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Offshore wind

Beneath the turbines: reducing underwater noise.

Offshore wind farms are growing in number and size. This means cleaner energy, but also potentially a greater environmental impact below the sea’s surface. Expertise in underwater acoustics is ever-more crucial for protecting marine life and gaining new project approval.

Overview

Offshore wind turbines create underwater noise, and this differs between a wind farm’s construction and operation.

Construction of popular turbine foundations commonly involves hammering a pile into the seabed. Acoustically speaking, this is like placing a large loudspeaker underwater. The impulsive noise is much louder than many other underwater noise sources (and the sea isn’t a quiet place naturally). As wind turbines increase in number and size, construction of future offshore wind farms will likely cause even higher noise levels.

Operating wind turbines also create underwater noise. The interaction between wind and blades generates the massive dynamic forces necessary to produce electricity. At the same time, the forces cause vibration throughout the turbine’s structure, radiating noise into the sea and seabed. While current knowledge suggests that offshore pile driving could significantly harm marine mammals, the operational effect of noise from wind turbines is less clear-cut. Noise levels are lower than during construction, but they are almost continuous throughout each turbine’s lifetime. Presently, experts think that there may be little conflict between marine mammals and operating wind farms here. But as the size and design of turbines increases, that view may change.

A rising general noise level in the sea

Pile driving and rotating turbines are not the only man-made noise affecting underwater environments. Industrial activity in the oceans is growing elsewhere. Shipping is steadily increasing. Then, there are subsea installations for oil and gas production and new wave and tidal energy devices, as well as seismic explorations.

These activities are potentially significant noise sources, but they have not all yet been fully described in terms of underwater acoustics. This means that their impact on marine life is only partially understood. What we do know is that many types of marine life rely on acoustics for communication, navigation and feeding, and man-made noise has the potential to interfere with these vital routines. Marine biologists are concerned about the physiological and behavioural impacts on sea creatures.

Birds-eye view of sound-field showing dependence on bathymetry (blue lines)
Birds-eye view of sound-field showing dependence on bathymetry (blue lines)

How marine life is being protected

Limiting underwater noise on an industrial scale is a relatively new development. In 2008, the EU introduced its Marine Strategy Framework Directive and one of the topics covered by the Directive was underwater noise and how to protect the marine environment.

In European and UK waters, an Environmental Impact Assessment (EIA) is required for the planning of any new large offshore wind farm. EIAs assess the potential impact of a proposed development on the physical, biological and human environment during construction, operation and decommissioning. An often important contribution to the EIA is a numerical prediction of the underwater noise, particularly from foundation construction activity, as this may potentially be harmful to marine life. Recent evidence shows that, as a rule, pile driving without noise-mitigating measures is very likely to exceed the strict German noise limits (in place for over a decade).

Modelling and managing underwater noise

For more than a decade, full-scale experience has been gained in the North and Baltic Seas. This has shown that it is possible to meet noise requirements, such as those in Germany. Additionally, numerical computer models can simulate the performance of mitigation techniques. It is possible to model noise radiation from both construction and operation stages, with and without noise mitigation, producing detailed noise maps. These can cover a radius of several kilometres away from the wind turbine.

Computer modelling of underwater noise from offshore wind is a complex, multidisciplinary effort with no off-the-shelf solution. Many organisations develop their own models and validate them against full-scale sea trial data. Modelling and managing underwater noise comprises four main steps, requiring multidisciplinary capabilities and reliable prediction tools.

  1. Data from previous geophysical surveys are input into an underwater acoustic prediction model, with a representation of the piling noise source. Depending on the project stage and design details, modelling can be done at varying approximation levels for a trade-off between accuracy and the model’s geographic coverage.
  2. The calculated sound field is combined with population distributions of noise sensitive species. From a comparison with noise exposure criteria for specific species, ‘effect zones’ can be identified (within these zones, the various thresholds are exceeded).
  3. Considering the animal behavioural response to the noise then enables an assessment of the risk of impact. Acoustic experts can collaborate with specialist marine biologists – bio-acousticians – here.
  4. Finally, various noise mitigation techniques and strategies are explored.

Design challenges

The main challenges in reducing underwater noise are design related. R&D initiatives for noise mitigation are underway, including using alternative pile hammering techniques, some based on vibration, through to resonator principles to mitigate noise. New turbine foundations are also maturing, from suction buckets to floating foundations.

Vertical slice of sound-field in water and sea bed for a point source. Black line marks the seabed
Vertical slice of sound-field in water and sea bed for a point source. Black line marks the seabed

A quieter outlook

In an age of sustainable energy, we can and should reduce underwater noise using specialist acoustic assessment and uniting expertise across the disciplines of acoustics, engineering, environmental science and biology. Both industry and shipping are likely to face increased regulation in this area in the future. Addressing this issue should also encourage the development of more environmentally friendly technologies. As the world demands increasing levels of low-carbon energy, the opportunities for enterprising engineers are below the water.

Talk to an expert

René Smidt Lützen

René Smidt Lützen

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