Wind Energy

56
Damage Detection on Structures of Offshore
Wind Turbines using Multiparameter
Eigenvalues
Johannes Reetz
56.1 Introduction
An efficient and reliable use of offshore wind-energy requires a condition controlled
maintenance. Thereby information about the state of damage and the
residual load carrying capacity of the structure has to be provided by means
of condition monitoring systems (CMS). Such a system is not yet available.
In the following, a new method is demonstrated treating this problem. The
method is based upon the fact that damages influence the dynamic behavior
of elasto mechanical structures. Turek et al. [1] show that with the aid of a
validated finite element model damage detection can be made by means of
measured modal quantities. However, their method does not provide information
about quantification and localisation of damage. By formulating the
identification problem as a multiparameter eigenvalue problem using the finite
element model with parametrised model matrices, damage can be quantified
and localised [2].
56.2 The Multiparameter Eigenvalue Method
Starting from the known linear elastic and lightly damped supporting structure
the first step is the approximation as a finite element model. Thereby
the model has to be validated with the real structure. It is advantageous to
apply pre-existing knowledge about structural area or parts, e.g. welded or
bolted joints or the splash zone, which are prone to damage. For this purpose
the portions of the model matrices – particularly of the stiffness matrix –
associated with these details are parametrised. Herewith, the mathematical
model characterising the dynamic behavior is given. On the other hand, the
information about the current dynamic behavior of the potentially damaged
structure is provided by the modal quantities. The modal quantities which
are most exactly measurable are the eigenfrequencies. These can be measured
by means of ambient excitation. The identification should provide the