1.4 Project description
1.4.1.1 This section of the NTS provides an outline description of the design of the Mona Offshore Wind Project infrastructure, as well as activities associated with the construction, operations and maintenance, and decommissioning of the Mona Offshore Wind Project.
1.4.1.2 It is important to note that the Mona Offshore Wind Project is in the early stages of the development process, therefore the project description is indicative and has been designed to include flexibility to allow for further project refinement during detailed design, post consent. Offshore wind is a continually evolving industry with a constant focus on cost reduction, therefore improvements in technology and construction methodologies occur frequently therefore flexibility is required to allow for the adoption of new technology and methods.
1.4.2 Infrastructure overview
1.4.2.1 The key components of the Mona Offshore Wind Project infrastructure are shown in Figure 1.2. The key components of the Mona Offshore Wind Project include:
- Offshore wind turbines
- Foundations (for wind turbines and Offshore Substation Platforms (OSPs)
- Inter-array cables linking the individual wind turbines to the OSPs
- Scour protection for foundations and cables where required
- High Voltage Alternating Current (HVAC) transmission system components including: OSPs
- Offshore interconnector cable(s).
- Offshore export cables
- Transition Joint Bays (connecting the offshore and onshore cables)
- Onshore export cablesOnshore Substation
- Connection into Existing Bodelwyddan National Grid Substation.
1.4.2.2 Construction of the Mona Offshore Wind Project is intended to commence in 2026, and the project is intended to be fully operational by 2030 in order to provide a vital contribution to the UK Government’s renewable energy targets.
Figure 1.2: Overview of the Mona Offshore Wind Project infrastructure
1.4.3 Pre-construction site investigation surveys
1.4.3.1 In addition to the work carried out to date, pre-construction site investigation surveys will be undertaken to provide detailed information on seabed conditions and to identify the presence/absence of any potential obstructions or hazards. Pre-construction site investigation surveys are likely to include geophysical and geotechnical surveys which will be conducted within the Mona Array Area and Mona Offshore Cable Corridor.
1.4.4 Wind turbines
1.4.4.1 The Mona Offshore Wind Project will consist of up to 107 wind turbines, the final number of wind turbines will be determined during the post-consent detailed design phase (see Table 1.1). The wind turbines will follow the traditional wind turbine design with a horizontal rotor axis with three blades.
| Parameter | Smallest wind turbine | Largest wind turbine |
|---|---|---|
| Number of turbines | 107 | 68 |
| Minimum height of lowest blade tip above Lowest Astronomical Tide (LAT) (m) | 34 | 34 |
| Maximum blade tip height above LAT (m) | 293 | 324 |
| Maximum rotor blade diameter (m) | 250 | 280 |
Table 1.1: Maximum design parameters: wind turbines
1.4.5 Offshore Substation Platforms
1.4.5.1 The OSPs will contain the equipment required to transform electricity generated at the wind turbines to a higher voltage for transportation through the offshore export cables to shore. They may also house secondary equipment and facilities for operating, maintaining and controlling the OSP. They are likely to have one or more decks, a helicopter platform, cranes and communication antenna.
1.4.5.2 Up to four separate OSPs will be required and they will all be located within the Mona Array Area. The exact locations will be determined during the post-consent detailed design phase. Locations will take into account the seabed conditions and the most efficient cable routing amongst other considerations. OSPs are generally constructed by installing the foundation structure, then the substation itself will be lifted from a transport vessel/ barge onto the foundation.
1.4.6 Foundations for wind turbines and OSPs
1.4.6.1 The wind turbines and OSPs will be attached to the seabed by foundation structures. The Applicant requires flexibility in foundation choice to ensure that anticipated changes in available technology can be accommodated within the Mona Offshore Wind Project final design.
1.4.6.2 The foundation types that are being considered for the Mona Offshore Wind Project are shown in Table 1.2 and Figure 1.3 to Figure 1.6.
| Foundation option | Wind turbines | OSPs |
|---|---|---|
| Maximum number of structures | 107 | 4 |
| Monopile | Yes | Yes |
| Pin piled 3-legged Jacket | Yes | Yes |
| Pin piled 4-legged Jacket | Yes | Yes |
| Pin piled 6-legged Jacket | No | Yes |
| Suction bucket 3-legged Jacket | Yes | Yes |
| Suction bucket 4-legged Jacket | Yes | Yes |
| Suction bucket 6-legged Jacket | No | Yes |
| Gravity base | Yes | Yes |
Table 1.2: Foundation options for wind turbines and OSPs
Figure 1.3: Schematic of an monopile foundation design
Figure 1.4: Schematic of a pin pile jacket foundation
Figure 1.5: Schematic of a suction bucket jacket foundation
Figure 1.6: Schematic of a gravity base foundation
1.4.7 Offshore export cables
1.4.7.1 Although the Mona Offshore Cable Corridor has been identified, the exact route of the offshore export cables is yet to be determined and will be based upon geophysical and geotechnical survey information.
1.4.7.2 Up to four offshore export cables, with a voltage of up to 275kV will be required for the Mona Offshore Wind Project. The Mona Offshore Cable Corridor will be up to 90km long and 1.5km wide. Each offshore export cable will also house a fibre optic cable for communication. Where possible, the cables will be buried below the seabed to the landfall.
1.4.8 Onshore export cables
1.4.8.1 The offshore export cables will connect to the onshore export cables at the transition joint bays and will transfer the electricity to the onshore substation. The onshore export cables will be buried for their entire length. Overhead lines are not proposed for the Mona Offshore Wind Project.
1.4.8.2 A maximum of four cable circuits has been assumed as the maximum design parameter for the environmental assessment. Each cable circuit will consist of three cables, giving a total of up to 12 cables. Once installed, the cables will occupy a corridor approximately 30m wide, although this width may change where obstacles are encountered.
1.4.8.3 The onshore export cables will be located within the Mona Proposed Onshore Development Area. At this time, it is anticipated that the cables would route south from the landfall and pass to the west of Abergele (Figure 1.7).
1.4.8.4 The Mona Onshore Cable Corridor will be approximately 18km in length. The cables will be buried underground at a target depth of 1.8m. This target burial depth may be exceeded where the route is required to cross beneath features such as pipelines, land drains, highways or rivers. The Mona Onshore Cable Corridor will be up to 100m wide (including the temporary construction width).
1.4.8.5 A further section of buried onshore export cabling is required to connect the Mona Onshore Substation with the existing National Grid substation at Bodelwyddan. This is referred to as the ‘Mona 400kV Grid Connection Cable’ and will be located within the Mona Proposed Onshore Development Area.
1.4.8.6 This section of cabling will be similar in design to the Onshore Cable Corridor: it will be approximately 3km and will be up to 60m wide (including the temporary construction width).
Figure 1.7: Mona Proposed Onshore Development Area
1.4.9 Onshore Substation
1.4.9.1 The Mona Offshore Wind Project will connect to the national grid at the Bodelwyddan substation, located south of Rhyl, North Wales. Two locations are proposed for consideration within the PEIR – Land Substation (LSS) Option 2 which is immediately south of the Bodelwyddan substation and LSS Option 7 which is east of the Bodelwyddan substation, near to Pen-rhew and southeast of St. Asaph town.
1.4.9.2 The Onshore Substation will contain the electrical components for transforming the power supplied from the offshore wind farm to 400kV and to adjust the power quality and power factor, as required to meet the UK Grid Code for supply to the national grid.
1.4.9.3 The Onshore Substation building substructures are likely to be composed of steel and cladding materials. The structural steelwork will be fabricated and prepared off site and delivered to site for construction.
1.4.9.4 A Hydrological, Ecological and Landscape Management Plan will be prepared for the onshore substation site that will set out the mitigation measures for screening, ecological habitats and the management of surface water runoff. This will be submitted with the application for consent.
1.4.10 Operations and maintenance phase
1.4.10. The overall operations and maintenance strategy will be finalised once the operations and maintenance base location and technical specification of the Mona Offshore Wind Project are known, including wind turbine type and final layout. The operations and maintenance requirements for the Mona Offshore Wind Project will be set out within an outline Operation and Maintenance Plan which will be submitted alongside the application for consent.
1.4.11 Decommissioning phase
1.4.11. The decommissioning sequence will generally be the reverse of the construction sequence and involve similar types and numbers of vessels and equipment. The Energy Act 2004 requires that a decommissioning plan must be submitted to the Secretary of State for the Department for Energy Security and Net Zero prior to the construction of the Mona Offshore Wind Project and is typically prepared post-consent. The Decommissioning Plan and programme will be updated during the Mona Offshore Wind Project lifetime to take account of changes in regulations, best practice and new technologies.
1.4.11. Wind turbines will be removed and any piled foundations are likely to be cut approximately 1m below the seabed.
1.4.11. Offshore export, inter-array and interconnector cables will be removed and disposed of onshore. At this time, it is difficult to foresee what techniques will be used to remove cables during decommissioning.
1.4.11. It is expected that the onshore export cables will be left in situ to minimise the environmental disturbance during decommissioning. The cable ends will be cut, sealed and securely buried as a precautionary measure, and any above ground infrastructure will be removed.
1.4.11. Decommissioning of the onshore substation will be reviewed in discussion with the transmission system operator and the regulator in the light of any other existing or proposed future use of the onshore substation. If complete decommissioning is required, then all of the electrical infrastructure will be removed and any waste arising disposed of in accordance with relevant regulations.
Figure 1.7: Mona Proposed Onshore Development Area