The impact of turbulence modelling on large offshore wind turbine response

In order to reduce climate change, a large up-scaling of renewable energy capacity is necessary in the near future. Offshore wind is expected to play a key role in this green shift. The wind conditions offshore are in general superior to onshore, and large areas are available. Furthermore, costs of offshore wind turbines have decreased drastically and are expected to decrease even further. This is partly due to the trend of increasing rotor sizes, which increases the energy extraction per installed wind turbine. In the design of wind turbines, it is crucial to simulate expected fatigue and power production. With increasing rotor sizes, the relative importance of the wind load increases, and its accurate characterization becomes essential. This, in turn, requires the proper understanding of the flow conditions over the whole rotor disk of large, modern wind turbines. With increasing turbine size in general, and for floating wind turbines with low eigen-frequencies in particular, larger time scales gain additional importance. The current wind turbine design standards recommend two simple turbulence models for estimating wind load in wind turbine design. These models are originally developed for small, onshore wind turbines, and do not account for variations in atmospheric stability. This thesis highlights the challenges of existing turbulence models used in wind turbine design. Wind fields obtained by the standard turbulence models are compared to each other, to quality assured, high-frequency measurements, and to wind fields simulated by more complex turbulence models. The two more complex models are based on measurements and on large eddy simulations. The main focus of the turbulence modelling is directed towards the estimation of coherent structures over the rotor plane. The different turbulence models are evaluated for various atmospheric stability conditions, and the obtained wind fields are used in aero-hydro-servo-elastic simulations of a large bottom-fixed and floating wind turbine. The resulting differences in wind turbine response are evaluated. As a first step, sonic anemometer measurements from an offshore meteorological mast are processed in this thesis, in order to obtain quality-assured, high-frequency measurements. The measurements are processed for wind energy applications, providing continuous and long enough time series for the proper consideration of the relevant lowfrequency range. It is, however, a challenge to achieve stationarity in longer time series of the wind speed, as the true wind conditions are continuously changing. The wind spectra obtained by the standard turbulence models are similar to each other. The standard turbulence models are unable to obtain low-frequency fluctuations in wind speed that are present in the measured spectra. It is known that an accurate modelling of low frequencies is important for floater response, but this thesis demonstrates the importance also for a bottom-fixed offshore wind turbine. An accurate representation of wind spectra is particularly important to the tower bottom fore-aft and blade root flapwise bending moments of the bottom-fixed wind turbine, and surge and pitch displacements of the floater. The standard turbulence models tend to underestimate these dynamic moments and motions due to their lack of large-scale fluctuations. Underestimation of loads in the design phase of wind turbines may in the worst case lead to reduced fatigue life and unexpected maintenance. The two standard turbulence models differ in their coherence estimates. For state-of-the-art rotor sizes, the coherence, representing the variation of wind speed over the rotor, is an important design parameter. As a consequence of modelling coherence differently, the wind turbine response varies significantly with the choice of turbulence model. With low coherence in the across wind direction, high dynamic yaw moments and motions are obtained. On the contrary, the dynamic thrust force is lower with lower coherence, and consequently also the moments and motions depending on it. Knowledge of the true offshore coherence is limited as measurements are scarce, especially in the across wind direction. It is therefore challenging to evaluate the accuracy of existing coherence models. With improved coherence measurements, one could easier evaluate the accuracy of turbulence models, and even find a more realistic turbulence model than the ones that are used today. The wind fields of the turbulence models based on measurements and large eddy simulation show significant differences with atmospheric stability conditions, which the standard turbulence models do not consider. These differences impact the dynamic response of the offshore wind turbines significantly. The more complex turbulence models have their own challenges, such as requirements of large amounts of high-frequency, quality assured measurements or high computational costs. Given the challenges of the turbulence models already discussed, one of the standard turbulence models was fitted to site specific measurements in this thesis. The resulting tower bottom and blade root response are more realistic, but the overall quantitative improvements are uncertain. This thesis shows how different turbulence models cause significant differences in the dynamic response of large bottom-fixed and floating wind turbines. It concludes that it is time for the wind turbine standards and industry to consider more advanced turbulence models that account for atmospheric stability in wind turbine design.For å redusere klimaendringane, er ei stor oppskalering i kapasitet av fornybar energi naudsynt i næraste framtid. Havvind er forventa å spele ei viktig rolle i dette grøne skiftet. Vindforholda til havs er generelt betre enn på land, og store område er tilgjengelege. Dessutan har kostnadane på vindturbinar til havs gått drastisk ned, og dei er forventa å gå vidare ned. Dette er delvis grunna tendensen til aukande rotorstorleik som aukar energiutvinninga per installert vindturbin. I design av vindturbinar er det avgjerande å simulere forventa utmatting og kraftproduksjon. Med aukande rotorstorleikar, aukar den relative viktigheita av vindlasten, og den nøyaktige karakteriseringa av vindlasten blir essensiell. Dette igjen, krev riktig forståing av strøymingsforholda over heile rotordisken til store, moderne vindturbinar. Med aukande turbinstorleik generelt, og for flytande vindturbinar med låge eigen-frekvensar spesielt, blir store tidsskala ekstra viktig. Dei aktuelle standardane for vindturbindesign anbefaler to enkle turbulensmodellar for å estimere vindlast i vindturbindesign. Desse modellane er i utgangspunktet meint for små vindturbinar på land, og tek ikkje omsyn til variasjonar i atmosfærisk stabilitet. Denne avhandlinga framhevar utfordringane til eksisterande turbulensmodellar som er brukt i vindturbindesign. Vindfelt som er generert av turbulensmodellane frå standardane er samanlikna med kvarandre, med kvalitetssikra, høg-frekvente målingar, og med vindfelt simulert med meir komplekse turbulensmodellar. Dei to meir komplekse modellane er basert på målingar og "large eddy simulation" (stor virvelsimulering). Hovudfokuset av turbulensmodelleringa er retta mot estimering av koherente strukturar over rotorplanet. Turbulensmodellane er vurdert for ulike atmosfæriske stabilitetsforhold, og dei genererte vindfelta er brukt i "aero-hydro-servo-elastic" simuleringar av ein stor botnfast og flytande vindturbin. Dei resulterande forskjellane i vindturbinrespons er vurdert. Som eit første steg, er målingar frå soniske anemometer på ein meteorologisk mast til havs prosesserte i denne avhandlinga, for å oppnå kvalitetssikra, høg-frekvente målingar. Målingane er prosesserte for vindenergiformål, med resulterande samanhengande og lange nok tidsseriar for å ta omsyn til det relevante låg-frekvente området. Det er ei utfordring å oppnå stasjonære, lange tidsseriar av vindfart, då dei sanne vindforholda endrast kontinuerleg. Vindspektruma frå turbulensmodellane frå standardane liknar på kvarandre. Turbulensmodellane frå standardane er ikkje i stand til å oppnå storskala svingingar i vindfart som er til stades i dei målte spektruma. Det er kjend at ei nøyaktig modellering av låge frekvensar er viktig for flytarrespons, men denne avhandlinga demonstrerer viktigheita òg for ein botnfast vindturbin til havs. Ein nøyaktig representasjon av vindspektruma er spesielt viktig for tårnbotn for-akter og bladrot klaffevis bøyemoment for den botnfaste vindturbinen, og jag- og stamp-forskyvingar for flytaren. Turbulensmodellane frå standardane har ein tendens til å underestimere desse dynamiske momenta og rørslene på grunna av deira mangel på stor-skala svingingar. Underestimering av lastar i designfasen av vindturbinar kan i verste fall føre til redusert utmattingsliv og uventa vedlikehald. Dei to standard turbulensmodellane er ulik i deira koherensestimat. For topp moderne rotorstorleikar er koherensen, som representerer variasjon i vindfart over rotoren, ein viktig designparameter. Som ein konsekvens av å modellere koherens ulikt, varierer vindturbinresponsen vesentleg med val av koherensmodell. Med låg koherens i retninga på tvers av vinden, oppnår ein høge dynamiske gir-moment og -rørsler. Til motsetning, er den dynamiske skyvkrafta lågare med lågare koherens, og følgjeleg òg momenta og rørslene som er avhengige av ho. Kunnskap om den sanne koherensen til havs er avgrensa, då tilgang på målingar er knapp, spesielt i retninga på tvers av vinden. Det er difor utfordrande å vurdere nøyaktigheita av eksisterande koherensmodellar. Med forbetra koherensmålingar, kan ein enklare vurdere nøyaktigheita av turbulensmodellar, og til og med finne ein meir realistisk turbulensmodell enn dei som blir brukt i dag. Vindfelta til turbulensmodellane som er baserte på målingar og "large eddy simulation" viser betydelege forskjellar med atmosfæriske stabilitetsforhold, som turbulensmodellane frå standardane ikkje tek omsyn til. Desse forskjellane har betydeleg innverknad på den dynamiske responsen til vindturbinar til havs. Dei meir komplekse turbulensmodellane har eigne utfordringar, som behov for store mengder høg-frekvente, kvalitetssikra målingar eller høg kostnad for utrekningskraft. Gitt utfordringane til turbulensmodellane som allereie er diskutert, er ein av turbulensmodellane frå standardane tilpassa lokasjonsspesifikke målingar i denne avhandlinga. Den resulterande responsen i tårnbotn og bladrot er meir realistisk, men dei overordna kvantitative forbetringane er usikre. Denne avhandlinga viser korleis ulike turbulensmodellar forårsakar betydelege forskjellar i dynamisk respons til store botnfaste og flytande vindturbinar. Ho konkluderer med at det er på tide at vindturbinstandardane og -industrien vurderer meir avanserte turbulensmodellar som tek omsyn til atmosfærisk stabilitet i vindturbindesign.Doktorgradsavhandlin

Quasi-static response of a bottom-fixed wind turbine subject to various incident wind fields

In the design of offshore wind farms the simulated dynamic response of the wind turbine structure includes loading from turbulent wind. The International Electrotechnical Commission (IEC) standard for wind turbine design recommends both the Mann spectral tensor model and the Kaimal spectral model combined with an exponential coherence formulation. These models give deviating wind loads. This study compares these two models to a large eddy simulations model and a model based on offshore wind measurements. The comparisons are performed for three situations, covering unstable, neutral and stable atmospheric conditions. The impact of the differences in the wind fields on the quasi-static response of a large bottom-fixed wind turbine is investigated. The findings are supported by an assessment of the impact of individual wind characteristics on the turbine responses. The wind model based on measurements causes high tower bottom and blade root flapwise bending moments due to a high wind load at very low frequencies. Low and negative horizontal coherence is obtained using the Mann spectral tensor model. This causes relatively large yaw moments as compared to the results using the other wind models. The largest differences in response are seen in the stable situation. We furthermore show that the quasi-static wind load has great impact on the total damage equivalent moments of the structure. From the results, we conclude that in the design of large offshore wind turbines one should carefully consider the structure of the turbulent wind. Further, longer simulations than recommended by the standards should be used to reduce uncertainty in estimated response.publishedVersio

Processing of sonic anemometer measurements for offshore wind turbine applications

Quality assured measurements from offshore masts may provide valuable information of the characteristics of the offshore wind field, which is of high relevance for simulations of offshore wind turbines' dynamic response. In order to obtain these high quality data sets, a processing procedure tailored to offshore wind turbine applications must be followed. In this study, existing quality control routines applied in literature are evaluated, and a complete procedure is developed for sonic anemometer measurements. This processing procedure is applied to measurements at three heights from 16 months of measurements at FINO1. The processing procedure results in a data set of more than 6 000 30-minute periods of high quality time series showing a large variety in terms of wind speed and turbulence intensity. Together with an assessment of the stationarity, this processed data set is ready for use in offshore wind turbine research.publishedVersio

Sensitivity of the dynamic response of a multimegawatt floating wind turbine to the choice of turbulence model

In the design of offshore wind turbines, it is important to make a realistic estimate of the wind load. This is particularly important for floating wind turbines, having natural frequencies in a frequency range where the wind loads are high and large turbulent structures exist. This study shows that turbulence modelling greatly impacts the response of a 15-MW floating wind turbine. The turbulence models recommended by the International Electrotechnical Commission (IEC) are challenged by considering two additional models: Large Eddy Simulations (LES) and an approach using input from offshore wind measurements (TIMESR). The two standard models, the Kaimal spectrum with IEC coherence model (Kaimal) and the Mann spectral tensor model (Mann), differ in their coherence formulation. This results in higher standard deviations for the surge and pitch motions, and lower for the yaw motion, when applying Kaimal in comparison to Mann. For the specific floater of this study, more damage is obtained in the mooring lines when applying Kaimal. Applying the more realistic models, LES and TIMESR, increases the range of response further, concluding that the two standard turbulence models may lead to incorrect estimations of the response of a floating wind turbine. LES and TIMESR take atmospheric stability into account, which is proven to alter the response significantly.publishedVersio

Analysis of turbulence models fitted to site, and their impact on the response of a bottom-fixed wind turbine

This study compares a wind field recommended by the wind turbine design standards to more realistic wind fields based on measurements. The widely used Mann spectral tensor model with inputs recommended by the standard, is compared to FitMann, the Mann model with inputs fitted to measurements and TIMESR; using measured time series combined with the Davenport coherence model. The Mann model produces too low energy levels at the lowest frequencies of the wind spectra, while the wind spectra generated by FitMann approaches the measured values. TIMESR reproduces the measured spectral values at all frequencies. The different models give similar vertical coherence, while the Mann and FitMann models give lower horizontal coherence than TIMESR. Investigating the wind loads on a bottom-fixed 10-MW wind turbine, the spectra for the tower bottom fore-aft and blade root flapwise bending moment follow the shape of the wind spectra closely at low frequencies. The low-frequency range is important for the blade root and in particular the tower bottom bending moment. Thus, the TIMESR model, followed by FitMann, is assumed to give the most accurate fatigue estimates. For the specific situation analysed in this study, the FitMann model gives only 18 and 5 % lower estimates than TIMESR of the tower bottom and blade root damage equivalent bending moments, while the Mann model gives 27 % and 12 % lower estimates. The tower top yaw and fore-aft bending moments depend on the wind coherence. For the specific situation analysed in this study, the FitMann model gives 9 and 5 % higher estimates of the tower top yaw and tower top damage equivalent (bending) moments compared to TIMESR, while the Mann model gives 23 % and 18 % higher estimates. Since only measurements of the vertical coherence are available, it is not clear which model is superior for the tower top moments. However, the importance of a proper coherence model is documented.publishedVersio

Evaluation of different wind fields for the investigation of the dynamic response of offshore wind turbines

As the size of offshore wind turbines increases, a realistic representation of the spatiotemporal distribution of the incident wind field becomes crucial for modeling the dynamic response of the turbine. The International Electrotechnical Commission (IEC) standard for wind turbine design recommends two turbulence models for simulations of the incident wind field, the Mann spectral tensor model, and the Kaimal spectral and exponential coherence model. In particular, for floating wind turbines, these standard models are challenged by more sophisticated ones. The characteristics of the wind field depend on the stability conditions of the atmosphere, which neither of the standard turbulence models account for. The spatial and temporal distribution of the turbulence, represented by coherence, is not modeled consistently by the two standard models. In this study, the Mann spectral tensor model and the Kaimal spectral and exponential coherence model are compared with wind fields constructed from offshore measurements and obtained from large‐eddy simulations. Cross sections and durations relevant for offshore wind turbine design are considered. Coherent structures from the different simulators are studied across various stability conditions and wind speeds through coherence and proper orthogonal decomposition mode plots. As expected, the standard models represent neutral stratification better than they do stable and unstable. Depending upon the method used for generating the wind field, significant differences in the spatial and temporal distribution of coherence are found. Consequently, the computed structural design loads on a wind turbine are expected to vary significantly depending upon the employed turbulence model. The knowledge gained in this study will be used in future studies to quantify the effect of various turbulence models on the dynamic response of large offshore wind turbines.publishedVersio

Sensitivity of the dynamic response of a multimegawatt floating wind turbine to the choice of turbulence model

In the design of offshore wind turbines, it is important to make a realistic estimate of the wind load. This is particularly important for floating wind turbines, having natural frequencies in a frequency range where the wind loads are high and large turbulent structures exist. This study shows that turbulence modelling greatly impacts the response of a 15-MW floating wind turbine. The turbulence models recommended by the International Electrotechnical Commission (IEC) are challenged by considering two additional models: Large Eddy Simulations (LES) and an approach using input from offshore wind measurements (TIMESR). The two standard models, the Kaimal spectrum with IEC coherence model (Kaimal) and the Mann spectral tensor model (Mann), differ in their coherence formulation. This results in higher standard deviations for the surge and pitch motions, and lower for the yaw motion, when applying Kaimal in comparison to Mann. For the specific floater of this study, more damage is obtained in the mooring lines when applying Kaimal. Applying the more realistic models, LES and TIMESR, increases the range of response further, concluding that the two standard turbulence models may lead to incorrect estimations of the response of a floating wind turbine. LES and TIMESR take atmospheric stability into account, which is proven to alter the response significantly

Quasi-static response of a bottom-fixed wind turbine subject to various incident wind fields

In the design of offshore wind farms the simulated dynamic response of the wind turbine structure includes loading from turbulent wind. The International Electrotechnical Commission (IEC) standard for wind turbine design recommends both the Mann spectral tensor model and the Kaimal spectral model combined with an exponential coherence formulation. These models give deviating wind loads. This study compares these two models to a large eddy simulations model and a model based on offshore wind measurements. The comparisons are performed for three situations, covering unstable, neutral and stable atmospheric conditions. The impact of the differences in the wind fields on the quasi-static response of a large bottom-fixed wind turbine is investigated. The findings are supported by an assessment of the impact of individual wind characteristics on the turbine responses. The wind model based on measurements causes high tower bottom and blade root flapwise bending moments due to a high wind load at very low frequencies. Low and negative horizontal coherence is obtained using the Mann spectral tensor model. This causes relatively large yaw moments as compared to the results using the other wind models. The largest differences in response are seen in the stable situation. We furthermore show that the quasi-static wind load has great impact on the total damage equivalent moments of the structure. From the results, we conclude that in the design of large offshore wind turbines one should carefully consider the structure of the turbulent wind. Further, longer simulations than recommended by the standards should be used to reduce uncertainty in estimated response

Processing of sonic anemometer measurements for offshore wind turbine applications

Quality assured measurements from offshore masts may provide valuable information of the characteristics of the offshore wind field, which is of high relevance for simulations of offshore wind turbines' dynamic response. In order to obtain these high quality data sets, a processing procedure tailored to offshore wind turbine applications must be followed. In this study, existing quality control routines applied in literature are evaluated, and a complete procedure is developed for sonic anemometer measurements. This processing procedure is applied to measurements at three heights from 16 months of measurements at FINO1. The processing procedure results in a data set of more than 6 000 30-minute periods of high quality time series showing a large variety in terms of wind speed and turbulence intensity. Together with an assessment of the stationarity, this processed data set is ready for use in offshore wind turbine research