Müller, Nathalie (Fraunhofer-Institut für Windenergie und Energiesystemtechnik (IWES)) | Kraemer, Peter (University of Siegen) | Leduc, Dominique (Research Institute of Civil Engineering and Mechanics (GeM)) | Schoefs, Franck (Research Institute of Civil Engineering and Mechanics (GeM))
A fatigue test has been conducted on a large-scale offshore wind turbine grouted connection specimen at the Leibniz University of Hannover. For detecting damages in the grouted joint, a structural health monitoring (SHM) system based on fiber optic sensor-type fiber Bragg grating (FBG) has been implemented. By extracting the features of the FBG signal responses using the Wigner–Ville distribution (WVD) and one of its marginal properties, the energy spectral density (ESD), it is possible to detect the occurrence and the global severity of the damage. Some information about the local severity of the damage has also been obtained.
The grouted connection consists of the high-performance grout-filled space between the two structural steel components of respectively the sleeve and the pile of offshore wind turbines (OWTs). For monopile OWTs, it is located around the water level between the transition piece and the pile, whereas for jacket and tripod OWTs, it is located just above the seabed, between substructure and foundation pile. While grouted joints for monopiles are exposed to bending moments, grouted joints for latticed substructures (tripods and jackets) are exposed to predominant axial loadings and low torsional moments (Schaumann and Böker, 2005; Schaumann, Lochte-Holtgreven et al., 2010). It is a critical structural part of OWTs. In 2009–2010, engineers reported grouted connection failures causing slight and progressive settlement of turbines. The problem affected approximately 600 of the 988 monopile wind turbines in the North Sea, requiring further investigations concerning the design of the grouted connection (Rajgor, 2012). Since then, two grouted connection designs reducing the axial forces in this area have been recommended by Det Norske Veritas (2014): using a conical grouted connection (first design) or a tubular connection with shear keys (second design).
Wendt, Fabian F. (National Wind Technology Center, National Renewable Energy Laboratory) | Robertson, Amy N. (National Wind Technology Center, National Renewable Energy Laboratory) | Jonkman, Jason M. (National Wind Technology Center, National Renewable Energy Laboratory)
During the Offshore Code Comparison Collaboration, Continued, with Correlation (OC5) project, which focused on the validation of numerical methods through comparison against tank test data, the authors created a numerical FAST model of the 1:50-scale DeepCwind semisubmersible system that was tested at the Maritime Research Institute Netherlands ocean basin in 2013. The OC5 project revealed a general underprediction of loads and motions by the participating numerical models. This paper discusses several model calibration studies that were conducted to identify potential model parameter adjustments that help to improve the agreement between the numerical simulations and the experimental test data. These calibration studies cover wind-field-specific parameters (coherence, turbulence), and hydrodynamic and aerodynamic modeling approaches, as well as rotor model (blade-pitch and blade-mass imbalances) and tower model (structural tower damping coefficient) adjustments. These calibration studies were conducted based on relatively simple calibration load cases (wave only/wind only). The agreement between the final FAST model and experimental measurements is then assessed based on more complex combined wind and wave validation cases. The analysis presented in this paper does not claim to be an exhaustive parameter identification study but is aimed at describing the qualitative impact of different model parameters on the system response. This work should help to provide guidance for future systematic parameter identification and uncertainty quantification efforts.
Sun, Xiao-Qian (Zhong Neng Power-tech Development Co. Ltd.) | Cao, Shu-Gang (Zhong Neng Power-tech Development Co. Ltd.) | Chi, Yan (Zhong Neng Power-tech Development Co. Ltd.) | Zhu, Zhi-Cheng (Zhong Neng Power-tech Development Co. Ltd.)
This study investigated a vibration and tilt monitoring system for an offshore wind turbine constructed using a high-rise-pile- cap supporting foundation, which is the first offshore wind power project in South China with a batholith seabed. The analysis of data collected by the system during the 2016 typhoon Meranti showed that the typhoon significantly affected vibration and instantaneous tilt of the supporting system without any significant change to the first natural frequency. Additionally, it did not produce any permanent inclination, indicating that no serious structural failure occurred under the influence of the typhoon. However, during the typhoon, the vibration acceleration, vibration intensity, and the effective inclination of the high-rise-pile-cap supporting system using rock-socketed piles were smaller than those with driven frictional piles, indicating that the former is better than the latter in terms of resistance to vibration and tilt.
The construction of offshore wind power plants in China faces many challenges, including the raging typhoons in the East and South Seas. Each year, the Guangdong province experiences typhoons three times on average, accounting for 33% of the annual typhoons in China’s coastal areas. The proportions of typhoon episodes in Taiwan, the Hainan province, the Fujian province, and the Zhejiang province are 19%, 17%, 16%, and 10%, respectively (Wu and Li, 2012). The extreme vibration and abnormal inclination of the offshore wind turbine supporting system as a result of typhoons sometimes lead to structural failures and can even result in the collapse of the wind turbine structure into the ocean.
Wind turbine towers are being planned in ice covered regions subject to pressure ridges (e.g. the Great Lakes). Conical collars are often employed to reduce ice loads from level ice and their associated dynamics. For level ice, downward breaking cones have some advantages. It is not clear if this is the case for pressure ridges. This paper presents an improved method for ridge loads on wind turbines with downward breaking cones and makes comparisons with upward breaking cones.
First year pressure ridges can be formidable ice features and usually control design ice loads in the sub-Arctic. Important components of a ridge creating ice loads are the consolidated layer at the surface (which is considered as solid ice) and the ridge keel below consisting of ice rubble, but much thicker. The load due to the consolidated layer is usually derived as if it is thick level ice. On a cone, methods for level ice assume it can be idealized as a plate on an elastic foundation (the water) and equations have been developed for upward and downward breaking cones. But for a ridge on a downward cone, to break the consolidated layer downwards requires it to be pushed into the keel rubble below. This will have a different foundation modulus than water buoyancy. A method is developed to account for this difference. The method uses an iterative approach to determine the point of breaking of the consolidated layer (and associated load) accounting for the ridge geometry, keel rubble shear strength, the flexural strength of the consolidated layer and the buoyancy forces. The keel loads on the vertical shaft below the conical collar are based on the method currently in ISO 19906 (2010) but modified to add the effect of additional rubble in the keel from breaking the consolidated layer downwards.
In examples, it is shown that the breaking force can be about twice that of breaking the consolidated layer without the keel present. This might be seen as a disadvantage for downward breaking cones vs upward breaking. However, it is also shown that the clearing forces on an upward cone are higher; which tends to balance out the lower breaking force. Example loads are given on typical wind turbine bases due to typical ridges. Upward and downward breaking configurations are compared.
The paper provides new methods for ice loads due to ridges acting on wind turbine structures not currently covered by existing methods.
Wind turbine, an efficient way to sustainably generate electricity, of which the noise problem would affect the living environment adversely. This paper presents the results of the aerodynamic and aero-acoustic calculation of a vertical axis wind turbine. The IDDES technique and FW-H acoustic analogy are adopted to conduct all simulations. The results indicate that the combination of thickness and loading noise are the dominant noise sources at tonal peak frequency, and quadrupole noise has negligible influence. Rotational speed and receiver distance will significantly affect noise level. This work can be exploited to design quieter vertical axis wind turbines.
In recent years, the demands of renewable energy have attracted more public attention. As a clean and sustainable renewable energy, offshore wind energy has been utilized by wind turbines to generate electricity. However, one offensive problem, noise pollution, would affect the living environment of nearby creatures. Especially in several offshore wind turbine farms, birds and other animals, have left for new habitats. Therefore, it is an important issue to simulate and evaluate wind turbine noise.
According to the direction of rotation, wind turbines can be divided into two major categories: horizontal axis wind turbines (HAWT) and vertical axis wind turbines (VAWT) (Borg, Collu and Brennan, 2012). Since the wind turbines require further performance optimization to be competitive with other energy devices, the geometrical design, aerodynamic performance and optimal solutions are continuing to be investigated. Bae et al. (Y.H. Bae, M.H. Kim and H.C. Kim, 2017) studied a floating offshore wind turbine with broken mooring line. The power production and structural fatigue life were checked respectively, and some risk assessments were conducted. Rezaeiha et al. (Rezaeiha, Kalkman and Blocken, 2017) conducted researches on the effects of pitch angle on power performance and aerodynamics of a vertical axis wind turbine. A 6.6% increase in power coefficient could be achieved using a pitch angle of 2 degree at a tip speed ratio of 4 was shown in the results. MirHassani and Yarahmadi( MirHassani and Yarahmadi, 2017) investigated the wind farm layout optimization under uncertainty. A mixed integer quadratic optimization model is developed based on the interaction matrix for multi-turbine wake effects considering different hub height wind turbines. Compared to the conventional HAWTs, the VAWTs show many superiorities, including universal wind exposure, relatively simple blade structure, lower maintenance costs and lesser aerodynamic noise (Tjiu, Marnoto, Mat, Ruslan and Sopian, 2015).Although the noise generated by VAWT is lesser than that caused by HAWT, VAWT's noise is not negligible. Noise generated by operating wind turbines can be divided into mechanical noise and aerodynamic noise. Mechanical noise is generated by different machinery parts. Aerodynamic noise is produced from the moving blades and is mainly associated with the interaction of turbulence with the blade surface (Ghasemian, Ashrafi, and Sedaghat, 2017). Mechanical noise can be decreased by some engineering methods, while the reduction of aerodynamic noise is still a problem.
The support structure of offshore wind turbines is working in harsh ocean environments, where uncertainties exist and affect the performance of the whole system. This work presents an efficient methodology for the Reliability Based Design Optimization (RBDO) of the support structure of offshore wind turbines considering uncertainties. Reliability analysis is a feasible option in the absence of field measurement data. Monte Carlo simulations are robust and used as reliability analysis benchmark, but they are very computationally demanding for offshore wind turbine cases. Efficient Fractional Moment reliability analysis method was proposed. The results show that the proposed methodology can obtain a reliable design with better dynamic performance and less weight. Compared with the deterministic optimization, the presented dynamic RBDO of offshore wind turbines is more practical, and this methodology can be applied in the design of other similar offshore structures.
The support structure of offshore wind turbines is working in harsh ocean environments, reliability analysis is a feasible option in absence of field measurement data (Yang et al., 2017). To ensure that the proposed offshore wind turbine design is cost effective, it is necessary to check whether the decided support structures provides optimal life cycle cost.
For a reliable design, it is essential to consider various uncertainties in the dynamic analysis of offshore wind turbine (Xiao and Yang, 2014; Zhang et al., 2017). Due to the random nature of environmental parameters, wave, wind and currents must be modelled as stochastic process (Zhang and Yang, 2014). Hence, there is a need of stochastic dynamic analysis on one hand and the need of developing performance assessment, maintenance and optimization of the offshore wind turbine system with uncertainties. We try to answer the following questions: a) Can we formulate an efficient and accurate method for reliability analysis to replace Monte Carlo simulations which are robust but too time consuming; b) How to overcome computational challenges associated with reliability-based optimization methodology of offshore wind turbine system?
This paper presents the application of a risk- and reliability-based inspection planning framework for the InnWind 20 MW reference wind turbine jacket substructure. A detailed fracture mechanics-based fatigue crack growth model is developed and used as a basis to derive optimal inspection plans for the jacket substructure. Inspection plans for different inspection techniques are proposed, and recommendations on how to optimize inspection intervals are discussed.
Upscaling current wind turbines to very large wind turbines is considered as one of the important ways to decrease the levelized cost of energy (LCoE) of wind energy. Steel jacket structures are one possible type of support structure for very large offshore wind turbines and have been considered in the EU InnWind project, INNWIND.EU (http://www.innwind.eu). Reliability with respect to fatigue failure is generally driving the design of offshore wind turbine jacket structures and is being considered in this paper in combination with applications of reliability-based inspection planning.
In this work, we present four different methodologies for reducing the computational effort of fatigue assessment for offshore wind turbine support structures. To test these methods, we use them to predict the total fatigue damage of several modified support structure designs based on subsets that represent a reduction of about 6-17 times the original size of the load case set. Three of the methods are able to give quite accurate predictions, with expected errors of no more than 4-8%, though there are some limitations due to the variance inherent in some of the methods.
One of the main challenges for the design of offshore wind turbines support structures is the complexity of both the structure itself and the offshore environment. This complexity means that assessing the performance of the structure requires not only the use of detailed models, but also investigating a large number of different scenarios. Specifically, with reference to the standards that the design must conform to (e.g. International Electrotechnical Commision (2009)), there are literally thousands of different design load cases (DLCs) that must be assessed for any given structure, covering both all the various environmental states one expects to encounter at a given site and all the various scenarios that the structure is likely to experience. To summarize, we need to run detailed models and we need to run them many times. For one single assessment of a design, this can be accommodated by ever improving computer hardware and increased access to computer clusters for both institutions and individuals. However, for those wishing to run either probabilistic assessments or to optimize the design (or worse still, both of these at the same time), the large number of DLCs remains an important challenge. One that should be addressed not just by improved hardware, but by improved methodology. This is the main topic of the work to be presented below.
As it stands, it is not possible to completely replace the standard assessment with something new. Rather, one seeks to approximate the results of such full assessments by a less computationally demanding procedure. If the approximation is good enough, it may then serve well as a replacement for the conventional procedure when small deviations from the true estimates (e.g., fatigue damage) are allowable. Especially in a context like optimization, simplifications leading to such small deviations are often expected and, if the size of the deviations can be estimated, one may even incorporate these as modeling errors in a probabilistic analysis. Previous work attempting to find approximate simplified assessments have encountered some success, but have tended to be very simplified (for example in terms of the types of DLCs studied), have struggled to get a sufficiently accurate approximation while also getting a sufficient decrease in analysis time or have faced a combination of these issues. One approach is to completely abandon the time domain and instead attempt to analyze the structure in the frequency domain (see e.g. van der Tempel (2006)), but this approach has its own set of issues and we will here focus on methods in the time domain.
This paper presents an analytical study on the risk and vulnerability assessment of an Offshore Wind Turbine (OWT) subjected to coupled hydrodynamic and aerodynamic loads. The Computer Aided Engineering (CAE) tool FAST v8 simulator, developed by National Renewable Energy Laboratory (NREL), is used for the multi-hazard simulation of a “NREL offshore 5-MW baseline wind turbine”. FAST is able to incorporate non-linearity coupled with both hydro and aero dynamic effects resulting from wind-and-wave loading scenarios. Site characteristics of the OWT are considered based on Nantucket Sound, Massachusetts, the United States, which is an ideal site for a future U.S. wind farm. The target site that belongs to the east coast is regarded to be a more hurricane-prone region; thus, this paper utilizes an extreme turbulent model (ETM) coupled with irregular waves determined based on Pierson-Moskowitz spectrum. The OWT supported by a fixed-bottom foundation is modeled with multi-degree-of-freedom modules enabling the time-domain coupled analysis. The OWT is simulated, considering the extreme loading scenarios specified by the International Electrotechnical Commission (IEC 61400-3) design standard that takes variability of both wind and waves into consideration. Structural responses of the OWT subjected to coupled wind and wave loads are captured at various critical locations across the overall system, and the flexural demands of the OWT at the mudline are found to be critical in evaluating its failure mechanism. Peak flexural demand quantities are then employed for the development of vulnerability functions for variations in wind and wave characteristics, including wind speed and wave height. The limit state function pertaining to flexural failure mode used for the vulnerability determination is based on First Order Reliability Method (FORM). The analysis of the resulting vulnerability data reveals that the exceeding probability increases due to increase in both wind speed and wave height, especially beyond 12m/s, while the wave height has less impact on the probability than the wind speed until the wave height of 10 m is reached.
The paper presents seismic fragility curves and the seismic reliability assessments for a wind turbine support structure. A National Renewable Energy Laboratory (NREL) 5 MW reference wind turbine finite element model is first built by beam and shell elements using ANSYS finite element package to conduct seismic response spectrum analysis (RSA). The realistic earthquake data reported from Pacific Earthquake Engineering Research Center (PEER) ground motion database are taken into account in this work. Totally 500 earthquake response spectra derived by 100 Taiwan Chi-Chi earthquake data combined with magnification factors of 1,2, 4, 7 and 10 are imposed on the wind turbine support structure model as the loading conditions. Through Monte Carlo simulations (MCS), the peak displacements and peak stresses can be obtained from FEM corresponding to the earthquake peak ground acceleration (PGA) for further analyses. Then the fragility curves could be numerically derived from FEM results using maximum likelihood estimator (MLE). In addition, the Taiwan seismic hazard maps in terms of 475 and 2475 years return period are further considered to estimate the statistical parameters for annual maximum PGA distribution functions. Finally, the annual reliability index of the wind turbine support structure can be thus obtained from the fragility curves and annual PGA distributions.
Wind Energy becomes a swift-growing industry for electricity production to contribute the potential renewable energy. The development of wind turbine as shown in Fig. 1 (Ambrose, 2017) in production capability and in facility size has been rising rapidly in past decades. The World Energy Outlook 2016 report published by International Energy Agency have predicted that wind generation capacity would increase from about 384GW in 2015 to 1,452 GW in 2040 according to the New Policies Scenario (NPS) that incorporates existing energy policies as well as some assessment results risen from possible implementation of announced intentions. It is likely to turn out that the wind turbine facility will become more enormous consecutively.