An Introduction to Operational Modal Identification of Offshore Wind Turbine Structures

The present technical memorandum “An Introduction to Operational Modal Identification of Offshore Wind Turbine Structures” is prepared in connection with an ongoing Ph.D study at Aalborg University. The memorandum is intended for use in the civil engineering field and may serve as an inspiration to experimental modal analysis of complex dynamically loaded structures.

A huge number of papers have been published in the last years within traditional experimental modal analysis. However, for large civil engineering structures accurate model identification under actual operating conditions is difficult to extract by traditional experimental technologies. Operational modal analysis allows extracting modal parameters based on measuring only the response of a structure and using the ambient or natural operating forces as unmeasured input. Originally, operational modal analysis was developed for modal estimation of civil engineering structures like buildings, towers and bridges. Today, the technology is used for mechanical structures like ships, aircraft, vehicles and wind turbines. However, the loading forces for mechanical structures are complex. For a wind turbine structure a combination of harmonic excitation originating from the rotating rotor and broadband excitation from air turbulence is present, which sets limitations on the applicability of operational modal analysis to wind turbine structures.

The technical memorandum contains an introduction to the theory within experimental modal analysis and, in particular, to operational modal identification. Examples of structural assessment of wind turbines by operational identification are included.