7.10. Measures Adopted as Part of the Array

  1. As part of the Array design process, a number of designed in measures have been proposed to reduce the potential for impacts on physical processes (see Table 7.12   Open ▸ ). They are considered inherently part of the design of the Array and, as there is a commitment to implementing these measures, they have been considered in the assessment presented in section 7.11 (i.e. the determination of magnitude and therefore significance assumes implementation of these measures). These designed in measures are considered standard industry practice for this type of development.

 

Table 7.12:
Designed In Measures Adopted as Part of the Array

Table 7.12: Designed In Measures Adopted as Part of the Array

7.11. Assessment of Significance

  1. Table 7.7   Open ▸ summarises the potential impacts arising from the construction, operation and maintenance and decommissioning phases of the Array, as well as the MDS against which each impact has been assessed. An assessment of the likely significance of the effects of the Array on the physical processes receptors caused by each identified impact is given below.

Increase in Suspended Sediment Concentrations and associated Deposition and Sediment Transport due to Operation and Maintenance Activities

  1. An increase in SSCs and associated deposition may arise during the operation and maintenance phase of the Array, which may impact on the sediment transport regime within the physical processes study area. The potential of an increase in SSCs may arise as a result of mooring lines or cables making contact with and moving on the seabed, disturbing seabed materials and causing scouring and increased SSCs within the water column, which may have direct impacts to physical processes receptors.

                        Operation and maintenance phase

                        Magnitude of impact
  1. The majority of mooring lines on the seabed during operation and maintenance will remain largely static with movement predominately around the touchdown point. The greatest potential for the increase in SSCs due to mooring lines will be from catenary moorings which have the greatest length of mooring lines in contact with the seabed. The MDS is considered to be the foundations with the greatest length of mooring line on the seabed per foundation, rather than over the site boundary as a whole, as the effects are considered to be very localised, with no interactions between adjacent foundations. Therefore, semi-submersible foundations with up to nine catenary mooring lines have been considered. Movement on the seabed by inter-array cables will be limited to a small section between the touch down point and the point where the cable becomes static, resulting in minor increases to SSCs in the vicinity of the touchdown point only.
  2. The mooring line radius for the MDS is 700 m, with a touchdown distance of between 25 m and 150 m from the foundation, and overall length of 750 m. During operation approximately 680 m of the caternary mooring line will be in contact with the seabed which amounts to 6,120 m per foundation. The tidal range at the Ossian site is less than 4 m, therefore it is not anticipated that tidal movements will result in substantial horizontal and vertical movements. As a result, the mooring lines are not considered to notably increase the SSCs under standard operating conditions.
  3. Under storm conditions, the dynamic interaction between the mooring lines and the seabed will increase with intensity and direction of the storm. Horizontal movement of the floating foundations may result in the lifting of the mooring lines located on the windward side of the wind turbine, as tension on these mooring lines increases. Mooring lines on the leeward side would experience the opposite effect, whereby the length of mooring line in contact with the bed increases as they slacken, up to a maximum of 710 m for some mooring lines in the most extreme storm conditions. The length where disturbance is likely to occur will be less, as this will be greater closer to the touchdown point and negligible towards the anchor point. Furthermore, the dimensions of the mooring lines are considered to be small, with a chain thickness of 185 mm, and horizontal diameter of 620 mm, which will limit the volumes of seabed material they have the potential to disturb, even if they were to become completely embedded.
  4. With regard to inter-array cables, the total length of the dynamic inter-array cables will be 116 km with a  maximum external cable diameter of 300 mm. Movement of the inter-array cables may be reduced through the use of buoyancy modules and clump weights (subject to engineering design) thus limiting movement on the seabed to a very small proportion of the total dynamic cable length between the touchdown point and where it transitions to a static cable. Static inter-array and interconnector cables on the seabed will be buried or fixed with cable protection where target burial depths cannot be achieved. Thus the potential disturbance area is restricted to small areas in the vicinity of up to two dynamic cable touchdown points per wind turbine. Increased SSCs would therefore be spatially limited, smaller and adjacent to any disturbance resulting from the mooring lines, of which there are up to nine per floating foundation.
  5. The spacing between the 130 floating foundations under assessment is at a minimum 1.4 km, which is large enough for any impacts to SSCs to be considered as isolated, considering the low current speeds and sediment transport rates in the physical processes study area. Any dynamic interactions between the seabed and mooring lines or dynamic cables will likely be experienced similarly at adjacent foundations under tidal and storm conditions, with the foundations moving in the same direction and orientated the same way as their neighbouring foundations. Thus storm conditions will not impact upon minimum foundation spacing and seabed disturbance areas from mooring lines are considered sufficiently far apart to be isolated even under storm conditions.
  6. Variation in seabed composition is limited across the Array, with sand accounting for most of the seabed substrate, with small amounts of mud and gravel (paragraph 32). Disturbed materials are more likely to move along the seabed, rather than becoming fully suspended in the water column and due to the low nearbed current speeds, will not be transported for any significant distance before being re-deposited on the seabed. The baseline dominant current direction within the site boundary is to the south or south-south-west, with dominant wind directions also from the south-west. Therefore, disturbed sediments from mooring lines and cabling are likely to move towards the north-east, however, there may also be some effect from littoral currents produced by the dominant wave direction from the north.
  7. As discussed within the physical processes technical report (volume 3, appendix 7.1), movement would only occur during a small proportion of the tidal cycle, due to the reduction in current speeds, therefore material will settle within a few minutes to hours, depending on tidal state and be deposited close to the area of disturbance. Therefore, the potential for changes to the overall sediment transport regime in the physical processes study area is unlikely, particularly considering the small quantities of material with potential to be disturbed. There is a low potential to directly impact physical features within the site boundary from the increase in SSCs, however due to the isolated volumes of potential materials to be disturbed and the low sediment transport rates in the area, the impact can be considered to be relevant within the Array only. This direct impact would occur intermittently for short durations of the tidal cycle and would be greatest during storm conditions. Baseline TSSs were assessed as likely below 10 mg/l during a winter storm, and any increase as a result of the mooring lines and cabling are not expected to exceed this. Seabed scouring from movement of mooring lines and cabling on the bed during storm events will be limited due to the ongoing sediment transport processes.
  8. The impact is predicted to be of local spatial extent, short term duration, intermittent and high reversibility. It is predicted that the impact will affect the receptor directly. The magnitude is therefore considered to be low.
                        Sensitivity of the receptor
  1. As there are no designated sites within the physical processes study area, the receptor to the impact of increases in SSCs and sediment transport is the low value seabed morphology within the physical processes study area. As discussed within section 7.7.1 and in more detail in the physical processes technical report (volume 3, appendix 7.1), the seabed is located in deep waters with an average depth within the site boundary of 74.5 m. The bathymetry consists of gentle seafloor gradients, with some localised steeper ripple areas. Megaripples, sand waves, boulders, soft sediment deposits and deep channel structures with sedimentary infill are all present within the site boundary. The presence of megaripples and sand waves across the site boundary indicates mobile sediments, although sediment transport rates are low in the area. Sediment composition was relatively consistent across the site boundary and was dominated by sand, with diamicton and gravel deposits.
  2. Any increase in SSCs and associated deposition will include native material only, and although comprises predominantly mobile sand material, the low rates of sediment transport, will ensure it is redeposited close by after a short period of suspension, thus not impacting significantly on seabed morphology. Any significant changes to the seabed morphology will not recover immediately, due to the low rates of sediment transport, however the evidence of mobile sediments implies any impacts will be fully recoverable after some time.
  3. The seabed morphology is deemed to be of low vulnerability, medium recoverability and low value. The sensitivity of the receptor is therefore considered to be low.
                        Significance of the effect
  1. Changes to SSCs and associated deposition and sediment transport due to operation and maintenance activities do not extend to any designated areas, therefore the significance of the effect is discussed in terms of the effect on low value seabed morphology within the physical processes study area.
  2. The magnitude of the increase in SSCs and associated deposition is low, anticipated to occur only during extreme storm conditions. Low sediment transport rates will ensure any disturbed native materials are redeposited locally after a short period of suspension, thus not impacting significantly on seabed morphology or the overall sediment transport regime.
  3. Any changes to the seabed morphology as a result of the Array may not recover immediately, due to the low rates of sediment transport, although it is likely that baseline sediment transport will be increased during storm conditions and changes to seabed morphology will be dominated by the storm conditions rather than by the impact from the Array. Nonetheless, the evidence of mobile sediments within the baseline survey (volume 3, appendix 8.1, annex A) implies any impacts will be fully recoverable after some time.
  4. Potential increased SSCs as a result of seabed preparation, foundation installation and cable installation were assessed through a detailed modelling study as part of the nearby Berwick Bank Offshore Wind Farm EIA and showed only negligible to minor adverse significance (SSER, 2022). The volumes of sediments assessed were much greater than anticipated for any sediments disturbed by the mooring lines or cabling of the Array, for example 500 mg/l peak plume concentrations during Berwick Bank Offshore Wind Farm cable installation. Even though the operational impact of the Array has potential to occur over a greater period of time than the construction period of Berwick Bank Offshore Wind Farm, the impacts are considered to be temporally isolated, as any increase in SSCs would occur intermittently for short durations of the tidal cycle, before returning to ambient values. Therefore it can be inferred that there should be no significant effects for this impact.
  5. Furthermore, the Suspended Sediment Climatologies report (Cefas, 2016), describes two test cases of the large wind farms Walney and Greater Gabbard, located in the Irish and North Seas, respectively. It was noted that at the spatial scale of the sites, no significant effect on non-algal SPM was detected whilst using monthly averages (Cefas, 2016).
  6. Overall, the magnitude of the impact is deemed to be low and the sensitivity of the receptor is considered to be low. The effect will, therefore, be of negligible adverse significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No physical processes mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.

Impacts to the Wind Field due to the Presence of Infrastructure

  1. There is potential for the presence of infrastructure within the Array to alter the wind field, potentially impacting on mixing and stratification. The MDS is considered to be the greatest number of wind turbines within the Array, as that will produce the biggest impact over the physical processes study area. This impact is relevant to the operation and maintenance phase of the Array and may cause direct impacts to receptors.

                        Operation and maintenance phase

                        Magnitude of impact
  1. Wind turbines operate by converting kinetic energy from the wind into electricity with a generator. This process results in less kinetic energy in the atmosphere and a localised reduction in wind speed behind the wind turbine rotor. This reduction in wind speed is known as a “wake.”
  2. A recent study funded by the European Union Horizon 2020 project, made use of satellite-borne synthetic aperture radar (SAR) to conclude that this reduction in downstream wind speeds was in the region of 2% to 10% at 10 m above MSL with average wakes persisting for 20 km to 40 km (Owda and Badger, 2022). This study was based upon offshore wind farms or clusters of offshore wind farms with between 80 and 240 wind turbines in operation (Owda and Badger, 2022). A further study using SAR on two large offshore wind farms (circa 80 wind turbines) showed a decrease in wind speed in the lee of the wind turbines, with a velocity deficit of 8% to 9% immediately downstream of the wind turbines, recovering to within 2% over a distance of 5 km to 20 km (Christiansen and Hasager, 2005). This was validated using results from wake modelling and in situ measurements (Christiansen and Hasager, 2005).
  3. As would be expected, these wakes vary in both intensity and dimensions, and are highly dependent on a variety of factors, such as ambient wind speed, wind turbine size and layout (i.e. direction and spacing) of the wind turbine array (Barthelmie et al. 2010). Typically, reductions in wind speed increase with the number of wind turbines within an offshore wind farm up to a certain threshold (Christiansen and Hasager, 2005) and wakes will persist further in more stable atmospheric conditions (Platis et al., 2018). In the majority of weather situations, where unstable conditions are present, wind turbine wakes are typically localised within the offshore wind farm (Platis et al., 2018). This is due to the atmospheric turbulence aiding the recovery of the wake from vertical layers (Platis et al., 2018).
  4. The MDS for assessment included up to 265 floating wind turbines, with hub height at 148 m above LAT and a maximum rotor diameter of 236 m. Based on the information in paragraph 82, it is assumed that there will be a reduction in downstream wind speeds by up to 10% at 10 m above MSL due to the large number of wind turbines within the MDS for the Array, however designed in measures such as wind turbine spacing and wake modelling will in reality likely yield a lesser reduction at this altitude. Furthermore, the percentage reduction in wind speed will reduce further at the water surface. Wake distances are anticipated to extend beyond the Array due to the offshore location and stable atmospheric conditions, however, as outlined in paragraph 83, wake effects beyond 5 km to 20 km are considered to be very limited. This is even more applicable at the sea surface, where the effect of turbulence is greater.
  5. The impact is predicted to be of local spatial extent, long term duration, continuous and high reversibility. It is predicted that the impact will affect the receptor directly. The magnitude is therefore considered to be medium.
                        Sensitivity of the receptor
  1. Due to the offshore, exposed location of the Array, any notable changes to the wind field will not affect the overall wind regime within the physical processes study area significantly, and will be localised, with only limited changes persisting beyond the Array. Any changes to the wind field would be fully recoverable by the removal of the infrastructure.
  2. The wind field is deemed to be of low vulnerability and highly recoverable. The sensitivity of the receptor is therefore considered to be negligible.
                        Significance of the effect
  1. Changes to the wind field due to the presence of infrastructure during the operation and maintenance phase of the Array have been estimated to be less than 10% of baseline wind speed at 10 m above MSL, with the greatest reduction in wind speeds likely towards the centre of the Array due to wake interactions. Only limited wake effects are anticipated to be evident beyond the Array.
  2. Overall, the magnitude of the impact is deemed to be medium and the sensitivity of the receptor is considered to be negligible. The effect will therefore be of minor adverse significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No physical processes mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.

Impacts to Seasonal Stratification due to the Presence of Infrastructure

  1. Under certain circumstances, interactions of the OSP foundations, wind turbines and their foundations and associated infrastructure (including cable protection, scour protection and anchor mooring lines) on the wind field, wave climate and tidal regime could alter seasonal stratification (where water density varies with depth) within the water column. This impact is relevant to the operation and maintenance phase of the Array and may cause direct impacts to receptors.

                        Operation and maintenance phase

                        Magnitude of impact
  1. Downstream reductions in the wind field and the knock-on effect on waves and tides, along with the impact on waves and tides due to the presence of infrastructure within the water column may alter seasonal stratification. Therefore, it was considered that the largest surface obstruction and volume within the water column has the potential to cause the greatest impact to stratification. This is produced by 265 semi-submersible wind turbines, with a surface obstruction of 3,789,500 m2 with a corresponding draft of 25 m.
  2. As the wind turbines will be based on floating foundations, with only the OSPs using fixed foundations, and situated in relatively deep water, the infrastructure within the water column and on the seabed will likely be smaller and with less impedance than that of fixed foundation projects, such as Seagreen 1 Offshore Wind Farm (Seagreen, 2012) and Berwick Bank Offshore Wind Farm, for which computational modelling predicted no significant impacts (SSER, 2022).
  3. A recent study by Dorrell et al. (2022) suggests potential impacts arising from various offshore wind turbines upon seasonal stratification (including both fixed and floating infrastructures). A non-trivial effect on mixing may be caused by semi-submersible floating infrastructure, or other designs with small drafts, by intersecting the thermocline (Dorrell et al., 2022). Baroclinic effects will likely enhance drag by up to two orders of magnitude in the case of semi-submersible structures, whereby mixing will occur via shed lee waves, internal waves, blockage effects and wake-wake interactions (Dorrell et al., 2022). This means that water flowing past the semi-submersible structures may generate wakes that can double the natural turbulent mixing.
  4. Surveys on two non-operational, fixed foundation offshore wind farms in the North Sea were undertaken, showing a consistent weakening of stratification near the centre of the wind farms, which extended into the surrounding area by circa half a tidal excursion (Floeter et al., 2017). However, results were inconclusive as to the cause of the effect being due to the infrastructure in the water column. Furthermore, measurements of stratified wake from an offshore monopile at the DanTysk Offshore Wind Farm, showed a reduction in the potential energy anomaly by up to 65% and demonstrated that turbulence generated by monopiles reduces stratification (Schultze et al., 2020). There is however limited survey information available on how floating foundations may impact on stratification.
  5. The proposed technology for the Array will be smaller and with less impedance on prevailing flow and wave climate than the infrastructure relating to fixed foundations or floating technology considered in Dorrell et al. (2022), Floeter et al. (2017) and Schultze et al. (2020). A recent review by Farr et al. (2021) stated that floating offshore wind farms in deeper water are expected to be less disruptive to ocean currents and waves (and hence seasonal stratification) than wind turbines with fixed foundations and in shallower water. In terms of the presence of the Array floating foundations with a surface obstruction of 3,789,500 m2 over the Array, this is only 0.44% of the total Array area of 858 km2. The 25 m draft of these structures in the water column is much less than for the fixed foundations discussed in paragraph 96 and would equate to a water column obstruction volume of circa 861,250 m3 over the Array, in addition to the small number of OSPs. Unlike fixed foundations, this wind turbine foundation draft would lie entirely within the upper stratified layer, as discussed within paragraph 28. Within the Project Description (volume 1, chapter 3), the maximum potential draft is described as 40 m, which has potential to penetrate the thermocline. However, as the surface obstruction over the Array is much smaller than that presented by the option with 265 wind turbines, it was not selected as the MDS for impact on seasonal stratification.
  6. With regard to the effect of winds on waves and currents and the effect on mixing and seasonal stratification, assuming a maximum of circa 10% reduction in wind speeds at 10 m above the sea surface, as discussed within paragraph 85, this will be further reduced at the water surface, as demonstrated by the baseline description of reduction in horizontal wind speeds between hub height and sea level (paragraph 20). Studies in the North Sea have shown wind speed reductions at the surface due to these wakes in the order of 0.1 m/s to 0.5 m/s, depending on a range of factors including but not limited to the season and density of wind turbines (Akhtar et al., 2021; Christiansen et al., 2022).
  7. Christiansen et al. (2022) noted that “as a result of constantly changing wind directions, pronounced wake patterns disappear when averaging over time”. In context of prevailing physical processes at the site boundary, speed reductions of 0.1 m/s to 0.5 m/s within the surface layer would be considered to be of low magnitude, particularly given that these patterns would likely disappear when averaging over time owing to naturally varying wind speeds and directions across the site boundary. Thus, the limited reduction in wind speed due to the presence of the infrastructure is not considered to have a marked effect on waves and currents within the Array, which are predominantly determined from other factors, such as swell (as evidenced from the long period waves in the baseline environment) and the large scale tidal regime. Furthermore, from baseline evidence, as summarised within paragraph 28 and detailed in the Technical Report (volume 3, appendix 7.1), it has been shown that the impact of wind on seasonal stratification through the water column at the Array is negligible.
  8. If any impact exists, it is likely that this will involve a reduction to stratification due to the presence of the foundations within the upper stratified layer of the water column. This would likely be countered by any potential increase to stratification caused by a decrease in wind speeds, as the two impacts would likely have opposing effects. Furthermore, any increase in seasonal stratification due to climate change would counteract stratification reduction due to water column infrastructure.
  9. The impact is predicted to be of local spatial extent, long term duration, intermittent and high reversibility. It is predicted that the impact will affect the receptor directly. The magnitude is therefore considered to be medium.
                        Sensitivity of the receptor
  1. The receptor to changes to seasonal stratification due to the presence of infrastructure is considered to be the tidal front in the physical processes study area, as the baseline identified the physical processes study area as being subject to weak seasonal stratification. Frontal positions are predominantly controlled by tidal mixing, however any changes to seasonal stratification would be fully recoverable by the removal of the infrastructure.
  2. The tidal front is deemed to be of low vulnerability and highly recoverable. The sensitivity of the receptor is therefore considered to be negligible.
                        Significance of the effect
  1. Previous modelling studies and offshore wind developments in the North Sea, based on fixed wind turbine foundations, have demonstrated that there are no significant impacts on waves and tides (Dudgeon, 2009; Arcus, 2012; Repsol and EDP Renewables, 2013; MORL, 2014). Furthermore, the Cefas (2005) study has demonstrated that there are no post-construction impacts, and the MMO (2014) review concluded that current and wake monitoring was included in licences for early offshore wind farms (such as Burbo Bank Offshore Wind Farm) but had been removed from later licences due to insignificant effects. Subsequently, this evidence outlines that there will be very limited impacts on wind, waves and tidal flows, thus in turn, stratification from these pathways.
  2. Furthermore, effects to stratification fronts were assessed to be negligible and of minor significance due to insignificant changes to the tidal regime at Beatrice Offshore Wind Farm and Moray East Offshore Wind Farm, which are situated within the Moray Firth (Arcus, 2012; MORL, 2014).
  3. A further study by Carpenter et al. (2016) concluded that there is expected to be very little impact on large-scale stratification at the current offshore wind farm capacity in the North Sea. This study provided a comparison of the estimated timescales of mixing and advection for water bodies with offshore wind farms against baseline stratification (Carpenter et al., 2016). Although further research is required on the impact on large scale stratification due to the increase in leased capacity in the North Sea, the impact of the Array is predicted to be of local spatial extent, which is supported by the conclusion of the study by Carpenter et al. (2016).
  4. Due to the scale of the Array and the designed in measure of sufficient spacing between wind turbines, the impact will be insignificant in terms of the effect on waves and tides over the Array as a whole and will not be expected to change the wave or tidal regime in the physical processes study area. Therefore, there is unlikely to be any knock-on impact to stratification, with the semi-submersible structures likely to lie completely within the stratified layer, without penetration of the thermocline.
  5. With regard to wind effects on seasonal stratification, a reduction in wind wake which occurs primarily at hub height is anticipated to have very limited effect on stratification though the water column. Any increase to stratification would likely be countered by a reduction to stratification due to the presence of infrastructure within the water column. Any changes to seasonal stratification are considered to be highly localised and will not result in widescale changes to the tidal front.
  6. Overall, the magnitude of the impact is deemed to be medium and the sensitivity of the receptor is considered to be negligible. The effect will therefore be of minor adverse significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No physical processes mitigation is considered necessary because the likely effect in the absence of mitigation is not significant in EIA terms.