Effectiveness of viscous damper on vibrations control of full -height lattice tower under wave and wind excitations in time-domain

as the expansion of offshore wind farms in the world and offshore wind turbines to generate electricity, it is significant to consideration the support structures of wind turbines and also to improve their productivity by considering the irregular and accidental conditions of offshore areas. In this study, the efficiency of viscous dampers, that known as one of the passive control methods in vibrations control system of structures as well as offshore platforms, in improving the response of full-height lattice tower for an offshore wind turbine in the time-domain analysis is investigated. For this purpose, several dampers are used diagonally on the platform and also along the legs to determine its best efficiency.

The structure, which is the platform of a 5 MW wind turbine in the North Sea, was modeled using Fast, Abaqus, and Sap2000 software, as well as the Simulink model was used to solve the equations of motion and determine the response of the lattice tower structure, which was considered as a shear model. structural responses were calculated using time-domain analysis and under wave and wind excitations. The Kaimal and Pierson-Moskowitz spectrum were used to apply the wind and wave forces, The wave-induced hydrodynamic forces were calculated based on the airy linear wave theory and the wind-induced aerodynamic forces were calculated based on the blade element momentum theory.

Based on the structure responses in different sea states and considering the condition with a significant wave height of 1.99354 meters and wind speed of 12 meters per second, which has led to the most vibrations in the structure, The results obtained from the responses of the structure equipped with viscous dampers show that if the diagonal brace elements at certain elevations are removed from the structure and the viscous dampers are replaced diagonally, the structure vibrations have not been reduced significantly and the drift of the structure where the viscous damper is located has increased compared to the structure without the damper,so that in one case which equipped with dampers The RMS displacement of the wind turbine Nacelle has increased to 19% compared to structures without dampers, while the results show that considering viscous dampers vertically and along the legs of lattice tower, the wind turbine Nacelle level displacement has been significantly reduced compared to the model without dampers.

Due to the geometry and cantilever behavior of the full-height lattice tower in which the relative displacement of the floors is due to the structure rotation, by viscous dampers placement diagonally in which reduces the rigidity of the platform structure due to the removed brace elements do not have been seen a significant effect on energy dissipation, while by the viscous damper placement along the legs of the structure, a greater effect have been observed on energy dissipation and vibration control of this structure. The use of various types of systems in vibration control to improving the response of the full-height lattice tower structure that support the structure of wind turbine can be investigated in future research.