Abstract
This report presents modelling of the acoustic output of operational off-shore wind turbines and its dependence on the type of foundation structure used. Three foundation types are examined: jacket, monopile and gravity foundation. The acoustic output from each of these foundation types is then compared to curves representing the hearing and behavioural response of marine species likely to come into contact with off-shore wind farms in Scottish Waters. The marine species examined are minke whales, harbour porpoise, grey seals, harbour seals, bottlenose dolphins, European eels, allis shad, sea trout and Atlantic salmon.
Vibration produced by a generic 6 MW wind turbine was modelled across the 10 Hz to 2 kHz frequency band. The generic wind turbine was placed on the three different foundation types and the variation of the sound field in the marine environment around each foundation was modelled to a distance of 40 m from the foundation. The resulting sound fields tend to be strongly tonal with sound pressure level (SPL) peaks associated with gear meshing frequencies in the gearbox and electromagnetic interactions in the generator.
The monopile produced the highest SPL of the foundations at lower frequencies (<200 Hz), with levels of 149 dB re 1 μPa within 5 m of the foundation at 560 Hz. The jacket produced the highest SPL at high frequencies (>500 Hz) with 177 dB re 1 μPa at 700 Hz and 191 dB re 1 μPa at 925 Hz within 5 m of the jacket. These high SPL at high frequency produced by the jacket are associated with structural resonances for which the high SPL is strongly localised to volumes very close to the jacket and dissipate rapidly moving away from the foundation.
The sound field modelled within 40 m of each foundation type was extended to a range of 20 km using a beam trace model. Beam trace models of 16 turbines were combined to determine the sound field surrounding wind farms set out in a diamond and square pattern. Negligible difference in the sound field was found between the two wind farm layouts. The acoustic output at different wind speeds (5, 10 & 15 ms-1) and associated power generation was compared to the background noise to determine the range at which noise produced by the wind farm would be masked by the background noise. The monopile is audible above the background noise at least 20 km from the wind farm in all wind conditions. The gravity foundation is masked at low frequency (<000 Hz) at 5 ms-1, but becomes audible at 10 and 15 ms-1. The jacket is only audible above the background noise at frequencies higher than 400 Hz.
The modelled noise levels are likely to be audible to marine mammals particularly at 15 ms-1 when the generic wind turbines are producing maximum power. Jacket foundations generate the lowest marine mammal impact ranges compared to monopile and gravity foundations. Species with hearing specialised to low frequency, such as minke whales, may in certain circumstances detect the wind farm at least 18 km away and are the species most likely to be affected by noise from operational wind turbines. Harbour seals, grey seals and bottlenose dolphins are not considered to be at risk of displacement by the operational wind farm modelled.
Atlantic salmon and European eels are able to detect the presence of monopiles at greater ranges than gravity bases, though this may not affect their behaviour. Allis shad and sea trout appear to not be able to detect noise produced by operational wind turbines except at close range (<100 m).