Flexibility Of Tower Research Articles

Tower loads characteristics of a semi-submersible floating wind turbine: An experimental study

For a Floating Wind Turbine (FWT), the tower emerges as a critical structure that connecting the Rotor-Nacelle Assembly (RNA) and the supporting platform, facilitating the transmission of both aerodynamic and hydrodynamic loads. This paper deduces the distinct responses of the bending moment generated by the motion of a FWT compared with a fixed wind turbine. Due to the complexity of the tower load responses, a comprehensive experimental investigation was conducted on a semi-submersible FWT. Stiffness-matched and geometry-matched tower models were designed and fabricated. These models accurately simulate the tower's flexibility and windward areas, impacting the global dynamic response. The model test precisely replicates the marine environmental conditions, incorporating the interactions between wind, waves, and current. It is particularly worth noting that, a multi-fans large-scale Wind Generation System (WGS) with rectifier networks was improved and fabricated to provide a reliable wind field. Results indicate that the tower load responses under the conditions with wind exhibit distinct nP component characteristics. Notably, 1P and 3P responses dominate most tower load responses, with the 3P response typically increasing with wind speed. The extreme value of My.b (y-axis moment at tower-base) is approximately 10 times larger than that of My.t (y-axis moment at tower-top), signifying a significant magnitude difference and reflecting the response characteristics of the tower loads. And the responses of My.b and Fx.t (x-axis force at tower-top) at the natural frequency of pitch motion are notably suppressed by the aerodynamic damping effect of wind. Furthermore, Fx.b (x-axis force at tower-base) and My.b exhibit obvious responses at the natural frequency of pitch motion. As the wave height increases, the wave-frequency response of the tower base load also strengthens, but the high-frequency response still accounts for a certain proportion of energy in the tower top load.

An experimental study on the influence of wind-wave-current coupling effect on the global performance of a 12 MW semi-submersible floating wind turbine

Investigating the coupling effect between different marine environmental conditions is essential to understand the dynamic response of wind turbines. This paper presents the study findings from the model tests conducted in a wave basin, exploring the coupling effect of wind, waves, and currents on the global performance of a 1:70 scale model 12 MW semi-submersible floating wind turbine (FWT). A multi-blade large-scale wind generation system with a rectifier network was improved and fabricated to provide a reliable wind field for the experiment. The stiffness-matched and geometry-matched tower models were designed and fabricated, achieving an accurate simulation of the tower's flexibility and windward areas and their impact on the global dynamic response. Following the calibration of marine environmental conditions and the characterization of the model FWT system, the study focused on the operational and extreme responses of the FWT concept. The aerodynamic damping effect of wind in pitch and surge motions is remarkable, especially in pitch motion, primarily manifesting by reducing the standard deviation of motions. Mooring load responses are predominantly induced by the hydrodynamic effect, with the wave-frequency response gradually becoming dominant as wave parameters increase. Tower-top load responses are primarily induced by the hydrodynamic effect of waves and the aerodynamic load effect of wind. Furthermore, the phenomenon where the action of current strengthens the aerodynamic damping effect of wind is observed. The coupling effect between wind and current amplifies surge motion while diminishing pitch motion, effectively reducing the tower-top and tower-base load responses.

Numerical investigation of wind turbine wake characteristics using a coupled CFD-CSD method considering blade and tower flexibility

With the increase in wind turbine sizes, careful investigation into wind turbine wake effect is crucial for operational safety. This study proposes a two-way fluid-structure interaction (FSI) model combining large eddy simulation (LES) and computational structural dynamics (CSD) to investigate wind turbine wake characteristics. Examining the effect of blade and tower flexibility and inflow conditions, the study reveals negligible influence of blade and tower flexibility on mean velocity deficit under both inflow conditions. However, they significantly impact turbulence intensity under uniform inflow condition. The tower motion produces a maximum increase of approximately 45% while the blade flap-wise motion has the second-largest impact, increasing the maximum turbulence intensity by approximately 33% when compared with the fully rigid case. Additionally, the study quantitatively examines wake vortex inclination based on the inclination angle of the second-circle wake vortex structure. Finally, the study concludes that the streamline near the blade tip follows the blade surface, while the flow separation occurs near the blade root, generating two vortices on the suction side. As the tower motion is considered, the larger vortex exhibits alternate vortex shedding while the smaller vortex remains stable in both uniform and turbulent winds.

Fluid-structure interaction analysis of wind turbine aerodynamic loads and aeroelastic responses considering blade and tower flexibility

With the increasing size of wind turbines, the aeroelastic phenomenon plays an essential role in the safety of wind turbines. A fluid-structure interaction (FSI) analysis for wind turbine by integrating the LES turbulent model and a structural dynamic model is carried out to investigate the aerodynamic loads and aeroelastic responses considering different inflow conditions, and blade and tower flexibility. For the first time, the time series and statistical features of aerodynamic loads, the exceedance probability, the equivalent fatigue loads, and the time series and statistical features of aeroelastic responses of wind turbine are calculated by the two-way FSI model. Furthermore, effects of inflow wind conditions and the blade and tower flexibility are investigated. It is noted that the turbulent winds result in increased fluctuations of aerodynamic loads and the equivalent fatigue load. The flap-wise motions of blades are more critical to load magnitude than the edgewise motions, and the inclusion of tower motion mitigates this aerodynamic load due to the coupling effects. Finally, it is also observed that the tower motion has a noticeable impact on the flap-wise displacement at the blade tip, and the flap-wise motions of blades significantly affect the fore-aft displacement at the tower top.

Variable cross-sectional effect on bi-directional blades–tower–soil–structure dynamic interaction on offshore wind turbine subject to wind–wave loads

BackgroundThis study introduces a numerical model designed to simulate interactions occurring between a wind turbine's tower and the surrounding soil, as well as between the nacelle, blades, and the surrounding environment. This simulation accounts for both fore–aft and side-to-side movements. To describe these interactions, the model leverages the Euler–Lagrange equations. It calculates wave loads utilizing the Morison equation, with wave data generated based on the JONSWAP spectrum. Furthermore, aerodynamic loads are determined using the blade element moment theory, and the wind spectrum is generated using the Von Karman turbulence model. The tower is represented as a variable cross-sectional beam, employing a two-noded Euler beam element with two degrees of freedom: transverse displacement and rotation, and utilizing Hermite polynomial shape functions.ResultsIn a comparative analysis against experimental data, this modified model demonstrates significant enhancements in accurately reproducing the dynamic behavior of wind turbines with variable cross-sectional towers, outperforming models that approximate the tower with a constant cross section. Our findings reveal that the modified model achieves a remarkable improvement of 15% in replicating the tower's dynamic response when compared to the constant cross-sectional models. As a case study, a 5 MW monopile wind turbine with a flexible foundation, specifically the one provided by the National Renewable Energy Laboratory (NREL), is employed to simulate its dynamic response.ConclusionsThis research presents a robust numerical model for simulating wind turbine behavior in various environmental conditions. The incorporation of variable cross-sectional tower representation significantly improves the model's accuracy, making it a valuable tool for assessing wind turbine dynamics. The study's findings highlight the importance of considering tower flexibility in wind turbine simulations to enhance their real-world applicability.

LES simulation study of wind turbine aerodynamic characteristics with fluid-structure interaction analysis considering blade and tower flexibility

As wind turbine blades become longer and more flexible, aerodynamic characteristics become increasingly complex and require complete investigations. In this study, a two-way fluid-structure interaction (FSI) model is proposed to analyze the effects of inflow conditions, including uniform and atmospheric boundary layer (ABL) winds, as well as blade and tower flexibility, on the aerodynamic characteristics of wind turbine blades. A 4.5 MW real wind turbine with the rotor diameter of 152 m and the hub height of 94 m is considered. Results show that the edgewise blade motion can be disregarded, while the motion of the tower and the flap-wise motion of blade significantly influence the aerodynamic characteristics of blades. The separation flow at the blade root suction side is more distinct and becomes less significant with an increase in spanwise distance, with stagnation points primarily distributed along the flow separation boundary at 0.45–0.95L (L is the blade length). The three-dimensional rotational effect of the blade induces notable fluctuations in the pulsating pressure coefficient, particularly within this region. Notably, a prominent peak is observed at the location of 0.65L. Consequently, it is recommended to allocate additional focus on this specific region to mitigate potential fatigue loads.

Analytical study on the aerodynamic and hydrodynamic damping of the platform in an operating spar-type floating offshore wind turbine

Accurate evaluation of aerodynamic damping and hydrodynamic damping is critical to response prediction and vibration control of spar-type floating offshore wind turbines. This paper presents a comprehensive analytical study of these two damping sources with respect to platform motions. Aerodynamic damping is evaluated by a newly derived aerodynamic damping matrix based on linearisation of aerodynamic resultant forces at tower top. Both radiation and viscous drag effects are considered for hydrodynamic damping. The former is analytically expressed in the form of a radiation damping matrix, while the latter is derived based on Morison's equation and strip theory. A simplified model is established based on the analytical damping expressions and successfully verified against FAST and Aqwa. Under various operational conditions, the damping ratios for different degrees of freedom of the platform are estimated using complex modal analysis. It is found that the modal damping ratios arising from different sources can be very different for different vibration modes of the platform, and the tower flexibility is proved to have negligible impact on the platform damping. For a typical operational state, surge, sway, pitch and yaw motions are highly damped, with the roll motion intermediately damped and the heave motion nearly undamped.

An aero-hydro coupled method for investigating ship collision against a floating offshore wind turbine

Ship collision hazards with floating offshore wind turbines (FOWTs) has been identified and the dynamic responses of FOWT under ship collision scenarios are necessary to be investigated. This paper develops a numerical solver to address the challenge of considering wind-wave loads in the ship-FOWT collision analysis. The user-defined load subroutine, LOADUD, in LS-DYNA is utilized to model the wind, wave and mooring loads. A simplified aerodynamic model considering blade pitch strategy is used and the hydrodynamic loads are calculated based on potential-flow theory with a linear wave kinematic model. The mooring forces are considered with a linearized model. The implementation of the coupled solver is verified by conducting free decay test, wave-only and wind-only analysis, respectively and comparing them with the reference values. Good agreement has been achieved, indicating that the aero-hydro coupling effects can be accurately simulated by the numerical solver. With the developed method and solver, cases of ship collision with a spar-type floating wind turbine are simulated and both local structural deformation and global motions in 6DOF are obtained. The influence of velocity, flexibility of tower, deformability of the ship, wind-wave loads are further discussed. It is found that the impact velocity can greatly affect the structural responses and a high-speed impact can directly lead to tower collapse. By comparing the cases of ship collision with FOWT, and those with fixed-support wind turbine, the flexibility of wind turbine tower is found to rarely influence the energy dissipation under collision scenario of FOWT, while it is important for the collision scenario of fixed-support wind turbine. The wind-wave condition can be more hazardous as a smaller impact velocity can lead to structures collapsing. This is because, even though the structure can withstand the ship collision, the residual strength may not be enough for the following wind loads, which can result in the structure to finally collapse. Additionally, the proposed fully coupled method is used to validate one of the authors’ previous studies, which is a decoupled method regarding the ship-FOWT collision analysis [1]. The external dynamic results from two methods are compared and discussed. Good agreement is reached between two methods for low-velocity impact but for high-speed collision or collisions with serious deformation, limitations of decoupled method can be found because some basic assumptions of them are violated. The proposed method and solver can be used to evaluate the crashworthiness of FOWT during the engineering design phase.

Aeroelastic analysis of wind turbines under turbulent inflow conditions

Abstract. The aeroelastic response of a 2 MW NM80 turbine with a rotor diameter of 80 m and interaction phenomena are investigated by the use of a high-fidelity model. A time-accurate unsteady fluid–structure interaction (FSI) coupling is used between a computational fluid dynamics (CFD) code for the aerodynamic response and a multi-body simulation (MBS) code for the structural response. Different CFD models of the same turbine with increasing complexity and technical details are coupled to the same MBS model in order to identify the impact of the different modeling approaches. The influence of the blade and tower flexibility and of the inflow turbulence is analyzed starting from a specific case of the DANAERO experiment, where a comparison with experimental data is given. A wider range of uniform inflow velocities are investigated by the use of a blade element momentum (BEM) aerodynamic model. Lastly a fatigue analysis is performed from load signals in order to identify the most damaging load cycles and the fatigue ratio between the different models, showing that a highly turbulent inflow has a larger impact than flexibility, when low inflow velocities are considered. The results without the injection of turbulence are also discussed and compared to the ones provided by the BEM code AeroDyn.

Flexible multibody dynamic modeling of a floating wind turbine

This paper presents a modeling approach for the nonlinear flexible dynamics simulation of a floating offshore wind turbine (FOWT). The system consists of a floating platform, the moorings, the wind turbine tower, nacelle and the rotor. The floating platform is modeled as a six-degrees-of-freedom (6-DOF) rigid body subject to buoyancy, hydrodynamic and mooring loads. The wind turbine tower is modeled as a three-dimensional (3D) damped tapered Euler–Bernoulli beam undergoing coupled general rigid body and elastic motions. The beam bending-bending and twist motions are considered. The nacelle is modeled as a rigid body attached to the tower tip. The rotor is also modeled as rigid body spinning around its axis and subject to aerodynamic load. The generator torque control is integrated into the model to capture the rotor spin dynamics. The equations of coupled rigid body and elastic motions are derived using Lagrange’s Equation in terms of the generalized flexible coordinates and the platform quasi coordinates. The external loads are formulated and incorporated into the system equations of motion. The dynamic model is extensively validated against the most popular FOWT simulation tools with an excellent agreement. Finally, the influence of the tower flexibility on the FOWT dynamic behavior is investigated.

Natural characteristic analysis of wind turbine drivetrain considering flexible supporting

The mechanical system of wind turbine is much complicated and can be divided into the drivetrain and supporting portions. The drivetrain consists of wheel, main shaft, gearbox, generator, etc. and the supporting portion mainly consists of a tower and a cabin. In order to reduce the unit cost of electricity, the capacity and size of wind turbine are increased gradually in the past years. Meanwhile, with the increase of the wind turbine height, the tower actually becomes more flexible as the supporting part. And the influence of the supporting tower flexibility becomes stronger due to the varying wind loads both in magnitude and direction. Using the rigid–flexible coupling multibody dynamic theory, the coupled dynamic model of the wind turbine drive train was developed considering the flexible supporting. Then the natural characteristics of the system were computed and investigated. For the dynamic modeling, the blades, the tower and main shaft were modeled as flexible bodies, while the other components, such as the hub and the gearbox, were modeled as rigid bodies. The potential resonance frequencies of the system were discussed through the Campbell diagram and the modal energy distribution analysis. The results show that the natural frequency of swing mode shapes for the tower was 0.399 Hz and 0.405 Hz. The first natural frequency of drivetrain, which represented a torsional vibration mode, was 1.64 Hz. From the Campbell diagram and the modal energy distribution analysis, resonances would not occur within the normal operating speed range for the drivetrain. And a comparison analysis indicated that the flexible supports would increase the bearing loads along axial direction and radial direction, especially in main shaft and torque arm, but that influence was not obvious at parallel stage. However, to some extent, the flexible supports can decrease the loads fluctuation of drivetrain. Finally, the online vibration experiments were carried out in the wind field. The vibration characteristics of the wind turbine drivetrain were analyzed and the experimental results also compared well with the theoretical dynamic results.

A practical soil-structure interaction model for a wind turbine subjected to seismic loads and emergency shutdown

In seismically active areas the design of wind turbine must be verified for seismic load combinations. These consider a certain likelihood of earthquake occurrence during normal operation, which could eventually lead to an emergency stop. In order to simulate these scenarios, the computational model should take into account the aerodynamics of the rotor, the flexibility of tower and soil, transient operational phases and, first and foremost, the interrelation of all these aspects. At the same time, the complexity should be reduced in order to avoid enormous computational costs, especially when the soil-structure interaction is taken into account. This study presents a practical model for the analysis of the soil-structure interaction effects on the seismic behavior of wind turbine, during normal power production and emergency shutdown. The presented model is based on simplified lumped parameter model for the soil-foundation subsystem and is validated against a detailed model, based on a 3D coupled finite element method and scaled boundary finite element method approach. This article shows a demonstrative example for a reference 5MW wind turbine subjected to a seismic event which triggers an emergency shutdown. The application of the lumped parameter model allows a significant model size reduction and accurate approximation of the soil-structure behavior in time domain.

Fully Coupled Three-Dimensional Dynamic Response of a Tension-Leg Platform Floating Wind Turbine in Waves and Wind

A dynamic model for a tension-leg platform (TLP) floating offshore wind turbine is proposed. The model includes three-dimensional wind and wave loads and the associated structural response. The total system is formulated using 17 degrees of freedom (DOF), 6 for the platform motions and 11 for the wind turbine. Three-dimensional hydrodynamic loads have been formulated using a frequency- and direction-dependent spectrum. While wave loads are computed from the wave kinematics using Morison's equation, the aerodynamic loads are modeled by means of unsteady blade-element-momentum (BEM) theory, including Glauert correction for high values of the axial induction factor, dynamic stall, dynamic wake, and dynamic yaw. The aerodynamic model takes into account the wind shear and turbulence effects. For a representative geographical location, platform responses are obtained for a set of wind and wave climatic conditions. The platform responses show an influence from the aerodynamic loads, most clearly through quasi-steady mean surge and pitch responses associated with the mean wind. Further, the aerodynamic loads show an influence from the platform motion through a fluctuating rotor load contribution, which is a consequence of the wave-induced rotor dynamics. Loads and coupled responses are predicted for a set of load cases with different wave headings. Further, an advanced aero-elastic code, Flex5, is extended for the TLP wind turbine configuration and the response comparison with the simpler model shows a generally good agreement, except for the yaw motion. This deviation is found to be a result of the missing lateral tower flexibility in the simpler model.

Fundamental Mode Estimation for Modern Cable-Stayed Bridges Considering the Tower Flexibility

The design of cable-stayed bridges is typically governed by the dynamic response. This work provides designers with essential information about the fundamental vibration modes, proposing analytical expressions based on the mechanical and geometrical properties of the structure. Different bridge geometries are usually considered in the early design stages until the optimum solution is defined. In these design stages, the analytical formulation is advantageous, because finite-element models are not required and modifying the bridge characteristics is straightforward. The influence of the tower flexibility is included in this study, unlike in previous attempts on mode estimation. The dimensions and proportions of the canonical models proposed in the analytical study stem from the previous compilation of the dimensions of a large number of constructed cable-stayed bridges. Five tower shapes, central or lateral cable-system layouts and box- or U-shaped deck sections, have been considered. The vibration properties of more than 1,000 cable-stayed bridges with main spans ranging from 200 to 800 m long were extracted within an extensive parametric analysis. The Vaschy-Buckingham theorem of dimensional analysis was applied to the numerical results to propose the formulation for period estimation. Finally, the formulas were validated with the vibration properties of 17 real cable-stayed bridges constructed in different countries. The importance of the tower flexibility is verified, and the errors observed are typically below 15%, significantly improving the estimations obtained by previous research works.

Modeling and Parameter Analysis of the OC3-Hywind Floating Wind Turbine with a Tuned Mass Damper in Nacelle

Floating wind turbine will suffer from more fatigue and ultimate loads compared with fixed-bottom installation due to its floating foundation, while structural control offers a possible solution for direct load reduction. This paper deals with the modelling and parameter tuning of a spar-type floating wind turbine with a tuned mass damper (TMD) installed in nacelle. First of all, a mathematical model for the platform surge-heave-pitch motion and TMD-nacelle interaction is established based on D’Alembert’s principle. Both intrinsic dynamics and external hydro and mooring effects are captured in the model, while tower flexibility is also featured. Then, different parameter tuning methods are adopted to determine the TMD parameters for effective load reduction. Finally, fully coupled nonlinear wind turbine simulations with different designs are conducted in different wind and wave conditions. The results demonstrate that the design of TMD with small spring and damping coefficients will achieve much load reduction in the above rated condition. However, it will deteriorate system performance when the turbine is working in the below rated or parked situations. In contrast, the design with large spring and damping constants will produce moderate load reduction in all working conditions.

Discussion on "Rotor-floater-mooring coupled dynamic analysis of mono-column-TLP-type FOWT (Floating Offshore Wind Turbine)"

The paper (Bae and Kim 2011) presented the theory and methodology for the coupled dynamic analysis among wind turbine, tower, floater, and mooring lines including aerodynamics, active control of blades, tower-blade elasticity, and floater-mooring dynamics in winds, waves, and currents for a 1.5MW mono-column TLP FOWT. Authors coupled the aero-elastic-control program FAST developed by NREL (e.g., Jonkman 2009) with the floater-mooring program CHARM3D/ HARP developed by Professor Kim’s research group (e.g., Kim et al. 2001, 2005, 2009) during the past decade to analyze full dynamic coupling of multi-floaters and mooring-riser. This kind of full dynamic coupling including all the aspects of FOWT is still very rare in the open literature and under continuous development in many countries. The most important differences between the FOWT and conventional floating offshore oilproduction platform are tower flexibility, rotational-blade inertia, active blade control, and the corresponding aero-dynamic loading. They influence the floater motions and mooring dynamics and, in turn, the platform motions affect the control scheme and aerodynamic/elastic loading. The two parts can be interfaced at each time step, as detailed in the paper. One of the important design concerns is the maximum acceleration at the nacelle. The height and tip-mass of FOWTs are typically large, so high acceleration there may cause very large inertia loading on the tower and floater. It can also greatly reduce the system’s fatigue life. In this regard, the correct estimation of the maximum acceleration at the nacelle position is very critical in the design of FOWTs. In case of a rigid floating body, the horizontal acceleration at the top consists of two parts. The first one due to horizontal motions (surge-sway) and the second-one due to rotational motions (pitch-roll) through the moment arm l from the center of rotation. In case of a flexible tower, there is additional contribution from the additional flexible modes. If we introduce the additional elastic modes, then the original rigid-body pitch natural frequency is slightly shifted due to dynamic x·· lθ ··

Periodic Disturbance Accommodating Control for Blade Load Mitigation in Wind Turbines

Performance of a model-based periodic gain controller for wind turbines is presented using Disturbance Accommodating Control (DAC) techniques to estimate fluctuating wind disturbances. The control objective is to regulate rotor speed at above-rated wind speeds while mitigating cyclic blade root loads. Actuation is via individual blade pitch, and sensors are limited to rotor angle and speed. The modeled turbine is a two-bladed, downwind machine with simple blade and tower flexibility having four degrees of freedom. Comparisons are made to a time-invariant DAC controller and to a proportional-integral-derivative (PID) design. Simulations are performed using a fluctuating wind input and a nonlinear turbine model. Results indicate that the state-space control designs are effective in reducing blade loads without a sacrifice in speed regulation. The periodic controller shows the most potential because it uses a time-varying turbine model to estimate unmeasured states. The use of additional sensors to help reconstruct the blade flap rate can significantly improve the level of load attenuation, as witnessed in full-state feedback results.