Frequency Domain Fatigue Analysis of Offshore Wind Monopile Support Structure

Frequency Domain Fatigue Analysis of Offshore Wind Monopile Support Structure: For the Purpose of Offshore Wind Farm Optimization

Markolefas, Panagiotis (TU Delft Electrical Engineering, Mathematics and Computer Science)

Zaayer, M.B. (mentor)

Delft University of Technology

Electrical Engineering | Sustainable Energy Technology

2020-11-27

The preliminary design of wind turbine support structures within an offshore wind farm plays a significant role in decreasing the costs involved and is pivotal in establishing the wind sector as a leading contributor towards a sustainable future. During the early design stages, the optimization of structural dimensions, which involves a large number of iterations, is an essential step in determining the optimal offshore wind farm layout. Aiming to assist in the time and cost efficiency of such procedures, this thesis project develops a fatigue analysis model of offshore wind turbine monopile structures in the frequency domain. The structure is modeled using the finite element method and consists of Euler-Bernoulli beam elements. The soil-structure interaction is approximated by the effective fixity length and the rotor-nacelle assembly is incorporated as a point mass, contributing its inertial properties to the tower’s top node. Transfer functions are developed to relate the input environmental force spectra to the total stress response of the structure. Time series of wind thrust on rotor are converted to aerodynamic power spectral densities (PSDs). The hydrodynamic spectra are calculated based on nodal wave forces obtained by the Morison equation, with the wave environment being defined by mathematical models for wave particle kinematics and wave elevation spectra. The mode superposition method is used to enable the addition of aerodynamic and hydrodynamic modal responses. In order to retrieve the stress range variations from the total stress response spectra, the Dirlik method is applied. Finally, the fatigue damage accumulation is calculated by the Palmgren-Miner’s rule. The proposed model (PM) is capable of calculating the fatigue at any location along the structure and for any environmental state. It produces very accurate results for the 1st fore-aft (F-A) mode shape deflection. It also demonstrates accuracy in calculating the natural frequencies of the 1st and 2nd F-A, as well as of the 1st side-to-side. The considerable deviation of the 2nd side-to-side natural frequency is attributed to the simplified RNA modelling. The inclusion of a 6% aerodynamic damping to couple the wind and wave responses has a significant effect on the dynamic response, decreasing its peak by almost an order of magnitude. The dynamic response and the environmental load spectra are also shown to be mesh independent, demonstrating the model’s reliability. The mesh sensitivity analysis shows that the maximum stresses and, as a consequence, the stress response spectra produce erroneous results for low mesh sizes, which is anticipated. The fatigue damage quickly converges to an asymptotic value for increasing mesh sizes. The fast convergence rates allow the model to produce adequate results for low mesh sizes, achieving very low simulation times. Special attention is given to the effect that the hydrodynamic cross-power spectral densities (CSDs) have on the fatigue. The wave CSDs increase the fatigue due to wind and wave PSDs by about 9%, suggesting that their incorporation could prove significant.

Fatigue Analysis
Offshore Wind Turbines
Frequency Domain
Support structure
Monopile foundation

http://resolver.tudelft.nl/uuid:5a272b62-3082-4ac4-8078-03ffe9a863c1

Student theses

master thesis

© 2020 Panagiotis Markolefas