One year monitoring of an offshore wind turbine: Variability of modal parameters to ambient and operational conditions

•Paper highlights instrumentation and monitoring of the first US offshore wind farm.•Modal parameters of the OWT are reported over one year of continuous monitoring.•Modal parameters show correlation with operational and environmental conditions.•The side-to-side modal properties are generally stable with little variability.•The fore-aft modal properties vary significantly with wind speed and turbine operation. This paper presents the variation of identified modal parameters of an offshore wind turbine (OWT) over one year of continuous monitoring. The considered OWT is a Haliade 150–6 MW GE turbine on a jacket substructure at the Block Island Wind Farm in Rhode Island, U.S. The turbine was instrumented with a monitoring system including accelerometers, strain gauges, inclinometers, and a data acquisition system with automatic data transfer to cloud storage. Continuous vibration measurements of the OWT have been collected over one year from April 2021 to April 2022. An automated output-only operational modal identification approach is developed to extract modal parameters of the OWT over this one-year period using windows of 10-min data. The identified modal parameters of the first few vibration modes show variability and correlation with several ambient and operational conditions. It is observed that the identified natural frequencies and damping ratios of the first side-to-side (SS) mode are stable over time and have very small variation, but those of the first fore-aft (FA) mode show significant variability. The second FA/SS modes can only be identified with reasonable accuracy when the turbine is in idle condition. Similar to the first FA/SS modes, the second FA mode has larger variability in frequencies and damping ratios than the second SS mode because of the aerodynamics effects. The system identification results over one full year verify the effectiveness and robustness of the proposed approach for long-term monitoring of this OWT and serve as the reference database.