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Noise impact

Offshore wind farms are located far away from human populations, which are not affected by the noise generated by wind turbines. The marine animals could be affected by the underwater noise generated during the construction and operation of wind turbines. Any effects of the noise will depend on the sensitivity of the species present and their ability to habituate to the noise.

The procedures to measure the acoustic noise from offshore wind turbines should include the next inputs :

• Wind turbine parameters: rated power, rotor diameter, etc.
• Type of foundation, material, pile depth, etc.
• Effective pile driving and/or vibration energy.
• Period of construction phase and blow or vibrator frequency.
• Depth of water at the site.

Construction and decommissioning noise

The noise generated comes from machines and vessels, pile-driving, explosions and installation of wind turbines. Measurements carried out by the German Federal Ministry of Environment on two platforms, reaching peak levels of 193 dB at 400 m of distance from the pile (North Sea) and 196 dB at 300m (Baltic Sea). Nedwell reports peaks up to 260 dB to foundation construction and 178 dB to cable lying, at 100 m of distance of the sound source (Gill, 2005). These high sound levels may cause permanent or temporary damage to the animal acoustics systems located in the vicinity of the construction site. However, there is not enough scientific knowledge to determine the maximum thresholds permitted for certain effects. Close collaboration between physicists, engineers and biologists is necessary to get relevant information and obtain standardisation of the measurement procedures in offshore developments.

The spectrum obtained of measurements from FINO-1 at 400 m of distance from source revealed maximum peaks of 180 dB. The measurements carried out during construction of North Hoyle wind farm in U.K. indicate that:

• The peak noise of pile hammering at 5 m depth was 260 dB and at 10 m depth was 262 dB.
• There were no preferential directions for propagation of noise.
• The behaviour of marine mammals and fish could be influenced several kilometres away from the turbine. The next table shows the avoidance reaction expected to occur due to piling at the North Hoyle wind farm construction.

Table 2.4: Calculated ranges for avoidance distance for different marine species.

The behaviour of marine organisms may be modified by the noise, resulting in an avoidance of the area during construction. The possible effects on sealife will depend on the sensitivity of the species present in the area and will be reduced when the noise decreases at the end of the construction (or decommissioning) phase.

Different working groups are currently discussing mitigation measures to reduce damage on sealife:

• Soft start in the ramp-up procedure, slowly increasing the energy of the emitted sound.
• Using air-bubble curtain around the pile, could result in a decrease of 10-20 dB.
• Mantling of the ramming pile with acoustically-insulated material such as plastic could result in a decrease of 5 -25 dB in source level.
• Extending the duration of the impact during pile-driving could decrease of 10-15 dB in source level.
• Using acoustic devices which emitted sounds kept away mammals during ramp-up procedure. Several pingers might be necessary at different distances from sound source.

Operational noise

In the operation phase, the sound generated in the gearbox and the generator is transmitted by the tower wall resulting in sound propagation underwater. Measurements of the noise emitted into the air from wind turbines and transformers have shown a negligible contribution to the underwater noise level. The underwater noise from wind turbines is not higher than the ambient noise level in the frequency range above approximately 1 kHz, but it is higher below approximately 1 kHz. The noise may have an impact on the benthic fauna, fish and marine mammals in the vicinity of wind turbine foundations.

Operational noise from single turbines of maximum rated power of 1.5 MW, was measured in Utgruden, Sweden at 110 m distance by Thomsen et al, 2006. At moderate wind speeds of 12,/s, the 1/3 octave sound pressure levels was between 90 and 115 dB.

The anthropogenic noise may produce both behavioural and physiological impacts for sealife. Impacts on behaviour include:

• Attraction or avoidance in the area.
• Panic.
• Increases in the intensity of vocal communication.

Reports about noise impact on fish have shown a range of effects, from avoidance behaviour to physiological impacts. Changes in behaviour could make fish vacate feeding and spawning areas and migration routes. Studies about noise impact on invertebrates and planktonic organisms result in a general consensus of very few effects, unless the organisms are very close to the powerful noise source. Measurements from one 1,500 kW wind turbine carried out by the German Federal Ministry of Environment has found that operational noise emissions do not damage the hearing systems of marine sealife. About the behaviour, the same study stated that it is not clear whether noise from turbines has an influence on marine animals.

Ships are involved in the construction of wind parks and also during the operation phase through maintenance of wind turbines and platforms. The noise from ships depends on ship size and speed, although there are variations between boats of similar classes. Ships of medium size range produce sounds with frequency mainly between 20 Hz and 10 kHz and levels between 130 and 160 dB at 1 m.

Standardised approaches to obtain noise certificates, similar to those existing onshore, are the more urgent tasks.

Electromagnetic fields on marine organisms

The electricity produced by offshore wind turbines is transmitted by cables over long distances. The electric current generated produce magnetic fields. Studies about possible effects of artificial static magnetic fields have been carried out on various species under various experimental conditions. Artificial electromagnetic fields could interact with marine organisms to produce detectable changes. Usually, only very slight differences in control groups have been recorded.

The magnetic field may affect mainly molluscs, crustaceans, fish and marine mammals that use the earth's magnetic field for orientation during navigation. But it is still unknown whether the magnetic fields associated with wind turbines influence on marine organisms

Elasmobranches, one of the species more electro-sensitive, are attracted by electrical fields in the range of 0.005-1 µV cm1 and avoid fields over 10 µV cm1.

Electro-sensitive species could be attracted or repelled by the electrical fields generated by submarine cables. Special attention must be paid in areas of breeding, feeding or nursing because of the congregation or dispersion of sensitive individuals in the benthic community.

Experimental analysis on several benthic organisms (see table 2.5) exposed to static magnetic fields of 3.7 mT for several weeks have shown no differences in survival between experimental and control populations. Similarly, mussels living under these static magnetic field conditions for three months during the reproductive period do not present significant differences with the control group. The conclusions are that static magnetic fields of power cable transmissions don't seem to influence the orientation, movement and physiology of the tested benthic organisms.

The results from the study carried out on Nysted about the influence of electromagnetic fields on fish are not conclusive. Some impact on fish behaviour has been recorded, but it was not possible to establish any correlation. There is not enough knowledge about this topic and additional research is needed.

The magnetic fields of both types of cable (bipolar and concentric) used in marine wind farms, are small or zero. The Greenpeace study concludes that the electromagnetic fields of submarine cables have no significant impacts on marine environment. Studies at long-term perspective are necessary to confirm the negligible impact of electromagnetic fields of wind energy on marine ecosystems.