This report has been prepared by Subacoustech Ltd for Collaborative Offshore Wind Research Into the Environment (COWRIE).
A series of measurements were made of the underwater noise during underwater pin pile drilling operations as part of the installation of the SeaGen tidal turbine device in Strangford Lough, Northern Ireland. The data are presented in unweighted noise metrics such as RMS, Sound Pressure Level and Sound Exposure Level. The data are also presented as weighted levels above hearing threshold (dBht) for specific fish and marine mammal species.
Measurements of background underwater noise during periods when no drilling was being carried out have indicated high levels of high frequency background noise are present in the Strangford Narrows region of Strangford Lough. It is thought that this is due to the high speed of tidal flow in the region generating noise by interaction of turbulent water with the sea bed and at the water surface.
Measurements at ranges of 28 m to 2130 m from the drilling operation indicated that the unweighted, one second RMS Sound Pressure Levels varied from 105 to 139 dB re. 1 µPa. A fit to the measured data has indicated a Source Level for the drilling noise of 162 dB re. 1 µPa @ 1 m. The levels of underwater noise from the drilling operation are comparable with small vessel noise, and considerably lower than the levels of noise generated by other piling techniques such as impact piling or vibro-piling.
High levels of underwater noise can have an effect on underwater species, in that they are known to cause a behavioural avoidance response in fish and other marine animals, and in some cases where the level is very high can cause physical injury. The measured levels of underwater noise from the pin pile drilling indicate that the noise levels are very much lower than those that may cause fatality, physical injury or audiological injury to the species of fish and marine mammal considered.
Comparison of the measured background noise data with the hearing sensitivity of the harbour porpoise has indicated that this region is a noisy environment for marine animals that are sensitive to high frequency noise. The data for the drilling noise indicates that these species are unlikely to be able to hear noise from the drilling operation over the high levels of perceived background noise. This conclusion highlights the importance of considering the spectral perception of underwater noise by marine animals when estimating its impact.
The likelihood of avoidance of the drilling noise by species of fish and marine mammal has been assessed by using the dBht approach (Nedwell et al, 2007b) which estimates the perception or “loudness” of noise by weighting the measured noise levels by the published hearing threshold data for the species considered. The data indicates that the noise does not exceed the 90 dBht level, at which strong and sustained avoidance is expected, at any measured range. The 50 dBht level, at which a mild and brief reaction is expected in a minority of individuals, extends to a maximum range of 115 m.
The data therefore indicates that the species of fish and marine mammals considered are unlikely to be disturbed by the drilling noise unless they are in the close vicinity of the drilling operation.