The Effects of Nonproportional Damping on the Identification of Offshore Wind Turbine Foundation Properties

There is a consistent discrepancy between the predicted and measured dynamic responses of in situ offshore wind turbine (OWT) structures. Underestimation of the foundation soil stiffness is thought to contribute significantly to this difference. Identification of the in situ foundation properties of OWT from monitoring data would reduce this uncertainty, providing critical feedback on foundation design methods and aiding lifetime reassessment. In this study, a system identification framework for estimating the in situ foundation stiffness of a parked OWT is presented using a model updating approach applied to simulated data. The results are shown to accurately replicate the behaviour of the true foundation. The study also demonstrates that the nonproportional nature of the aerodynamic damping causes the structure to exhibit mode shapes whose real parts do not correspond to those of the undamped system. A normalisation technique is applied that obtains a close approximation of the undamped mode shapes from the complex damped mode shapes. It is demonstrated that large errors are introduced in the identified foundation behaviour if this normalisation is not employed. Such errors can result in misleading interpretations of the foundation or superstructure properties of the OWT.