1.3. Project Description

1.3.1. Introduction

  1. This section provides a summary of the infrastructure and the construction, operation and maintenance, and decommissioning activities associated with the Array, which is further detailed in volume 1, chapter 3.
  2. The Array EIA Report has followed the Project Design Envelope (PDE) approach meaning that parameters for the Array included in this section present the maximum extents of the design in order to assess the likely significant most adverse effects of the Array. It should be noted that for some technical topics the most adverse case might be a combination of parameters, not just the maximum parameter, as explained and assessed in volume 2, chapters 7 to 20.

1.3.1. Array Infrastructure

  1. The Array will be located within the site boundary (shown in Figure 1.1   Open ▸ ), 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 1.1   Open ▸ ).
  2. The main components of the Array will include:
  • up to 265 floating wind turbines each comprising a tower section, nacelle, hub and three rotor blades, and associated floating foundations which will support the wind turbines;
  • mooring and anchoring systems for each floating foundation which will connect the floating foundation to the seabed;
  • connectors and ancillaries for mooring and anchoring systems, including buoyancy elements and clump weights;
  • up to six large OSPs, or up to three large OSPs and up to 12 small OSPs with fixed jacket foundations;
  • scour protection for wind turbine anchoring systems;
  • scour protection for small and large OSP fixed foundations as required;
  • a network of dynamic/static inter-array cabling linking the individual floating wind turbines to OSPs, and interconnector cables between OSPs (approximately 1,261 km of inter-array cabling and 236 km of interconnector cabling); and
  • discrete condition monitoring equipment (such as sensors, cameras, dataloggers etc.), as required for safe and efficient operation of the Array infrastructure.
  1. Floating wind turbines will comprise a tower section, nacelle, hub and three rotor blades, and will be attached to a floating foundation. The maximum rotor blade diameter will be no greater than 350 m, with a maximum blade tip height of up to 399 m above Lowest Astronomical Tide (LAT) and minimum blade clearance of 36 m above LAT. The hub height will be no greater than 224 m above LAT ( Figure 1.2   Open ▸ ). The Applicant will develop and agree a scheme for wind turbine lighting and navigation marking with the relevant consultees post-consent decision, for approval by Scottish Ministers after consultation with appropriate consultees.

Figure 1.2:
Indicative Schematic of a Semi-Submersible Floating Wind Turbine

Figure 1.2: Indicative Schematic of a Semi-Submersible Floating Wind Turbine

 

  1. The wind turbine layout will be developed to effectively make use of the available wind resource and suitability of seabed conditions, whilst ensuring that the environmental effects and potential impacts on other marine users (e.g. fisheries and shipping routes) are reduced. If required by consent conditions, confirmation of the final layout of the wind turbines will occur at the final design stage (post-consent) in consultation with relevant stakeholders and submitted to the MD-LOT for approval.
  2. The floating foundations will be connected to the seabed via mooring and anchoring systems. Mooring lines run from the floating foundations, through the water column, to an anchoring system which maintains station (i.e. position) of the floating foundation. The mooring line will connect to the floating foundation at a connection point which is located just below the sea surface.
  3. Anchoring systems fix the mooring lines to the seabed and may include various solutions, such as driven piles (i.e. foundations which are driven into the seabed using a pile-driving hammer), or embedded anchor types such as suction anchors (installed by pumping water out of a capped steel cylinder, resulting in this being sucked into the seabed) and Drag Embedment Anchors (DEA) (dragged across the seabed until required depth and holding capacity is reached). The Applicant is considering installation of up to nine anchors per floating foundation. The final anchoring solution selected may vary across the Array and will take account of the seabed conditions, detailed analysis of geotechnical data to inform engineering design, and environmental impacts. Five specific scenarios are outlined for the anchoring systems, details of which can be found in volume 1, chapter 3.
  4. The use of a number of different connectors and ancillaries may be required for the mooring and anchoring systems which alter the mooring system behaviour, for example, to secure different sections of mooring lines to each other and to the anchoring system, to suspend and/or provide tension in sections of the mooring line, and to weigh down sections of the mooring line which reduces mooring line radius and limits movement of the floating foundation.
  5. The OSPs will transform the electricity generated by the wind turbines to a higher voltage and/or to direct current allowing the power to be efficiently transmitted directly to shore or to a wider offshore grid network. OSPs are comprised of a topside, which sits above sea level, attached to a jacket foundation which will be fixed to the seabed using driven piles. The Applicant has defined two options for OSP arrangements to be considered within the Array EIA Report. The exact number and size of OSPs will be subject to National Grid ESO final design recommendations and detailed design, however, the overall size, footprint, piling parameters and key design features will remain within the representative OSP design scenarios considered within the Array EIA Report. Further details can be found in volume 1, chapter 3.
  6. Natural processes at sea, such as waves, currents, and storms, can lead to seabed erosion and ‘scour hole’ formation around anchor and mooring systems, and foundation structures. Scour protection in the form of concrete mattresses (large articulated concrete blocks, linked by a rope lattice and placed on and/or around structures) or rock (either layers of graded stones placed on and/or around structures, or rock filled mesh fibre bags) will be used to mitigate scour around foundations. The type and volume of scour protection required for the Array will vary depending on the various wind turbine anchoring options and offshore platform options considered, and the final parameters will be decided once the design of these is finalised.
  7. Up to 1,261 km of inter-array cables carry the electrical current produced by wind turbines to an OSP. As the floating foundations will move with waves and currents, it is proposed that dynamic inter-array cables will be used. There are several cable designs which may be used, however, the most likely to be used for the Array is a ‘lazy-S’ configuration which allows extension of the cables in response to the floating foundation movements. Where the inter-array cable meets the seabed, this portion is termed ‘static’ and will be protected either through burial methods at a minimum target burial depth of 0.4 m, or via external cable protection such as use of rock or concrete mattresses ( Figure 1.3   Open ▸ ).
  8. Up to 236 km of interconnector cables will be installed within the Array. It is anticipated that cables will be protected via burial methods at a minimum target depth of 0.4 m. External cable protection will be used in areas where minimum target burial depth cannot be achieved to restrict movement and prevent exposure over the lifetime of the Array. This will protect cables from activities such as fishing, anchor placement or dropped objects, and limit effect of heat and/or electromagnetic fields.
  9. Up to 12 inter-array cable crossings and up to 12 interconnector cable crossings may be installed across the Array. This will be facilitated by the installation of standard cable crossing designs, likely to be comprised of concrete mattresses, rock placement, or other methods as noted in volume 1, chapter 3.

Figure 1.3:
Indicative Schematic of Dynamic/Static Inter-Array Cable System

Figure 1.3: Indicative Schematic of Dynamic/Static Inter-Array Cable System

 

1.3.2. Site Preparation Activities

                        Pre-construction surveys

  1. Pre-construction surveys, including geophysical and geotechnical surveys, may be carried out to provide further information of:
  • seabed conditions and morphology;
  • soil conditions and properties;
  • presence or absence of any potential obstructions or hazards; and
  • to inform detailed design for the Array.

                        Clearance of unexploded ordnance

  1. UXOs are explosive weapons, from World War I and II for example, that did not explode when they were employed and still pose a risk of detonation. The presence of UXO poses a health and safety risk where it coincides with the planned location of infrastructure and associated vessel activity, and therefore it is necessary to survey for and carefully manage UXOs.
  2. Where it is not possible to avoid or relocate a UXO, the preferred method for UXO clearance is for a low order technique called deflagration which causes the UXO to burn out without detonating, therefore reducing impacts to marine fauna and health and safety risks. The Applicant assumed that up to 15 UXOs may require clearance based upon the desk-based study (Ordtek, 2022) and experience from other offshore wind farms in the region. As a risk remains that unintended high order detonation may occur, 10% of clearance events have been assumed to have the potential to result in high order detonation (see volume 1, chapter 3 and volume 2, chapter 10).

                        Sand wave and boulder clearance

  1. Sand waves are a low ridge of sand formed through the action of the wind or water (through waves or tidal currents). Existing sand waves may need to be cleared in some areas of the Array to provide a relatively flat surface for installation of inter-array and interconnector cables and ensure they can be buried at target burial depth and remain buried for the lifetime of the Array. Sand wave clearance may be undertaken in specific areas of the Array (e.g. along inter-array and interconnector cables), as required, and may occur throughout the construction phase.
  2. A boulder is defined as a rock fragment which is over 256 mm (Wentworth Scale) in diameter and/or length. Boulder clearance may be required in some areas of the Array prior to installation of offshore infrastructure, in particular, along inter-array cables and interconnector cables, to increase the success rate for achieving minimum target burial depth during cable burial, and reduce the risk of cable damage during installation and subsequent burial. It may also be required in the vicinity of the OSP jacket foundation locations (including within the jack-up vessel zone around the OSP foundation locations), to avoid disruption to installation activities and to ensure stability where the jack-up vessel legs touchdown on the seabed. A Dynamic Positioning (DP1) vessel (one which uses a computer control system to automatically maintain the vessels own position and heading) is likely to be used to undertake the boulder clearance campaign. Boulders may be cleared using a plough or boulder grab, however, the geophysical and pre-construction surveys, and the parameters of any boulders present (e.g. size, density and location of boulders), will inform the methodology to be used.

1.3.3. Construction Phase

  1. Construction of the Array is expected to occur over a period of eight years cumulatively (including site preparation works) aligning with the following indicative construction series:
  • step 1 –anchoring and mooring installation;
  • step 2 – OSP topsides and fixed jacket foundations installation/commissioning;
  • step 3 – inter-array and interconnector cables installation, including cable burial and/or protection, where required; and
  • step 4 – floating wind turbine and floating foundation installation/commissioning.
  1. A number of installation vessels will be used during the construction phase including main installation vessels, support vessels, tugs and anchor handlers, cable installation vessels, guard vessels, survey vessels, Crew Transfer Vessels (CTVs) and scour/cable protection installation vessels. Helicopters may also be used for crew transfers.

1.3.4. Operation and Maintenance Phase

  1. The operational life of the Array is expected to be 35 years. The overall operation and maintenance strategy will be finalised once the operation and maintenance base location and technical specification of the Array are known, including wind turbine type, electrical export option and final project layout. Volume 1, chapter 3 provides a description of the foreseeable planned and unplanned maintenance activities.
  2. Routine operation and maintenance works will be conducted using support vessels, CTVs, and/or Remotely Operated Vehicles (ROVs). Divers and Dive Support Vessels (DSVs) may be utilised if required, although it is anticipated that diverless operations will be utilised as far as practicable. For infrequent major operation and maintenance works, including major component replacements, the floating wind turbines and associated floating foundations will be detached from their mooring and anchoring systems and towed to a suitable port facility. Jack-up vessels will be used for infrequent major maintenance campaigns associated with the OSPs. ROVs will be used to inspect foundations, mooring and anchoring systems, and cabling.

1.3.5. Decommissioning Phase Methodology

  1. At the end of the Array’s operational lifetime, it is expected that all structures above the seabed (with the exception of driven piles and DEAs (depending upon anchor system used), scour protection and cable protection) will be fully removed where feasible. Driven piles and/or DEAs installed as part of the wind turbine anchoring system, static portions of inter-array cables, interconnector cables, scour protection and cable protection are either expected to remain in situ or method of decommissioning is yet to be determined. Legislation, guidance and good practice will be kept under review throughout the lifetime of the Array and will be followed at the time of decommissioning. Environmental conditions and sensitivities will also be considered since removal of structures may result in greater environmental impacts in comparison to leaving in situ.
  2. The sequence of decommissioning is likely to be the reverse of the construction sequence, and similar types and numbers of vessels and equipment are expected to be involved.

1.3.6. Repowering

  1. ‘Repowering’ of the Array, which involves replacing old wind turbines with more efficient and powerful ones, at or near the end of its design life may be considered suitable, for example, where new technology becomes available. In this example, wind turbines and/or foundations may be reconstructed and replaced with those of a different specification or design. If the decision was taken to repower the Array at or near the end of its design life, this may be subject to further consent(s) (and potentially an EIA Report), depending on specifications and designs of the new wind turbines and/or foundations required to repower the Array.