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, 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?
Nowadays, the pile-supported structures have been applied in many offshore industries, especially in the wind energy field. As a new type of stationary pile-supported structure, high-rise pile-cap structures are attracting more attention. In the present study, a fully nonlinear solver based on Navier-Stokes equations is established for the investigation into a ten-pile cap structure. The wave loads are obtained by integrating pressure over the surface of each part of the structure. The effects of the interaction between the piles and the cap are investigated. In some cases that the cap is on the top, the wave loads on the piles are 30 percent larger than that without the cap. This is caused by the effect of impact. It means that the wave loads are underestimated significantly by simply using Morison equation. The relationships between the wave loads on piles and the gap between the cap and still, the wave height, and the wavelength are also discussed in detail.
In the recent decades, the pile-supported structures are commonly found in the costal and offshore environment, especially in the wind energy field for Offshore Wind Turbine (OWT). They are generally built by means of a group of piles in different arrangements.The long-term safety of the pile group-supported structures is still a major concern in coastal engineering. As a consequence, the prediction of wave loads on offshore pile group- supported structures is of great importance.
Nowadays, we have already had some typical types of pile-supported structures, such as monopile, suction pile, pile cap and so on (Ryu,et,al.,2012). As a new type of stationary pile group-supported structure, high-rise pile-cap structure is attracting more attention. It has lots of advantages, such as high stiffness, reasonable cost, easy mounting, etc. (Chen & Zhou.et al.,2016). Generally, for pile group-supported structures, the total wave force could be obtained by calculating each single cylinder pile with Morison equation (Morison, J.et al., 1950). However, the effect of different interference parameters is difficult to estimate, especially the pile group effect. Zdravkovich (2003) discussed the effects of the interference parameters on the overall drag forces for pile groups. Through some small-scale and large-scale experiments, Bonakdar et al (2012) and Bonakdar (2014) reviewed that the interference parameters such as the relative spacing between piles and the number of neighbouring piles could noticeably affect the wave loads on every single pile. Anew characteristic geometric scale called effective diameter is introduced by Qu et al (2017), and the total inline wave force on different complex pile-supported structures could be estimated by the unified empirical formulas.Moreover, the interaction between the cap and the piles should also be taken into consideration when the wave hits the cap. Therefore,the Morison Equation is no longer appropriate for estimating the inline wave force of the high-rise pile-cap structure. Meanwhile, the cap is much larger than the other parts of the structure and it will also lead to a larger lift force on the structure.
Wind power generation is gradually expanding from land to sea. In floating offshore wind turbine, the position of floater is maintained by the mooring line, and it is difficult to predict the behavior by the combined action of wind and wave forces. To create optimal design conditions of floater and mooring line, a combined analysis of aerodynamics and hydrodynamics should be performed. Therefore, in this study, we have analyzed the behavior of the floater under wind and wave loads considering the aerodynamic damping effects of rotating blade. We used FAST provided by NREL for numerical simulations. Experiments were carried out in the Ocean Engineering Wide Tank, University of Ulsan, using the reduced OC3 Spar type platform model.
Since the Paris Conference of the Parties in 2015, interest in renewable energy around the world is higher than ever. Among them, wind power generation has achieved remarkable growth so that power generation cost can compete with the coal-fired power plant. Wind turbines on the land has a disadvantage of making noise caused by the blades, and it is difficult to secure a tract of land. On the other hand, offshore wind turbine is relatively easy to secure a large area, and it is free from noise even in the enlargement of the turbine. Also, it can obtain more persistent and stronger wind than on land. Therefore, wind power generation is gradually expanding from land to sea. In floating offshore wind turbine, to create optimal design conditions of floater and mooring line, a combined analysis of aerodynamics and hydrodynamics should be performed. Therefore, in this study, we have analyzed the behavior of the floater under wind and wave loads considering the aerodynamic damping effects of rotating blade. There are several papers (Salzmann and Temple, 2005; Myers and Valamanesh, 2014) that have already been studied. In this paper, presents the estimated results from the numerical simulations and the model scale experiments of the OC3-Hywind 5-MW Floating Offshore Wind Turbine. With a 1/128 scale ratio, model tests were carried out in the Ocean Engineering Wide Tank of University of Ulsan. Numerical simulations done by NREL-FAST v8. FAST v8 was performed according to following FAST User's Guide (Buhl and Jonkman, 2005). The objective is to analyze motion of floater considering aerodynamic damping effect at the combined wind/wave conditions.
Based on the CFD-CSD (Computational Fluid Dynamics-Computational Structure Dynamics) coupled model, the antiliquefaction effect of stone layers on liquefiable seabed is studied. Under the extreme waves, the dynamic response of the seabed around the wind turbine in the Xiangshui area of Jiangsu Province was simulated. By comparing the seabed response results of four tests with cover stones under the same wave conditions, it shows that the thickness and porosity of the cover stones are two important parameters of anti-liquefaction capacity.
The stability of composite bucket foundation of offshore wind turbine under wave action is very important for the development of offshore wind power technology (Zhang et al., 2016). Under the action of wave, the seabed liquefaction occurs due to periodic changes in pore water pressure and effective stress in the seabed. It is of great scientific significance and great engineering value to study the liquefaction and anti-liquefaction measures of the seabed soil under the action of waves for the stability of the offshore wind turbines, especially for the steady development of the offshore wind power foundation.
Based on the analytical method, experimental study and numerical simulation, the study of liquefaction and stability of seabed under wave action is mainly concentrated on three aspects: (1) the pore water pressure in the time and space, the effective stress state and shear strength in the seabed are analyzed (Hsu and Jeng, 1994; Jeng, 1997; Jeng, 2013; Ye et al., 2018); (2) the wave- structure-seabed interaction is investigated (Sumer, 2014; Ye et al., 2015; Zhang et al., 2016; Sui et al., 2017); (3) in order to accurately analyze the liquefaction of seabed, a series of seabed liquefaction standards have been put forward (Ye, 2012), and a series of anti-liquefaction methods have been studied (Yang et al., 2004; Susana and Rafeal, 2006; Sumer et al., 2010; Zhang et al., 2014; Huang et al., 2015).
Currently, the development of renewable energy has become a trend with the increasing demand for energy. Wind energy, as a renewable source of energy, is also getting more attention. Increasing effort is devoted to developing floating offshore wind turbines in deep water. In this paper, a V-shaped semisubmersible floating wind turbine was adopted to investigate the dynamic response of the system. Numerical simulations are conducted using aero-hydro coupled analysis in a time domain. The performance of the V-shaped semisubmersible floating wind turbine with respect to global platform motion, mooring line tensions and tower base moment is evaluated in this study. It turns out that the V-shaped semisubmersible offshore wind turbine is a promising concept that provides a good practice for the application of wind energy in deep water in the future.
Currently, due to energy deficiency, many countries are devoted to developing renewable energy to meet energy demands. According to the Chinese 13th renewable energy development five year plan, by 2020, total electric from renewable energy will grow to up to 27% of the total electricity generated(NDRC, 2017). Wind energy, one of the promising renewable energies, has attracted more and more attention because of its low environmental pollution. Compared with onshore wind energy, offshore wind energy has better wind condition, unlimited sites and negligible environmental impact. Especially in China, the area with rich onshore wind resource is far from the energy consumption center, which is located near the eastern coastline (Li et al., 2012). A number of studies have been carried out for offshore wind turbine analysis (Jiang et al., 2015; Shi et al., 2016; Shi et al., 2014). The bottom-fixed wind turbine is not suitable for deep water due to increase in cost (Shi, 2015). Therefore, the floating offshore wind turbine (FOWT) is becoming one of the promising solutions.
According to the offshore oil and gas industry, several different foundations are suitable for FOWT: spar buoy, tension leg platform (TLP), semi-submersible platform and barge. In particular, the semisubmersible platform, compared with spar buoy and TLP, has more feasibility in various water depth, seabed conditions and low installation costs due to the simpler installation (it is fully constructed onshore). The semi-submersible platform can also avoid the main energy range of the waves because of its relatively large natural period. The OC4 semi-submersible offshore wind turbine was simulated by Bayati (2014) to focus on the impact of second-order hydrodynamics on semi-submersible platforms. Moreover, the second-order hydrodynamic force can stimulate the oscillation of the platform and further cause fatigue damage to the structure. How the mooring systems influence the motion of the FOWT (Masciola and Robertson, 2013) determined by using coupled and uncoupled model on DeepCwind semi-submersible FOWT. Luan et al. (2016) employed a braceless semi-submersible platform to establish a numerical model and performed extreme sea states analysis on a braceless semi-submersible platform. The results showed that the platform has good stability under extreme sea and is a good design concept. A 5 WM wind turbine was employed by Kim et al. (2017), and WindFloat and OC4 floating platform were carried out to focus on the motion of FOWT and evaluating the mooring system force by using FAST (Jonkman, 2005) code.
Chen, Ling (University of Chinese Academy of Sciences, Beijing) | Zhou, Jifu (Chinese Academy of Sciences, Beijing) | Wang, Xu (Chinese Academy of Sciences, Beijing) | Wang, Zhan (Chinese Academy of Sciences, Beijing)
A new type of bottom-fixed structure, the so-called high-rise pile cap foundation, has been proposed and used to support offshore wind turbines in the Donghai Bridge Wind Farm, China. Engineers are unaware of the wave load mechanisms for this new structure. Using the Navier–Stokes equations and volume of fluid technique, a fully nonlinear numerical wave tank is established to investigate free surface wave loads and moments for the new structure. The interaction between the cap and piles are discussed in detail. In the case of fully nonlinear waves, the maximum horizontal wave load on all the piles with the cap can increase by 30% compared with those without the cap, and the maximum horizontal wave load on a single pile is nearly doubled. The horizontal wave load on the cap with the piles can increase by about 15%, while the vertical wave load decreases slightly. The conventional Morison formula and diffraction theory generally underestimate the wave loads on the piles and the cap as well.