Modal properties of floating wind turbines: Analytical study and operational modal analysis of an utility-scale wind turbine

Abstract

Structural health monitoring through Operational Modal Analysis based on output-only methods has already a long term successful history of applications in conventional civil engineering structures and, more recently, onshore and offshore bottom fixed wind turbines. Although the former application has been proven as a reliable and relevant source of information regarding the dynamic response of the structure and ultimately in the definition of operation strategies, its application to floating offshore structures is yet to be explored, a gap that is tackled in this work. Since the introduction of a floating device, typically associated with very low frequency motions, impacts the relevant modal properties for the tower design, a preliminary and simplified analytical approach is presented, where these effects are properly characterised. The derived expressions, that are ready for design purposes, are validated based on numerical simulations in OpenFAST. After that, experimental data from a full scale floating wind turbine is used as input for Operational Modal Analysis for the first time. Here, not only some key features that are intrinsically associated with these type of structures are unveiled, but also the applicability of output-only methods for dynamic identification is confirmed.