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Utformingen av et drivverk med middels hastighet for 10-MW referanse offshore vindturbin for Danmarks Tekniske Universitet (DTU). Det foreslås en fire-punkts drivlinjeoppsett som er utstyrt med en girkasse med to planetrinn og ett parallelt trinn. Deretter er drivkomponentene designet på grunnlag av designbelastninger og kriterier som er anbefalt i relevante internasjonale standarder. Til slutt oppnås en optimalisert drivstasjonsmodell via en iterativ designprosess som minimerer vekt og volum. En high-fidelity numerisk modell etableres via multikroppssystemtilnærmingen. Deretter sammenlignes den utviklede drivlinjemodellen med den forenklede modellen som ble foreslått av DTU, og de to modellene stemmer bra. I tillegg, en resonansevaluering av drivlinje er utført basert på Campbell-diagrammer og modal energidistribusjon. Detaljerte parametere for drivverkdesign og dynamisk modellering er gitt for å støtte reproduksjonen av drivstoffmodellen. En avkoblet tilnærming, som består av global aero-hydro-servo-elastisk analyse og lokal drivanalyseanalyse, brukes til å bestemme drivverkets dynamiske respons. De 20 år lange utmattelsesskadene på gir og lagre beregnes ut fra fordelingen av spenning eller belastningstid, Palmgren-Miner lineær akkumulerende skadeshypotese og langsiktige fordeling av miljøtilstand. Deretter etableres et inspeksjonsprioritetskart basert på feilrangeringen av drivlinjekomponentene, som støtter drivstoffinspeksjon og vedlikeholdsvurdering og videre modelloptimalisering. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2020 The Authors. Wind Energy Published by John Wiley & Sons, Ltd.

Interest in the exploitation of offshore wind resources using floating wind turbines has increased. Commercial development of floating horizontal axis wind turbines (FHAWTs) is emerging because of their commercial success in onshore and near-shore areas. Floating vertical axis wind turbines (FVAWTs) are also promising because of their low installation and maintenance costs. Therefore, a comparative study on the dynamic responses of FHAWTs and FVAWTs is of great interest. In the present study, a FHAWT employing the 5MW wind turbine developed by the National Renewable Energy Laboratory (NREL) and a FVAWT employing a Darrieus rotor, both mounted on the OC3 spar buoy, were considered. An improved control strategy was introduced for FVAWTs to achieve an approximately constant mean generator power for the above rated wind speeds. Fully coupled time domain simulations were carried out using identical, directional aligned and correlated wind and wave conditions. Because of different aerodynamic load characteristics and control strategies, the FVAWT results in larger mean tower base bending moments and mooring line tensions above the rated wind speed. Because significant two-per-revolution aerodynamic loads act on the FVAWT, the generator power, tower base bending moments and delta line tensions show prominent two-per-revolution variation. Consequently, the FVAWT suffers from severe fatigue damage at the tower bottom. However, the dynamic performance of the FVAWT could be improved by increasing the number of blades, using helical blades or employing a more advanced control strategy, which requires additional research. Copyright © 2016 John Wiley & Sons, Ltd.

Floating vertical-axis wind turbines (FVAWTs) provide the potential for utilizing offshore wind resources in moderate and deep water because of their economical installation and maintenance. Therefore, it is important to assess the performance of the FVAWT concept. This paper presents a stochastic dynamic response analysis of a 5 MW FVAWT based on fully coupled nonlinear time domain simulations. The studied FVAWT, which is composed of a Darrieus rotor and a semi-submersible floater, is subjected to various wind and wave conditions. The global motion, structural response and mooring line tension of the FVAWT are calculated using time domain simulations and studied based on statistical analysis and frequency-domain analysis. The response of the FVAWT is compared under steady and turbulent wind conditions to investigate the effects of turbulent wind. The advantage of the FVAWT in reducing the 2P effect on the response is demonstrated by comparing the floating wind turbine with the equivalent land-based wind turbine. Additionally, by comparing the behaviour of FVAWTs with flexible and rigid rotors, the effect of rotor flexibility is evaluated. Furthermore, the FVAWT is also investigated in the parked condition. The global motions and structural responses as a function of the azimuthal angle are studied. Finally, the dynamic response of the FVAWT in selected misaligned wind and wave conditions is analysed to determine the effects of wind-wave misalignment on the dynamic response. Copyright © 2016 John Wiley & Sons, Ltd.

This paper presents detailed descriptions, modeling parameters and technical data of a 5MW high-speed gearbox developed for the National Renewable Energy Laboratory offshore 5MW baseline wind turbine. The main aim of this paper is to support the concept studies and research for large offshore wind turbines by providing a baseline gearbox model with detailed modeling parameters. This baseline gearbox follows the most conventional design types of those used in wind turbines. It is based on the four-point supports: two main bearings and two torque arms. The gearbox consists of three stages: two planetary and one parallel stage gears. The gear ratios among the stages are calculated in a way to obtain the minimum gearbox weight. The gearbox components are designed and selected based on the offshore wind turbine design codes and validated by comparison to the data available from large offshore wind turbine prototypes. All parameters required to establish the dynamic model of the gearbox are then provided. Moreover, a maintenance map indicating components with high to low probability of failure is shown. The 5 MW reference gearbox can be used as a baseline for research on wind turbine gearboxes and comparison studies. It can also be employed in globalmore » analysis tools to represent a more realistic model of a gearbox in a coupled analysis.« less

This paper presents an approach for performing a long-term fatigue analysis of rolling element bearings in wind turbine gearboxes. Multilevel integrated analyses were performed using the aeroservoelastic code HAWC2, the multibody dynamics code SIMPACK, the three-dimensional finite element code Calyx and a simplified lifetime prediction model for rolling contact fatigue. The National Renewable Energy Laboratory’s 750 kW wind turbine and its planetary bearing were studied. Design load cases, including normal production, parked and transient load cases, were considered. To obtain the internal bearing load distribution, an advanced approach combining a finite element/contact mechanics model and a response surface model were used. In addition, a traditional approach, the Harris model, was also applied for comparison. The long-term probability distribution of the bearing raceway contact pressure range was then obtained using Weibull and generalized Gamma distribution functions. Finally, we estimated the fatigue life of the bearing, discussed the differences of the methods used to obtain the bearing internal loads and analyzed the effects of the environmental conditions and load cases on the results. The Harris model may underestimate the inner raceway life by 55.7%, which can cause large load fluctuations along the raceways. The bearing fatigue life is very sensitive to the wind distribution and less affected by the transient and parked load cases. Copyright © 2014 John Wiley & Sons, Ltd.

This article deals with the atmospheric ice accumulation on wind turbine blades and its effect on the aerodynamic performance and structural response. The role of eight atmospheric and system parameters on the ice accretion profiles was estimated using the 2D ice accumulation software lewice Twenty-four hours of icing, with time varying wind speed and atmospheric icing conditions, was simulated on a rotor. Computational fluid dynamics code, FLUENT, was used to estimate the aerodynamic coefficients of the blade after icing. The results were also validated against wind tunnel measurements performed at LM Wind Power using a NACA64618 airfoil. The effects of changes in geometry and surface roughness are considered in the simulation. A blade element momentum code WT-Perf is then used to quantify the degradation in performance curves. The dynamic responses of the wind turbine under normal and iced conditions were simulated with the wind turbine aeroelastic code HAWC2. The results show different behaviors below and above rated wind speeds. In below rated wind speed, for a 5 MW virtual NREL wind turbine, power loss up to 35% is observed, and the rated power is shifted from wind speed of 11 to 19 m s−1. However, the thrust of the iced rotor in below rated wind speed is smaller than the clean rotor up to 14%, but after rated wind speed, it is up to 40% bigger than the clean rotor. Finally, it is briefly indicated how the results of this paper can be used for condition monitoring and ice detection. Copyright © 2012 John Wiley & Sons, Ltd.

Wind turbines must be designed in such a way that they can survive in extreme environmental conditions. Therefore, it is important to accurately estimate the extreme design loads. This paper deals with a recently proposed method for obtaining short-term extreme values for the dynamic responses of offshore fixed wind turbines. The 5 MW NREL wind turbine is mounted on a jacket structure (92 m high) at a water depth of 70 m at a northern offshore site in the North Sea. The hub height is 67 m above tower base or top of the jacket, i.e. 89 m above mean water level. The turbine response is numerically obtained by using the aerodynamic software HAWC2 and the hydrodynamic software USFOS. Two critical responses are discussed, the base shear force and the bending moment at the bottom of the jacket. The extreme structural responses are considered for wave-induced and wind-induced loads for a 100 year return-period harsh metocean condition with a 14.0 m significant wave height, a 16 s peak spectral period, a 50 m s − 1 (10 min average) wind speed (at the hub) and a turbulence intensity of 0.1 for a parked wind turbine. After performing the 10 min nonlinear dynamic simulations, a recently proposed extrapolation method is used for obtaining the extreme values of those responses over a period of 3 h. The sensitivity of the extremes to sample size is also studied. The extreme value statistics are estimated from the empirical mean upcrossing rates. This method together with other frequently used methods (i.e. the Weibull tail method and the global maxima method) is compared with the 3 h extreme values obtained directly from the time-domain simulations. Copyright © 2012 John Wiley & Sons, Ltd.

Floating wind turbines can be the most practical and economical way to extract the vast offshore wind energy resources at deep and intermediate water depths. The Norwegian Ministry of Petroleum and Energy is strongly committed to developing offshore wind technology that utilises available renewable energy sources. As the wind is steadier and stronger over the sea than over land, the wind industry recently moved to offshore areas. Analysis of the structural dynamic response of offshore wind turbines subjected to stochastic wave and wind loads is an important aspect of the assessment of their potential for power production and of their structural integrity. Of the concepts that have been proposed for floating wind turbines, spar-types such as the catenary moored spar (CMS) and tension leg spar (TLS) wind turbines seem to be well-suited to the harsh environmental conditions that exist in the North Sea. Hywind and Sway are two examples of such Norwegian concepts; they are based on the CMS and TLS, respectively. Floating wind turbines are sophisticated structures that are subjected to simultaneous wind and wave actions. The coupled nonlinear structural dynamics and motion response equations of these turbines introduce geometrical nonlinearities through the relative motions and velocities. Moreover, the hydrodynamic and aerodynamic loading of this type of structure is nonlinear. A floating wind turbine is a multibody aero-hydro-servo-elastic structural system; for such structures, the coupled nonlinear equations of motion considering nonlinear excitation and damping forces, including all wave- and wind-induced features, should be solved in the time domain. In this thesis, the motion and structural responses for operational and extreme environmental conditions were considered to investigate the performance and the structural integrity of spar-type floating wind turbines. The power production and the effects of aerodynamic and hydrodynamic damping, including wind-induced hydrodynamic and wave-induced aerodynamic damping, were investigated. Negative damping adversely affects the power performance and structural integrity. In this thesis, the controller gains were tuned to remove servo-induced instabilities. The rotor configuration effect on the responses and power production was investigated by comparing the upwind and downwind turbines. To develop robust design tools for offshore wind power, the competencies of the offshore technology and wind technology must be combined. Both the offshore and wind energy industries have begun to extend their existing numerical codes to account for the combined aerodynamic and hydrodynamic effects on the structure. As a result verifications of extended codes by doing experiments and code-to-code comparisons are needed. One of the aspects of the present research was to fill this gap by performing hydrodynamic and hydro-elastic comparison between commercial codes. For both CMS and TLS concepts, the comparisons were carried out prior to using the tools to study the behaviour of the CMS and TLS under wave- and wind-induced loads. Offshore structures encounter a variety of operational and harsh environmental conditions. Limit states such as ultimate, fatigue, accidental collapse and serviceability limit states (ULS, FLS, ALS and SLS) are defined as the design criteria for offshore structures. In performing realistic ultimate limit state analysis, the extreme responses of a floating wind turbine over its life should be estimated. This estimation requires detailed analysis of the extreme response. In the present thesis, extreme value analysis for spar-type wind turbines subjected to simultaneous wave and wind actions was preformed. The structural responses and the effect of modelled forces such as turbulence on these responses were investigated. The joint distribution of the environmental characteristics of the wave and wind was applied through the contour surface method. Stochastic wave and wind analysis showed that, while rigid body modelling was sufficient for obtaining accurate motions, consideration of the elastic behaviour of the tower/support structure was necessary to predict structural responses. The blades structural responses were found to be significantly affected by the turbulent wind. However, the mean and standard deviation of global motion and structural responses were not affected by the turbulence. Thus, to reduce the simulation time in fatigue analysis, a constant wind speed model can be applied. The CMS and TLS wind turbines are inertia-dominated structures, and the hydrodynamic viscous drag did not affect their wave-induced responses, while an increase in viscous drag could effectively reduce the resonant responses of such turbines. Under operational conditions, aerodynamic damping was found to be active in reducing both wave frequency and resonant responses. The results showed that, for a floating wind turbine, extreme response could occur in survival conditions, while for a fixed wind turbine, the extreme response occurs in operational cases related to the rated wind speed. To estimate the extreme value responses, extrapolation methods were used to reduce the sample size in Monte Carlo simulations. The accuracy of methods to estimate the extreme responses as a function of sample size and methods applied was investigated. The normalized responses for both CMS and TLS offshore wind turbines were presented to draw more generalized conclusions.

There have been some recent efforts to numerically model and analyse the wind turbine gearbox. To date, much of the focus has been on increasing model refinement and demonstrating its added value. This paper takes a step back and examines in detail the modelling and analysis of an important wind turbine gearbox component, the planet carrier, in a multi-body setting. The planet carrier studied in this work comes from the 750 kW wind turbine gearbox used in the National Renewable Energy Laboratory's Gearbox Reliability Collaborative project. The study is performed in two parts. First, the influence of subcomponents mated to the planet carrier in the gearbox assembly is investigated in detail. These components consist of the planet pins, bearings and the main shaft. In the second part of the study, the flexible body modelling of the planet carrier for use in multi-body simulations is examined through the use of condensed finite element and multi-body simulation models. Both eigenvalue analyses and time domain simulations are performed. Comparisons are made regarding the eigenfrequencies, categorized mode shapes and the maximum and minimum planet carrier rim deflections from the time domain simulations. The mode shapes are categorized into seven distinct deformation patterns. An actual load case from the dynamometer tests, a 100% rated torque loading, is used in the time domain simulations. The results from this comprehensive study provide an insight into the proper modelling of a wind turbine planet carrier in a multi-body setting. Copyright © 2012 John Wiley & Sons, Ltd.

If a vertical axis wind turbine is mounted offshore on a semi-submersible, the pitch motion of the platform will dominate the static pitch and dynamic motion of the platform and wind turbine such that the effect of tower tilting on the aerodynamics of the vertical axis wind turbine should be investigated to more accurately predict the aerodynamic loads. This paper proposes certain modifications to the double multiple-streamtube (DMS) model to include the component of wind speed parallel to the rotating shaft. The model is validated against experimental data collected on an H-Darrieus wind turbine in skewed flow conditions. Three different dynamic stall models are also integrated into the DMS model: Gormont's model with the adaptation of Strickland, Gormont's model with the modification of Berg and the Beddoes–Leishman dynamic stall model. Both the small Sandia 17 m wind turbine and the large DeepWind 5 MW are modelled. According to the experimental data, the DMS model with the inclusion of the dynamic stall model is also well validated. On the basis of the assumption that the velocity component parallel to the rotor shaft is small in the downstream part of the rotor, the effect of tower tilting is quantified with respect to power, rotor torque, thrust force and the normal force and tangential force coefficients on the blades. Additionally, applications of Glauert momentum theory and pure axial momentum theory are compared to evaluate the effect of the velocity component parallel to the rotor shaft on the accuracy of the model. Copyright © 2013 John Wiley & Sons, Ltd.

This study focuses on the dynamic responses of land-based and floating wind turbines under blade pitch system fault and emergency shutdown conditions. The NREL 5 MW turbine is studied. A hydraulic pitch system is considered, and the faults under study are events with a seized blade or a blade running out of control. Emergency shutdown is defined as a fast pitch-to-feather maneuver of the blades. Load cases with power production and grid fault with ensuing shutdown are also analysed for comparison. The fault scenarios and the blades’ fast pitching activity are simulated using HAWC2 through external Dynamic Link Libraries. On the basis of the time-domain simulations, the response characteristics of the land-based and the floating turbines in the four design load cases are compared. The load effects from the fault conditions are compared with the operational cases. Strong system dynamics and resonant responses, such as the tower elastic mode and the yaw resonant response, are elicited during shutdown. If the pitch system has a fault and one blade is hindered from normal pitching, the uneven load distribution of the blades leads to large structural and motion responses. For both turbines, the response maxima vary cyclically with the instantaneous azimuth when the blades start pitching to feather. For the floating wind turbine, the interaction of waves and wind also affects the results. The effect of the pitch rate during shutdown is analysed. The responses of the land-based turbine in grid loss and shutdown conditions are proportional to the pitch rate, whereas decreased sensitivity is found in the cases with pitch system faults. For the floating turbine, the effect of the pitch rate is small, and reduced pitch and yaw motion extremes are observed as the pitch rate increases. Copyright © 2013 John Wiley & Sons, Ltd.