On the Load Impact of the Helix Approach on Offshore Wind Turbines

Abstract

To accelerate the growth of offshore wind power, its Levelised Cost Of Energy (LCOE) must be reduced. A means of reducing the LCOE of offshore wind power is by mitigating the adverse wake effect by employing Wind Farm Control (WFC) for power maximization. A recent WFC technology is the helix approach, which uses Individual Pitch Control (IPC) to initiate early wake mixing. Research regarding the helix approach is scarce, yet early results suggest that significant power gains are achievable. Simultaneously, increases in wind turbine loads are expected. To aid future researchers in making a trade-off between increased loading and performance, an extensive load analysis is conducted, aiming to analyze and quantify the load impact of the helix approach on the SG 11.0-200 DD offshore wind turbine.

A sensitivity analysis is conducted to investigate the influence of the helix parameters on the load impact on the wind turbine tower, blades, main bearing and pitch bearing. It is found that the load impact on all components is mostly sensitive to the pitch amplitude. Most components show some sensitivity to the Strouhal number as well, except for the main bearing. For the blade pitch bearings, an upward and downward trend can be observed for Counter-Clockwise (CCW) and Clockwise (CW) helix directions, respectively, which are related to the pitch frequency.

The helix approach is part of the plans for the Hollandse Kust Noord (HKN) wind farm. For this reason, a lifetime fatigue analysis is conducted to quantify the load impact on a wind turbine at the HKN wind farm over its full lifetime for several helix implementations. The fact that the helix approach is only employed under certain conditions is taken into account. It was found that the load impact on the pitch bearings is significant. The load impacts on the tower and main bearing become considerable for helix implementations with a large pitch amplitude. All other components are mildly affected.

Finally, the load impact of the novel concept of Closed-Loop Helix Control (CLHC) is compared to conventional helix. This approach considers asymmetric loading on the rotor (caused by e.g. wind shear and tower shadow) when employing helix. Results found that CLHC reduces blade loads with respect to the helix approach. However, since the blades are mildly affected by the helix approach in general, choosing CLHC over conventional helix is not imperative.