Miljøprojekt, 1016 – Noise from offshore wind turbines

Noise from offshore wind turbines

Summary and conclusions

Noise assessment of wind turbines through calculations based on sound power levels measured according to e.g. [1] is used world wide. With larger wind turbines and calculation distances some of the calculation models developed for industry noise tend to give erroneous results.

The wind turbines are getting larger and are going offshore. Some offshore locations will be relatively close to coastlines with residential or recreational areas, and the environmental impact from e.g. noise can prove to be a subject to debate. The distances from offshore wind farms to the coastline can vary from a few kilometres, e.g. Middelgrunden outside Copenhagen Harbour, to more than 10 km, e.g. Horns Rev west of Blåvandshuk in Jutland.

Noise propagation from offshore wind turbines is different from propagation from land based wind turbines. Due to reflections from the acoustically hard sea surface, the noise is reduced at a slower rate than sound propagating over acoustically soft grassland, and what is supposed to be a safe distance for a wind farm on land may not be so for offshore wind farms.

For that reason it is important to be able to make valid noise assessments for offshore wind farms. This includes reliable noise calculations and the possibility to check the specified noise data through measurements. Noise measurements offshore are more difficult than on land, and it is necessary to change the mounting of the microphone from a position on a plate on the ground. DELTA has tested an alternative method where the microphone is placed on board a vessel 3 to 5 m above sea level and has found it applicable. The main problem with the method is that background noise from waves hitting the vessel may mask the noise from the wind turbine and make measurements impossible.

A few more tests in different situations should be made before the method is recommended as a general method.

Prediction of noise from offshore wind turbines involves propagation of noise over large distances. This means that small inaccuracies in the prediction models can become significant. Most important is the ground effect, but also air absorption at low frequencies has to be considered.

The Danish dB(A) model assuming a hard ground overestimates the levels of noise propagating over ground, but gives reasonable results offshore for limited distances up to 500 m. In the octave band version the reliable distance extends to 2 to 5 kilometers. The models fail at large distances because of multiple reflections from the sea surface building up and leading to cylindrical spreading of the sound energy.

NORD2000 includes this effect. It is, however, necessary to achieve a better understanding of at which distances and frequencies the effect of multiple reflections is active, that is when to change from coherent to incoherent summation of the reflections.

Air absorption coefficients at low frequencies should be included in all models.

For situations without screening different wind directions and wind speeds give only small variations in downwind directions and only slightly more in cross wind. When calculating the annual average noise level, variations in the noise level during varying meteorological situations become important.