Vibroacoustic Underwater Noise from Fixed and Floating Offshore Wind Turbines

This study presents a physics-based vibroacoustic framework to predict and compare the operational underwater noise emissions of 10 MW bottom-fixed and floating offshore wind turbines, revealing that floating configurations generate higher low-frequency sound levels and more complex, direction-dependent radiation patterns than monopile structures, while water depth significantly influences overall propagation and sound levels.

The Gist

Imagine the ocean as a giant, silent concert hall. For years, we've worried about the loud, sudden "bangs" of construction work (like hammering piles into the seabed) that disturb marine life. But this new study is concerned with the continuous hum that wind turbines make while they are actually running and generating electricity.

The researchers wanted to answer a simple question: Does it matter if the wind turbine is stuck to the bottom of the ocean or floating on the surface?

To find out, they built a sophisticated "digital twin" of a massive 10-megawatt wind turbine. They simulated how the wind pushes the blades, how the gears inside the turbine rattle, and how those vibrations travel down the tower and into the water. They then compared two versions:

1. The "Stuck" Version (Monopile): A giant steel pole driven deep into the seabed.
2. The "Floating" Version: A massive platform that bobs and sways on the surface, anchored by cables.

Here is what they discovered, explained through everyday analogies:

1. The "Heavy Swimmer" vs. The "Stiff Pole"

Think of the Floating Turbine like a heavy swimmer in a pool. Because the platform is huge and free to move, it sways, rolls, and bobs with the waves. This movement creates a lot of low-frequency noise (a deep, rumbling sound).

• The Finding: The floating version is much louder in the deep, rumbling range (below 10 Hz). It's like a bass drum that keeps thumping. The study found it can be up to 15 dB louder than the fixed version at these low frequencies because the whole platform is moving like a giant, vibrating drum skin.

Think of the Fixed Turbine (Monopile) like a stiff pole planted in concrete. It can't sway. Instead, the vibrations from the spinning gears and shafts travel straight down the pole.

• The Finding: The fixed version is actually quieter in the deep rumble, but it gets louder at higher pitches (the "whine" of the gears). Because the pole is stiff, it transmits those higher-frequency mechanical vibrations very efficiently into the water, like a tuning fork.

2. The Shape of the Sound

Sound doesn't just go straight out; it spreads in patterns.

• The Fixed Turbine: The sound spreads out fairly evenly, like ripples from a stone dropped in a calm pond. It's predictable and symmetrical.
• The Floating Turbine: The sound is chaotic and directional. Because the floating platform has three legs and crossbeams that move in complex ways, the sound creates a "lumpy" pattern. It shoots loud beams of sound in some directions and leaves quiet spots in others. It's less like a ripple and more like a flashlight beam that flickers and points in different directions.

3. The "Room Size" Effect (Water Depth)

The depth of the water acts like the size of the room the sound is playing in.

• Shallow Water (The Small Room): In shallow water, the sound bounces between the surface and the bottom, getting trapped. This makes the sound travel further and stay louder, especially for the floating turbines. It's like shouting in a small bathroom; the sound gets trapped and echoes.
• Deep Water (The Big Hall): In deep water, the sound can spread out in all three dimensions (up, down, and sideways). This causes the energy to dissipate faster. The study found that moving a floating turbine from shallow to deep water can drop the noise level by about 9 dB, simply because the sound has more room to spread out and fade.

4. Who Can Hear It?

The researchers compared their noise maps to the hearing ranges of sea creatures.

• The Fixed Turbine: Its higher-pitched "gear whine" overlaps significantly with the hearing range of seals, dolphins, and porpoises. This means these animals are more likely to hear and be disturbed by the fixed turbines at closer ranges.
• The Floating Turbine: Its deep "rumble" is mostly below what most marine mammals can hear. However, the study notes that this deep rumble is often drowned out by natural ocean noise (like wind and waves) anyway, so it might be less of a problem for animals than the high-pitched noise of the fixed turbines.

The Bottom Line

This study provides a new "calculator" for engineers. Before building a wind farm, they can now use this tool to predict exactly how loud the underwater noise will be.

• If you build on a fixed pole, expect a louder high-pitched whine that travels well in shallow water.
• If you build on a floating platform, expect a deeper, bass-heavy rumble that behaves differently depending on how deep the water is and which direction the sound is traveling.

The goal isn't to say one is "bad" and the other is "good," but to understand the difference so we can design wind farms that are kinder to the ocean's acoustic environment.

Technical Summary

Technical Summary: Vibroacoustic Underwater Noise from Fixed and Floating Offshore Wind Turbines

Problem Statement
Anthropogenic underwater noise from offshore wind turbines is an emerging environmental concern, particularly as the sector scales toward larger capacities and deeper waters using both bottom-fixed (monopile) and floating technologies. While impulsive noise from construction (e.g., pile driving) is well-regulated, continuous operational noise remains less understood. This noise originates from two pathways: airborne aeroacoustics (often neglected due to air-water impedance mismatch) and structure-borne vibroacoustics. The latter arises from fluctuating aerodynamic thrust, drivetrain excitation, and hydrodynamic loading, which induce structural vibrations radiating into the water. Current environmental assessments often rely on post-hoc measurements or simplified empirical scaling, lacking physics-based predictive tools capable of quantifying noise during the early design phase. Furthermore, the distinct dynamic mechanisms of floating platforms—characterized by rigid-body motions and hydro-elastic coupling—may alter spectral content and radiation efficiency compared to monopiles, yet these differences are not fully quantified.

Methodology
The study presents a physics-based vibroacoustic framework to predict operational underwater noise for a 10 MW DTU reference turbine in two configurations: a monopile foundation (30 m water depth) and a semi-submersible floating platform (tested at 30 m and 350 m depths). The methodology integrates three sequential stages:

1. Structural Dynamics (Time-Domain): Simulations are performed using OpenFAST to solve coupled aero-hydro-servo-elastic equations. The model includes tower and blade bending modes, substructure elasticity (via SubDyn), and hydrodynamic loads (via HydroDyn). For the floating configuration, six rigid-body degrees of freedom (surge, sway, heave, roll, pitch, yaw) are included. To account for drivetrain noise, which OpenFAST does not resolve internally, synthetic tonal excitations representing shaft harmonics and gear-mesh frequencies are introduced at the nacelle, scaled by modal amplification factors.
2. Source Reconstruction: Nodal accelerations from submerged structural nodes are converted into equivalent dipole force sources. A radiation-impedance correction is applied to account for the finite cross-sectional dimensions of structural members, addressing high-frequency attenuation and phase shielding effects that simple line-source models miss.
3. Acoustic Propagation (Frequency-Domain): The acoustic pressure field is computed by solving the linear Helmholtz equation using analytical Green's functions. The model employs the method of images to account for boundary conditions: a pressure-release free surface and a partially reflecting seabed (sandy sediment). Convergence analysis confirms that 30 image reflections are sufficient for accuracy.

Key Contributions
The paper makes three specific scientific contributions:

1. Integrated Modeling Framework: It formulates a unified approach coupling aeroelastic loading, structural dynamics, fluid–structure interaction, and underwater acoustic propagation, moving beyond separate source and propagation modeling.
2. Measurable Descriptors: It defines acoustic descriptors (narrowband SPL, OASPL, directivity patterns) enabling consistent comparison between fundamentally different support structures.
3. Mechanistic Identification: It identifies the physical mechanisms driving differences in noise signatures between bottom-fixed and floating turbines, specifically isolating the roles of rigid-body motion, structural mass, and water depth.

Results
The comparative analysis yields several distinct findings regarding spectral content, directivity, and propagation:

• Spectral Characteristics:
  • Low Frequencies (< 10 Hz): Floating configurations exhibit significantly enhanced emissions (up to 15% or ~15 dB higher OASPL) compared to monopiles. This is attributed to additional rigid-body degrees of freedom and the larger structural mass of the floating platform, which increase radiation efficiency at very low frequencies.
  • Mid-to-High Frequencies (> 10 Hz): Monopile structures radiate more efficiently. The rigid connection to the seabed facilitates the transmission of drivetrain-induced vibrations and local structural modes, resulting in higher sound pressure levels in this range compared to floating platforms, which distribute excitation over a larger, more compliant structure.
• Directivity and Spatial Distribution:
  • Monopile: Displays a smooth, nearly axisymmetric response at higher frequencies and a clear dipolar pattern aligned with the wind direction at low frequencies.
  • Floating: Exhibits complex, anisotropic 3D radiation patterns. At low frequencies, the main emission axis rotates due to coupled rigid-body motions. At higher frequencies (100–1000 Hz), floating platforms show strong direction-dependent variations (20–25 dB) with multi-lobed patterns, unlike the monopile's uniformity.
• Propagation and Depth Effects:
  • Water depth critically influences propagation regimes. Shallow water (30 m) constrains propagation to a quasi-2D waveguide, leading to slower attenuation rates. Deep water (350 m) allows for full 3D geometric spreading, resulting in lower received levels.
  • Floating configurations in shallow water show OASPL variations of up to 7% (~9 dB) compared to their deep-water counterparts, highlighting the sensitivity of floating systems to bathymetry.
• Turbulence Impact: Introducing atmospheric turbulence broadens sharp tonal peaks into wider spectral humps and increases overall SPL by up to 10 dB at low frequencies, though the fundamental trends regarding support structure typology remain unchanged.

Significance and Claims
The authors assert that this work provides a predictive tool for assessing underwater acoustic impacts during the design phase, rather than relying solely on post-installation measurements. By linking measurable structural response parameters to radiated sound levels, the framework supports:

• Environmentally Informed Design: Allowing developers to optimize turbine selection, spatial configuration, and structural detailing with explicit consideration of the acoustic footprint.
• Regulatory Support: Providing traceable metrics for standardization, compliance verification, and the development of science-based regulatory thresholds.
• Mechanistic Understanding: Clarifying that floating turbines should not be treated as simple extensions of fixed-bottom systems; their unique dynamics introduce distinct low-frequency sources and complex 3D radiation patterns that require specific assessment.

The study concludes that while floating platforms may concentrate energy in frequency bands less sensitive to many marine species (very low frequencies), monopiles pose a higher risk in the mid-to-high frequency bands where many marine vertebrates have sensitive hearing. The framework enables a quantitative assessment of these trade-offs to support future mitigation strategies.