Model Development and Load Analysis of an Offshore Wind Turbine

by Mohammad Masoomi

Institution: University of Toledo
Department: Mechanical Engineering
Degree: MS
Year: 2014
Keywords: Mechanical Engineering
Record ID: 2025929
Full text PDF: http://rave.ohiolink.edu/etdc/view?acc_num=toledo1404552688


The purpose of this thesis is to investigate the loadings on and motion of an offshore wind turbine. The three major components of an offshore wind turbine are the wind turbine, the platform and the mooring lines. The effects of these components on the motion of the wind turbine were investigated. The NREL-5 MW offshore baseline model wind turbine was chosen for this study. The selected platforms are the surface and submerged TLP platforms suggested by MIT/NREL.Loadings on an offshore wind turbine are the result of three primary forces. Aerodynamic forces, hydrodynamic forces and forces from the mooring lines. To start with the loadings from the waves, hydrodynamic coefficients were determined. This was accomplished by using the WAMIT (Wave Analysis at MIT) computer program to determine the hydrodynamic coefficients. The computer code FAST (Fatigue, Aerodynamics, Structures, and Turbulence) was then used to determine the effects of the wind turbine and the wave loads. An equation of motion was developed that incorporated these coefficients. The steady aerodynamic force was derived using Bernoulli’s equation. In order to add the force from the mooring lines, a new code had to be written. This code assumed that each mooring line remained in the elastic region. Moreover, a stiffness matrix was computed and incorporated into the equation of motion. In the final step, the three primary forces were combined to calculate the wind turbine motion for six degrees of freedom with and without aerodynamic loads.To validate the theoretical results scale model tests were performed within an existing water channel. A wide range of waves with different amplitudes and frequencies were generated within the channel. Various model platforms were constructed and the hydrodynamic coefficients were derived to compare with theoretical results. Finally, a comparison was made of platform configurations with different aspect ratios and designs. It was found that a submerged platform has less movement compared with a surface-based platform with the same shape. Also, the effects of wind speed and water depth on the motion of wind turbine were studied. It was found that at larger depth, the maximum amplitude occurs at higher frequency. The justification being that the stiffness of the mooring line will decrease as depth increases.