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Natural frequency of offshore wind turbines on rigid and flexible monopiles in cohesionless soils with linear stiffness distribution |
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| Institution: | 1. Department of Civil and Environmental Engineering, Florida A&M University, Florida State University College of Engineering, Tallahassee, FL, USA;2. Department of Civil Engineering, Dong-A University, Busan, Korea;3. Civil, Structure & Architectural Engineering Team, Global Engineering Technology, Seoul, Korea;1. University of Bristol, United Kingdom;2. Chair in Geomechanics, University of Surrey, United Kingdom;1. Faculty of Mechanical and Energy Engineering, Shahid Beheshti University, A.C., Tehran, Iran;2. Mechanical and Materials Engineering Department, University of Western Ontario, Canada;1. D’Appolonia S.p.A. – Marine & Offshore Engineering, Via Martiri di Cefalonia 2, 20097 San Donato Milanese, Italy;2. Geo-Engineering Section/Offshore Engineering Section, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands |
| Abstract: | A simplified method is introduced to obtain the fundamental frequency of offshore wind turbines supported by monopile foundations. Soil-pile interaction is modeled based on Winkler approach and concept of beam on elastic foundation. The soil is considered to have linearly varying modulus of subgrade reaction along depth which is a typical assumption for cohesionless soils. Rayleigh method which is based on conservation of total energy of the system is utilized. Firstly the natural frequency of the system with rigid pile is derived and then an innovative procedure is introduced to take pile flexural stiffness into consideration. Comparison between results of the present method with those of a numerical FE model for a typical 2 MW wind turbine structure shows excellent agreement for rigid pile and flexible pile with small value of slenderness ratio. The agreement is also good for flexible pile with higher slenderness ratios. A parametric study is carried out to investigate the effect of important parameters of foundation including pile slenderness ratio, pile aspect ratio and pile mass on the system natural frequency. |
| Keywords: | Natural frequency Monopile Subgrade reaction Slenderness ratio Rayleigh method |
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