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Offshore

 

Offshore wind energy is a renewable technology capable of supplying significant energy in a sustainable way. According to EWEA estimates, between 20 GW and 40 GW of offshore wind energy capacity will be operating in the European Union by 2020. This capacity could meet more than 4 per cent of EU electricity consumption (Edge, 2007). The total offshore installed capacity in Europe at the end of 2007 was almost 1,100 MW, distributed in the coastal waters of Denmark, Ireland, The Netherlands, Sweden and the UK, representing almost 2 per cent of the total wind energy (56,536 MW) in the EU.

Offshore wind projects are more complex than onshore ones. Offshore developments include platforms, turbines, cables, substations, grids, interconnection and shipping, dredging and associated construction activity. The operation and maintenance activities include the transport of employees by ship and helicopter and occasional hardware retrofits.

From an ecological point of view, shallow waters are usually places with high ecological value and are important habitats for breeding, resting and migratory seabirds. Close participation and good communication between the countries involved in the new developments is essential to reduce environmental impacts from several wind farms in the same area.

Most of the experience gained in offshore wind energy comes from several years of monitoring three wind farms in Denmark (Middelgrunden, Horns Rev and Nysted) installed between 2001 to 2003. Valuable analysis has also been carried out by the Federal Environment Ministry (BMU) of Germany through technical, environmental and nature conservation research about offshore wind energy foundations.

 

VISUAL IMPACT

Offshore wind farms usually have more and bigger turbines than onshore developments. However, visual impact is lower due to the greater distance from the coastline. Nevertheless, the coastal landscape is often unique and provides some of the most valued landscapes, thus special attention could be required (SDC, 2005).

The visual impact of offshore wind farms can affect three components of the seascape:

1. An area of sea;
2. A length of coastline; and
3. An area of land.

Figure 2.1: Components of Seascape.

Source: Wratten et al. (2005)

 

Offshore wind farms involve several elements which have influence on the character of the produced visual impact (Wratten et al., 2005):

 

 

• The site and size of wind farm area;
• The wind turbines: size, materials and colours;
• The layout and spacing of wind farms and associated structures;
• Location, dimension and form of ancillary onshore (substation, pylons, overhead lines, underground cables) and offshore structures (substation and anemometer masts);
• Navigational visibility, markings and lights;
• The transportation and maintenance boats;
• The pier, slipway or port to be used by boats; and
• Proposed road or track access, and access requirements to the coast.

Just as for onshore developments, ZTV zones, photomontages and video-montages are tools used to predict the potential effects of new offshore wind developments.

The potential offshore visibility depends on topography, vegetation caver and artificial structures existing on the landscapes. The visibility assessment of offshore developments includes the extent of visibility over the main marine, coastline and land activities (recreational activities, coastal populations and main road, rail and footpath). The effects of the curvature of the earth and lighting conditions are relevant in the visibility of offshore wind farms (Wratten et al, 2005). Rainy and cloudy days result in less visibility. Experience to date on Horns Rev proves that a wind farm is much less visible than the 'worst case' clear photomontage assessment, due to prevailing weather conditions and distance (IEA, 2005).

The magnitude of change in the seascape with the construction of a new offshore wind farm is dependent of several parameters such as distance, number of turbines, the proportion of turbine that is visible, weather conditions and the navigational lighting of turbines. The distance between observer and wind farm usually has the strongest influence on the visual impact perception. Nevertheless, changes in lighting and weather conditions vary considerably the visual effects at the same distance.

The indicative thresholds established for highly sensitive seascapes during the DTI study on three SEA areas in UK are shown in Table 2.3.

Table 2.3: Thresholds for Seascapes

Thresholds

<13km possible major visual effects

13-24km possible moderate visual effects

>24km possible minor visual effects.

Sourse: Wratten et al. (2005)

More recently research on visual assessment by Bishop and Miller (2005) found that distance and contrast are very good predictors of perceived impact. The study, based on North Hoyle wind farm at 7 km off the coast of Wales, showed that in all atmospheres and lighting conditions (except a stormy sky), visual impacts decreased with distance. However, visual impact increased with increasing contrast. Further research is needed to analyse the dependence of visual effects on turbine numbers, orientation and distribution.

Cumulative effects may occur when several wind farms are built in the same area. The degree of cumulative impact is a product of the number of wind farms and  the distance between them, the siting and design of the wind farms, the inter-relationship between their ZTVs, and the overall character of the seascape and its sensitivity to wind farms (Wratten et al, 2005).

The Danish Energy Agency (DEA) has reported an absence of negative press during the development of Nysted and Horns Rev offshore wind farms. Opinion polls showed better acceptance levels for the projects in the post-construction phase (IEA, 2005).

 

 

NOISE IMPACT

Offshore wind farms are located far away from human populations, which are not affected by the noise generated by wind turbines. However, marine animals could be affected by the underwater noise generated during the construction and operation of wind turbines (Koeller et al, 2006; Thomen et al, 2006). Any effects of the noise will depend on the sensitivity of the species present and their ability to adjust to it.

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

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

(Koeller et al, 2006)

 

Construction and Decommissioning Noise

Construction and decommissioning noise 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 reached peak levels of 193 dB at 400m from the pile (North Sea) and 196 dB at 300m (Baltic Sea). Nedwell reports peaks up to 260 dB in foundation construction and 178 dB in cable lying at 100m from the sound source (Gill, 2005). These high sound levels may cause permanent or temporary damage to the acoustics systems of animals in the vicinity of the construction site (Gill, 2005; Koeller et al, 2006). However, there is not enough scientific knowledge to determine the maximum thresholds permitted for certain effects (Koeller et al, 2006; Thomsen et al, 2006). Close collaboration between physicists, engineers and biologists is necessary to get relevant information and obtain standardisation of the measurement procedures in offshore developments (Koeller et al, 2006).

The measurements from FINO-1 at 400 m of distance from source revealed maximum peaks of 180 dB (Thomsen et al, 2006). The measurements carried out during construction of North Hoyle wind farm in UK indicate that:

• The peak noise of pile hammering at 5m depth was 260 dB and at 10m 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. 

(Nedwell et al, 2004; Thomsen et al, 2006)

Table 2.4 shows the avoidance reaction expected to occur due to pile driving during the North Hoyle wind farm construction.

Table 2.4: Calculated Ranges for Avoidance Distance for Different Marine Species

Species	Distance
Salmon	1,400 m
Cod	5,500 m
Dab	100 m
Bottlenose dolphin	4,600 m
Harbour porpoise	1,400 m
Harbour seal	2,000 m

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 (Gill, 2005).

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

• Soft start in the ramp-up procedure, slowly increasing the energy of the emitted sound;
• Using an air-bubble curtain around the pile, which 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.

(Thomsen et al, 2006)

 

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 (Greenpeace, 2005).

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

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

• Attraction to or avoidance of the area;
• Panic; and
• 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 of noise impact on invertebrates and planktonic organisms have a general consensus of very few effects, unless the organisms are very close to the powerful noise source (Greenpeace, 2005). Measurements from one 1,500 kW wind turbine carried out by the German Federal Ministry of the Environment has found that operational noise emissions do not damage the hearing systems of marine sealife. Concerning behaviour, the same study stated that it is not clear whether noise from turbines has an influence on marine animals (Koeller et al, 2006).

Ships are involved in the construction of wind parks and also during the operation phase for 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 a frequency mainly between 20 Hz and 10 kHz and levels between 130 and 160 dB at 1 m (Thomsen et al, 2006).

Standardised approaches to obtain noise certificates, similar to those existing onshore, are necessary.

 

 

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