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Underwater Piling Noise During Nearshore Wind Farm Construction In UK Waters
Ford, B., Jiang, J, Todd, V, and Todd, I (2017): Measurements of underwater piling noise during nearshore windfarm construction in UK: potential impact on harbour porpoise (Phocoena phocoena) and compliance with German UBA limit: 24th International Congress on Sound and Vibration, 23-17th July, London, UK. For a similar study performed in German waters, see www.osc.co.uk.
According to the United Kingdom (UK) renewable energy strategy, 15% of electricity will be generated with renewable sources by 2020 (Higgins and Foley, 2014). Offshore wind farms are currently the main source of renewable energy, and the number of turbines in UK waters is increasing each year. Foundations for wind turbines vary in construction, but most necessitate piles being driven into the seabed using a hydraulic hammer, which produces high levels of impulsive noise.
Underwater noise can cause disturbance to marine species including marine mammals (Richardson et al., 1995), fish (Scholik and Yan, 2001) and marine birds (Hansen et al., 2017). Disturbance ranges from masking of their own, or other underwater sounds, to permanent injury or death. One benchmark which assesses impacts of underwater noise on marine mammals, are the Southall criteria (Southall et al., 2007), which provides thresholds for different marine mammal species based on source type (single pulses, multiple pulses, or non-pulsed).
Sound propagating from pile installation is influenced by, inter alia, pile size, hammer, energy and geological properties at the site. Although only a few countries, including Germany, Belgium and Denmark, have set a threshold for underwater noise levels during offshore construction, sound measurements of the first four pile installations are often required. The German Federal Environment Agency (UBA) set a limit of 160 dB Sound Exposure Level (SEL) and 190 dB Peak-to-Peak Level (Lpp) at 750 m from the sound source. In the UK, no legislation is enforced for sound measurements; however, in accordance with Joint Nature Conservation Committee (JNCC) guidelines (JNCC, 2010), Marine mammal Observers (MMOs) and Passive Acoustic Monitoring (PAM) are usually required to ensure no marine mammals are within injury range before pile driving can commence, and additional noise modelling is sometimes included in licence conditions.
During construction of a nearshore wind farm in UK waters, OSC performed noise and environmental measurements at 750 m distance during pile driving of two monopiles. Using a Kraken underwater noise propagation model, noise level indices we calculated and Transmission Loss (TL) predicted. Results were compared to German UBA limits and JNCC legislation in UK waters.
The main energy of piling noise reached frequencies of up to 30 kHz. Sound Exposure Level (SEL) for each single hammer strike exceeded 160 dB at 750 m, but Lpp only exceeded 190 dB on some occasions.
To understand potential effects on marine mammals, SEL was weighted using the Southall criteria (Southall et al., 2007) for low, mid and high frequency marine mammals and pinnipeds.
In the UK, where these measurements were taken, there is currently no underwater noise threshold enforced, but visual and/or acoustic monitoring of a dedicated exclusion zone, usually 500 m around the sound source, is often required. In this study, we predicted auditory impact ranges for marine mammals by calculating distances from the sound source, which were likely to cause Permanent Threshold Shift (PTS) and Temporary Threshold Shift (TTS) for both monopiles. Impact range for PTS for all marine mammals were within the 500 m exclusion zone (<320 m) for both piles, and impact range for TTS was within the 500 m zone for cetaceans; however for pinnipeds, it was 560 m and 540 m for piles one and two respectively. Consequently, it was concluded that the JNCC 500 m exclusion zone could potentially offer protection for most marine mammal species during offshore wind farm construction.
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