Underwater noise emissions from offshore wind turbines

Vibration of the turbine’s gear box and generator is guided downwards and radiated as sound from the tower wall (Figure 1). Sound radiation by surface waves is difficult to compute and to predict, in particular for complicated boundary conditions. Hence, measurements on an already existing offshore wind turbine were made. The setup is shown in Figure 2. Since access to the turbine is only possible at low wind speeds, an automatic recording was made over a one month period. At every full hour, 20 minutes of underwater sound and tower wall vibration were recorded to hard disk. The accelerometer position – approx. 10 m above sea level and perpendicular to the wall – was choosen after preliminary measurements with several sensor positions above and below sea level [1]. Wind and electric power values were taken from the turbine’s routine log files.