Finite Element Modelling and In Situ Modal Testing of an Offshore Wind Turbine

Abstract

Purpose Generating electricity from wind turbines is currently a viable option to meet the electric power requirements in many countries. The structure of offshore wind turbines is constantly subjected to external dynamic loads due to wind, waves and rotor loads due to the rotation of blades. The frequency content of these dynamic loads is in the range of natural frequencies of the wind turbine. Therefore, determining the in situ dynamic characteristics of a wind turbine is very beneficial, as it can lead to further improvements in its design, performance and safety. Methods In this paper, the dynamics of the structure of an offshore wind turbine is investigated numerically and experimentally. The finite element (FE) model of structural components is developed based on design specifications. The model takes the effect of rotor-nacelle assembly into account by considering its mass and moment of inertias relative to the top centre of the tower. In situ modal tests using impulse excitations were carried out on the actual wind turbine when the rotor blades were not rotating to identify the modal parameters. Results This paper presents predicted natural frequencies from the FE model based on design conditions and those identified from the modal and ambient excitation tests. Conclusion It was observed that the first two bending modes are close to the natural frequencies identified experimentally by the in situ modal tests. It was also observed that the in situ modal tests were not able to excite other higher natural frequencies of the structure.

Seismic excitation of offshore wind turbines and transition piece response

“…Dynamic analyses were performed using implicit-explicit switching, with an initial implicit analysis to preload the structure, followed by dynamic excitation using the explicit solver, and finally followed by an implicit analysis to obtain the post-earthquake monotonic response. This numerical modelling scheme was chosen following work by Asnaashari et al 22 and Vieira et al 23 who demonstrated the effectiveness of 3D FE analysis to model grouted connections under static loads. Natural frequencies and modeshapes were determined using the Block Shift and Invert Lanczos method.…”

A Review of Recent Advancements in Offshore Wind Turbine Technology

“…It has been advocated that Rayleigh's energy method is a close approximation of the coupled FSI modelling, and thus, it can be used as a precursor to high-fidelity numerical modelling. Asnaashari et al [98] have numerically analyzed the frequency spectrum of an offshore wind turbine with a monopile foundation. Euler-Bernoulli beam element has been employed for carrying out FEA of the turbine tower and the transition piece.…”