Aspects of Operational Modal Analysis for Structures of Offshore Wind Energy Plants

Offshore wind energy plants (OWEP) are dynamic systems excited by stochastic loading due to wind, waves and operational interaction between the blades, machine and tower. For ensuring reliable operability of OWEP the knowledge of the dynamic characteristics are therefore of major importance. Furthermore, dynamic properties like eigenfrequencies and mode shapes can be used as damage sensitive features for structural health monitoring algorithms or model validation. This paper starts with a discussion of the basic dynamic characteristics of the 5MW-OWEP (Multibrid M5000-2 with tripod foundation in Bremerhaven, Germany) and its environment by means of various measured signals. Subsequently, the results of an operational modal analysis (OMA) on this OWEP are presented. The extraction of the eigenfrequencies, damping ratios and mode shapes of the structure is performed automatically by means of Vector Autoregressive Models (ARV). The modal analysis algorithm is applied on measured acceleration signals from the tower over a period longer than seven month at different environmental and operational conditions (EOC). Finally, the results of the OMA are discussed with respect to some relationships between the changing EOC and variations of the dynamic properties of the structure.