Wind Turbine Structures Research Articles

A novel physics-informed framework for real-time adaptive modal parameters estimation of offshore structures

Singular value decomposition provides a rigorous mathematical foundation for these monitoring methods, such as modal analysis, damage detection, etc. As substantial limitations including computational efficiency and adaptive capabilities have been identified in the matrix process, data-driven algorithms with enhanced efficiency and accuracy to adapt the rapid development of information and technologies have been highly required in offshore structures monitoring. To meet this demand, a novel physics-informed frame work enabling the real-time adaptive monitoring has been proposed in this paper. The working principle of the developed framework has been represented by the smart conversion of physics-informed modal identification into the optimal process of fast and accurate solving an eigensystem governed by differential equations via recurrent neural network. The ingenious design of the proposed framework has complied with the rule of singular value decomposition used for modal identification and therefore, learning capabilities of the physics-informed framework have been remarkably enhanced by successfully addressing two bottleneck problems including proper initialized input values and the optimal time increment of the developed recurrent neural network. Both numerical simulations and the field-data based study of a mono-pile offshore wind turbine structure have been presented to examine the superior performance of the proposed framework. Results have shown that the proposed framework has the ability to adaptively identify modal parameters with a higher level of computational efficiency as compared with traditional methods. Furthermore, the computational advantage of the develop framework has demonstrated the potential to integrate with sensor networks and edge computing for smart monitoring and maintenance in various engineering subjects.

A review of piezoelectric energy harvesters for harvesting wind energy

Microelectromechanical systems (MEMS) powered by conventional batteries are disadvantaged in terms of scope of application and environmental friendliness because their power sources need to be replaced regularly and have the risk of polluting the environment. Piezoelectric technology provides a solution for harvesting clean energy such as wind energy from the environment, achieving self-supply of electrical energy for MEMS, and meeting the requirement of without management and maintenance after installation. An overview of piezoelectric wind energy harvesting can help to connect the MEMS field and meet its self-supply needs. This paper presents a comprehensive review of state-of-the-art advances in piezoelectric wind energy harvesters (PWEH). The classification of piezoelectric materials is briefly introduced. The principle of its operation is introduced. It is divided into five categories by structure: bluff body, airfoil, flag, wind concentrator, and wind turbine structures. The research status of harvester has been discussed from four perspectives: structure, application, theoretical modeling, and signal processing. The existing literature is integrated, and the future development directions in three aspects: expanding application area, improving adaptive capabilities, and application-oriented system design are proposed. This paper provides references for people in the industry who are committed to structural innovation and performance enhancement.

Average model of wind energy system dedicated to optimal design of the global system

This paper presents an average model based on justified simplifications dedicated to the design and optimization of wind energy systems. Indeed, the classic models of wind energy systems are complex and their use is not efficient for the optimal design of the components of the power chain given the complexity, the significant time of resolution and the strong correlation of the physical parameters of these models. For these reasons, a model based on the theory of average values with reduced simulation time of a wind turbine structure is developed. This model is validated against the classic model of the wind chain using the SimPowerSystem library of power component models integrated under the Matlab-Simulink simulation environment.

State of the art in structural health monitoring of offshore and marine structures

This paper deals with state of the art in structural health monitoring (SHM) methods in offshore and marine structures. Most SHM methods have been developed for onshore infrastructures. Few studies are available to implement SHM technologies in offshore and marine structures. This paper aims to fill this gap and highlight the challenges in implementing SHM methods in offshore and marine structures. The present work categorises the available techniques for establishing SHM models in oil rigs, offshore wind turbine structures, subsea systems, vessels, pipelines and so on. Additionally, the capabilities of proposed ideas in recent publications are classified into three main categories: model-based methods, vibration-based methods and digital twin methods. Recently developed novel signal processing and machine learning algorithms are reviewed and their abilities are discussed. Developed methods in vision-based and population-based approaches are also presented and discussed. The aim of this paper is to provide guidelines for selecting and establishing SHM in offshore and marine structures.

Calculation method for the interface contact stress of steel tube-to-sleeve grouted connections subjected to bending moments

Steel tube-to-sleeve grouted connections (GCs) have been widely used for offshore wind turbine structures due to its excellent structural behaviour. Interfacial contact stress is a crucial indicator for evaluating the ultimate stress state of the GCs. However, the information is still limited regarding the calculation formulas for contact stresses considering the effects of friction coefficient, elastic modulus of grout, and cross-sectional dimensions of steel tube and sleeve. An extensive FE analysis on a total of twenty full-scale grouted connection specimens under bending moments was performed in this paper. Based on the parametric analysis results, the contact stress distribution pattern was obtained and the design formulas were derived for calculating the contact stress. In these formulas, the cross-sectional influence factor (CIF) is proposed to evaluate the effect of cross-sectional dimensions of steel tube and sleeve. The proposed formulas were featured with a reasonable accuracy and could be adopted in the flexural design of GCs.

Evolution mechanism of wind turbine wake structure in yawed condition by actuator line method and theoretical analysis

Through deflecting the wind turbine wake and reducing the influence of wake effect, wind farm cooperative control can enhance the total power generation of wind farm. In addition to wake deflection, the wake of yawed wind turbine shows complex distortion characteristics, which cannot be explained sufficiently by traditional momentum conservation. In this paper, numerical simulation using Actuator Line Method and theoretical analysis were adopted to investigate the wake characteristics of NREL 5 MW wind turbine in yaw. The wake of yawed wind turbine shows a velocity deficit distribution with kidney-shape. Due to the periodic change of tip vortex shedding velocity, the vorticity carried by tip vortex changes periodically, leading to the formation of an asymmetric counter-rotating vortex pair (CVP), which induced the complex wake structure of yawed wind turbine. On this basis, a new theoretical model for the wake structure of yawed wind turbine is proposed, which can accurately predict the main characteristics of the CVP. This paper reveals the wake evolution mechanism of yawed wind turbine and establishes the corresponding theoretical model, which is conducive to the development of high-precision engineering wake model and the optimization of wind farm cooperative control.

Study on simulation of real‐time hybrid model test for offshore wind turbines

AbstractAs an emerging energy source with great potential, offshore wind power is supported by development policies in an increasing number of countries. However, the performance evaluation of floating offshore wind turbine (FOWT) structures is challenging due to the complexity of wind and wave loads and the difficulty in reproducing such loading conditions for downscaled FOWTs in laboratories. In physical model tests of FOWTs, the Froude's similarity law is adopted for the foundation, while the Reynolds similarity law is adopted for the blades, thus there is an issue with uncoordinated scaling laws. The FOWTs real‐time hybrid test uses loading device to simulate air loads, offering a new way to avoid scaling issues and reduce cost barriers. In this study, taking the 5 MW Braceless FOWT as the research object, a conceptual design of a real‐time hybrid model test system for Braceless offshore wind turbines is proposed. Six sets of working conditions are set for simulation to study the motion and force performances of the hybrid system. The performance requirements of the Braceless structure for the loading device in the real‐time hybrid test are quantified. Simulate different errors in the test to analyse the sensitivity of the response of the Braceless offshore wind turbine to various errors. The results show that the Braceless FOWT is relatively sensitive to motion and force errors. Due to the low‐frequency characteristics of offshore wind turbines and external load, the delay error has relatively little effect. The method results in this paper can provide theoretical support and design information support for the implementation of real‐time hybrid model tests in the future.

Research on the along-wind aerodynamic damping and its effect on vibration control of offshore wind turbine

The aerodynamic damping of the offshore wind turbine structure (OWT), which can be divided into along-wind aerodynamic damping and across-wind aerodynamic damping, is complex, changeable and closely related to the operation status of the wind turbine, the adopted control strategy and the wind speed. Further, the along-wind aerodynamic damping always plays a significant role in an operational OWT, which can effectively mitigate the dynamic response of the OWT structure. In this research, a new theoretical calculation method of the along-wind aerodynamic damping was firstly derived considering the contributions resulted from the changes in rotor speed with respect to wind speed. Then, this new proposed theoretical calculation process was verified by the simulation method in open-source software FAST and the parameters, which affect the along-wind aerodynamic damping such as wind speed, rotor speed and pitch angle, were also investigated. It is shown that the along-wind aerodynamic damping increases as the value of above influencing factors increases until the rotor speed approaches the rated rotor speed and the pitch angle is greater than the critical angle. Finally, the effect of aerodynamic damping on the performances of tuned mass damper (TMD) for an 5 MW OWT under operational condition was studied. The results demonstrate that the effectiveness of TMD is weakened with the increase of aerodynamic damping. Specifically, when the along-wind aerodynamic damping of the 5 MW OWT increases from 4.97% to 6.87%, the additional damping of the TMD decreases from 7.301% to 6.386%.

Study of the Interaction between Regular Waves and a Wind-Fish Combined Structure

The combination of a wind turbine and fish cage (wind-fish) is able to solve the complex issue of power supply for deep-sea aquaculture. Nevertheless, the hydrodynamic coupling between the fish cage and the wind turbine foundation is increasingly complex, and there is little research on the related hydrodynamics. In this paper, the hydrodynamic research on the wind turbine foundation and fish cage structure is carried out using numerical simulation, in which the porous medium model is adopted to simulate the effects of the fish cage on the flow field. By analyzing the hydrodynamic characteristics of the combined structure of an offshore wind turbine and aquaculture cage under the action of regular waves, the load on both the cage in the combined structure and the wind turbine foundation, and the overall load on the combined structure of offshore wind turbine and cage are obtained. The effects of wave parameters, structural dimensions, cage submergence depth, and biological attachment on cage load, wind turbine foundation load, and overall load of the combined structure are analyzed. The results demonstrate that the wind turbine foundation is persistently the central part of the combined structure. The wave height, water depth, and degree of attachment of marine organisms all have different effects on the amplitude of the structural load. In contrast, the wave period, cage geometry, and cage submergence depth have no significant effects on the amplitude of the structural load. In addition to the water depth, other factors will affect the proportion of the load on the wind turbine foundation and the total load as well.

Dynamic analysis of an integrated offshore structure comprising a jacket-supported offshore wind turbine and aquaculture steel cage

A novel integrated offshore design, JOWT-SC, which combines a jacket-supported offshore wind turbine structure (JOWT) with an aquaculture steel cage (SC), is proposed in this paper. The enclosed steel cage in the integrated design can adjust to changes in water depth, preventing the fish from escaping. The jacket structure is equipped with two decks and a 10 MW reference wind turbine that provide steady electricity for the offshore fish farm. The numerical modeling and time-domain simulations were conducted in software SACS, and the dynamic responses of the JOWT-SC were also compared with the baseline JOWT, that is without a steel cage. The considered environmental conditions include waves measured from the southeast open sea area of Fujian in China, several wind speeds that represent normal operating conditions for the reference wind turbine, and a constant current flow. The results of the study demonstrate that the aquaculture steel cage significantly influences the dynamic responses of the integrated JOWT-SC substructure, and the pile-foundation shear responses are more sensitive to the wave frequency. In particular, the cage increases the hydrodynamic responses of the JOWT-SC at the mudline under the combined action of winds, waves, and currents, compared with the case where the JOWT-SC is only subjected to wind loads. Overall, the results of this study can be used as a reference for the realization of novel designs in practical engineering.

The C1 wedge connection in towers for wind turbine structures, tensile behaviour of a segment test

This paper shows a part of the analysis of the development of the second generation of the C1 wedge connections for use in offshore wind turbine supporting towers. The novelty of this connection is that bolt failure is avoided under static and fatigue loads. This study aims to investigate the tensile behaviour of the connection by combining the findings of experiments and finite element (FE) analysis. Two specimens subjected to uniaxial and cyclic tensile loading tested until failure are used for illustration. Advanced quasi-static FE analysis results, considering the most detailed geometry and using an explicit dynamic solver, are compared to the experimental results. The FE analysis results agree well with the experimental results. Based on the FE model, a parametric study is carried out to analyse the influence of the bolt grade, friction coefficient between contact surfaces, and preloading force level on mechanical behaviour. Failure modes, bolt force development, and the evolution of gap opening between contacted segments are analysed. Results demonstrate that the tensile fracture of the C1 wedge connection mainly appears in the lower segment. All the investigated parameters have a negligible effect on the connection's ultimate resistance and failure mode. However, the friction coefficient between contact surfaces and bolt preload level significantly affects the connection's local deformation capacity and the response of the bolt stress range. The FE simulation provides practical guidance for designing this connection without bolt failure.

Numerical study of an ice-offshore wind turbine structure interaction with the pile-soil interaction under stochastic wind loads

In order to better understand the ice-induced collision/vibration mechanism of offshore wind turbines (OWTs) with ice-breaking cones, the Tsai-Wu yield criterion of ice for bending failure is coupled to the nonlinear collision simulation tool ANSYS/LS-DYNA in this paper, and a numerical approach is adopted to predict dynamic ice force. Through comparisons of ice force between simulations and physical tests, the accuracy of dynamic ice force simulated by the proposed approach is verified. Meanwhile, to consider the pile-soil interaction (PSI) effect, the Euler-Bernoulli Beam theory and the Timoshenko Beam theory are used, respectively, and the dynamic characteristics of the OWT are compared and analyzed. The discrepancies between the calculation results of the two methods and the influence of PSI on OWTs under combined wind-ice conditions are discovered. Further, the fully coupled interaction model between ice and flexible OWT with PSI under wind and ice loadings is established. Based on the full three-dimensional (3D) interaction model, the influence of the ice velocities, ice thicknesses, and cone angles on the dynamic ice force and structural response is investigated. The area damage rate and fatigue damage rate of the monopile foundation are proposed to further carry out damage and fatigue analyses under wind and ice loads. Higher fatigue damage is associated with smaller area damage rate. In addition, the ice thickness has a more significant effect on OWT damage and fatigue compared with the ice velocity.

New Converter Solution With a Compact Modular Multilevel Structure Suitable for High-Power Medium-Voltage Wind Turbines

Modern wind turbines with increasing power levels are continuously emerging. These power levels are reaching a point where excessively high currents are obtained if the traditional low-voltage wind-turbine structure is adopted. High currents lead to excessively high losses and the need for bulky and heavy conductors to carry them. A medium-voltage structure should be a more competitive alternative to be adopted in high-power wind turbines. In this article, a new converter solution with a modular multilevel structure suitable for driving modern/future high-power medium-voltage wind turbines is proposed. This converter topology has desirable features common to modular multilevel converters, such as the improved reliability at high voltage levels, and the possibility to synthesize high-power-quality staircase-shape voltages, which leads to low requirements for harmonic and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${dv}/{dt}$</tex-math></inline-formula> filters. Furthermore, the proposed converter presents reduced capacitance and inductance requirements compared with other well-established modular multilevel converter topologies, which should result in a lighter and more compact solution. The proposed solution could potentially allow for the reduction of the overall costs of the supporting structure required to withstand the weight of the wind turbine. Simulation and experimental results are presented in this article to validate the proposed topology.

Efficient optimization design method of jacket structures for offshore wind turbines

With the gradual implementation of offshore wind energy production, the future tendency is to expand into the deeper water. The jacket foundations will take the place of the present monopile foundations when the water depth increases. The foundations account for the majority of the construction cost for offshore wind farms, and the structural optimization of jackets will bring lucrative economic benefits. Structural optimization is a complex iterative process that requires huge computing costs. Therefore, this paper proposes a structural optimization method based on surrogate models to solve this problem effectively and swiftly obtain optimized design schemes of lightweight jackets for offshore wind turbines. The structural responses of jacket wind turbine systems under the equivalent static extreme loads with a recurrence period of 50 years are mainly considered in structural optimization design, and the key optimization variables of jackets are determined by parameter sensitivity analysis. The finite element models of jackets are transformed into surrogate models, and the genetic algorithm is employed to optimize the surrogate models directly. The optimized jackets are additionally verified through coupled dynamic analysis, besides, buckling strength and fatigue life are also checked. And local refined optimizations are carried out for the failure members. According to the optimized design schemes of lightweight jackets for 30 m, 50 m and 70 m water depths, it is demonstrated that the structural optimization design method is adequate and efficient for jackets of wind turbines. Parameter sensitivity analysis can cut the number of optimization variables in half to improve the optimization efficiency. Furthermore, the application of surrogate models can significantly speed up the optimization process by saving about 98.61% of the original time consumed. The optimization design method of the jackets for offshore wind turbines proposed in this paper is suitable for practical engineering, with high precision and efficiency.

An equation-based method for fully coupled analyses of floating offshore wind turbine based on Modelica

This paper presents a fully coupled aero-hydro-servo-elastic method to simulate floating offshore wind turbines (FOWT) based on Modelica language and AeroDyn co-simulations. The main features of this method are as follows. (1) The features of Modelica language (equation-based, object-oriented, and hierarchy structure) afford extreme flexibility and efficiency in the modeling process, and wind turbine structures or theoretical methods can be easily upgraded or replaced. (2) AeroDyn, a relatively precise aerodynamic load calculation module, is integrated into the system; thus, the model is more accurate than the previous Modelica library in calculating the responses of FOWTs. First, this paper introduces the theory of the proposed model in Modelica and AeroDyn module to calculate offshore FOWT response. Second, the implementation method for Modelica with AeroDyn is introduced. A dynamic link library (DLL) is programmed as the channel for exchanging data, and a controller to coordinate the stepping relationship between Modelica and AeroDyn is established. Third, code-to-code comparisons are performed based on International Energy Agency (IEA) Wind Task 23 Subtask 2. The results show excellent agreements between the proposed method and FAST in predicting dynamic responses of the rotor, tower, platform, and mooring system under both steady and turbulent winds combined with wave conditions. Finally, a case about high altitude work is carried out to demonstrate the flexibility of modeling processes. A single pendulum model constrained at the bottom is used to model the motion of the worker at a high altitude, coupled to the motion of a FOWT. The results show that the motion along the Y-axis is effectively suppressed as the damping force at the bottom increases, while the suppression is less pronounced on the X-axis. This method comprising the structure properties of blade and time domain potential flow hydrodynamic loads is more complete than the previous method using Modelica language to construct the fully coupled FOWT simulation model. Besides, it provides a more flexible modeling and simulation tool for some engineering practices, such as FOWT combined with fish cages, and FOWT operation and maintenance.

Switching Kalman Filtering-Based Corrosion Detection and Prognostics for Offshore Wind-Turbine Structures

Since manual inspections of offshore wind turbines are costly, there is a need for remote monitoring of their health condition, including health prognostics. In this paper, we focus on corrosion detection and corrosion prognosis since corrosion is a major failure mode of offshore wind turbine structures. In particular, we propose an algorithm for corrosion detection and three algorithms for corrosion prognosis by using Bayesian filtering approaches, and quantitatively compare their accuracy against synthetic datasets having characteristics typical for wall thickness measurements using ultrasound sensors. We found that a corrosion prognosis algorithm based on the Pourbaix corrosion model using unscented Kalman filtering outperforms the algorithms based on a linear corrosion model and the bimodal corrosion model introduced by Melchers.

Multi-objective optimization for simultaneously designing active control of tower vibrations and power control in wind turbines

For wind turbines operating in the full load region, stabilizing the generated power at its nominal value is a key objective. Furthermore, the control of tower vibrations is a major constraint in wind turbine structures because diverse loading sources can induce fatigue damage. These goals are often conflicting, and a trade-off solution must be found. To reduce tower vibrations, an additional damping method can be implemented by ensuring coordination between the pitch and torque control scheme. This study proposes a control scheme that balances these two objectives: power generation and tower vibration reduction. The control design is based on a collective pitch combined with two active tower damping controls: one for frontal oscillations to the wind, which generates an extra pitch component, and another for lateral displacements, which produces an additional component of the generator torque. The control parameters are tuned through multi-objective optimization and multi-criteria decision methods with FAST software along with MATLAB/Simulink. The proposed control scheme was simulated according to the extreme wind direction change in the IEC 61400–1 standard. The results show that the proposed procedure achieves a notable reduction in tower vibrations, whereas the generated power is almost unaffected. To support this conclusion, a set of performance indices and the time and frequency responses are analyzed from the simulations Comparisons with other control schemes illustrate the superior performance of the proposed methodology.

Investigation of the effect of pitting corrosion on the fatigue strength degradation of structural steel using a short crack model

Steel support structures of offshore wind turbines (jackets and monopiles) undergo both fatigue and corrosion damage, impacting their lifetime. This paper investigates how pitting corrosion, caused by being exposed to the marine environment, affects the fatigue strength of structural steel. A short fatigue crack model is used to estimate the minimum required applied load amplitude which causes a growing crack emanating from the bottom of a semi-elliptical pit. Pit growth rate data, reported in literature, is employed to update the pit size and its sharpness at each time step. The modelling results show the fatigue strength degradation as a function of the exposure time to the corrosive environment. As exposure time increases, it is observed that degradation happens more quickly in the early years followed by a gradually decreasing degradation rate of the fatigue strength in the following years.

Integrated models for rocking of offshore wind turbine structures

Integrated models for rocking of offshore wind turbine structures

A method for structural damage detection considering scour depth under the pile-soil interaction

The structure of an offshore wind turbine accumulates damage during its service life, while its surrounding soil is continually scoured. However, the existing damage detection technique based on structural mode ignores scouring, which will cause damage misjudgment in engineering application. Therefore, the effect of scour depth on structural damage detection need to be studied urgently. In this paper, the pile and its surrounding soil are constructed into a vibration system, and the change of soil constraint state caused by scouring is reflected in structural mode. Based on that, a new structural damage detection method considering the pile-soil interaction is developed, which is suitable for engineering reality. Then, from a comparison between the calculated result and preset damage in the numerical simulations and model experiments, the proposed method is validated. The results show that the interference factors caused by scouring increase with the increase of scour depth, which causes the deterioration of damage identification effects gradually, and even masks the preset damage in the end. Finally, an optimized FE model based on the actual scour depth is presented, which reduces the number of interference factors and improves the effect of damage detection significantly.