On the Torsional–Translational Response of Wind Turbine Structures

Dynamic analysis of offshore steel wind turbine towers subjected to wind, wave and current loading during construction

Wind turbine towers experience complex dynamic loads during actual operation, and these loads are difficult to accurately predict in advance, which may lead to inaccurate structural fatigue and strength assessment during the structural design phase, thereby posing safety risks to the wind turbine tower. However, online monitoring of wind loads has become possible with the development of load identification technology. Therefore, an identification method for wind load exerted on wind turbine towers was developed in this study to estimate the wind loads using structural strain, which can be used for online monitoring of wind loads. The wind loads exerted on the wind turbine tower were simplified into six equivalent concentrated forces on the topside of the tower, and the initial mathematical model for wind load identification was established based on dynamic load identification theory in the frequency domain, in which many candidate sensor locations and directions were considered. Then, the initial mathematical model was expressed as a linear system of equations. A numerical example was used to verify the accuracy and stability of the initial mathematical model for the wind load identification, and the identification results indicate that the initial mathematical model combined with the Moore–Penrose inverse algorithm can provide stable and accurate reconstruction results. However, the initial mathematical model uses too many sensors, which is not conducive to engineering applications. Therefore, D-optimal and C-optimal design methods were used to reduce the dimension of the initial mathematical model and determine the location and direction of strain gauges. The C-optimal design method adopts a direct optimisation search strategy, while the D-optimal design method adopts an indirect optimisation search strategy. Then, four numerical examples of wind load identification show that dimensionality reduction of the mathematical model leads to high accuracy, in which the C-optimal design algorithm provides more robust identification results. Moreover, the fatigue damage calculated based on the load identification wind loads closely approximates that derived from finite element simulation wind load, with a relative error within 6%. Therefore, the load identification method developed in this study offers a pragmatic solution for the accurate acquisition of the actual wind load of a wind turbine tower.

In current design practice, the dynamic wind loads are described in terms of the equivalent static wind loads based on the gust response factor. This approach results in a distribution of the equivalent static loading similar to the mean static wind load distribution, which may not always be a physically meaningful and realistic load description. In this paper, the equivalent static load representation for multimode buffeting response of bridges is formulated in terms of either a weighted combination of modal inertial load components, or the background and resonant load components. The focus of the present study is on the determination of weighting factors of equivalent static load components in which the correlation among modal response components due to structural and aerodynamic coupling effects is taken into consideration. It is noteworthy that the equivalent static load distributions vary for each response component. The proposed approach particularly helps in extracting design loads from full aeroelastic model test results by expressing the dynamic loads in terms of the equivalent static loads. This facilitates in drawing useful design input from full aeroelastic tests, which have been employed mostly for monitoring the response of bridge models at selected locations. A simplified formulation is also presented in a closed form when wind loading information is available and coupling in modal response components is negligible, which can be very attractive for the preliminary design application. Examples are presented to illustrate modeling of the equivalent static loading and to demonstrate its effectiveness in bridge design.

Wind loads on structures are difficult to directly measure, so it is practical to identify structural wind loads based on the measurements of structural responses. However, this inversed problem is challenging compared with conventional load identification as wind loads are time-space coupled and spatially distributed dynamic loads on structures. An improved method is proposed for identifying wind loads on structures using only partial measurements of structural acceleration responses in this paper. First, the wind loads on a structure are decomposed by proper orthogonal decomposition as a series of time-space decoupled sub-distributed dynamic loads with independent basic spatial distribution functions and time history functions. Herein, structural modes are adopted as the basic spatial distribution functions and structural modes of discretized and continuous structural systems are investigated. Then, a history function of the decomposed wind load is identified in the modal domain based on modal Kalman filter with unknown inputs, which is proposed by the authors. Finally, the distributed wind loads are reconstructed for discrete or continuous structural systems. The feasibility of the proposed algorithm is verified by two numerical examples of identification of wind loads on a discrete shear frame and a wind turbine tower, respectively.

This study presents a dynamic response analysis of operational and parked wind turbines in order to gain better understanding of the roles of wind loads on turbine blades and tower in the generation of turbine response. The results show that the wind load on the tower has a negligible effect on the blade responses of both operational and parked turbines. Its effect on the tower response is also negligible for operational turbine, but is significant for parked turbine. The tower extreme responses due to the wind loads on blades and tower of parked turbine can be estimated separately and then combined for the estimation of total tower extreme response. In current wind turbine design practice, the tower extreme response due to the wind loads on blades is often represented as a static response under an equivalent static load in terms of a concentrated force and a moment at the tower top. This study presents an improved equivalent static load model with additional distributed inertial force on tower, and introduces the square-root-of-sum-square combination rule, which is shown to provide a better prediction of tower extreme response.

Digital twin technology for wind turbine towers based on joint load–response estimation: A laboratory experimental study

Reliability-based design optimisation framework for wind turbine towers

In current design practice, spatiotemporally varying wind loads on buildings are modeled as equivalent static wind loads. This loading description serves as pivotal information for estimating response under the combined action of wind and other loads. This paper presents a framework for evaluating the equivalent static wind load for any given peak response of buildings with uncoupled responses in the three primary directions. A new description of the background loading based on the gust loading envelope/peak dynamic loading is presented. The resonant loading is expressed in terms of the inertial load following the respective fundamental structural mode. The equivalent static wind loading for the total peak response is then expressed as a linear combination of the background and resonant components. Following this framework, closed-form formulations using an analytical wind loading model are presented. The gust response factors and the equivalent static wind loads for various alongwind response components ...

Abstract: W ind turbine tower has a very important role in wind turbine system as one of the renewable energy that has been attracting attention worldwide recently. Due to the growth of wind power market, advance and development of offshore wind system and getting huger capacity is inevitable. As a result, the vibration is generated at wind turbine tower by receiving constantly dynamic loads such as wind load and wave load. Among these dynamic loads, the mechanical load caused by the rotation of the blade is able to make relatively periodic load to the wind turbine tower. So natural frequency of the wind turbine tower should be designed to avoid the rotation frequency of the rotor according to the design criteria to avoid resonance. Currently research of the wind turbine tower, the precise research does not be carried out because of simplifying the structure of the other upper and lower. In this study, the effect of blade modeling differences are to be analyzed in natural frequency of wind turbine tower.

This paper presents a new method for determining rocking ground motion considering the effects of both time delay and loss of coherency in spatially variable seismic ground motions. Acceleration, velocity, and displacement response spectra resulting from the rocking component are also derived. Next, new seismic intensity parameters are proposed to estimate the combined action of the middle-field horizontal and rocking motions on the earthquake excitation of structures. These seismic intensity parameters are revised forms of the widely used ones for characterizing translational components, namely, peak ground velocity, peak ground displacement, cumulative absolute displacement, acceleration response spectrum, acceleration spectrum intensity, Fajfar intensity, and Kappos spectrum intensity. In addition, an expression for the estimation of the effect of the base tilt due to the rocking ground motion on the structural response is presented. The main advantage of the proposed relations is that they can...

Effect of monopile foundation modeling on the structural response of a 5-MW offshore wind turbine tower

In the present paper the continuous model method is applied to the prototype of a wind turbine tower in order to perform its modal structural analysis. This mathematical analysis is used as an alternative approach to the modal analysis method that uses discrete models. It is well known that in discrete models with high-level discretization and a large number of finite elements, several open questions on the accuracy, the convergence and the stability of the solution arise during the modal or response history analysis. In this sense, the results of the analysis by means of discrete modeling are in several cases doubtful and, therefore, a modal analysis by applying a continuous model as an effective alternative is recommended. To this end, the present paper proposes a continuous model approach to calculate the eigen-frequencies, periods and mode shapes to a wind turbine tower prototype. Starting from the equilibrium of forces on a differential element of the structure, the equation of motion of the tower is formulated and using in turn the known boundary conditions at the two ends of the wind tower, the tower eigen-problem is numerically treated and solved. The action of the higher mode-shapes is very important and may become critical in the case that the tower is subjected to strong dynamic loading (cf. e.g. wind) and simultaneously is excited by a strong seismic motion.

Wind turbine technology has developed tremendously over the past years. In Egypt, the Zafarana wind farm is currently generating at a capacity of 517 MW, making it one of the largest onshore wind farms in the world. It is located in an active seismic zone along the west side of the Gulf of Suez. Accordingly, seismic risk assessment is demanded for studying the structural integrity of wind towers under expected seismic hazard events. In the context of ongoing joint Egypt–US research project “Seismic Risk Assessment of Wind Turbine Towers in Zafarana wind Farm Egypt” (Project ID: 4588), this paper describes the dynamic performance investigation of an existing Nordex N43 wind turbine tower. Both experimental and numerical work are illustrated explaining the methodology adopted to investigate the dynamic behavior of the tower under seismic load. Field dynamic testing of the full-scale tower was performed using ambient vibration techniques (AVT). Both frequency domain and time domain methods were utilized to identify the actual dynamic properties of the tower as built in the site. Mainly, the natural frequencies, their corresponding mode shapes and damping ratios of the tower were successfully identified using AVT. A vibration-based finite element model (FEM) was constructed using ANSYS V.12 software. The numerical and experimental results of modal analysis were both compared for matching purpose. Using different simulation considerations, the initial FEM was updated to finally match the experimental results with good agreement. Using the final updated FEM, the response of the tower under the AQABA earthquake excitation was investigated. Time history analysis was conducted to define the seismic response of the tower in terms of the structural stresses and displacements. This work is considered as one of the pioneer structural studies of the wind turbine towers in Egypt. Identification of the actual dynamic properties of the existing tower was successfully performed based on AVT. Using advanced techniques in both the field testing and the numerical investigations produced reliable FEM specific for the tested tower, which can be further used in more advanced structural investigations for improving the design of such special structures.

The effects of structural and aerodynamic non-linearity on dynamic wind loads on overhead wires have been investigated. According to the Japanese design standards for transmission structures, wind loads on overhead wires are determined using equivalent static wind loads that can be used to estimate the maximum responses under dynamic loads. Some assumptions of linear theory are necessary to derive the equivalent static wind loads, and they have been validated only in the case of strong winds. To derive equivalent static wind loads in the case of weaker winds for snow-accreted conditions, time history response analyses of overhead wires have been performed. Because the turbulence intensity becomes higher in weaker winds, aerodynamic non-linearity causes the wind loads on the wires to become larger. Furthermore, structural non-linearity causes the tension in the wires to become greater. The contribution of wire resonance to dynamic load increases when the wind speed is low, and the gust response factor becomes greater than the value derived considering only the quasi-static response caused by wind turbulence. Taking into consideration the two major effects of aerodynamic and structural non-linearity, a modified method is proposed to enable the use of a design method based on equivalent static wind loads.

With the trend of larger wind turbine rotors to utilize more wind energy, wind turbine towers are becoming consequently higher and more flexible. Thus, the dynamic response of towers appears more significant and needs to be controlled to avoid potential resonance and even damage. This paper proposed a new type of double-track nonlinear energy sink (NES) to explore its mitigation effects on dynamic response of wind turbine towers. Firstly, the double-track NES is designed with optimal parameters. Subsequently, dynamic response of the onshore wind turbine tower with the double-track NES installed are investigated experimentally in wind tunnel. A comparison of dynamic response of wind turbine tower with or without double-track NES control is presented to prove that the novel NES can rapidly attenuate the response of the underlying tower, for both in-flow and cross-flow directions. This study reveals the potential application of the double-track NES in dynamic response control of wind turbine tower.