2343
Time-Varying Dynamic Properties of Offshore Wind Turbines Evaluated by Modal
Testing
Étude
expérimentale de l’évolution temporelle des propriétés dynamiques d’éoliennes
maritimes
Damgaard M., Andersen J.K.F.
Vestas Turbines R&D, Denmark
Ibsen L.B., Andersen L.V.
Department of Civil Engineering, Aalborg University, Denmark
ABSTRACT: Modal frequencies and damping ratios of civil engineering structures are often used as damage-sensitive features, since
changes in the dynamic characteristics of the structures may indicate structural damage. For offshore wind turbine structures, the
modal parameters are influenced by environmental impacts that change boundary conditions, irreversible soil deformations and
inherent structural properties. The excitation frequencies related to the environmental loads and the passage of blades past the tower
are so low that a proper estimate of the modal parameters are needed in order to avoid strong resonance of the wind turbine structure.
In this paper, free vibration tests and a numerical Winkler type approach are used to evaluate the dynamic properties of a total of 30
offshore wind turbines located in the North Sea. Analyses indicate time-varying eigenfrequencies and damping ratios of the lowest
structural eigenmode. Isolating the oscillation oil damper performance, moveable seabed conditions may lead to the observed time
dependency.
RÉSUMÉ: Les fréquences modales et les taux d'amortissement des structures de génie civil sont souvent utilisés comme indicateur de
dommages car
l’évolution de la réponse
dynamique des structures peut indiquer des dégâts structuraux. Pour des structures comme les
éoliennes maritimes, les paramètres modaux sont influencés par la déformation irréversible des sols, les propriétés structurelles
inhérentes et les conditions environnementales qui peuvent changer les conditions aux limites.
Les fréquences d’excitations liées aux
charges environnementales et aux passages des palles
sont si basses qu’une estimation correcte des paramètres modaux est nécessai
re
pour éviter une forte résonance de la structure de l'éolienne. Dans cet article, des tests vibratoires et une approche numérique du type
Winkler sont utilisés
afin d’évaluer les propriétés dynamiques de 30 éoliennes
maritimes situées en mer du Nord. Les analyses
révèlent le changement des fréquences propres et des taux d'amortissement de la plus basse fréquence propre structurelle en fonction
du temps. En isolant la performance de l'amortisseur oscillant à huile, les changements de conditions du fond marin peuvent
démontrer une dépendance temporelle.
KEYWORDS: Free vibration; modal; offshore wind turbine;
p-y
curve; scour; winkler approach.
1 INTRODUCTION
Recently, offshore wind turbine towers and blades have
increased significantly in height and length, respectively, with
only a small increase in weight. Therefore, the dynamic
response of the wind turbine structure occurs in a frequency
range close to the excitation frequencies related to
environmental and structural harmonic loads. In this context,
sufficient geometrical and material damping in the structure and
soil are required to counteract large amplitudes of vibration.
Especially for wind parks characterised by a large degree of
wind-wave misalignment, a proper estimate of the inherent
damping is needed due to low aerodynamic forces out of the
rotor plane.
The aim of this paper is to investigate the time-varying
eigenfrequency
f
1
and inherent modal
damping δ
1
of the lowest
eigenmode
Φ
(1)
for offshore wind turbines installed on a
monopile foundation. Experimental modal analysis of offshore
wind turbines have been studied by several researchers. Based
on free vibration tests, Tarp-Johansen
et al.
2009 and Damgaard
et al.
2011 have used
“ro
tor-
stop” tests to determine each
damping contributor to the measured inherent modal damping
δ
1
of an offshore wind turbine. Versteijlen
et al.
2011 and
Devriendt
et al.
2012 used the same modal approach to obtain
reliable damping estimates. In addition, Versteijlen
et al.
2011
considered operational modal analysis in order to include the
aerodynamic effects on the structure. The theory has been
widely used for civil engineering structures like bridges and
buildings. However, in the last years the application of
operational modal analysis on wind turbines has been published
in many excellent papers, see for instance Hansen
et al.
2006
and Tcherniak
et al.
2010. A thorough data processing of more
than 650 free vibration tests on 30 offshore wind turbine
structures are presented in the paper. The variation in the
dynamic properties is supported by a numerical Winkler
approach that estimates the modal parameters for different
environmental conditions.
2 STRUCTURE AND SITE CONDITIONS
A total of 30 Vestas V90-3MW turbines located in the North
Sea are considered. Each tower is installed on a monopile
connected by a grouted transition piece to the tower base. The
tower height is approximately 60 m, the monopile diameter 4.3
m and the water depth 8 m w.r.t. LAT. For each turbine an
oscillation damper is placed in the top of the tower. It consists
of a pendulum partly immersed in highly viscous oil, capable of
oscillating in the horizontal directions. The soil consists mainly
of cohesionless soil in the top layers
with friction angles φ
k
higher than 30° followed by cohesive soils with undrained shear
strength
c
u
higher than 90 kPa.
3 MODAL PARAMETER ESTIMATION
By use of two accelerometers placed in the nacelle, the modal
parameters of each wind turbine are experimentally estimated
