Mitigating Underwater Noise from Offshore Wind Turbines via Individual Pitch Control

TL;DR

The paper tackles the underwater acoustic footprint of offshore wind turbines by quantifying blade‑aerodynamic noise transmission through the air–water interface using a Snell cone framework and a BPM noise model. It introduces an open‑loop individual pitch control strategy that modulates blade pitch at the blade passing frequency to suppress OSPL and amplitude modulation, and benchmarks it against conventional pitch schemes. Across three reference turbines (NREL 5 MW, DTU 10 MW, IEA 22 MW), the IPC achieves up to about 5 dB OSPL reduction with ~3–5° pitch changes, at a modest 5–10% energy penalty, and shows clearer benefits in mid/low‑frequency ranges relevant to marine mammals. The study also maps effects to marine mammal hearing groups, stresses the need for site‑specific ecological assessments, and suggests future integration with closed‑loop reinforcement learning for adaptive noise control.

Abstract

This paper proposes a pitch control strategy to mitigate the underwater acoustic footprint of offshore wind turbines, a measure that will soon become necessary to minimize impacts on marine life, which rely on sound for communication, navigation, and survival. First, we quantify the underwater acoustic signature of blade-generated aerodynamic noise from three reference turbines, the NREL 5 MW, DTU 10 MW, and IEA 22 MW, using coupling blade element momentum and coupled air-water acoustic propagation modeling. Second, we propose and implement an open-loop individual pitch control (IPC) strategy that modulates the pitch of the blade at the blade passing frequency to attenuate the overall sound pressure level (OSPL) and the amplitude modulation (AM) of the transmitted noise. Third, we benchmark IPC performance against conventional pitch schemes. The results indicate that up to 5 dB reductions in OSPL and a decrease in AM depth 20% can be achieved with a pitch variation of $Δθ\approx 5^\circ$, with small losses (5-10%) in energy capture. These findings highlight a previously underappreciated noise pathway and demonstrate that targeted blade-pitch modulation can mitigate its impact.

Figures (6)

• Figure 1: Illustration of air–water sound refraction effects. On the left, refraction of acoustic rays from a blade-tip noise source governed by Snell’s law. On the right, the effect of distance on the Snell incidence angle $\phi$.
• Figure 2: Observers configuration employed to estimate $\overline\text{OSPL}$ at certain time instant for each wind turbine blade. $N_c = 20$ observation points are uniformly distributed along the Snell Cone border. The wind direction is from $-x$ to $+x$, as indicated, and the vertical axis corresponds to $z$. The view is from above the rotor plane.
• Figure 3: Wind turbine noise behavior for different observer positions and blade pitch angle values over two rotor revolutions. Results presented for the NREL 5 MW wind turbine.
• Figure 4: Pitch law for each blade depending on its phase.
• Figure 5: Comparison of different pitch strategies on the $\overline\text{OSPL}$ for the DTU 10 MW wind turbine for two rotor revolutions.