The Response and Optimisation of Hybrid Wind Turbine Towers
Citation:
KENNA, ALAN PATRICK, The Response and Optimisation of Hybrid Wind Turbine Towers, Trinity College Dublin.School of Engineering, 2019Download Item:
AKenna_PhD Thesis_Response & Optimisation of Hybrid Wind Turbine Towers.pdf (PhD Thesis) 5.771Mb
Abstract:
This thesis investigates the response and optimisation of wind turbine tower
structures with a particular emphasis on hybrid steel-concrete towers. The
wind turbine blades with tower interaction are represented through equations
of motion where mass, stiffness and damping properties have a time
varying component. This thesis investigates the response and optimisation
of these towers in a global and local sense through review of global tower
top behaviour as well as the response at selected local locations from around
the tower shell. Structural models of the tower are presented and used in
analysing the response. An exact, closed form analytical model was developed
using classical beam bending theory, with boundary and compatibility
conditions imposed to generate a system of homogeneous linear equations
with non-trivial solutions. Approximate, Finite Element models of the tower
were constructed using both modified Euler-Bernoulli beam elements and
also Reissner-Mindlin shell finite elements of varying numbers of degrees of
freedom.
Two reduced order dynamic multi-degree of freedom (MDOF) models for
an overall wind turbine assembly are then presented using a mixed formulation
approach including Finite Element models incorporated into Euler-
Lagrangian based systems. Discrete, global interpolation functions are used
to reduce the total number of degrees of freedom (DOF) of the tower models
to a selected reduced number of DOF. Continuous mode shapes are used to
reduce the blade elements to selected DOF. Rotating blades are exposed
to time varying load application through aerodynamic load and periodicity
introduced by gravity. Axial effects through gravity and centrifugal stiffening
act on the blades to vary their stiffness. Aerodynamic loading has been
simulated using the modified blade element momentum (BEM) algorithm
which accounts for the angle of attack, blade pre-twist, pitch angle and wind
shear. Turbulence was generated from a Kaimal spectrum.
The closed form analytical model was used to assess tower free vibration
response and MDOF dynamical models were used to investigate forced vibration
response of towers of varying properties. The nacelle mass and
hybrid interface height had the most significant impact on the first natural
frequency of the tower. This was observed through free vibration response
but also through frequency domain review of the forced response. Hybrid
interface height was strongly correlated with the mean displacement but to
a lesser extent on the velocity and acceleration response. Concrete compressive
strength and structural damping properties had an influence on the
tower response. The first and second natural frequency of the tower was
slightly reduced when introducing and increasing a prestress into the models.
The global forced vibration response of the tower showed insignificant
change as a result of the introduction of prestress. The effect of prestress was
more significant at a local finite element level on review of strain response in
three principal directions. Separately, the frequency content of local finite
element strain response was significantly different to the frequency content
of the global tower top response. This was deemed to be due to the effects
of combined deformation through all tower global DOF.
A methodology has been proposed for the optimisation of hybrid concretesteel
wind turbine towers. This methodology incorporates the generalisation
of free and forced vibration results of such towers using a configuration
of Artificial Neural Networks, which are embedded within an optimisation
algorithm which itself is a hybrid of a Genetic Algorithm and a Pattern
Search Algorithm. Objective functions are defined in terms of both structural
and non-structural criteria. Fundamental fore-aft frequency was maximised,
peak tower displacement was minimised, as was a weighted sum of
concrete and steel stress utilisation ratios. Levelised Cost of Energy (LCoE)
was set as an objective and was minimised for a series of load cases and
hub heights. Concrete and prestressed reinforcement contributed most significantly
to the breakdown of LCoE. The Climate Change Potential (CPP)
was also set as an objective to be minimised and followed similar patterns
to the LCoE in terms of sensitivity to change in wind speed and height.
Contributions to the overall CCP are much more equally spread than was
the case in LCoE, with each contributing similar amounts. Multi-objective
optimisation was carried out using the epsilon constraint method.
A method was proposed to utilise and process spatial strain and acceleration
signals as a means of damage detection around the shell of the finite
element model of the wind turbine tower. Processing involved passing the
signals through the Discrete Wavelet Transform (DWT) signal processing
technique. The spatial signals were all transformed and co-efficients were
found for low and high frequency components. GIS spatial images were
presented to represent aerodynamic loading and tower responses generated
using BEM and the 11 DOF structural models described earlier in the thesis.
By generalising the loading and response quantities as a function of spatially
distributed environmental exposure conditions, it is possible to plot these
loading and response quantities spatially.
Sponsor
Grant Number
Bord na Mona
Trinity College Dublin (TCD)
Description:
APPROVED
Author: KENNA, ALAN PATRICK
Advisor:
Basu, BiswajitPublisher:
Trinity College Dublin. School of Engineering. Disc of Civil Structural & Environmental EngType of material:
ThesisCollections:
Availability:
Full text availableLicences: