Publications

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Various sources of risk exist for all civil structures, one of which is seismic risk. As structures change in scale, the magnitude of seismic risk changes relative to risk from other sources. This paper presents an introduction to seismic hazard as applied to wind turbine structures. The existing design methods and research regarding seismic risk for wind turbines is then summarized. Finally a preliminary assessment is made based on current guidelines to understand how tower moment demand scales as rated power increases. Potential areas of uncertainty in the application of the current guidelines are summarized.

This report documents the status of the Sandia National Laboratories' Wind Plant Reliability Database. Included in this report are updates on the form and contents of the Database, which stems from a fivestep process of data partnerships, data definition and transfer, data formatting and normalization, analysis, and reporting. Selected observations are also reported.

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When a system design approach is applied to wind turbine blades, manufacturing and structural requirements are included along with aerodynamic considerations in the design optimization. The resulting system-driven design includes several innovative structural features such as flat-back airfoils, a constant thickness carbon spar-cap, and a thin, large diameter root. Subscale blades were manufactured to evaluate the as-built integrated performance. The design resulted in a 22% reduction in mass, but withstood over 300% of its design load during testing. Compressive strains of nearly 0.9% were measured in the carbon spar-cap. The test results from this and an earlier design are compared, as are finite element models of each design. Included in the analysis is a review of the acoustic emission events that were detected through the use of surface mounted microphones.

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An estimate of the distribution of fatigue ranges or extreme loads for wind turbines may be obtained by separating the problem into two uncoupled parts, (1) a turbine specific portion, independent of the site and (2) a site-specific description of environmental variables. We consider contextually appropriate probability models to describe the turbine specific response for extreme loads or fatigue. The site-specific portion is described by a joint probability distribution of a vector of environmental variables, which characterize the wind process at the hub-height of the wind turbine. Several approaches are considered for combining the two portions to obtain an estimate of the extreme load, e.g., 50-year loads or fatigue damage. We assess the efficacy of these models to obtain accurate estimates, including various levels of epistemic uncertainty, of the turbine response.

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International standards for wind turbine certification depend on finding long–term fatigue load distributions that are conservative with respect to the state of knowledge for a given system. Statistical models of loads for fatigue application are described and demonstrated using flap and edge blade–bending data from a commercial turbine in complex terrain. Distributions of rainflow–counted range data for each ten-minute segment are characterized by parameters related to their first three statistical moments (mean, coefficient of variation, and skewness). Quadratic Weibull distribution functions basedon these three moments are shown to match the measured load distributions if the non–damaging lowamplitude ranges are first eliminated. The moments are mapped to the wind conditions with a two-dimensional regression over ten–minute average wind speed and turbulence intensity. With this mapping, the short–term distribution of ranges is known for any combinationof average wind speed and turbulence intensity. The long-term distribution of ranges is determined by integrating over the annual distribution of input conditions. First, we study long-term loads derived by integration over wind speed distribution alone, using standard-specified turbulence levels. Next, we perform this integration over both wind speed and turbulence distribution for the example site. Results are compared between standard-driven and site-driven load estimates. Finally, using statistics based on the regression ofthe statistical moments over the input conditions, the uncertainty (due to the limited data set) in the long-term load distribution is represented by 95% confidence bounds on predicted loads. © 2001 by ASME.

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The benefit of introducing carbon fibers in a wind turbine blade was evaluated. The SERI-8 wind turbine blade was used as a baseline for study. A model of the blade strength and stiffness properties was created using the 3D-Beam code; the predicted geometry and structural properties were validated against available data and static test results. Different enhanced models, which represent different volumes of carbon fibers in the blade, were also studied for two design options: with and without bend-twist coupling. Studies indicate that hybrid blades have excellent structural properties compared to the all-glass SERI-8 blade. Recurring fabrication costs were also included in the study. The cost study highlights the importance of the labor-cost to material-cost ratio in the cost benefits and penalties of fabrication of a hybrid glass and carbon blade.