A study of the dynamic performance of the multi-system involved in the offshore floating type wind turbine

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As the utilization of the renewable energy sources has significantly increased over the past decades, offshore wind turbines have been noted for their advantages of more power output and less space limitation than the onshore wind turbines. New prototypes of offshore wind turbines have been developed; not only has the power output been increased but also their sizes have become larger. However, the distinct offshore environmental loads may significantly affect the performances of the offshore wind turbine. The aims of this Ph.D research are to investigate the vibration behaviours of the multi-system involved in a spar type offshore direct-drive wind turbine. First of all, the overview of the main components involved in the offshore direct-drive wind turbine and their dynamic performances related to the mechanical vibration behaviour are presented. Different types of topology of the direct-drive wind turbines are briefly discussed and a comparison of all generator topologies is made based on the criteria of their efficiency, weight and cost. The supporting structures for the wind turbine have been developed from land to transitional-water, then the deep-water. Complex designs have been used to meet the environment requirements. Moreover, the consideration of the nacelle-blade system has been developed from a non-rotating mass to a rotating system with interaction when studying the tower response. The excitation conditions have become more complex. Not only the environmental excitations, such as the aerodynamics and hydrodynamic excitations should be considered, but also the internal excitations, such as the unbalanced magnetic pull force in the generator or tower shadow can be involved. Three new models about the main components: direct-drive drive-train system, floating platform and the tower structure are presented in this thesis. To start with, the effect of rotor position and weight adjustment on the vibration behaviour of the drive-train system within a 5 MW direct-drive wind turbine is studied by considering the unbalanced magnetic pull forces. The direct-drive wind turbine, different from the standard geared wind turbine, uses a direct-drive generator to avoid the gearbox failures, in which the direct-drive permanent-magnet generator has been widely used. The unbalanced magnetic pull (UMP) force which is caused by the eccentricity of both rotor and stator of the generator can have an impact on the vibration behaviour of its drive-train system, an up to 30% difference of the tolerance allowable for a safe operation in the generator can be found from the simulation results. The drive-train system which consists of the main shaft, rotor, hub and blades is modelled as a four degree-of-freedom (DOF) nonlinear system. In the present drive-train system, the location of its centre of gravity can be moved along its rotating axis by doing the rotor adjustment in terms of rotor position and mass ratio. Both rotor displacement and bearing forces are obtained for a wide range of rotor positions and weight under different rotating speeds. Such results would provide optimized rotor position and mass ratio to improve the performance of the drive-train system. Then a combined model of a spar-type floating platform wind turbine under deep-sea conditions is developed. The spar-type supporting platform with tower structure is modelled as a rigid body, while the nacelle is considered as a point mass attached on the top of the tower. Then the dynamic interaction between the drive-train system and the nacelle is considered by incorporating the modelling of the direct-drive drive-train system. The hydrodynamic and aerodynamic excitations on the structures are considered, including current, wave, and wind excitations as well as buoyant forces. With the help of the time history and FFT spectrum, the effects of both hydrodynamic and aerodynamic excitations along with the dynamic interaction between the drive-train system and tower structure on the dynamic behaviour of the spar-type floating platform are investigated under different operating sea conditions. At last, the tower structure is developed using a flexible model. An analytic solution for the free-vibration of the tower structure based on Euler-Bernoulli beam-column theory is presented. The tower structure is modelled as a free-free beam with two end mass components. The platform and the nacelle are considered as two large mass rigid components connected by torsion springs at two tower ends. The stiffness at the connections can be different due to the different joint conditions between the tower and two mass components, such as joint type or crack. The effects of system parameters on the natural frequencies are investigated under a range of variables, including the tower structure parameters, platform and nacelle parameters, and the connecting types. Uniform tower model is presented firstly and then the non-uniform structure is discussed. The non-linear relationships between those variables and the natural frequency of the whole system are numerically found and some design issues are discussed for the spar-type floating wind turbines.
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