Frequency-domain modelling of floating wind turbines
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Authors
Advisors
Langley, Robin
Date
2015-04-28Awarding Institution
University of Cambridge
Author Affiliation
Department of Engineering
Pembroke College
Qualification
Doctor of Philosophy (PhD)
Language
English
Type
Thesis
Metadata
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Lupton, R. (2015). Frequency-domain modelling of floating wind turbines (Doctoral thesis). https://doi.org/10.17863/CAM.14119
Abstract
The development of new types of offshore wind turbine on floating
platforms requires the development of new approaches to modelling the
combined platform-turbine system. In this thesis a linearised
frequency-domain approach is developed which gives fast but
approximate results: linearised models of the structural dynamics,
hydrodynamics, aerodynamics and control system dynamics are brought
together to find the overall response of the floating wind turbine to
harmonic wind and wave loading.
Initially, a nonlinear flexible multibody dynamics code is developed and
verified, which is then used to provide reference nonlinear simulation
results. The structural dynamics of a wind turbine on a moving platform
are shown to be nonlinear, but for realistic conditions the effects are small.
An approximate analysis of the second-order response of floating
cylinders to hydrodynamic loads suggests slow drift motion may be
relatively small for floating wind turbines, compared to other floating
offshore structures.
The aerodynamic loads are linearised using both harmonic and tangent
linearisation approaches; the harmonic linearisation gives improved
results when stall occurs. The wake dynamics can also be included. The
control system behaviour is linearised using the same method, which
works well when the wind speed is far from the rated wind speed; close to
the rated wind speed the nonlinearity is stronger, but further
improvement should be possible.
These sub-models are combined to give a simple but complete model of a
floating wind turbine, with flexible blades and a flexible tower, but
neglecting the control system behaviour, wake dynamics and nonlinear
hydrodynamic loads. For the OC3-Hywind turbine, the accuracy of the
results is assessed by comparison to nonlinear time-domain simulations
using the commercial code Bladed. Peak-peak errors of less than 5 % are
achievable for many harmonic wind and wave inputs, but certain
conditions lead to larger errors. The effect of including linearised control
system behaviour is demonstrated for a subset of conditions. Overall, the
results are promising but more work is needed for practical application.
Keywords
Research Subject Categories::TECHNOLOGY::Engineering mechanics, Research Subject Categories::TECHNOLOGY::Engineering mechanics::Mechanical and thermal engineering::Mechanical energy engineering, floating wind turbines, wind energy, frequency-domain modelling, harmonic linearisation, equivalent linearisation, wind turbines
Sponsorship
This work was supported by GL Garrad Hassan.
Rights
Attribution-NonCommercial-NoDerivs 2.0 UK: England & Wales
Licence URL: http://creativecommons.org/licenses/by-nc-nd/2.0/uk/
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