Extreme Operating Gust Research Articles

A Comparative Study on Load Assessment Methods for Offshore Wind Turbines Using a Simplified Method and OpenFAST Simulations

Simplified methods are often used for load estimations during the initial design of the foundations of offshore wind turbines (OWTs). However, the reliability of simplified methods for designing different OWTs needs to be studied. This paper provides a comparative study to evaluate the reliability of simplified approaches. The foundation loads are calculated for OWTs at the mudline level using a simplified approach and OpenFAST simulations and compared. Three OWTs, NREL 5 MW, DTU 10 MW, and IEA 15 MW, are used as reference models. An Extreme Turbulence Model wind load at a rated wind speed, combined with a 50-year Extreme Wave Height (EWH) and Extreme Operating Gust (EOG) wind load and a 1-year maximum wave height are used as the load combinations in this study. In addition, the extreme loads are calculated using both approaches for various metocean data from five different wind farms. Further, the pile penetration lengths calculated using the mudline loads via two methods are compared. The results show that the simplified method provides conservative results for the estimated loads compared to the OpenFAST results, where the extent of conservativism is studied. For example, the bending moment and shear force at the mudline using the simplified approach are 23% to 69% and 32% to 53% higher compared to the OpenFAST results, respectively. In addition, the results show that the simplified approach can be effectively used during the initial phases of monopile foundation design by using factors such as 1.5 and 2 for the shear force and bending moment, respectively.

A detached-eddy simulation study on assessing the impact of extreme wind conditions on load and wake characteristics of a horizontal-axis wind turbine

This study utilizes improved delayed detached-eddy simulation (IDDES) to investigate the impact of extreme wind conditions on a horizontal-axis wind turbine, as specified by the IEC-61400 standard. The employed IDDES has a hybrid formulation, where SST K-ω is used to model the turbulence in the boundary layers and large-eddy simulation (LES) is used to resolve turbulence in separated flow regions and the wake area. It also allows for switching to a wall-modeled LES approach, depending on the incoming turbulence content. The research focuses on extreme operating gust (EOG), extreme horizontal shear (EHS), and extreme vertical shear (EVS) conditions at two Tip Speed Ratios (TSR). The loadings and wake characteristics are compared to uniform inlet velocity results to highlight the effects of extreme winds. The novelty of this research lies in its comprehensive investigation of wake behavior under extreme wind conditions, providing valuable insights for improving wind farm layout design and reliability in challenging operational environments. The analysis shows that the impact of the TSR on wind turbine loadings during EOG, EVS, and EHS events is significant. During EOG events for a turbine operating at low TSR, the thrust force and bending moment reach 2.38 and 2.57 times their respective values in uniform inflow conditions, while at the optimal TSR, the thrust force and bending moment increase by only 25%. In all extreme wind cases, there is an escalation in turbulence within the wake, and wake meandering is observed in the EHS and EVS cases.

Probabilistic Design Methods for Gust-Based Loads on Wind Turbines

The IEC 61400-1 standard specifies design load cases (DLCs) to be considered in the design of wind turbine structures. Specifically, DLC 2.3 considers the occurrence of a gust while the turbine shuts down due to an electrical fault. Originally, this load case used a deterministic wind event called the extreme operating gust (EOG), but the standard now also includes an approach for calculating the extreme response based on stochastic simulations with turbulent wind. This study presents and compares existing approaches with novel probabilistic design approaches for DLC 2.3 based on simulations with turbulent wind. First, a semiprobabilistic approach is proposed, where the inverse first-order reliability method (iFORM) is used for the extrapolation of the response for electrical faults occurring at a given rate. Next, three probabilistic approaches are formulated for the calculation of the reliability index, which differs in how the aggregation is performed over wind conditions and whether faults are modeled using a Poisson distribution or just by the rate. An example illustrates the methods considering the tower fore-aft bending moment at the tower base and shows that the approach based on iFORM can lead to reductions in material usage compared to the existing methods. For reliability assessment, the probabilistic approach using the Poisson process is needed for high failure rates, and the reliabilities obtained for designs using all semiprobabilistic methods are above the target level, indicating that further reductions may be obtained via the use of probabilistic design methods.

Effect of Macroscopic Turbulent Gust on the Aerodynamic Performance of Vertical Axis Wind Turbine

Vertical Axis Wind Turbines (VAWTs) have proven to be suitable for changing wind conditions, particularly in urban settings. In this paper, a 2D URANS (Unsteady Reynolds-Averaged Navier Stokes) numerical analysis is employed for an H-Darrieus VAWT. A turbulent domain is created through systemically randomising the inlet velocity to create macro-turbulence in front of the VAWT. The parameters for spatial and temporal randomisation of velocity and its effects on the turbine performance are studied for a mean free stream velocity, U∞ = 10 m/s, and a tip speed ratio (TSR) of 4.1. The mean Coefficient of power (Cp) for randomised fluctuation of 2 m/s and half-cycle randomisation update frequency is 0.411 and for uniform inlet velocity is 0.400. The Cp vs. Tip Speed ratio plot suggests that the optimal tip speed ratio for operation is around 4.1 for this particular wind turbine of diameter 1 m, chord 0.06 m, and NACA 0018 airfoils. The effect of randomisation for tip speed ratio λ = 2.5, 3.3, 4.1, and 5.3 on the performance of the turbine is studied. Turbine wake recovers at a faster rate for macro-turbulent conditions and is symmetric when compared to wake generated by uniform velocity inlet. The maximum velocity deficit for a distance behind the turbine, x/d = 8 at TSR (λ) = 4.1 is 46% for randomised inlet and 64% for uniform inlet. The effect of randomisation for λ = 2.5 to 5.3 on the performance of the turbine is analysed. A time-varying gust based on International Electrotechnical Commission (IEC) Extreme Operating Gust is used to study the effect of fluctuating wind conditions in a turbulent environment. Since real-time conditions often exceed gust factors mentioned by IEC, winds with large gust factors such as 1.50, 1.64, and 1.80 are analysed. With an increase in gust amplitude, Ugust = 6 m/s to Ugust = 12 m/s on a free stream velocity of U∞ = 10 m/s, the mean Cp decreases from 0.41 to 0.35 since the wind turbine operates under tip speed ratios outside optimal range due to large fluctuations in incoming velocity.

Experimental assessment of a blended fatigue‐extreme controller employing trailing edge flaps

AbstractThis paper presents an experimental assessment of a blended fatigue‐extreme controller for load control employing trailing edge flaps on a lab‐scale wind turbine. The controller blends between a repetitive model predictive controller that targets fatigue loads and a dedicated extreme load controller, which consists of a simple on‐off load control strategy. The Fatigue controller uses the flapwise blade root bending moments of the three blades as input sensors. The Extreme controller additionally uses on‐blade angle of attack and velocity measurements as well as acceleration measurements to detect extreme events and to allow for a fast reaction. The experiments are conducted on the Berlin Research Turbine within the large wind tunnel of the TU Berlin. In order to reproduce test cases with deterministic extreme wind conditions that follow industry standards, the wind tunnel was redesigned. The analyzed test cases are extreme direction change, extreme coherent gust, extreme operating gust and extreme coherent gust with direction change. The test cases are analyzed by on‐blade angle of attack and velocity measurements. The load control performance of the Blended controller is compared to the pure fatigue oriented and the pure extreme load controller. The Blended controller achieves a maximum flapwise blade root bending moment reduction of 23%, which is comparable to the reduction achieved by the Extreme controller.

Mechanism of mooring line breakage and shutdown opportunity analysis of a semi-submersible offshore wind turbine in extreme operating gust

One of the most dangerous conditions for wind turbines is extreme operating gust (EOG), which is characterized by transient increases in wind speed. As for a floating offshore wind turbine (FOWT), however, it is unclear the potential risks and fracture mechanisms associated with the mooring system under the coupling conditions of EOG, waves, and currents. There is no published rule regarding FOWT design for reference at this time. In this study, a full aero-hydro-servo-elastic numerical model was developed based on blade element momentum (BEM) theory with correction factors, potential flow theory, Morison formulation, and finite element analysis model for mooring lines. The model incorporates the dynamic interactions caused by extreme gusts, waves, currents, control, and shutdown on FOWT responses. This is beneficial for revealing the fully coupled risk mechanism of mooring line breakage. The numerical simulation is conducted on a semi-submersible OC4-DeepCwind wind turbine with the designed mooring system in a 50 m-depth ocean area. First, the hazardous EOG parameters on mooring line safety are identified, including rise time and wind speed amplitude. And the reason of largely increased mooring tension is explained by analyzing the aerodynamic and hydrodynamic response mechanisms of the FOWT system. Additionally, the response quantities, including platform motions, tower loads, and mooring tensions, are examined under EOG-current-wave conditions with the identified rise time, wind speed amplitude, and various start times. In the study, we demonstrate that only specific coupling effects between wind and wave loads can cause transiently increased over-limit mooring tension, and we explain the essential risk mechanism of mooring line breakage. EOG has been found to induce continuous mooring line breakage, long-distance platform drift, power grid damage, and even platform capsizing. There is about 55 s between the starting time of EOG and the occurrence of the mooring line breakage incident, providing an excellent opportunity for an emergency shutdown strategy. Afterwards, the optimal response period for a shutdown is identified and proven to effectively reduce the likelihood and severity of the accident. Results of this study may serve as references for the design of floating offshore wind turbines, the optimization of emergency strategies, and the formulation of design rules.

Comparison of aerodynamic models for horizontal axis wind turbine blades accounting for curved tip shapes

AbstractCurved tip extensions are among the rotor innovation concepts that can contribute to the higher performance and lower cost of horizontal axis wind turbines. One of the key drivers to exploit their advantages is the use of accurate and efficient computational aerodynamic models during the design stage. The present work gives an overview of the performance of different state‐of‐the‐art models. The following tools were employed, in descending order of complexity: (i) a blade‐resolved Navier Stokes solver, (ii) a lifting line model, (iii) a vortex‐based method coupling a near‐wake model with a far‐wake model, and (iv) two implementations of the widely used blade element momentum method (BEM), with and without radial induction. The predictions of the codes were compared when simulating the baseline geometry of a reference wind turbine and different tip extension designs with relatively large sweep angle and/or dihedral angle. Four load cases were selected for this comparison, to cover several aspects of the aerodynamic modeling: steady power curve, pitch step, extreme operating gust impact, and standstill in deep stall. The present study highlighted the limitations of the BEM‐based formulations to capture the trends attributed to the introduction of curvature at the tip. This was true even when using the radial induction submodel. The rest of the computational methods showed relatively good agreement in most of the studied load cases. An exception to this was the standstill configuration, as the blade‐resolved Navier‐Stokes solver was the only code able to capture the highly unsteady effects of deep stall.

A source-term formulation for injecting wind gusts in CFD simulations

The objective of the present paper is to develop a methodology to inject strong wind gusts into the computational domain in order to efficiently simulate their effect on the fluid flow. The design of the methodology based on a source-term formulation takes the feedback effect of the resulting turbulent flow (and, if present, the impacted structure) on the wind gust itself into account. Since the injection of the wind gusts can be carried out close to the region of main interest, CPU-time intensive methods to ensure a proper transport of the gust through the flow field can be avoided. The methodology is mainly intended for the application within eddy-resolving simulations (e.g., LES), but it is not restricted to this class of simulation approaches. For the description of the gusts classical shape functions such as the Extreme Coherent Gust (ECG) and the Extreme Operating Gust (EOG) as well as a newly derived C2-”1-cosine” shape are applied. Two scenarios are taken into account to assess the proposed gust injection technique. On the one hand a (laminar) undisturbed flow field is considered and the effect of different time and length scales of the gusts on their evolution and propagation through the flow field is studied in detail. On the other hand a turbulent background flow is assumed demonstrating that the methodology suggested is also applicable for practically relevant turbulent flows.

A study of dynamic response of a wind turbine blade based on the multi-body dynamics method

The geometric nonlinear problem caused by large deformation and the aeroelastic issue under extreme wind conditions are becoming more and more prominent, as the flexibility of wind turbine blades enhances. To solve this problem, a model of Blade analysis with Multi-Body (BaMB) method is built by the rigid multi-body dynamics method in absolute coordinates. The model describes geometric nonlinearity and complex geometry of blades, and accurate results can be obtained by the model after blades are reasonably divided. Based on this model, the geometric nonlinearity of a 100 kW blade under static loading and the aeroelastic response under extreme operating gust (EOG) condition are investigated numerically and compared with experimental results. The results show that BaMB model is able to predict more accurate deformation than beam models under static loading when the deformation of the blade increases, especially when the blade is loaded with more than 120% of the maximum design load and the tip deflection is larger than 15.36% of the blade spans. At 210% of the maximum design load, the BaMB model can predict the deformation at the accuracy of 1.48% with respect to the test; however, the accuracies of the Euler-Bernoulli beam and the Timoshenko beam are 18.3% and 16.79%, respectively. Even in the case of large deformation, the BaMB model can reach the accuracy close to the finite element method (FEM) with high computational efficiency. The aeroelastic response of the blade under EOG condition is analyzed, and the results show that the BaMB model predicts reliable aeroelastic characteristics of the blade compared with commercial software.

What are unsafe strategies to pick up a light load according to graduates/students in physiotherapy and medicine?

The installation of small wind turbines and solar photovoltaic (PV) modules in the built environment requires a detailed understanding of wind resources to maximise turbine power output and minimise damage due to wind loading. While urban sites are known to be highly turbulent, less is known about the composition and frequency of gust events. To date a number of catastrophic wind turbine and solar PV structural failures have resulted from inadequate design for extreme gust events. In this study a 12 month wind resource measurement campaign was undertaken at an urban site. Gust events that satisfied the IEC 61400.2–2013 definition of an annual extreme operating gust were found to occur 100 times during the 12 months interval. These events had a 23% higher mean amplitude, and 21% shorter rise-and-fall time than that assumed by the standard. In addition, a site gust factor of 1.76 was determined, a value 26% higher than given in IEC 61400.2–2013. Consequently, IEC 61400.2–2013 appears not to predict the frequency and amplitude of extreme gust events at this given site suggesting there exists a risk that structural components may be under-designed for installation in urban sites.

Measured gust events in the urban environment, a comparison with the IEC standard

The installation of small wind turbines and solar photovoltaic (PV) modules in the built environment requires a detailed understanding of wind resources to maximise turbine power output and minimise damage due to wind loading. While urban sites are known to be highly turbulent, less is known about the composition and frequency of gust events. To date a number of catastrophic wind turbine and solar PV structural failures have resulted from inadequate design for extreme gust events. In this study a 12 month wind resource measurement campaign was undertaken at an urban site. Gust events that satisfied the IEC 61400.2–2013 definition of an annual extreme operating gust were found to occur 100 times during the 12 months interval. These events had a 23% higher mean amplitude, and 21% shorter rise-and-fall time than that assumed by the standard. In addition, a site gust factor of 1.76 was determined, a value 26% higher than given in IEC 61400.2–2013. Consequently, IEC 61400.2–2013 appears not to predict the frequency and amplitude of extreme gust events at this given site suggesting there exists a risk that structural components may be under-designed for installation in urban sites.

Characteristics of Near Surface Winds over Different Underlying Surfaces in China: Implications for Wind Power Development

Accurate wind and turbulence information are essential to wind energy research and utilization, among which wind shear and turbulence intensity/scale have seldom been investigated. In this paper, the observational data from the 100-m high wind towers in Xilinhot in Inner Mongolia (2009–10; grassland region), Huanghua in Hebei Province (2009–10; coastal flat region), and Xingzi County in Jiangxi Province (2010–11; mountain-lake region) are used to study the variations in near surface winds and turbulence characteristics related to the development of local wind energy over different underlying surfaces. The results indicate that (1) the percentage of the observed wind shear exponents exceeding 0.3 for the grassland region is 6%, while the percentage is 13% for the coastal flat region and 10% for the mountain-lake region. In other words, if the wind speed at 10 m is 10 m s−1, the percentage of the wind speed at 100 m exceeding 20 m s−1 for the grassland region is 6%, while the percentage is 13% for the coastal flat region and 10% for the mountain-lake region. (2) In terms of the turbulent intensity in the zonal, meridional, and vertical directions (Iu, Iv, and Iw, respectively), the frequencies of Iv /Iu < 0.8 in the grassland, coastal flat, and mountain-lake regions are 23%–29%, 32%–38%, and 30%–37%, respectively. Additionally, the frequencies of Iw / Iv < 0.5 in the grassland, coastal flat, and mountain-lake regions are 45%–75%, 52%–70%, and 43%–53%, respectively. The frequencies of Iv /Iu < 0.8 and Iw /Iu < 0.5 in each region mean that Iu is large and the air flow is unstable and fluctuating, which will damage the wind turbines. Therefore, these conditions do not meet the wind turbine design requirements, which must be considered separately. (3) At 50- and 70-m heights, the value of the turbulence scale parameter Λ in the grassland region is greater than that in the coastal flat region, and the latter is greater than that in the mountain-lake region. Therefore, under the same conditions, some parameters, e.g., the extreme directional change and extreme operating gust at the hub height in the grassland region, are greater than those in the coastal flat region, which are greater than those in the mountain-lake region. These results provide a reference for harnessing local wind energy resources and for the selection and design of wind turbines.

Simulation of transient gusts on the NREL 5 MW wind turbine using the URANS solver THETA

Abstract. A procedure to propagate longitudinal transient gusts through a flow field by using the resolved-gust approach is implemented in the URANS solver THETA. Both the gust strike of a 1−cos⁡() gust and an extreme operating gust following the IEC 61400-1 standard are investigated on the generic NREL 5 MW wind turbine at rated operating conditions. The impact of both gusts on pressure distributions, rotor thrust, rotor torque, and flow states on the blade are examined and quantified. The flow states on the rotor blade before the gust strike at maximum and minimum gust velocity are compared. An increased blade loading is detectable in the pressure coefficients and integrated blade loads. The friction force coefficients indicate the dynamic separation and re-attachment of the flow during the gust. Moreover, a verification of the method is performed by comparing the rotor torque during the extreme operating gust to results of FAST rotor code.

Small wind turbines: A numerical study for aerodynamic performance assessment under gust conditions

Abstract In the present work, a numerical study aimed to analyse the effect of an extreme loading event on a Horizontal Axis Wind Turbine (HAWT) is performed. A 3D unsteady CFD model of the NREL Phase VI small-sized wind turbine is validated against experimental data, with the incompressible solver of ANSYS Fluent and an unstructured moving mesh strategy. Then the Extreme Operating Gust (EOG) model from IEC 61400-2 is considered as the inlet condition. The results of the aerodynamic response and of the structural ultimate check, based on the IEC guidelines, are presented for both the operating and the parked turbine, in order to underline the benefit of the safety position in terms of lower stress transferred to the critical root section.

Wind turbine control in computational fluid dynamics with OpenFOAM

In this article, the implementation of the pitch-control option is described within the OpenFOAM computational fluid dynamics library supported by the preliminary results for a few representative test cases. The proposed pitch-control logic represents a simplified version of the typically model available for the load simulation based on blade element momentum theory codes. The control variable for the blade pitch regulation is the rotor rotational speed, for which an additional torque-speed controller determining the equilibrium between the aerodynamic and the electrical torque is required. The simulations are performed on the 80-m-diameter turbine used as a reference design within the SmartBlades project. The unsteady test cases are based on the assumption of rigid, not pre-bend blade, and refer to an emergency stop and an extreme operating gust. Results verified the correctness of the implemented control logic and its sensitivity to the control parameters, that is, the key factors for a successful regulation of power production and load mitigation in wind turbine design. The implemented simulation methodology constitutes the first step toward the reproduction of very complex operating conditions for wind turbines by means of accurate computations.

Vertical Axis Wind Turbine Design Load Cases Investigation and Comparison with Horizontal Axis Wind Turbine

The paper studies the applicability of the IEC 61400-1 ed.3, 2005 International Standard of wind turbine minimum design requirements in the case of an onshore Darrieus VAWT and compares the results of basic Design Load Cases (DLCs) with those of a 3-bladed HAWT. The study is based on aeroelastic computations using the HAWC2 aero-servo-elastic code A 2-bladed 5 MW VAWT rotor is used based on a modified version of the DeepWind rotor For the HAWT simulations the NREL 3-bladed 5 MW reference wind turbine model is utilized Various DLCs are examined including normal power production, emergency shut down and parked situations, from cut-in to cut-out and extreme wind conditions. The ultimate and 1 Hz equivalent fatigue loads of the blade root and turbine base bottom are extracted and compared in order to give an insight of the load levels between the two concepts. According to the analysis the IEC 61400-1 ed.3 can be used to a large extent with proper interpretation of the DLCs and choice of parameters such as the hub-height. In addition, the design drivers for the VAWT appear to differ from the ones of the HAWT. Normal operation results in the highest tower bottom and blade root loads for the VAWT, where parked under storm situation (DLC 6.2) and extreme operating gust (DLC 2.3) are more severe for the HAWT. Turbine base bottom and blade root edgewise fatigue loads are much higher for the VAWT compared to the HAWT. The interpretation and simulation of DLC 6.2 for the VAWT lead to blade instabilities, while extreme wind shear and extreme wind direction change are not critical in terms of loading of the VAWT structure. Finally, the extreme operating gust wind condition simulations revealed that the emerging loads depend on the combination of the rotor orientation and the time stamp that the frontal passage of gust goes through the rotor plane.

Aerodynamic Performance of Small-Scale Horizontal Axis Wind Turbines Under Two Different Extreme Wind Conditions

Aerodynamic performance of two small-scale horizontal axis wind turbines are analyzed under the extreme operating gust and extreme direction change conditions with initial wind speeds of 7, 10, 13, 15 and 20 m/s. Performance predictions are performed using computational fluid dynamics, and time variations of shaft torque and hub bending moment produced by the turbines are presented and compared with each other. Sectional flow field and sectional blade loading details along with surface skin friction line predictions are also presented in order to explain the loading behavior of the turbine blades at the mentioned extreme wind conditions. Predictions show that variations in wind speed and blade loadings are similar at low wind speeds, however, this similarity degrades as the wind speed increases. Also compared to wind speed changes, aerodynamic forces are shown to adapt more slowly to wind direction changes

Assessment of Gearbox Operational Loads and Reliability under High Mean Wind Speeds

This paper investigates the dynamic loads occurring in the drivetrain of wind turbines with a focus on offshore applications. Herein a model of the gearbox of the 5 MW wind turbine is presented. The model is developed in a multi-body framework using commercial software MSC ADAMS. Validation of the model was based on the experimental data provided by NREL for 750kW prototype gearbox. Failures of gearboxes caused by high dynamic loads have a significant influence on the cost of operation of wind farms. For these reasons in the study, the probability of failure of the gearbox working in an offshore wind turbine that operates in storm conditions with mean wind speeds less than 30 m/s is presented. In the study, normal shut-downs of a wind turbine in storm conditions were investigated. The analysis were conducted for two storm control strategies and different wind conditions from an extreme operating gust, normal turbulence model and extreme turbulence model. In the paper, loads in the planetary gear are quantified as well as the torsional moments in the main shaft. On the basis of simulation results the annual probability of failure of the gearbox in a wind turbine with soft storm controller is calculated, and compared with the one had the gearbox working in a wind turbine operating with hard storm controller. In the study, it was found that normal shut-downs do not have a significant influence on the ultimate loads in the gearbox, since they are related mostly to the gusts occurring during turbulence. Application of the storm controller with reduction of the wind turbine power allowed the decrease of the probability of failure for ultimate stresses.

Wind Turbine and Photovoltaic Hybrid Generations under Extreme Operating Gust

A micro-grid consisting of a 300kW wind turbine and a 40kW photovoltaic array is investigated under extreme operating gust (EOG) wind scenarios. The micro-grid is connected to the 380-V utility through a utility-side converter using natural-frame control, which provides a constant voltage of the DC link between the micro-grid and utility. The impact of the extreme gust wind variations on the micro-grid performance is studied for variable speed wind energy system equipped with a squirrel-cage induction generator. The Hurghada city, Red Sea, Egypt is taken as a case study for the wind speed profile. A detailed model of extreme gust-wind speed variation is implemented and simulated using PSIM commercial software package, based on climate characteristics of Hurghada city. The indirect rotor field oriented control (FOC) method is implemented to the generator-side converter to keep the system stable under the extreme gust wind conditions and to control the squirrel-cage induction generator (SCIG) speed for maximum power-point tracker (MPPT) regime. Power quality of the utility-side converter in terms of operation at different power factors, voltage value and THD are verified.

Rapid optimization of stall-regulated wind turbine blades using a frequency-domain method: Part 2, cost function selection and results

A fast and effective frequency-domain optimization method was developed for stall-regulated blades. It was found that when using linearized dynamics, typical cost functions employing damage-equivalent root bending moments are not suitable for stall-regulated wind turbines: when the cost function is minimized, the edgewise damping can be low, and the flapwise damping can approach zero during an extreme operating gust. A new cost function is proposed that leads to nicely balanced stall behavior and damping over the entire operating windspeed range. The method was used to design the blades of two multi-MW, stall-regulated, offshore wind turbines, comparable with the NREL 5 MW and NTNU 10 MW pitch-regulated turbines. It is shown that the optimal stall-regulated blade has a unique aerodynamic profile that gives high flapwise and edgewise damping and a uniform mean power output above the rated windspeed. The blades are described in sufficient detail that they can be used in further aeroelastic analyses, to compare large stall-regulated and pitch-regulated turbines. Copyright © 2014 John Wiley & Sons, Ltd.