completed.

Scaled Wind Farm Technology Facility Overview.

In the past decade wind energy installations have increased exponentially driven by reducing cost from technology innovation and favorable governmental policy. Modern wind turbines are highly efficient, capturing close to the theoretical limit of energy available in the rotor diameter. Therefore, to continue to reduce the cost of wind energy through technology innovation a broadening of scope from individual wind turbines to the complex interaction within a wind farm is needed. Some estimates show that 10 40% of wind energy is lost within a wind farm due to underperformance and turbine-turbine interaction. The US Department of Energy has recently announced an initiative to reshape the national research focus around this priority. DOE, in recognizing a testing facility gap, has commissioned Sandia National Laboratories with the design, construction and operation of a facility to perform research in turbine-turbine interaction and wind plant underperformance. Completed in 2013, the DOE/SNL Scaled Wind Farm Technology Facility has been constructed to perform early-stage high-risk cost-efficient testing and development in the areas of turbine-turbine interaction, wind plant underperformance, wind plant control, advanced rotors, and fundamental studies in aero-elasticity, aero-acoustics and aerodynamics. This paper will cover unique aspects of the construction of the facility to support these objectives, testing performed to create a validated model, and an overview of research projects that will use the facility.

Aeroelastic Modeling of Large Offshore Vertical-axis Wind Turbines: Development of the Offshore Wind Energy Simulation Toolkit.

The availability of offshore wind resources in coastal regions makes offshore wind energy an attractive opportunity. There are, however, significant challenges in realizing offshore wind energy with an acceptable cost of energy due to increased infrastructure, logistics, and operations and maintenance costs. Vertical-axis wind turbines (VAWTs) are potentially ideal candidates for offshore applications, with many apparent advantages over the horizontal-axis wind turbine configuration in the offshore arena. VAWTs, however, will need to undergo much development in the coming years. Thus, the Offshore Wind ENergy Simulation (OWENS) toolkit is being developed as a design tool for assessing innovative floating VAWT configurations. This paper presents an overview of the OWENS toolkit and provides an update on the development of the tool. Verification and validation exercises are discussed, and comparisons to experimental data for the Sandia National Laboratories 34meter VAWT test bed are presented. A discussion and demonstration of a “loose” coupling approach to external loading modules, which allows a greater degree of modularity, is given. Results for a realistic VAWT structure on a floating platform under aerodynamic loads are shown and coupling between platform and turbine motions is demonstrated. Finally, future plans for development and use of the OWENS toolkit are discussed.

An evaluation of wind turbine blade cross section analysis techniques.

The blades of a modern wind turbine are critical components central to capturing and transmitting most of the load experienced by the system. They are complex structural items composed of many layers of fiber and resin composite material and typically, one or more shear webs. Large turbine blades being developed today are beyond the point of effective trial-and-error design of the past and design for reliability is always extremely important. Section analysis tools are used to reduce the three-dimensional continuum blade structure to a simpler beam representation for use in system response calculations to support full system design and certification. One model simplification approach is to analyze the two- dimensional blade cross sections to determine the properties for the beam. Another technique is to determine beam properties using static deflections of a full three-dimensional finite element model of a blade. This paper provides insight into discrepancies observed in outputs from each approach. Simple two-dimensional geometries and three-dimensional blade models are analyzed in this investigation. Finally, a subset of computational and experimental section properties for a full turbine blade are compared.

Experimental Results of Structural Health Monitoring of Wind Turbine Blades

A 9 meter TX-100 wind turbine blade, developed under a Sandia National Laboratories R&D program, was recently fatigue tested to blade failure at the National Renewable Energy Laboratories, National Wind Technology Center. The fatigue test provided an opportunity to exercise a number of structural health monitoring (SHM) techniques and nondestructive testing (NDT) systems. The SHM systems were provided by teams from NASA Kennedy Space Center, Purdue University and Virginia Tech (VT). The NASA and VT impedance-based SHM systems used separate but similar arrays of Smart Material macro-fiber composite actuators and sensors. Their actuator activation techniques were different. The Purdue SHM setup consisted of several arrays of PCB accelerometers and exercised a variety of passive and active SHM techniques, including virtual and restoring force methods. A commercial off-the-shelf Physical Acoustics Corporation acoustic emission (AE) NDT system gathered blade AE data throughout the test. At a fatigue cycle rate around 1.2 Hertz, and after more than 4,000,000 fatigue cycles, the blade was diagnostically and visibly failing at the blade spar cap termination point at 4.5 meters. For safety reasons, the test was stopped just before the blade completely failed. This paper provides an overview of the SHM and NDT system setups, and some test results.

Decades of Wind Turbine Load Simulation.

A high-performance computer was used to simulate ninety-six years of operation of a five megawatt wind turbine. Over five million aero-elastic simulations were performed, wit h each simulation consisting of wind turbine operation for a ten minute period in turbulent wind conditions. These simulations have produced a large database of wind turbine loads, including ten minute extreme loads as well as fatigue cycles on various turbine components. In this paper, the extreme load probability distributions are presented. The long total simulation time has enabled good estimation of the tails of the distributions down to probabilities associated with twenty-year (and longer) return events. The database can serve in the future as a truth model against which design-oriented load extrapolation techniques can be tested. The simulations also allow for detailed examination of the simulations leading to the largest loads, as demonstrated for two representative cases.