On the Kite-Platform Interactions in Offshore Airborne Wind Energy Systems: Frequency Analysis and Control Approach

TL;DR

The paper analyzes deep offshore airborne wind energy systems where a tethered kite pumps power from high-altitude winds while connected to a spar-buoy platform. It develops a coupled 6-DoF platform model based on linear potential flow and a nonlinear tethered kite model, enabling frequency-domain analysis of kite–platform interactions. A frequency-aware control strategy is proposed to steer the kite along trajectories with a target frequency $f_{traj}^*$, thereby avoiding resonance with platform sway and reducing fatigue loads. Simulation results show that adjusting the kite path length via target-point selection can maintain tether-force frequencies away from platform resonances and substantially damp platform oscillations, enhancing offshore viability. The work lays groundwork for more robust offshore AWES designs, with future validation in more diverse sea states and experimental settings.

Abstract

This study investigates deep offshore, pumping Airborne Wind Energy systems, focusing on the kite-platform interaction. The considered system includes a 360 m2 soft-wing kite, connected by a tether to a winch installed on a 10-meter-deep spar with four mooring lines. Wind power is converted into electricity with a feedback controlled periodic trajectory of the kite and corresponding reeling motion of the tether. An analysis of the mutual influence between the platform and the kite dynamics, with different wave regimes, reveals a rather small sensitivity of the flight pattern to the platform oscillations; on the other hand, the frequency of tether force oscillations can be close to the platform resonance peaks, resulting in possible increased fatigue loads and damage of the floating and submerged components. A control design procedure is then proposed to avoid this problem, acting on the kite path planner. Simulation results confirm the effectiveness of the approach.

Figures (10)

• Figure 1: Conceptual layout of the system, with the four considered reference frames.
• Figure 2: Tether force spectrum with tether length $L=900\,$m. (a) Onshore. (b) Offshore - Wave A. (c) Offshore - Wave B.
• Figure 3: Pulling force in N with tether length $L=900\,$m. (a) Onshore. (b) Offshore - Wave A. (c) Offshore - Wave B.
• Figure 4: Bode diagram of the frequency response of $x_P$ w.r.t the traction force $x$-component, and spectrum of the latter for wave A and tether length $L=900\,$m.
• Figure 5: Bode diagram of the frequency response of $y_P$ w.r.t the traction force $y$-component, and spectrum of the latter for wave A and tether length $L=900\,$m.