Virtual sensing techniques for response estimation and fatigue assessment of offshore wind turbines

The foundations of offshore wind turbines (OWTs) will ultimately determine the lifetime of an offshore wind farm. With uncertainties in earlier designs and consequent design conservatism there is potential room for life-time extension of these foundations. To assess remaining useful life and support end-of-life decisions, one thus needs to adequately compare the design assumptions with the actual in-situ conditions. As fatigue life is an important design consideration for the most common monopile foundation of OWTs, it is necessary to assess the stress history of individual turbines.
OWTs are subjected to both quasi-static and dynamic loads, both of which contribute to its fatigue life progression. The quasi-static strain originates from variations in the thrust loading, either caused by the wind or the turbine controller. The dynamic, or high frequent strains originate from wave loading and the turbine’s dynamics.
As monitoring solution one could measure the stress history at fatigue critical locations using a direct (strain) measurement. However, for OWTs on monopile foundations often the fatigue critical locations are located below the seabed. Installing strain sensors at these hotspots is therefore impossible for existing wind turbines. This measurement restriction motivates the use of a virtual sensing technique to estimate the strains at the hotspots without the necessity to install sensors on the hotspot itself. The full-field response of the structure is thus reconstructed based on a limited number of accelerometers and strain sensors (installed at a few easily accessible locations) and a calibrated Finite Element Model (FEM) of the system. The system model uses a multi-band modal expansion approach constituted of the quasi-static and dynamic contributions. These contributions are superimposed to reconstruct the stress history at all degrees of freedom of the FEM and subsequent assess fatigue life consumption at all fatigue hot spots of the OWT.
In the framework of the performed research, the developed virtual sensing technique is validated for the first time ever with long term measurements from an actual OWT subjected to multiple variations of environmental and operating conditions as well as sudden events. This innovative virtual sensing scheme constitutes a robust structural health monitoring tool with the ability to interrogate entire OWT structures of multiple foundation types (monopiles and jackets) and accurately assess fatigue life consumption and remaining useful life at all their fatigue hotspots.