The potential impact of noise from Offshore Wind Farms

By Eoin King, Ryan Institute, University of Galway

Ireland’s position on the edge of the Atlantic places it in an ideal position to harness energy from renewable sources, particularly from offshore wind power. Irelands National Mitigation Plan (2017) recognises that we have one of the best offshore renewable energy (wind, wave and tidal) resources in the world, and there is very significant potential in utilising these resources to generate carbon-free renewable electricity. This is recognised in the current programme for government, ratified in 2020, setting an ambition of at least 3.5 GW of offshore wind power by 2030, as well as taking advantage of the potential for at least another 30GW of offshore floating wind power in the deeper waters in the Atlantic. There are clear targets for Ireland regarding decarbonized energy systems, and there is a greater understanding of the role that offshore wind energy can play in achieving these targets. 

While the potential behind offshore wind energy is massive, there are some concerns over the environmental impact that may arise from widespread adoption, including increased noise levels above and below the sea, disturbances to habitats, and potential pollution from the release of contaminants from seabed sediments. The collection of wind turbines in farms, as opposed to standalone units, will increase noise generation. Further, with the increasing height of the wind turbine towers, and the increasing size of the offshore wind farms, these environmental impacts are becoming significant [1]. This is particularly important in the case of low frequency noise. Larger turbines have significantly more low frequency noise compared to smaller turbines.

While most discussions around noise from Offshore Wind Farms are concerned with the impact (underwater) noise may have on marine life, this article considers the potential for the impact of audible noise on coastal communities. Of course, while underwater noise is a significant concern and will require more research to assess likely impacts, there has been minimal consideration (in the academic literature) of the impacts of audible noise on coastal communities.

As it currently stands there is no reliable method to ascertain the noise impact of offshore wind turbine developments. A standard simply does not exist, so in the relatively few cases where a noise impact assessment is performed, developers often apply standards that are used for onshore developments. Several EISs for planned developments in Ireland (and the UK) have used ISO 9613 in the assessment of operational noise. This standard is not designed for propagation over water, and its use may significantly under-predict the true impact of a development. Propagation over water usually means that the sound will travel over longer distance, especially at lower frequencies. There is little published research or guidance in Ireland or the UK on the propagation of noise over water. Further, there is currently no recommended calculation method for the assessment of noise from offshore wind farms.

There is some evidence that, when wind turbine noise is propagating over water, there is a 3 dB decrease in sound level for each doubling of distance (cylindrical propagation) instead of the more usual 6 dB (spherical propagation) used for on-shore calculations [2]. This would mean noise over water will be louder over twice the distance compared to propagation over land, and could lead to significant exposure issues related to low frequency noise. Any exposure assessment for offshore wind turbines may significantly under-predict the level of noise, and, as low frequency noise transmits more efficiently through walls, these underpredictions may lead to significant adverse issues in homes. 

While the issue of sound propagation over water needs to be further investigated in itself, the interaction between the water surface and the meteorological conditions at sea is another important issue in need of further research. Meteorological conditions, water surface roughness, and the water/land border, are also important to consider, and their effects are not currently well defined. Further, when a sound wave propagating over the sea reaches the shoreline, a variety of changes happen all at once, including changes to the ground impedance and the wind and temperature gradients. This will alter the sound speed profile and will induce some refraction. Only a few studies have been made of the shoreline effect for acoustical propagation [3], but one study suggests a possible attenuation of only 3dB [4]. The current noise prediction tools commonly used to assess the acoustic impact of wind farm developments are only partially taking these parameters into account. A reliable method to capture the effect of these parameters on noise propagation from offshore is required, as a failure to take these issues into account may result in a significant underprediction of the impact of offshore wind turbines.

This is further complicated in a marine environment. The marine boundary layer (MABL), the part of the atmosphere in contact with the ocean, is directly influenced by the ocean, and is shallower than the atmospheric boundary layer. Further the MABL is where the ocean and atmosphere exchange large amounts of heat, moisture, and momentum, primarily via turbulent transport. Consequently, wind speeds on sea for a given height are higher, and turbulence intensities are lower, than on land. It is further complicated by the potential presence of low-level jets, which are winds of high speed occurring at a relatively low height and are usually formed by diurnal changes in the thermal stratification of the surface layer. 

As sound is propagating from source to receiver, the manner in which it interacts with the ground surface also plays an important role. Some sound energy hits the ground surface and is absorbed or reflected. The spreading of the reflected sound is dependent on the surface roughness of the ground. It is generally assumed that a flat water surface will totally reflect sound. However, under real conditions on sea, water waves, with different amplitudes and wavelengths, will be present. The effect that water waves have on sound propagation is not fully understood. 

In discussions on the noise impact of offshore developments, many point to that fact the wind farms installation are located increasingly further offshore, typical several 10s of km offshore, which may make their acoustic emissions negligible onshore. However, there is always a certain degree of uncertainty, for reasons outlined above. Quite simply, it is not possible to make an assertion on impact one way or the other using today’s tools, as no validated model currently exists. Further, in the case of Ireland, of the six Phase One offshore Wind Farms under development, four are actually within approximately 10km of the shore at their closest point. It is not unreasonable to suggest that any of these proposed developments, as well as those further offshore, could be audible in coastal communities, and certainly warrants further investigation.

Should noise from offshore wind turbines be audible on the coast, it is highly likely they will be audible above the background noise sources. This could lead to complaints resulting in a lack of social acceptance for developments. The time has come to perform a detailed assessment of the acoustic impact of offshore developments, in order to avoid unnecessary noise-induced barriers to development in the future, and ensure we can harness the full potential of offshore wind energy. Research is being conducted in the area, both at the University of Galway, and across Europe through the work of IEA Wind TCP Task 39.

Sound propagating over water – some personal experience. 

Last summer, along with two close friends, I decided to take up sea swimming in the early morning hours. I started in June and continued until mid-November, at which time the water in the Atlantic Ocean was simply too cold for me to brave any longer. One particularly cold and calm morning, my friends and I were not feeling particularly brave, and instead of jumping into the sea, we chose to slowly descent at some steps a little distance from the main diving board area (to save ourselves from embarrassing ourselves in front of the seasoned swimmers). The water was so cold we uttered some choice words (along with the occasional shriek). But thankfully, as we were a little bit away from the experienced swimmers, we felt that no-one could hear us. That was until someone came upon us and started to poke some fun at us – he had been walking along the promenade probably a couple of hundred metres away, and heard every word we said, and much to our embarrassment repeated those words back to us. He told us that everyone along the prom could hear us, and we had brightened up everyone’s morning. It turns out that our voices carried over the calm water that morning, and anyone out for a walk heard us as we ‘acclimatised’ to the cold water

Sound can travel very efficiently over water, sometimes much more efficiently than you might expect!

References

[1] Dai K., Bergot A., Liang C., Xiang W., Huang Z., Environmental issues associated with wind energy – A review, Renewable Energy (75), 911-921, 2015.

[2] Keith, S.E., Daigle, G.A., Stinson, M.R., Wind turbine low frequency and infrasound propagation and sound pressure level calculations at dwellings. J. Acoust. Soc. Am.,144, 981–996, 2018.

[3] Boué, M., Final report for the Swedish Energy Agency project 21597-3 (TRANS), Long-range

sound propagation over the sea with application to wind turbine noise, KTH, Stockholm, 2007.

[4] L. Johansson Sound Propagation around off-shore Wind Turbines -Long-Range Parabolic

Equation Calculations for Baltic Sea Conditions, Licentiate Thesis, KTH, Stockholm 2003.