Abstract
Airicole, GE Wind Energy and SEAS/Energi E2 have initiated this project in order to achieve a better understanding on how offshore based wind farms effect the underwater noise. The main reason is to gain knowledge on how marine wildlife could be effected by this kind of installation.
The measurements were performed at Utgrunden wind farm that is situated at the reef Utgrunden on the Swedish southeast coast. The farm consists of seven 1,5 MW turbines. Three hydrophones registered the underwater sound and four accelerometers the tower vibrations. The measurement campaign was conducted during a period from November 2002 to February 2003. The objectives with this project is to answer the following issues and its results are:
1. What is the character of sound from a single power station?
-The turbines radiate sound mainly at a few dominating frequencies from 30 Hz up to 800 Hz. At frequencies below 3 Hz no contribution from the turbines can be detected due to the high background level from the waves and the low tower vibration level.
2. What are the sound generating mechanisms in the turbine?
-Gearbox mesh frequency vibrations that are transmitted via the tower structure and radiated out to the water mainly generate the sound. Airborne blade sound is effectively dampened in the transition from air to water.
3. How does the sound attenuate with increasing distance at different frequencies?
-The average attenuation per doubled distance for frequencies between 31 Hz and 722 Hz is approximately 4 dB in the measured positions. No clear frequency dependence could be found.
4. How does the sound pressure level vary with increasing wind speed?
-With increasing wind speed, the sound pressure level increases and the dominating frequencies move upward due increasing turbine rotational speed.
5. How does sound from different power stations interfere with each other and influence the over all sound image?
-No clear tendencies of interference could be observed in this study. This could be due to small variations in turbine speed and that the hydrophone positions needs to be less dominated by a single turbine.
6. How is a passing ship influencing the sound level in the farm?
-Passages of ship dominates the sound in the park for frequency higher than approx. 63 Hz. There is a strong dependence of type of ship, distance etc.