Abstract. For measurements taken over a decade at the coastal Danish site Høvsøre, we find the variance associated with wind speed events from the offshore direction to exceed the prescribed extreme turbulence model (ETM) of the International Electrotechnical Commission (IEC) 61400-1 Edition 3 standard for wind turbine safety. The variance of wind velocity fluctuations manifested during these events is not due to extreme turbulence; rather, it is primarily caused by ramp-like increases in wind speed associated with larger-scale meteorological processes. The measurements are both linearly detrended and high-pass filtered in order to investigate how these events – and such commonly used filtering – affect the estimated 50-year return period of turbulence levels. High-pass filtering the measurements with a cutoff frequency of 1∕300 Hz reduces the 50-year turbulence levels below that of IEC ETM class C, whereas linear detrending does not. This is seen as the high-pass filtering more effectively removes variance associated with the ramp-like events. The impact of the observed events on a wind turbine are investigated using aeroelastic simulations that are driven by constrained turbulence simulation fields. Relevant wind turbine component loads from the simulations are compared with the extreme turbulence load case prescribed by the IEC standard. The loads from the event simulations are on average lower for all considered load components, with one exception: ramp-like events at wind speeds between 8 and 16 m s−1, at which the wind speed rises to exceed rated wind speed, can lead to high thrust on the rotor, resulting in extreme tower-base fore–aft loads that exceed the extreme turbulence load case of the IEC standard.

Abstract. The present study introduces a new method to characterize ramp-like wind speed fluctuations, including coherent gusts. This method combines two well-known methods: the continuous wavelet transform and the fitting of an analytical form based on the error function. The method provides estimation of ramp amplitude and rise time, and is herein used to statistically characterize ramp-like fluctuations at three different measurement sites. Together with the corresponding amplitude of wind direction change, the ramp amplitude and rise time variables are compared to the extreme coherent gust with direction change from the IEC wind turbine safety standard. From the comparison we find that the observed amplitudes of the estimated fluctuations do not exceed the one prescribed in the standard, but the rise time is generally much longer, on average around 200 s. The direction change does however exceed the one prescribed in the standard several times, but for those events the rise time is a minute or more. We also demonstrate a general pattern in the statistical behaviour of the characteristic ramp variables, noting their wind speed dependence, or lack thereof, at the different sites.

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Leading-edge erosion (LEE) of wind turbine blades is caused by the impact of particles, for example, raindrops, and leads to a loss in the power production and high maintenance cost. Investigations have shown that a reduction in tip speed, so-called erosion-safe mode, increases the blade lifetime but the influence of different dropsize parameterizations and rain amounts on the blade lifetime is unclear. This study compares blade lifetime calculations using two different drop-size parameterizations, which both describe a characteristic drop size of a rain measurement. Furthermore, changes in blade lifetime in case rain amount is increased or decreased are investigated as well as the effect of different wind shear exponents. The blade-lifetime calculations are based on a kinetic-energy model and an accumulated rain model. The results show that a drop-size parameterization based on rain rate leads to 44 times longer blade lifetime compared with a parameterization using in situ drop-size measurements. This large difference is probably due to the underestimation of large drops of the first mentioned parameterization. A change in rain amount of about ±17 % results only in a marginal change in blade lifetime. For both cases and models, an extension of blade lifetime was calculated when reducing the tip speed during specific rain events. The change of wind shear exponents caused as well a considerable effect on the lifetime prediction. Overall, blade lifetime is primary depending on the chosen model, where the kinetic-energy model is highly sensitive to the drop-size parameterization.