Presentations

Abstract

Offshore wind turbines on monopile foundations have a finite lifetime. A significant contributor to fatigue life are sideways oscillations coming from the interaction between the structure's resonance frequency and (mis-aligned) waves . To mitigate sideways loads and to allow as such structural optimization on substructure side, turbine OEM's have developed Active Tower Dampers (ATD ), equally known as Side-Side Tower Dampers (SSTD), to counter sideways oscillations through applying torque to the rotor. However, as a side effect the activity of the ATD introduces oscillations in the power output of the turbine, at a frequency exactly equal to the wind turbine's resonance frequency. These forced oscillations in the power output are flagged by the TSOs that consider them as a risk factor for possible grid-wide power oscillations as the frequency is close to the EU interarea grid frequencies.. As such TSOs are cautious to permit the forced power oscillations coming from the ATD and favor to curtail their operation, even at WTG level. However, the wind turbines were designed with the unhindered ATD in mind. Thus curtailing the ATD might imply that the installed assets may no longer meet their designed lifetimes, and operators may be faced with early decommissioning. In addition, it appears to be overshooting to curtail at WTG level as the impact on grid-wide power oscillations should be considered at a windfarm level. In a unique experiment at the Seamade offshore wind farm, fatigue load measurements on both the substructure and blades were collected while the ATD was curtailed with varying levels. The experiment allowed to accurately quantify the impact of curtailing the ATD on the fatigue progression of both blades and substructure. While the impact on the blades was considered within natural variability, the detrimental impact on the substructure was clearly visible.