Ocean Wave Modulation of Offshore Wind Turbine Loads and Wake

The project Probabilistic Load Description, Monitoring and Reduction of the Loads on Future Offshore Wind Turbines(OWEA Loads) builds on the preceding project Verification of Offshore Wind Turbines. In this project, the evaluation of extensive data material aims to achieve the load-compliant and reliable design and operation of offshore wind turbines. The researchers want to investigate the dynamic interaction of the turbine with wind, waves, and the influence of the wake on the turbines behind it. The investigations were supported by measurements with lidar and UAV, numeric simulations and the evaluation of the measurement data. This determines characteristic extreme and fatigue loads and combined resultant loads of wind, turbulence and waves. The project’s findings could result in recommendations for sets of rules which could lead to a more efficient and economical design of offshore wind turbines.

Analysis of performance improvement methods for offshore wind turbine blades affected by leading edge erosion

Global warming may change the wind and wave pattern, thus altering the dynamic behavior of offshore wind turbine (OWT). This study examines the impact of climate change on the sustainable design and wind energy production of monopile-supported OWT in soft clay incorporating future wind speed, wave height, and period. Two offshore locations, namely, at east and west coasts in India, are selected. For both the locations, wind speed, wave height, and wave period are obtained from the buoy deployed by the Indian National Centre for Ocean Information Services (INCOIS) from 1998–2006. Statistical downscaling method is used to predict the future wind speed, wave height, and wave period due to climate change for the period of 2006–2040. The observed data is compared with the downscaled data from the National Centers for Environmental Prediction (NCEP) reanalysis data to calibrate the statistical downscaling model. The general circulation model (GCM) predictor variables for various global carbon dioxide emission scenarios are used for future climate change. This study shows that the modification in design of OWT is required due to change in dynamic behavior of OWT considering future climate.

Ice loads and ice-induced vibrations of offshore wind turbine based on coupled DEM-FEM simulations

This study addresses the critical engineering challenges in the structural design of offshore wind turbine towers, focusing on enhancing resistance to extreme environmental loads. As the demand for renewable energy increases, the design of mega offshore wind turbines requires robust solutions for structural reliability and longevity. Using finite element analysis (FEA), this research evaluates the effectiveness of various internal stiffener designs—ring stiffeners, skeletal-type stiffeners, and their combinations—in reinforcing cylindrical offshore wind turbine towers against wind and wave forces. Detailed simulations model the physical behavior of different stiffener configurations, assessing parameters such as von Mises stress distributions, displacement and buckling under load. The results indicate that towers with combined stiffener designs exhibit superior structural resistance and reduced stress concentrations compared to those with singular stiffeners or none. These findings have significant implications for the design and construction of mega offshore wind turbines, highlighting the importance of integrating advanced stiffener configurations to improve structural stability in harsh marine environments.

Issues related to environmental concern and fossil fuel exhaustion has made wind energy the most widely accepted renewable energy resource. However, there are still several challenges to be solved such as the integrated design of wind turbines, aeroelastic response and stability prediction, grid integration, offshore resource assessment and scaling related problems. While analyzing the market of wind turbines to find the direction of the future developments, one can see a continuous upscaling of wind turbines. Upscaling is performed to harness a larger resource and benefit from economy of scale. This will pose several fundamental implications that have to be identified and tackled in advance. This research focuses on investigating the technical and economical feasibility and limits of large scale offshore wind turbines using the current dominant concept, i.e. a three-bladed, upwind, variable speed, pitch regulated wind turbine installed on a monopile in an offshore wind farm. Thus, the objective of this research is to investigate how upscaling influences the offshore wind turbines. Specifically, following questions are of interest: 1. How do the technical characteristics of the larger scales change with size and can these technical characteristics appear as a barrier? 2. How does the economy of the future offshore wind turbines change with size? 3. What are the considerations and required changes for future offshore wind turbines? To address these questions, a more sophisticated method than the classical upscaling method should be employed. This method should provide the detailed technical and economical data at larger scales and address all the design drivers of such big machines to identify the associated problems. However, interdisciplinary interactions among structure, aerodynamics and control subject to constraints on fatigue, stresses, deflections and frequencies as well as considerations on aeroelastic instability make the development of such a method a cumbersome and complex task. Among many different methods, integrated aeroservoelastic design optimization is found to be the best approach. Therefore, the scaling study of this research is formulated as an multidisciplinary design optimization problem. This method enables the design of the future offshore wind turbines at the required level of details that is needed to investigate the effect of size on technical and economical characteristics at larger scales. Using this method, 5, 10 and 20 MW wind turbines are designed and optimized, including the most relevant design constraints and levelized cost of energy as the objective function. In addition to the design of these wind turbines, the method itself shows a clear way forward for the future offshore wind turbine design methodology development. Based on these optimized wind turbines, scaling trends are constructed to investigate the behavior of a wind turbine as it scales with size. These trends are formulated as a function of rotor diameter to properly reflect the scale. Loading, mass, cost and some other useful trends are extracted to investigate the scaling phenomenon. Blades and tower as the most flexible load carrying components are examined with more attention. Using these results, the challenges of very large scale offshore wind turbines up to 20 MW range are explored and identified. These results demonstrate that a 20 MW design is technically feasible though economically not attractive. Therefore, upscaling of the current wind turbine configurations seems to be an inappropriate approach for larger offshore wind turbines.

The offshore wind industry is growing significantly these and coming years at offshore places far away from the coasts, where wind and wave loads as well as fatigue, corrosion and wear of the substructures are main sources of uncertainty. These uncertainties are important for the design, installation, operation and maintenance of the offshore wind infrastructure. Due to the location, it is important to reach the optimal plans for operation of offshore wind turbines in order to reduce the costs, coming from inspection and maintenance activities. Offshore wind turbines (OWTs) are operated with specific power production specifications which are inherently linked with the reliability levels for the fatigue loads due to wind impact and depending of the component to be analyzed. When OWTs are operated over its design capacity influenced by changes of generator, gearbox, blades or simply by the control configuration, they will be imposed to higher wind velocities and obviously larger fatigue loads. Besides operating over the design power capacity, the location at wind farms can play a detrimental role for the fatigue performance of OWTs. Wind turbines with a wind farm will face up (larger) turbulence coming from wakes generated by surrounding OWTs. Therefore, the structural components of offshore wind turbines have to be able to withstand large fatigue loads during the design life. The operational conditions are characterized by the wind speeds at the site, the wind turbine capacity, the operational modes, and the wind farm layout. Changes in the control configuration of the operational states imply an influence on the fatigue loads of the structural components. Varying the production periods, operational wind speed and desired energy production may therefore have a significant effect on the reliability of the wind turbine components. This paper addresses the influence of the operational configuration on the structural reliability of offshore wind turbines by assessing its life cycle during operational periods with different operational configurations. This implies different influences on the load stress ranges at different wind speeds. The influence of the operational control on the load may be important when offshore wind turbines are intended to be fully exploited in their fatigue life. In the same manner, the wind farm location can vary the operational conditions and add fatigue load. The in-wind farm location and associated wake effects is taking into account by a code-based reliability approach. The stochastic model will be explained in detail and it will be shown how the influence of the operational conditions influence the fatigue reliability by setting up different load-stress ranges, wind intensities and wind turbulence for the case of single/alone and in-wind farm locations. Therefore, a reliability-based approach is used and a probabilistic model has been developed where strength and load uncertainties are described by stochastic variables. SN-curve / Miner’s rule and fracture mechanics approaches are considered to model the fatigue life. Design and limit state equations are established for the accumulated fatigue damage for single (no wake effects) and in-wind farm condition (with wake effects). The acceptable reliability level for optimal fatigue design of OWTs is discussed and results are presented. Further, the influence of inspections is considered in order to extend and maintain a given target safety level. The probabilistic basis for the analysis of fatigue reliability is using the fatigue model, proposed by Sorensen et al. (2008). Welded steel joints in the support structure are considered in this paper. An application example is described for the impact of the operation configuration (control configuration due to over-rate power production) in the life cycle reliability for single (no wake effects) and in-wind farm condition (with wake effects).

Far offshore wind conditions in scope of wind energy

Renewable energy provides a solution for complex current and future social and environmental problems whereas offshore industry has a large potential for providing renewable energy for future. Currently, offshore technology making use of wind for energy generation purpose becomes a hot spot with highly advanced research and development going on on one side and complex and critical problems present and difficult to solve on the other. This paper is trying to study problems related to the quantification of the hydrodynamic and aerodynamic loads for the design of offshore wind turbine support structures in the offshore wind farm. Both the hydrodynamic and aerodynamic conditions in the offshore site are extremely complex resulting in the difficulty of reasonable determination for the external loads on the wind turbine support structures. However, due to the increasing global demands for future energy solution, the design, analysis and optimization of offshore wind turbine is nevertheless an important issue. The paper first gives an introduction of the offshore wind farm and the complexity of the offshore environment. Wave load is explored with introduction of existing wave load models, comparison of their characteristics while the focus is placed on the nonlinear wave load by means of the Stokes higher order wave theory. Properties of a single regular wave based on methods of linear wave theory and Stokes higher order wave theory are compared which lead to differences in the results of wave load models when these two different methods are used. Wind load model is introduced briefly, followed by the introduction of current methods for determination or approximation of combined wave and wind load and also recommendations for practice. Park effect of the wind load and wave load is also introduced at limited depth in the latter stage as a direction for future research. Conclusion and recommendations based on all the above are therefore given at the last section of the paper.

This paper concerns the application of design standards to the design of offshore wind turbines, focusing on the United States. Offshore wind turbines (OWTs) are those wind turbines whose support structures are subject to hydrodynamic loading. A considerable number of OWTs have been installed in Europe, but so far none in the United States. Interest in offshore wind energy is growing in the U.S., and it is expected that projects will reach the design stage in the near future. This paper will facilitate development in the U.S. The paper consists in three highly related parts. The first part gives the background for the development of offshore wind in the U.S. in terms of external conditions and expected design standard requirements. The paper's second part provides a summary of the method and recommendations of the committee draft of the OWT design standard IEC 61400-3 [1] that is believed to represent state-of-the-art. It is placed within the context of other relevant standards. Of particular note are both wind turbine specific standards and standards that are presently used to design offshore structures in the U.S. and internationally. This summary is crucial for the appreciation of the principal differences between designing OWTs, where dynamic response to wind loads is most often dominant, and typical bottom fixed offshore structures. The last part of the paper discusses the level of structural reliability implied by the design rules of IEC 61400-3. There is a long tradition in design of wind turbines to use 50-yr return period values for extreme environmental conditions with associated load safety factors. The tradition in design of offshore structures is to use 100-yr return period values, with or without load factors depending on code format. When developing design standards for OWTs this difference has to be addressed. The paper explains how this has been done in the IEC 61400-3 CD and what the philosophy behind it is been in terms of choice of reliability level. This includes an estimation of the structural reliability under extreme North European environmental conditions. Especially the uncertainty model assumptions are presented in some detail. For hurricanes the uncertainty model will be different possibly implying different load factors if return period is maintained. The paper discusses what should be considered in this regard, e.g. if it is feasible to aim at the same reliability as for U.S. offshore structures. Some simple tools are provided that can be of use for initial assessments structural reliability acceptance criterion. Introduction The initial European work on offshore wind systems were concerned with the scaling and structural and economic optimization of bottom-mounted offshore wind farm systems. Two early examples of this type of design work include the Opti-OWECS (Optimization of Bottom-Mounted Offshore Wind Energy Converters) study [2] and the Dutch Offshore Wind Energy Converter (DOWEC) project [3].

Abstract. The use of meta-models (e.g. Kriging, artificial neural networks, and polynomial chaos expansion) as surrogate models of aeroelastic simulation models offers a good opportunity to perform lifetime calculations with a feasible computational effort. Meta-models for the approximation of fatigue loads, i.e. damage equivalent loads, of wind turbines in normal operation have been researched comprehensively in recent years. For offshore wind turbines in particular, however, downtimes, i.e. the times when wind turbines idle, also have a significant impact the lifetime. Currently, there are no meta-models of idling wind turbines available. However, it cannot simply be assumed that the findings from normal operation can be directly transferred to idling, as the structural behaviour differs significantly from normal operation due to the lack of aerodynamic damping and the resulting larger impact of the wave loads. For this reason, for the first time, the creation of meta-models, more precisely Kriging meta-models, for an idling offshore wind turbine is investigated comprehensively in this paper. The investigation of meta-modelling shows that for the approximation of the rotor blade root bending moments, two additional input parameters have to be considered in addition to the input parameters that are used for the creation of a meta-model for the same offshore wind turbine in normal operation. The comprehensive investigation of the Kriging meta-models shows that the meta-models trained with 2500 data points represent the simulation model with an acceptable approximation quality when choosing suitable Kriging settings.

Wind is a one of the clean resources of energy and has the ability to contribute a considerable share in growing world energy consumption. The small wind turbine plays a vital role in fulfillment of energy needs preferably for household purpose. In order to unleash the budding of applicability of small wind turbine, it is necessary to improve its performance. The performance of a small wind turbine can be distinguished by the manners in which power, thrust and torque vary with the wind speed. The wind power indicates the amount of energy captured by the wind turbine rotor. It is convenient to express the performance of small wind turbine by means of non-dimensional performance curves, therefore in this paper the most graphs are drawn to power, thrust and torque coefficients as a function of the tip speed ratio. This paper presents the effect of design parameters such as the tip speed ratio, angle of attack, wind speed, solidity, number of blades, etc. on the aerodynamic performance of small wind turbine and proposes the optimum values of these parameters for the newly designed blade. The new designed blade consists of two new airfoils and named as IND 15045 and IND 09848. This new profile blade is designed for a wind turbine of 1 kW rated power. The blade is divided into ten sections. The designed length of blade is 1.5 m and it is made using IND 15045 airfoils at three root sections and IND 09848 airfoils for remaining seven sections. Q-Blade is used for the numerical simulation of wind turbine airfoils and blade. It is integrated tool of XFOIL and blade element momentum theory of wind turbine blade design. Also the effect of constant rotational speed operation, effect of stall regulation effect of rotational speed change and the effect of solidity on the performance of wind turbine is discussed. This paper delivers a broad view of perception for design of small wind turbine and parameter selection for the new wind turbine blade. Also in this paper the effect of different losses viz. tip losses, drag losses, stall losses and hub losses on the small wind turbine are discussed. The efficiency of the small wind turbine varies significantly with wind speed, but it would be designed such a way that maximized efficiencies are achieved at the wind speed where the maximum energy is available.

In the middle of 2013, the offshore installed wind capacity was 6.5 GW in European countries while by 2020 the total installed capacity is expected to reach 43 GW mainly by application of offshore wind turbines; the growth of offshore wind industry necessitates introducing concepts with potential of being classed without being much affected by different water depths. Initially, recent patents of offshore wind turbines are presented in this article. Afterwards, stochastic dynamic motion responses and generated power of a semisubmersible floating wind turbine for different water depths of 100 m and 200 m are examined for selected environmental conditions. The mooring line design is correspondingly modified to provide proper station keeping as well as appropriate interactive influence on the dynamic responses of the semisubmersible wind turbine without affecting the generated power. Fully coupled aero-hydro-servo-elastic numerical models have been developed and integrated time-domain approach has been applied in order to investigate the dynamics of the semisubmersible floating wind turbine. The motion responses, mooring lines effective tension as well as wind turbine performance are compared for different water depths and environmental conditions. Hence, the present semisubmersible floating wind turbine can be classified which means an optimized design can easily be modified for new offshore site with different water depth without the need of major modifications. Keywords: Offshore floating wind turbine, Classification, wind and wave induced dynamics.

Most offshore wind turbines (OWTs) are designed according to the international standard International Electrotechnical Commission (IEC) 61400-3 which requires consideration of several design load cases under extreme sea state conditions during which the wind turbine is in survival mode (i.e. the rotor is parked and blades are feathered). Each of these load cases depends on combinations of two random variables, the mean wind speed and the significant wave height, both with a mean return period of 50 years. The response of an offshore wind turbine under wave loading is known to be sensitive to both the significant wave height and a frequency measure of the sea state such as the peak spectral period. The IEC standard states that design calculations for the extreme sea state should be based on values of peak spectral period which result in the highest loads acting on the structure, but does not provide additional guidance. The Standard does provide a deterministic range for the period of the extreme wave conditioned on the significant wave height, and this can be converted to a range of peak spectral period using published empirical relationships. This paper considers an offshore location off the coast of Georgia, where the National Oceanic and Atmospheric Administration (NOAA) buoy 41008 is located, and shows that a deterministic range of peak spectral period converted from the range provided in the IEC Standard may not accurately represent measured data. Moreover, the paper shows that the response of a hypothetical offshore wind turbine, installed at this location and supported by a monopile foundation, is sensitive to variation in the peak spectral period, emphasizing the importance of modeling the turbine for an appropriate and possibly site-specific range of peak spectral periods. A probabilistic approach is proposed to find an appropriate and site-specific range of the peak spectral period for the design of offshore wind turbines under the extreme sea state.

In addition to a carbon-neutral vision being recognized worldwide, the utilization of wind energies via horizontal-axis wind turbines, especially in offshore areas, has been intensively investigated from an academic perspective. Numerical simulations play a significant role in the design and optimization of offshore wind turbines. The current review focuses on studies concerning the numerical simulations of offshore wind turbine dynamics, including the modelling of the aerodynamic and hydrodynamic conditions of the environment and the reduced-order modelling of the wind turbine dynamic responses. In detail, the functions and mechanisms of each module in the numerical simulation of the wind turbine dynamics are articulated, which in turn demonstrates its importance for the design of offshore wind turbines, and hence the development of the offshore wind industry. Based on this review, it is argued that the vertical variations in wind velocities, the blade element momentum theory, the wave dynamic models, and the reduced-order model for structural dynamics are the major concerns for the numerical simulation of wind turbines. Consequently, such directions should be emphasized in future studies.