Assessment of damping in monopile-supported offshore wind turbines: Integrating operational modal analysis with regression techniques

Abstract

The identification of damping sources in the design of offshore wind turbine (OWT) structures has been approached in many different ways. Among all the methods, assessing damping sources based on monitoring data for OWT operations is more reliable compared to using experimental or empirical techniques. This study aims to perform an automated operational modal analysis of an instrumented OWT using detailed vibration monitoring to effectively assess different sources of damping. Firstly, continuous vibration acceleration of a monopile-supported utility-scale OWT is measured to obtain mode parameters by operational modal analysis. An automated modal identification method is designed to obtain mode parameters from a large of monitoring data. The natural frequency and damping ratio of both the first and second order bending modes of the OWT tower are observed. Besides, the characteristics of these mode parameters during the OWT operation are analysed. A data-driven damping assessment method is then introduced to access aerodynamic, hydrodynamic and structural damping. This assessment method is based on the identified damping from operational modal analysis, and it separates the different damping sources with regression techniques. The proposed assessment method provides a practical way to understand the damping sources of OWTs which is meaningful for the structural design. Furthermore, the method also holds potential applicability for damping assessment in various types of OWTs.