Dynamic behaviour of suction caissons

v
Summary in English
Offshore wind energy is a promising source of energy in the near future, and is rapidly
becoming competitive with other power generating technologies. The continuous improvement
in wind turbine technology means that the wind turbines have increased
tremendously in both size and performance during the last 25 years. In order to reduce
the costs, the overall weight of the wind turbine components is minimized, which means
that the wind turbine structures become more flexible and thus more sensitive to dynamic
excitation. Since the first resonance frequency of the modern offshore wind turbines is
close to the excitation frequencies of the rotor system, it is of outmost importance to be
able to evaluate the resonance frequencies of the wind turbine structure accurately as the
wind turbines increase in size. In order to achieve reliable responses of the wind turbine
structure during working loads it is necessary to account for the possibilities of dynamic
effects of the soil–structure interaction. The aim of this thesis is to evaluate the dynamic
soil-structure interaction of foundations for offshore wind turbines, with the intention
that the dynamic properties of the foundation can be properly included in a composite
structure-foundation system. The work has been focused on one particular foundation
type; the suction caisson.
The frequency dependent stiffness (impedance) of the suction caisson has been investigated
by means of a three-dimensional coupled Boundary Element/Finite Element
model, where the soil is simplified as a homogenous linear viscoelastic material. The
dynamic stiffness of the suction caisson is expressed in terms of dimensionless frequencydependent
coefficients corresponding to the different degrees of freedom. Comparisons
with known analytical and numerical solutions indicate that the static and dynamic behaviour
of the foundation are predicted accurately with the applied model. The analysis
has been carried out for different combinations of the skirt length, soil stiffness and the
Poisson’s ratio of the subsoil. Subsequently, the high-frequency impedance has been
determined for the use in lumped-parameter models of wind turbine foundations.
The requirement for real-time computations in commercial software packages for
performance and loading analysis of wind turbines, do not conform with the use of e.g. a
three-dimensional coupled Boundary Element/Finite Element Method, where the foundation
stiffness is evaluated in the frequency domain. For that reason, the dynamic stiffness
(impedance) for each degree of freedom have been formulated into lumped-parameter
models with frequency independent coefficients, suitable for implementation in standard
dynamic finite element schemes. The lumped-parameter models have been used to simulate
the soil–structure interaction within a numerical finite element model of a Vestas
V90 3.0 MW offshore wind turbine with a suction caisson foundation. The simulations
of the soil–structure interaction by means of lumped-parameter model approximations
of the impedance have shown that the concept is useful for use in applications where the
performance of the wind turbine are to be analysed.
Experimental modal analyses have been carried out with the intention of estimating
the natural frequencies of an existing Vestas 3.0 MW offshore wind turbine. The
experimental modal analysis of the wind turbine makes use of "Output-only modal identification"
which is utilized when the modal properties are identified from measured
responses only. The experimental modal analyses have shown that the approach is a
useful tool to estimate the response of the wind turbine.
December 4, 2006