The United Kingdom government’s renewable energy strategy has a target of providing 10% of the UK’s electricity from renewable sources by 2010, and 15% by 2015. A significant expansion of offshore wind energy is an important component of this strategy. The UK's first large-scale offshore wind farm, at North Hoyle in Wales, was commissioned in November 2003, and a second development at Scroby Sands is nearing completion. Further major developments are planned at the Thames Estuary, the Greater Wash and in the North West.
The wind energy industry has concerns as to whether underwater noise during the construction and operation of windfarms might have the capacity to cause environmental effects, such as avoidance by marine mammals and fish. Since these effects of underwater noise are not yet fully understood, the Collaborative for Offshore Wind Research Into the Environment (COWRIE) instructed Subacoustech Ltd (via the offices of The Crown Estate) to investigate these matters.
The purposes of the investigation reported herein was to provide measurements which would:
- evaluate the pre-existing background noise environment;
- measure and interpret the underwater noise from construction and operation of windfarms, and thus to
- interpret these measurements in terms of any potential for environmental effect, and thus to
- provide information allowing the wind energy industry to minimise any impact of noise during the lifecycle (construction, operation and decommissioning) of windfarms.
The report presents a significant body of underwater noise measurements taken in the period April 2003 to January 2004 at operational and construction stage windfarm sites in the UK. A detailed analysis of the measurements has been made which indicates the spatial, temporal and statistical properties of the noise. An estimation of the likely behavioural and physical effects on a selection of the most common species of fish and marine mammals is also presented using both conventional analysis and the dBht (species) scale.
The measurements showed:
- that the levels of background noise at typical windfarm sites are towards the upper bound of typical deep water background noise levels. The overall sound pressure level varies significantly more during the daytime than at other times of day, due to the higher number of short local ship movements. The noise levels are higher at low wind speeds, contrary to the normal assumption that they will rise with increasing wind speed. At North Hoyle pre-existing man-made noise is probably a significant contributor to the background noise level;
- that piling at North Hoyle gave a Source Level of 260 dB re 1 μPa @ 1 metre for 5 m (metres) depth, and 262 dB re 1 μPa @ 1 m at 10 m depth, associated with a Transmission Loss given by 22 log(R), where R is the range. Calculations using the dBht scale levels indicate that strong avoidance reaction by a range of species would be likely at ranges of up to several kilometres. The levels of sound recorded during piling are such that within perhaps a hundred metres they could cause injury. Measurements of piling at Scroby Sands were similar in level to those at North Hoyle, and similar conclusions pertain in respect of possible environmental effects;
- that cable trenching at North Hoyle gave a Source Level of 178 dB re 1 μPa @ 1 m if a Transmission Loss of 22 log(R) is assumed;
- that rock socket drilling produced a strong fundamental component at 125 Hz, and harmonics up to 1 kHz, but it was not possible to establish the Source Level and Transmission Loss. Tonal components of the drilling could, however, be identified at ranges of up to 7 km.
It is recommended that piling in particular should be regarded as capable of causing environmental effects, and planning of piling operations should take account of the effects of its noise on sensitive species. Mitigation measures that may be used to reduce the impact of piling are discussed.