3. Project Description

3.1. Introduction

3.1.1. Overview

  1. This chapter of the Array Environmental Impact Assessment (EIA) Report provides a description of the infrastructure including all construction, operation and maintenance, and decommissioning activities associated with the Array. This chapter is informed by design work undertaken to date and current understanding of the environment associated with the Array from site-specific survey work undertaken by Ossian Offshore Wind Farm Limited (OWFL) (hereafter referred to as the ‘Applicant’).  
  2. As noted in volume 1, chapter 1, this EIA Report has been prepared for the Array only; the Proposed offshore export cable corridor(s) and Proposed onshore transmission infrastructure (comprising the Proposed onshore export cable corridor(s), Proposed onshore converter station and the Proposed landfall location(s)) associated with Ossian will be subject to a separate EIA Scoping Report(s), EIA Report(s) and consent application(s) in the future.
  3. The specific conditions and environmental factors within the Array have influence on the design and engineering options available. The Applicant has carried out several studies in the early development stage to address existing unknowns and to refine design parameters. Further studies are expected to be completed beyond the planning phase and into procurement and contracting to gain further site-specific information to inform the final Array design. This includes final wind turbine number and size, floating foundation design (including mooring and anchoring systems), Array layout, and the exact locations, numbers, and types of Offshore Substation Platforms (OSPs). The detailed design will be confirmed post-consent, subject to further site investigation.
  4. The Project Design Envelope (PDE) approach (also known as the 'Rochdale Envelope') (Scottish Government, 2022a) has been followed by the Applicant, meaning that parameters for the Array included in this chapter present the maximum extents of the design in order to assess the likely significant adverse effects of the Array. It should be noted that for some technical topics the Maximum Design Scenario (MDS) might be a combination of parameters, not just the maximum parameter, as explained and assessed in volume 2, chapters 7 to 20. The ‘maximum design envelope’ presented in this chapter defines the maximum range of design parameters. Within the EIA, the Applicant has determined the maximum impacts that could occur for given receptor groups, selecting these from within the range of the ‘maximum design envelope’ to define the MDS for that receptor group. As a result, for each topic-specific assessment, the predicted effects for any alternative parameter within the range will be no greater than those assessed.
  5. The final detailed design will be further developed as additional information becomes available from site investigations and on the commercial availability of technologies. It should be noted that the final detailed design for the Array will be within the PDE parameters presented in this chapter. This is a standard approach for large scale energy projects such as the Array.

3.1.2. Purpose of Chapter

  1. The purpose of this Project Description chapter is:
  • to provide the maximum PDE for the Array, comprising information on the site, design, size and other relevant features of the Array, based on preliminary conceptual design principles (section 3.1.4) and current understanding of the environment;
  • to set out the individual components of the Array, as well as the main activities associated with the construction, operation and maintenance and decommissioning phases; and
  • to provide the basis for the assessment of effects included in volume 2, chapters 7 to 20.

3.1.3. Project Design Envelope

  1. The PDE approach allows for flexibility in the final Array design to account for supply chain constraints, and selection of the most appropriate technology for the site and conditions, while ensuring all likely significant effects (LSE1) (beneficial or adverse) are assessed and reported within the Array EIA Report. The PDE presents a range of potential parameter values up to and including the maximum Array design parameters.
  2. The Array’s PDE has been designed to allow for sufficient flexibility in the final project design options and further refinement during the final design stage, where the full details of a project are not known at the point of Application submission. For each of the impacts assessed within the technical assessments (volume 2, chapters 7 to 20), the PDE has been reviewed and the MDS has been identified from the range of potential options for each parameter. The MDS approach allows the Applicant to retain some flexibility in the final design of the Array and associated offshore infrastructure, but certain maximum parameters are set and are assessed in this Array EIA Report. The maximum scenario, as per the PDE, is presented within this chapter. Anything less than that set out within this chapter and assessed within the technical assessments (volume 2, chapters 7 to 20) will have a lesser impact.
  3. This approach aligns with Scottish Government (2013) guidance, which states that “by applying the principles of an approach commonly known as the 'Rochdale Envelope' it is possible to undertake an environmental assessment which takes account of the need for flexibility in the future evolution of the detailed Project proposal, within clearly defined parameters. In such cases, the level of detail of the proposals must be sufficient to enable a proper assessment of the likely significant environmental effects, and any resultant mitigation measures - if necessary, considering a range of possibilities”. The approach also complies with the guidance prepared by Marine Scotland and the Energy Consents Unit in June 2022 for applicants using the design envelope for applications under section 36 of the Electricity Act 1989 (Scottish Government, 2022a). Further information on the Rochdale Envelope approach and the site selection process for the Array is detailed in volume 1, chapter 4.
  4. The PDE describes a range of parameters that apply to a Project technology design scenario (e.g. largest wind turbine option). In this example, wind turbine size and wind turbine number are inherently correlated so if larger wind turbines are selected, fewer wind turbines are likely to be required. Therefore, each design parameter set out in this chapter is not considered independently. The PDE has been used to develop the MDS for each impact pathway in order to determine the parameters (or combination of parameters) which are likely to result in the maximum effect (e.g. the maximum design scenario) on a particular receptor, while adhering to the Project technology design scenarios (e.g. infrastructure parameters associated with the largest wind turbine size). It should be noted, however, that the largest parameters set out in this chapter will not necessarily comprise the MDS for any given receptor group and each of the impacts assessed within the technical assessments (volume 2, chapters 7 to 20).
  5. Since the submission of the Array EIA Scoping Report (Ossian OWFL, 2023), the Applicant has developed and refined the PDE for the Array EIA Report using the results of early engineering studies and information gained through consultation with stakeholders. A full description of PDE refinements for the Array is provided within volume 1, chapter 4, however, in summary, the following parameters have changed:
  • reduction in the number of floating wind turbines from 270 to 265;
  • refinement of floating wind turbine foundation anchoring options – piled anchor solutions, suction anchors, and drag embedment anchors retained in the PDE as these are considered to be suitable anchor solutions for the seabed conditions identified within the Array and with the most mature supply chain. Innovative technologies described under paragraphs 41 to 43 will be considered where commercially available and if there is opportunity to further reduce environmental impacts;
  • removal of floating OSP foundation option;
  • additional option for OSPs – either up to six large OSPs, or up to three large OSPs and 12 small OSPs;
  • refinement of piling parameters for floating wind turbine foundations and OSPs; and
  • refinement of inter-array and interconnector cable lengths – changed from 1,515 km (total inter-array and interconnector cable length) to 1,497 km (total inter-array and interconnector cable length).
  1. The Habitats Regulations Appraisal (HRA) process includes derogation provisions which may require the Applicant to provide compensatory measures to compensate for the potential adverse effects on the integrity of European sites resulting from the Array, either alone, or in combination with other plans and projects. The Applicant has undertaken an appraisal of the potential impacts of the compensatory measures proposed (without prejudice to the HRA to be conducted by the competent authority). An EIA and HRA has been undertaken on the proposed compensation measures for the Array and are provided as part of the Application.

3.1.4. Location and Site Information

  1. The Array will be located within the site boundary, located off the east coast of Scotland, approximately 80 km south-east of Aberdeen from the nearest point, and comprising an area of approximately 859 km2 ( Figure 3.1   Open ▸ ).
  2. In January 2022, as part of the ScotWind Leasing Round, the Applicant was awarded an Option to Lease Agreement to develop Ossian, an offshore wind farm project within the E1 Plan Option (PO) Area (please see volume 1, chapter 4 for further information on the site selection process). Figure 3.1   Open ▸ presents the location of the Array.

                        Water depths and seabed within the Array

  1. A geophysical survey covering the area within the site boundary was conducted between March 2022 and July 2022 to collect geophysical and bathymetric data. Across the site boundary, the maximum water depth was recorded at 88.7 m lowest astronomical tide (LAT), and the shallowest area was recorded at 63.8 m LAT. The seabed across the site boundary slopes gently downwards in an approximately north-west to south-east direction (Ocean Infinity, 2022).
  2. Seabed sediments within the site boundary are significantly dominated by deep circalittoral sand, with one area of limited extent comprised of deep circalittoral coarse sediment within the northern part of the site (EUSeaMap, 2021). The geophysical survey indicated that the seabed comprises mainly of sand, with areas of gravel in the west of the site boundary (Ocean Infinity, 2022). The seabed within the site boundary is generally flat, with mega-ripples and sand waves observed in the north-west of the site. Furrows were observed occasionally across the site boundary, more commonly in the west (Ocean Infinity, 2022).
  3. Further details of the bathymetry and seabed composition are presented within volume 2, chapters 7 and 8.

Figure 3.1:
Location of the Array

Figure 3.1: Location of the Array