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Offshore wind-turbine (OWT) support structures are subjected to cyclic dynamic loads with variations in loadings from wind and waves as well as the rotation of blades throughout their lifetime. The magnitude and extent of the cyclic loading can create a fatigue limit state controlling the design of support structures. In this paper, the remaining fatigue life of the support structure for a GE Haliade 6 MW fixed-bottom jacket offshore wind turbine within the Block Island Wind Farm (BIWF) is assessed. The fatigue damage to the tower and the jacket support structure using stress time histories at instrumented and non-instrumented locations are processed. Two validated finite-element models are utilized for assessing the stress cycles. The modal expansion method and a simplified approach using static calculations of the responses are employed to estimate the stress at the non-instrumented locations—known as virtual sensors. It is found that the hotspots at the base of the tower have longer service lives than the jacket. The fatigue damage to the jacket leg joints is less than 20% and 40% of its fatigue capacity during the 25-year design lifetime of the BIWF OWT, using the modal expansion method and the simplified static approach, respectively. [ABSTRACT FROM AUTHOR]

Accurate estimation of offshore wind turbine (OWT) modal parameters has a prominent effect on the design loads, lifetime prediction, and dynamic response of the system. Modal parameters can vary during the operation of OWTs. This paper studies the variation and sensitivity analysis of an OWT's modal parameters with respect to operational and environmental conditions. Three finite element models of a jacket-supported OWT at the Block Island Wind Farm are created within the OpenSees, SAP2000, and OpenFAST platforms and validated using experimental measurements. The OpenFAST model is used to simulate the modal parameters of the turbine under various wind speed, rotor speed, power, yaw angle, mean sea level, blade pitch angle, and soil spring values. The model-predicted modal parameters of the first fore–aft (FA) and side–side (SS) modes are compared to those identified from experimental measurements. Results from the simulations show that the first FA natural frequency and damping ratio mostly depend on the rotor speed and wind speed, respectively, while yaw angle and mean sea level do not have a visible effect. It is observed that there is about 8% stiffening in the first FA frequency and an aerodynamic damping of 7.5% during the operation of the OWT. [ABSTRACT FROM AUTHOR]

• 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. [ABSTRACT FROM AUTHOR]

This paper presents an assessment of foundation damping in piled offshore wind jacket structures through a case study of the Block Island Wind Farm (BIWF). Foundation damping is one of several sources of energy dissipation which can improve fatigue performance in offshore wind structures. Damping was quantified through operational model analysis (i.e. system identification) using structural health monitoring data combined with foundation damping modeling. The results showed that for the 1st bending mode the hysteretic damping ratio of the pile under axial cyclic loading was estimated to range from 0.2% to 1.5%. This range was comparable in magnitude to the foundation damping ratios documented for operational offshore wind monopiles under lateral loading. • Damping plays an important role in the fatigue performance of offshore wind structures. • Foundation damping was estimated for a piled offshore wind jacket structure during a storm event. • Foundation damping was estimated to be between 0.2% and 1.5% and was proportional to the cyclic load levels. • Foundation damping was comparable in magnitude to that observed in offshore wind monopiles. [ABSTRACT FROM AUTHOR]