Wind Turbine Structures Research Articles

Optimal maintenance planning and resource allocation for wind farms based on non-dominated sorting genetic algorithm-ΙΙ

The complex structure and harsh working environment of wind turbines cause frequent failures and unavailability of these turbines in wind farms. To promote the long-term stable development of wind power and enhance its market competitiveness, the reduction of operation and maintenance costs is particularly important, which are estimated to account for approximately 1/3 of the total life cycle cost. With the continuous increase in the size and number of wind turbines, wind farm maintenance tasks and resources are increasing and becoming unpredictable. The realization of the dynamic scheduling of maintenance tasks and resources under various constraints has become vital. In this study, an optimal multi-objective model of maintenance planning and resource allocation for wind farms is established. The maintenance tasks are obtained according to the preset maintenance strategy and current operating status of the wind turbine components. The dynamic requirements of maintenance planning and resource allocation for different wind farms in adjacent areas are periodically generated, and the Non-dominated sorting genetic algorithm-ΙΙ (NSGA-ΙΙ) is adopted to conduct a combinatorial optimization process. The validity of the proposed model are verified by a corresponding case study, along with a comparative analysis with other optimization algorithms and a sensitivity study of different parameters.

Field Monitoring of Offshore Wind Turbine Foundations in Ice Regions

Yu, S.S.; Zhang, D.Y.; Wang, S.F.; Wang, G.J.; Wang, G.; Yue, Q.J., and Li, G., 2020. Field monitoring of offshore wind turbine foundations in ice regions. In: Guido Aldana, P.A. and Kantamaneni, K. (eds.), Advances in Water Resources, Coastal Management, and Marine Science Technology. Journal of Coastal Research, Special Issue No. 104, pp. 343–350. Coconut Creek (Florida), ISSN 0749-0208.The offshore wind turbine foundation typically has a slender and flexible structure. Because of the uncertainty of ice load and ice-induced vibration, the structural characteristics may lead to many unclear risks. At present the research on the interaction between offshore wind turbine structures and sea ice is relatively scarce, and lack of measured data. In this paper, based on comprehensive field monitoring (FM) for ice zone offshore wind turbine foundations in the Northern Yellow Sea, information about the interaction between sea ice and the structures, and the structural vibration response was obtained. The results show that the ice load acts on the wind turbine foundations in the form of dynamic, and the risks of ice-induced resonance may cause an obvious dynamic magnification on the top of the wind turbine foundation. The modified ice force period model supplemented the reference for the wind turbine foundation design and operation in ice regions.

Dynamic responses of offshore wind turbine considering soil nonlinearity and wind-wave load combinations

The existing p-y method that is capable of considering soil-structure interaction (SSI) might not be appropriate for large diameter piles widely used in offshore wind turbine structures. In addition, a simplified way is often adopted for the dynamic analysis of OWT, which simplifies or ignore the SSI, blades, ocean environmental loads and operational conditions. Considering these, a new finite element model (FEM) is described for monopile-supported offshore wind turbines, including components of soil reactions, wind and wave combinations, monopile-tower-blades system. This model allows us to simulate the interaction between the large diameter pile and its surrounding soil and reveals more real dynamic responses of OWT under stochastic ocean environmental loads. This paper discusses the difference of the dynamic responses between the OWT model with traditional p-y method and the new model, the numerical analysis shows that traditional p-y method overestimates the top displacements of OWT under stochastic wind and wave loads. Based on the proposed model, the effects of wind-wave combination, wind-wave misalignment angle and operational conditions on the structural responses are further investigated. The combination of 90° wind-wave angle and operating condition is found to be the most unfavorable situation.

Design Optimization of Conical Concrete Support Structure for Offshore Wind Turbine

Various types of support structures for offshore wind turbine have been developed, and concrete structures have attracted attention due to many advantages. Although many studies have been conducted on the design of the existing steel structures, information and research on the design of concrete support structures are insufficient. Therefore, in this paper, a structural analysis model of conical concrete support structure (CCSS) is established and design optimization is presented. A detailed performance evaluation and the design of prestressed concrete were performed under the marine conditions of Phase 1 test site of southwest offshore wind project in Korea. The fluid–soil–structure interaction (FSI) was applied using the added mass method and soil spring model to represent the effects of water and soil. With the result of quasi-static analysis, a post-tensioning design was implemented by applying prestressing steel, and CCSS showed sufficient rigidity. From the natural frequency analysis, CCSS has a dynamic structural stability, and, in response spectrum and time-history analyses, the CCSS was safe enough under the earthquake loads. The methods and conclusions of this study can provide a theoretical reference for the structural analysis and design of concrete support structures for offshore wind turbines.

Localizing and quantifying structural damage by means of a beetle swarm optimization algorithm

An efficient meta-heuristic algorithm, named beetle swarm optimization (BSO), is proposed to localize and quantify structural damage using limited vibration measurement data. The beetle antennae search (BAS) algorithm that imitats a random walking mechanism in nature was recently developed to solve the optimization problem. However, the ratio of convergence of this algorithm significantly relys on the random direction and deviation for high-dimensional problems. To overcome this shortcoming, the BSO inspired by the swarm intelligence strategy is proposed. In the iterative search process of the BSO, each beetle swarm moves in a random direction like the BAS and the swarm of beetles is cognitive with the optimal one for the searching behavior. Consequently, the optimal one is updated step by step until a better beetle appears. To demonstrate the capability and robustness of the BSO, numerical and experimental studies using limited vibration measurement data of an offshore wind turbine structure are carried out for structural damage identification. An novel objective function is established by combining natural frequencies with mode shapes of the structure. The numerical results show that the BSO can accurately localize and quantify various types of damage even in a noise and temperature variations polluted environment. Moreover, it has higher accuracy and faster convergence speed than the BAS and the particle swarm optimization (PSO) algorithms. These promising performances could contribute to establishing a structural monitoring system for safety assurance of wind turbine structures.

New structure of sliding mode control for variable speed wind turbine

This article investigates a new sliding mode control structure for a variable speed wind turbine, in order to maximize the captured wind energy and to reduce the transient loads (shafts vibration) caused by the fast fluctuation of wind speed. The variable speed wind turbine control presents a difficult problem because of the nonlinearities of equivalent mathematical models and the effects of external disturbances (wind speed fluctuation). The sliding mode technique is a nonlinear control, widely used in this field; this is due to its robustness in terms of stability and performance against disturbances and modeling uncertainties, and its simplicity of implementation. However, this technique has a major problem which is the chattering phenomenon caused by the discontinuous portion of its control law. A new structure of sliding mode technique is developed in this paper to overcome this phenomenon and maintain the effectiveness of traditional approach. The performance of the developed control will be offered by numerical simulations and will be compared to the basic method of sliding mode control based on a strong nonlinear model of wind turbine.

A general substructure-based framework for input-state estimation using limited output measurements

This paper presents a general framework for estimating the state and unknown inputs at the level of a system subdomain using a limited number of output measurements, enabling thus the component-based vibration monitoring or control and providing a novel approach to model updating and hybrid testing applications. Under the premise that the system subdomain dynamics are driven by the unknown (i) externally applied inputs and (ii) interface forces, with the latter representing the unmodeled system components, the problem of output-only response prediction at the substructure level can be tailored to a Bayesian input-state estimation context. As such, the solution is recursively obtained by fusing a Reduced Order Model (ROM) of the structural subdomain of interest with the available response measurements via a Bayesian filter. The proposed framework is without loss of generality established on the basis of fixed- and free-interface domain decomposition methods and verified by means of three simulated Wind Turbine (WT) structure applications of increasing complexity. The performance is assessed in terms of the achieved accuracy on the estimated unknown quantities.

Effect of wind directionality on fatigue life of monopile support structures for offshore wind turbines

The U.S. offshore wind industry can expect higher costs due to the lack of domestic experience with offshore wind technology. A key factor of the capital expenditure related to offshore wind farms is the cost of the support structures of offshore wind turbines. Therefore, improvements to the reliability of support structures under ultimate and fatigue loading conditions will help reduce the levelized cost of energy of offshore wind. This study presents a framework that accounts for the wind directionality by assuming a distinct and independent wind speed distribution per each wind direction and investigates its effect on the fatigue life of offshore wind turbine support structures. A monopile support structure in a potential wind site close to a National Oceanic and Atmospheric Administration buoy in the north-eastern US waters is used in this study. Fatigue damage assessment is performed for the normal operational condition of wind turbine, and the results are presented considering both cathodic protection and free corrosion conditions at the mudline level of the monopile. The location and extent of the predicted fatigue damages are found to vary due to accounting for the wind directionality.

Dynamic Performance Enhancement of Vertical Wind Turbine Using Composite Blades Reinforced by Zinc-Oxide Nanoparticles. (Dept. M.)

The Performance of Vertical Axis Wind Turbine (VAWT) is highly dependent on its blades strength and stability. This paper is mainly dealing with enhancing the blades' structure of Vertical Axis Wind Turbine which is suitable for countries that have a moderate wind speed. The suggested improvement entails employing reinforced light composite blades using Zinc-Oxide (ZnO) nanoparticles (NPs) synthesized by the polymer-pyrolysis route. To assess the performance of the proposed design, a multistage Savonius blades prototype is built to realize the essential ultrasonic and vibration tests. Five different composite blades with different ZnO volume percentages are tested for assessing their values of the attenuation coefficient and the phase velocity and evaluating the composite material robustness. A wind tunnel is also used to evaluate the prototype power coefficient. Besides, the effect of the shift-angles between the different blades' stages on the turbine efficiency has been evaluated. The theoretical analysis and experimental results of the natural frequencies ensured that the proposed model can avoid the resonance scenarios through working at considerably high rotating speeds.

Predicting Loads and Dynamic Responses of An Offshore Jacket Wind Turbine in A Nonlinear Sea

In order to improve the simulation efficiency, a novel transformed linear Gaussian model has been first proposed in this paper for generating equivalent “nonlinear” irregular waves. It is demonstrated by calculation examples that for obtaining equivalent “nonlinear” waves with the same accuracy, the transformed linear Gaussian model is about 2.7 times faster than the traditional nonlinear simulation method and is about 2.14 times faster than the method proposed by Agarwal and Manuel (2011). The loads and dynamic responses calculation results regarding an offshore jacket wind turbine in this paper demonstrate that nonlinearly simulated irregular waves with bottom effects should be considered in order to design an un-conservative support structure for the offshore wind turbine. Furthermore, by studying the calculation results in this article we have found that the loads and dynamic responses of the offshore wind turbine when inputting transformed linearly simulated waves with bottom effects are almost identical to the corresponding values when inputting nonlinearly simulated waves with bottom effects. All these calculation results clearly demonstrate the superiority and effectiveness of using our novel transformed linear Gaussian model for predicting the wave loads and dynamic responses of an offshore wind turbine operating in a realistic nonlinear sea with bottom effects.

Comparative analysis of offshore support structures for two-bladed large wind turbines (10+MW) in deep waters

The offshore wind market is developing towards exploiting wind resources in deeper water sites. Inevitably, this fuels new research and feasibility studies on alternative solutions for the wind turbine support structures. Within this context, this work aims at comparing three different support structure design concepts for a 14 MW two-bladed downwind wind turbine: an XXL monopile, a hybrid jacket-tower and a lattice tower. To ensure a fair comparability of the three design concepts, a load capacity analysis is first performed to assess the yield strength of each concept and guarantee a similar material utilization. The comparative analysis is then carried out in terms of total mass, dynamic behaviour of the interaction between rotor and support structure, soil-structure interaction and resulting hydrodynamic forces. Based on the given design constraints, the preliminary results of this study favour a lattice tower solution. The study also highlights peculiar dynamic phenomena such as veering, mode hybridization and mode coalescence for the dynamic interaction between the lattice tower and the two-bladed rotor which need to be taken into account during the design phase.

Tower shadow excitation of a downwind rotor blade of a turbine with a tubular tower

The downwind configuration of wind turbines offers benefits regarding the blade-tower clearance, as during operation the blade primarily bends away from the tower. Consequently, the blades can be designed with lower stiffness. For tubular towers, however, a significant deficit of the wind speed in the tower wake occurs, resulting in fatigue-inducing vibrations. For this reason, full-height lattice towers are considered the preferred support structures for wind turbines with a downwind rotor. This work estimates the tower shadow excitation of a downwind rotor blade from a tubular tower. To this end, the blade of a commercial 6 MW downwind turbine is modelled with finite-elements. The tower wake is described on the basis of Madsen’s model and for the unsteady aerodynamic interaction Küssner’s function is adopted. At below- and above-rated wind conditions, the tower wake-induced vibrations are compared with the response of a blade of an equivalent upwind rotor, considering both the tip deflections and the root moments, the latter on the basis of damage-equivalent moments, to obtain an indication of the expected difference in fatigue damage. The downwind blade experiences vibrations with considerable larger amplitudes, especially in the out-of-plane direction. From the damage-equivalent moments it can be inferred that the blades of the downwind rotor encounter a much faster accumulation of fatigue damage.

Conceptual Study of a Real-Time Hybrid Simulation Framework for Monopile Offshore Wind Turbines Under Wind and Wave Loads

As an attractive renewable energy source, offshore wind plants are becoming increasingly popular for energy production. However, the performance assessment of offshore wind turbine (OWT) structure is a challenging task due to the combined wind-wave loading and difficulties in reproducing such loading conditions in laboratory. Real-time hybrid simulation (RTHS), combining physical testing and numerical simulation in real-time, offers a new venue to study the structural behavior of OWTs. It overcomes the scaling incompatibilities in OWT scaled model testing by replacing the rotor components with an actuation system, driven by an aerodynamic simulation tool running in real-time. In this study, a RTHS framework for monopile OWTs is proposed. A set of sensitivity analyses is carried out to evaluate the feasibility of this RTHS framework and determine possible tolerances on its design. By simulating different scaling laws and possible error contributors (delays and noises) in the proposed framework, the sensitivity of the OWT responses to these parameters are quantified. An example using a National Renewable Energy Lab (NREL) 5-MW reference OWT system at 1:25 scale is simulated in this study to demonstrate the proposed RTHS framework and sensitivity analyses. Three different scaling laws are considered. The sensitivity results show that the delays in the RTHS framework significantly impact the performance on the response evaluation, higher than the impact of noises. The proposed framework and sensitivity analyses presented in this study provides important information for future implementation and further development of the RTHS technology for similar marine structures.

Grounding System Modeling and Evaluation Using Integrated Circuit Based Fast Relaxed Vector Fitting Approach, Considering Soil Ionization

Since high voltage transmission line towers or wind turbines structures are installed in high-altitude areas, it is essential to achieve a high overvoltage protection system against direct and indirect lightning strikes collisions. The lightning current must be discharged quickly into the protective earth, to prevent the dangerous over-voltages formation and define a reference voltage node. This paper presents a novel model to assess the behavior of the grounding system, based on Pocklington integral equations under lightning magnetic fields and variations in soil ionization, in which an explicit circuit-based vector fitting RLC admittance branches are proposed. The frequency-dependent behavior of grounding system frequency response and soil ionization effect is modeled in time domain, straightly to implement into the electro-magnetic transient program (EMTP). The model verification contains horizontal, vertical, and their combinations of grounding grids to represent the complete investigations under lightning strikes. The harmonic impedance mathematical formulations and principles are derived based on a rational function, that could be applicable on ground potential rise (GPR) investigation.

Response of an Annular Tuned Liquid Damper Equipped With Damping Screens

Abstract Annular tuned liquid dampers (TLDs) may be installed in slender structures with limited floor space, in which people and utilities must pass through the core, such as a wind turbine or observation tower. This study investigates an annular-shaped TLD equipped with damping screens. A linearized equivalent mechanical model capable of capturing the fundamental sloshing mode response of an annular TLD is presented. An experimental shake table testing program is completed to assess the performance of the model. Thirty-six frequency sweep tests consisting of various TLD configurations, excitation amplitudes, and excitation directions are completed. Good agreement is observed between the linearized equivalent mechanical model and experimental wave heights, sloshing forces, and energy dissipated per cycle that have been filtered to include only the fundamental sloshing mode response. The model is also observed to be in good agreement with experimental data for different excitation directions. The model is coupled to a generalized structure to investigate the response of a structure equipped with an annular TLD. The annular TLD is found to reduce the response of a generalized offshore wind turbine structure undergoing harmonic force excitation. The annular TLD provides performance comparable to an optimal linear tuned mass damper (TMD) with the same properties for a range of force excitation amplitudes.

Calculation of monopile foundation in sand to long-term lateral cyclic load

Monopiles are widely used to support the upper structures of wind turbines in offshore engineering. Due to harsh environmental conditions, they are subjected to long-term lateral cyclic loading from waves and winds during the design life. A robust beam on the Winkler foundation model is presented to analyze the responses of monopile foundation under long-term lateral cyclic load, incorporating cyclic stiffness evolution of sand based on the results of drained cyclic triaxial tests. The reasonability of the model is proved by comparison with a published centrifuge model test. Furthermore, a parametric analysis is performed to study the effects of pile diameter, pile eccentricity, embedment depth and cyclic load amplitude to the accumulation of deformation at the pile head during the long-term lateral cyclic load, based on which an empirical expression for estimating the accumulated displacements of a long-term lateral loaded monopile under cyclic loads is given.

Conceptual design and dynamic analysis of a novel passive floating offshore wind turbine structure

ABSTRACT In order to mitigate dynamic responses of floating offshore wind turbines (FOWTs) under combined wind-wave-current loading, employment of structural control techniques is often considered. In this paper, a novel concept of a passive FOWT structure is proposed to overcome the previous limitations on space and mass of tuned mass dampers (TMDs). The conceptual design is examined on the basis of a finite element model. The dynamic responses, mainly roll and pitch rotations, are compared between the controlled and uncontrolled FOWTs. Subsequently, a parametric study is conducted and the main parameters in the model are optimised accordingly. Finally, the optimal performance of the proposed FOWT structure demonstrates the effectiveness of the proposed design, and also its superiority over the existing concepts.

Fatigue reliability analysis of wind turbine tower under random wind load

It is important to understand the vibration response and fatigue characteristics of the wind turbine structure. The fatigue reliability of the tower flange and bolt is studied in this paper. Firstly, by using the orthogonal expansion method and the Numerical-Theoretical method of random dynamic action, the random fluctuating wind field is expanded into a set of scattered points to calculate the wind load. The numerical calculation of the dynamic response of the wind turbine is verified with the measured data. Then, a fatigue probability calculation method based on the probability density evolution method is proposed. It is an effective method to substitute the fatigue damage D obtained by a rain flow counting method into the probability density evolution equation to solve the probability density of fatigue damage. In the end, a 1.5 MW wind turbine tower was taken as an example for analyzing the fatigue damage of its flange and bolt which considers the wind speed and wind direction distribution. By using the probability density evolution method, the evolution probability density curve clusters of the structural responses can be given accurately and quantitatively; thus, the reliability of the structure under random load can be conveniently obtained.

Design and Performance Analysis of Distributed Equal Angle Spiral Vertical Axis Wind Turbine

Background: The wind turbine is divided into a horizontal axis and a vertical axis depending on the relative positions of the rotating shaft and the ground. The advantage of the choke wind turbine is that the starting torque is large and the starting performance is good. The disadvantage is that the rotation resistance is large, the rotation speed is low, the asymmetric flow occurs when the wind wheel rotates, the lateral thrust is generated, and the wind energy utilization rate is lowered. How to improve the wind energy utilization rate of the resistance wind turbine is an important issue to be solved by the wind power technology. Objective: The nautilus isometric spiral wind turbines studied in this paper have been introduced and analyzed in detail, preparing for the further flow analysis and layout of wind turbines, improving the wind energy utilization rate of wind turbines, introducing patents of other structures and output characteristics of its generator set. Methods: Combined with the flow field analysis of ANSYS CFX software, the numerical simulation of the new wind turbine was carried out, and the aerodynamic performance of the new vertical axis wind turbine was analyzed. The mathematical model and control model of the generator were established by the maximum power control method, and the accuracy of the simulation results was verified by the measured data. Results: The basic parameters of the new wind turbine tip speed ratio, torque coefficient and wind energy utilization coefficient are analyzed. Changes in wind speed, pressure and eddy viscosity were investigated. Three-dimensional distribution results of wake parameters such as wind speed and pressure are obtained. By simulating the natural wind speed, the speed and output current of the generator during normal operation are obtained. Conclusion: By analyzing the wind performance and power generation characteristics of the new wind turbine, the feasibility of the new wind turbine is determined, which provides reference and reference for the optimal design and development of the wind turbine structure.

A simplified structural model for monopile-supported offshore wind turbines with tapered towers

When estimating the natural frequency of wind turbine structures using the single-degree-of-freedom modeling, usually part of the tower mass is concentrated to the tower top. However, the ratio of concentration is usually specified without considering that tapered towers are common in practice, which may cause certain errors. Additionally, the behavior of wind turbine structures may also be considerably influenced by the foundation flexibility and the substructure of offshore wind turbines. In this paper, a rigid-base single-degree-of-freedom structural model was proposed for wind turbines with tapered towers based on Rayleigh’s method. Then, for structural modeling of monopile-supported offshore wind turbines, the concentrated spring model was introduced to simulate the flexibility of the monopile foundation as well as the transition piece, and inertial and flexural effects of the substructure were also included. Accordingly, the closed-form solution of the natural frequency of wind turbine structures was derived and was verified to be satisfactory compared to existing numerical and experimental studies. This simplified structural model was also shown to approximate the seismic response of wind turbine structures. It was further used to demonstrate the influence the foundation scour on the natural frequency of offshore wind turbine structures, exhibiting its applicability to scour monitoring.