Sectoral marine plan for offshore wind energy: strategic environmental assessment screening and scoping report

3 Renewable wind technologies and the potential for environmental effects

3.1 Introduction

3.1.1 To help inform the assessment, the following paragraphs set out an overview of possible technologies that could be deployed alongside a summary of the environmental effects that could arise as a result of their implementation. This overview is based on current technologies that have reached, or are anticipated to reach, technological and commercial readiness in time for the expected implementation of the Draft Plan.

3.1.2 The following paragraphs should not be viewed as an exhaustive list of renewable wind technologies but rather as an indicative summary. Beyond the technologies discussed below, it is possible that other designs could emerge and reach technological and commercial readiness in time for deployment. It should also be noted that it is not within the remit of the Draft Plan and the accompanying SEA to determine the specific technologies that will be installed in the sites arising from the areas of search.

3.1.3 The basic components of an offshore wind installation are ^[36] :

• wind turbine(s);
• turbine foundation(s), including both bottom-fixed and floating;
• cables, including export cables, array cables, and any associated cable protection;
• offshore substation; and,
• onshore substation.

3.1.4 The potential for impacts can differ depending on the stage of development. The following sections set out to capture effects over each potential stage, such as those that may arise during construction, operation, and decommissioning ^[37] .

3.2 Bottom-fixed technologies

3.2.1 Bottom-fixed foundations are likely to continue to remain viable options for deployment. The most common types are monopile, gravity-based, and space frame (jacket and tripod) foundations ^[38] .

3.2.2 Monopile foundations are commonly used within the offshore wind industry due to their straightforward design and ease of installation ^[39] . They comprise a cylindrical steel tube that is either driven directly into the seabed or inserted into drilled sockets and grouted into place, depending on local conditions ^[40] . Monopiles can also be adhered to the seabed via suction caissons ^[41] . To date, monopiles have typically been deployed in waters between 0-30m in depth ^[42] .

3.2.3 Gravity-based foundations have been most successful in shallow waters and in areas where environmental conditions are less harsh ^[43] . Their generalised design consists of a concrete structure that may be fringed with steel or concrete skirts and is ballasted by either sand, iron ore, or rock poured into the base ^[44] .

3.2.4 Jacket structure foundations are particularly suited to deep waters of up to 50m and for supporting larger turbines ^[45] . The Beatrice Demonstration, off the coast of Caithness, is the world’s deepest installation of a jacket foundation to date, with two turbines installed at depths of 45m ^[46] . Although there are many variants on their design, the concept centres around a three or four-legged steel jacket/lattice structure with corner piles interconnected with bracings. Similarly, tripod foundations are lightweight three-legged steel structures, with piles driven into the seabed at each leg end to secure the foundation ^[47] . As with jacket structures, tripod foundations can be installed in deeper waters ^[48] .

Potential environmental effects

3.2.5 One of the primary impacts associated with bottom-fixed offshore wind foundations is underwater noise and vibration generated by seismic surveying and pile driving activities. For example, loud underwater noises within a certain range can induce temporary or permanent hearing loss in marine mammals such as the harbor porpoise ( Phoecoena phocoena) ^[49] and have been observed to cause disturbance and long term displacement among some species ^[50] . These impacts could extend to both migratory and resident fish as well as invertebrates. Impacts such as disturbance may be limited to the construction phase and in some cases, species such as marine mammals could return to the area in greater numbers during the operational phase due to greater levels of food and restrictions on fishing activity ^[51] .

3.2.6 The potential impacts of offshore wind energy on bird populations include the risk of collision with turbines ^[52] and the introduction of barrier effects ^[53] that can impinge on migration routes and access to sites for foraging and reproduction ^[54] . The risk of collision could also be relevant to bat populations ^[55] . Habitat loss through both direct destruction of habitat as well as through displacement ^[56] can also arise. Cumulative impacts from interactions with other sea activities could further impact on habitat availability and exacerbate displacement from breeding and feeding areas both within wind arrays as well as within neighbouring sites ^[57] .

3.2.7 Bottom-fixed foundations have the potential to give rise to positive impacts on biodiversity. For example, fish species and benthic communities could benefit from the introduction of fishing and trawling prohibitions within the vicinity of wind installations. Bottom-fixed foundations can also provide hard substrate for colonising organisms ^[58] and result in the creation of artificial reefs. This can induce a ‘reef effect’ whereby higher trophic levels also increase in number in response to greater food availability, including epibenthic and demersal fauna as well as bentho-pelagic fish ^[59] . However, there is also a risk of invasive species becoming established ^[60] .

3.2.8 Offshore wind foundations can also function as Fish Aggregating Devices ^[61] , which may have mixed impacts on biodiversity. For example, positive impacts can arise by providing ‘safe zones’ for young fish. However, this could also increase catch volumes in areas where fishing activity is not excluded, leading to potentially negative impacts on fish populations. The latter impact will be dependent upon the level of fishing activity that is permitted within these zones.

3.2.9 The spatial footprint of cabling for offshore wind installations can be extensive. Additionally, cables can emit heat, warming surrounding waters and leading to changes in the benthic environment. This heat can also attract organisms to the area, increasing their exposure to electro-magnetic field ( EMF) radiation ^[62] . Cable landfall and the construction of an onshore substation and associated infrastructure can also lead to coastal effects and other onshore impacts such as alterations to the setting of historic features and disturbance to habitats.

3.2.10 The installation and decommissioning of offshore wind structures may influence oceanographic processes, such as downstream turbulence, surface wave energy, local scour, inflowing currents, and surface upwelling ^[63] , with the magnitude of impacts largely dependent on the size of the arrays. Alterations in hydrodynamics can in turn lead to altered patterns of suspended sediment dispersion and deposition patterns ^[64] . In addition, disturbance of sediments could compromise local water quality, with associated impacts on marine species but benthic organisms in particular (e.g. filter feeders ^[65] and fish eggs ^[66] ). Changes in coastal morphology could also result from cable landfall installation and maintenance as well as altered wave energy regimes ^[67] .

3.2.11 Underwater cultural heritage can be disturbed, destroyed, or buried through surveying and installation procedures ^[68] . Visual impacts on the landscape, coastline, and seascape can also arise though intrusion, obstruction, changes in the content and focus of views, changes in the reactions and experiences (i.e. attitudes and behaviours) of viewers, and overall changes in visual amenity ^[69] . Such impacts can extend to coastal monuments and to the setting of historic landscapes and seascapes ^[70] .

3.2.12 Offshore wind energy installations can pose navigational hazards, with the danger of collisions between vessels and turbines. Static structures may also pose an obstacle to maritime emergency operations ^[71] . It is also possible for wind turbines to lead to visual impacts such as shadow flicker ^[72] , although such effects may not be significant in the context of offshore wind due to their distance from human settlements ^[73] .

3.3 Floating technologies

3.3.1 At present, three floating offshore wind installations have received consent in Scotland: Hywind Scotland, Dounreay Trì, and Kincardine Offshore Floating Windfarm. Of these three, Hywind was officially opened and began delivering electricity to the Scottish grid on 18 October 2017 ^[74] .

3.3.2 Floating wind technologies are relatively recent innovations, and as such, are still undergoing a process of technological development. The Technology Readiness Level ( TRL) index places a technology along a development spectrum from preliminary research ( TRL 1) to comprehensive system demonstration ( TRL 9) ^[75] . Based on this metric, three floating foundation designs are classified as technologically mature and could therefore be considered as possible candidates for deployment in deep waters around Scotland: the spar buoy, tension leg platform, and semi-submersible. Variants on these also exist, including multi-turbine foundations ^[76] .

3.3.3 The spar buoy design consists of a large cylindrical body that relies on ballast to remain upright and stable ^[77] . Stability is achieved by situating the centre of gravity lower in the water than the centre of buoyancy, with heavier components to the bottom of the structure and lighter elements nearer to or above the surface ^[78] . The recently opened Hywind Scotland uses a spar buoy concept developed by Statoil ^[79] .

3.3.4 The tension leg platform involves tethering a highly buoyant platform to the seabed using tensioned tendons attached to a central column and arms ^[80] . The tendons are kept in place by suction or piled anchors ^[81] . The downward force of the tendons offsets the excessive buoyancy of the platform, keeping the installation steady in the water ^[82] .

3.3.5 The semi-submersible platform merges elements of the preceding two concepts by combining a structure made up of columns linked by connecting bracings and submerged pontoons with catenary or taut spread mooring lines and drag anchors ^[83] .

3.3.6 All three types of foundation are ‘turbine agnostic’, which means that theoretically they can accommodate any type of turbine ^[84] . However, research is currently being undertaken to determine if outfitting floating foundations with bespoke turbines could help optimise performance and reduce costs ^[85] .

3.3.7 Although these represent the most mature designs, it is possible that less advanced designs may achieve technological and commercial readiness in time for deployment. For example, by 2020, it is projected that three to five additional floating foundation designs will have undergone full-scale demonstration at 2 MW or more generating capacity ^[86] . Similarly, new concepts may emerge and evolve to become candidates for deployment.

Potential environmental effects

3.3.8 Many of the potential environmental effects associated with bottom-fixed foundations (and discussed above) remain relevant to floating technologies, particularly those relating to the presence and operation of turbines. It is anticipated that floating technologies will be capable of accommodating even larger turbines than onshore or shallow installations ^[87] , which could increase the risk of impacts associated with their installation and operation (e.g. bird strike).

3.3.9 Both bottom-fixed and floating offshore wind installations can lead to habitat loss, change, or fragmentation. These impacts can be both direct (e.g. destruction of benthic and intertidal habitats due to anchor installation and cable laying) or indirect (e.g. by prompting avoidance behaviour among marine organisms) ^[88] , as well as temporary or permanent.

3.3.10 The primary difference between bottom-fixed and floating wind technologies is that the latter require less invasive installation procedures ^[89] . However, benthic impacts could still arise from the placement and installation of cables and anchors.

3.3.11 In the case of floating wind technologies, impacts arising from noise from pile driving activities are removed due to the lack of a bottom-fixed foundation ^[90] . However, acoustic effects could still occur as a result of cable laying and other activities. In addition, novel sounds could be introduced such as mooring lines generating snapping or vibrating noises below water ^[91] . There is also potential for entanglement risk to marine animals from mooring lines.

3.3.12 Floating technologies require some cables to be suspended within the water column. This could induce EMF in surrounding waters which could impact upon electrosensitive species such as Atlantic salmon, sea trout, and European eel ^[92] , as well as cetaceans which use geomagnetic fields to navigate ^[93] .

3.3.13 There is comparatively little information on deep seas around Scotland ^[94] and as a result, the presence and spatial extent of sensitive habitats may either be poorly documented or largely unknown. As such, the risk of impacting upon sensitive habitats may be heightened if appropriate surveying activities are not carried out prior to siting and installing floating wind arrays ^[95] .