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    Condition monitoring of wind turbine blades and tower via an automated laser scanning system
    (Elsevier Sci Ltd, 2019) Dilek, Ahmet U.; Oguz, Ali D.; Satis, Furkan; Gokdel, Yigit D.; Ozbek, Muammer
    During its service life, a wind turbine has to withstand very challenging loads and severe environmental factors which can cause some deteriorations and cracks on structural components. Yearly periodic controls may not be sufficient to diagnose a fault at early stages. Therefore, wind turbines are very important systems which have to be monitored continuously. This work aims at developing an automated scanning system for IR (Infrared) laser vibrometers to conduct the tests and measurements, which are required for condition monitoring, in a more accurate and efficient manner. Unlike visible green or red lasers, IR laser is reflected by most surfaces (especially by blade material) with a very high intensity without needing a preparation or surface enhancement on the target. However, IR laser cannot be guided by refracting/reflecting the beam by lenses and mirrors used by conventional laser scanners. This work proposes a new methodology which is based on mounting the laser source on a motorized platform and then rotating the platform rather than guiding the laser beam by optical systems. The tests performed by using wind turbine models show that the proposed system is capable of guiding the laser to the desired measurement points with very high precision and efficiency. The results of the analyses also show that some important dynamic characteristics of the system such as Eigenfrequencies and mode shapes can be extracted very accurately.
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    Development of Automated Laser Scanning System for Structural Health Monitoring of Wind Turbines
    (IEEE, 2017) Dilek, A. Ugur; Oguz, Ali D.; Gokdel, Y. Daghan; Ozbek, Muammer
    In recent years, the growing interest and demand in wind energy requires developing new wind turbines with larger size and capacity. However, this situation also causes some new and challenging problems which have not been encountered before in design, operation and maintenance. Strong winds and severe environmental factors can cause cracks and damage on several turbine components such as blades and tower. Determination of the location and extent of this damage at early stages is essential to ensure safe and efficient operation of the turbine. This work aims at developing a new motorized laser Doppler scanning system which can rotate about two axes freely. This new device will enable structural dynamics of the turbine to be monitored remotely by using optical measurement techniques. The developed laser system will be used to take vibration measurements on the structure regularly. The acquired measurements will be compared with the reference measurements taken on the undamaged structure and be used to detect possible damage on the turbine while turbine is parked.
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    Feasibility of Offshore Wind Energy in Turkey; A Case Study for Gulf of Edremit at the Aegean Sea
    (Gazi Univ, 2021) Ozbek, Muammer; Tunc, Kerim Mehmet Murat
    This work aims to investigate the feasibility of offshore wind energy in Turkey by performing extensive simulations and analyses for a 50-90 MW capacity offshore wind park to be constructed at the Gulf of Edremit, a very high potential site located at the Aegean Sea, 3 km from the west coastline of Turkey. The location and layout of the farm is optimized by considering the sea depth data obtained from bathymetric maps and wind speed measurements acquired from the MERRA data and local met mast. Dynamic interactions among the turbines and the resulting wake losses are modeled by using two different site assessment programs. Capacity factors and the annual energy production yields are calculated for 8 different types of turbines with rated powers changing between 2 and 3.6 MW.
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    A new analysis methodology for estimating the eigenfrequencies of systems with high modal damping
    (Academic Press Ltd- Elsevier Science Ltd, 2016) Ozbek, Muammer; Rixen, Daniel J.
    Contemporary system identification algorithms are well proven to provide accurate eigenfrequency estimates in analyzing the systems with low modal damping. Since most engineering structures usually have low damping ratios the corresponding response characteristics can easily be obtained by conventional methods. Indeed, these modes can be extracted by using relatively short measurement durations (150-200 cycles of the lowest frequency included in the data block). However, some specific applications such as analyzing the in-operation vibration behavior of MW scale large wind turbines also require an accurate estimation of the modes with high damping. For a rotating wind turbine, some important turbine modes (e.g. flapwise rotor modes) have very high aeroelastic damping, which make them very difficult (if not impossible) to be detected. Extracting these high damping modes is a challenging task for almost all system identification techniques that are currently in use. In this work, a new method, which is based on Natural Excitation Technique (NExT), is proposed as an alternative approach for extracting the eigenfrequencies of high damping modes in an efficient way. NExT is a well established experimental dynamic analysis tool which was specifically developed to extract the dynamic characteristics of wind turbines in the early 90s. However, during the analyses it was observed that conventional NExT algorithm requires analyzing very long measurement durations (4500-5000 cycles) to be able to estimate the high damping modes accurately. A new method proposed in this work enables the eigenfrequencies of high damping modes to be estimated by using data series which are approximately 30 times shorter (around 150 cycles) than those required for a standard NExT algorithm. (C) 2015 Elsevier Ltd. All rights reserved.
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    Simulation of electricity generation by marine current turbines at Istanbul Bosphorus Strait
    (Pergamon-Elsevier Science Ltd, 2016) Yazicioglu, Hasan; Tunc, K. M. Murat; Ozbek, Muammer; Kara, Tolga
    In this work, several simulations and analyses are carried out to investigate the feasibility of generating electricity from underwater sea currents at Istanbul Bosphorus Strait. Bosphorus is a natural canal which forms a border between Europe and Asia by connecting Black Sea and Marmara Sea. The differences in elevation and salinity ratios between these two seas cause strong marine currents. Depending on the morphology of the canal the speed of the flow varies and at some specific locations the energy intensity reaches to sufficient levels where electricity generation by marine current turbines becomes economically feasible. In this study, several simulations are performed for a 10 MW marine turbine farm/cluster whose location is selected by taking into account several factors such as the canal morphology, current speed and passage of vessels. 360 different simulations are performed for 15 different virtual sea states. Similarly, 8 different configurations are analyzed in order to find the optimum spacing between the turbines. Considering the spatial variations in the current speed within the selected region, the analyses are performed for three different flow speeds corresponding to +/- 10% change in the average value. For each simulation the annual energy yield and cluster efficiency are calculated. (C) 2015 Elsevier Ltd. All rights reserved.
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    Stability Control of Wind Turbines for Varying Operating Conditions Through Vibration Measurements
    (Springer Int Publishing Ag, 2015) Ozbek, Muammer; Rixen, Daniel J.
    Wind turbines have very specific characteristics and challenging operating conditions. Contemporary MW-scale turbines are usually designed to be operational for wind speeds between 4 and 25 m/s. In order to reach this goal, most turbines utilize active pitch control mechanisms where angle of the blade (pitch angle) is changed as a function of wind speed. Similarly, the whole rotor is rotated toward the effective wind direction by using the yaw mechanism. The ability of the turbine to adapt to the changes in operating conditions plays a crucial role in ensuring maximum energy production and the safety of the structure during extreme wind loads. This, on the other hand, makes it more difficult to investigate the system from dynamic analysis point of view. Unexpected resonance problems due to dynamic interactions among aeroelastic modes and/or excitation forces can always be encountered. Therefore, within the design wind speed interval, for each velocity increment, it has to be proven that there are no risks of resonance problems and that the structure is dynamically stable. This work aims at presenting the results of the dynamic stability analyses performed on a 2.5-MW, 80-m-diameter wind turbine. Within the scope of the research, the system parameters were extracted by using the in-operation vibration data recorded for various wind speeds and operating conditions. The data acquired by 8 strain gauges (2 sensors on each blade and 2 sensors on the tower) installed on the turbine were analyzed by using operational modal analysis (OMA) methods, while several turbine parameters (eigenfrequencies and damping ratios) were extracted. The obtained system parameters were then qualitatively compared with the results presented in a study from the literature, which includes both aeroelastic simulations and in-field measurements performed on a similar size and capacity wind turbine.
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    Structural Health Monitoring of Multi-MW-Scale Wind Turbines by Non-contact Optical Measurement Techniques: An Application on a 2.5-MW Wind Turbine
    (Springer Int Publishing Ag, 2015) Ozbek, Muammer; Rixen, Daniel J.
    Optical measurement systems utilizing photogrammetry and/or laser interferometry are introduced as cost-efficient alternatives to the conventional wind turbine/farm health-monitoring systems that are currently in use. The proposed techniques are proven to provide an accurate measurement of the dynamic behavior of a 2.5-MW, 80-m-diameter wind turbine. Several measurements are taken on the test turbine by using four CCD cameras and one laser vibrometer, and the response of the turbine is monitored from a distance of 220 m. The results of the infield tests show that photogrammetry (also can be called as computer vision technique) enables the 3-D deformations of the rotor to be measured at 33 different points simultaneously with an average accuracy of +/- 25 mm while the turbine is rotating. Several important turbine modes can also be extracted from the recorded data. Similarly, laser interferometry (used for the parked turbine) provides very valuable information on the dynamic properties of the turbine structure. Twelve different turbine modes can be identified from the obtained response data. The measurements enable the detection of even very small parameter variations that can be encountered due to the changes in operation conditions. Optical measurement systems are very easily applied on an existing turbine since they do not require any cable installations for power supply and data transfer in the structure. Placement of some reflective stickers on the blades is the only preparation that is necessary and can be completed within a few hours for a large-scale commercial wind turbine. Since all the measurement systems are located on the ground, a possible problem can be detected and solved easily. Optical measurement systems, which consist of several CCD cameras and/or one laser vibrometer, can be used for monitoring several turbines, which enables the monitoring costs of the wind farm to reduce significantly.