5. Assessment of Construction Effects

5.1. Blue Carbon

  1. Emissions associated with disturbance to blue carbon stocks during the construction phase of the Array have been calculated based on the total area of seabed disturbance, sediment types present in the Array, published associated carbon stock values and published literature values of the effect of disturbance on remineralisation.
  2. During construction, it is anticipated that there will be disturbance from the installation of the anchors and mooring lines, OSP foundations, interconnector and inter-array cables, cable protection and scour. The total disturbance of the seabed within the Array is calculated to be 32.25 km2, informed by values provided in the Project Description (volume 1, chapter 3).
  3. Section 4.1 presents the sediment types and relative distribution across the Array. Blue carbon factors assigned to each sediment type, ranging from 7.3 tC/ha for slightly gravelly muddy sand to 1.8 tC/ha for sandy gravel (Smeaton et al., 2020) were scaled by the relevant areas, resulting in an average blue carbon content across the Array of 5.00 tonnes of carbon per hectare. It has been assumed that the composition of sediment types within the total area of disturbed seabed are consistent with the Array wide composition. When the average blue carbon content is scaled by the area of disturbed seabed, this corresponds to a total of 16,109 tonnes of carbon across the Array that has the potential to be disturbed during construction.
  4. While a maximum of 16,109 tonnes of blue carbon stock (equivalent to 59,067 tCO2 when converted from carbon to CO2) may be disturbed during the construction phase, not all carbon disturbed will be remineralised to CO2. Though estimates vary, the majority of blue carbon found in marine sediments further than 5 km from the shore is likely to be unreactive, and therefore unlikely to be remineralised following disturbance (Smeaton and Austin, 2022). Instead, following disturbance the blue carbon may be re-deposited elsewhere, with limited remineralisation of CO2. Approximately 20% of the blue carbon stocks in marine sediments may be reactive and likely to be converted to CO2 following disturbance (Smeaton and Austin, 2022).
  5. Uncertainty remains with the values presented above however, as alternative studies have suggested a higher percentage of conversion to CO2 (Cunningham and Hunt, 2023). Therefore, it is conservatively anticipated that between 20% and 100% of the blue carbon stock disturbed within the Array will be converted to CO2, corresponding to an emissions value of between 11,813 tCO2 and 59,067 tCO2.
  6. To provide a conservative assessment, the greatest emissions value of blue carbon has been reported below.

5.2. Embodied Carbon

  1. The following sections detail the methodology used to calculate the construction phase emissions associated with the Array. Each section groups relevant elements of the Array by methodology used to calculate resultant emissions.
  2. The construction phase emissions cover the LCA stages A1-A5, materials and construction, i.e. emissions associated with the extraction, processing and manufacturing of materials. In addition, emissions associated with the transport of materials and technology to site (within the UK) have been analysed.
  3. The materials involved in the offshore components of the Array are the initial elements to consider within the cradle-to-grave approach towards completing this LCA. Emissions are derived from the raw material production required to manufacture the wind turbine generators, floating platforms, anchors and mooring lines, OSPs and OSP foundations, interconnector and inter-array cables and it is often the phase where the majority of embodied carbon is emitted.

5.2.1. Wind Turbines, Offshore substation platforms and Cables

  1. The construction phase emissions associated with the following elements of the Array have been calculated using approximate material quantities, and relevant material emission factors:
  • wind turbines (including floating platforms, anchors and mooring lines);
  • OSP topside structures and foundations;
  • interconnector and inter-array cables (including cable protection);
  • inter-array cable junction boxes; and
  • scour protection.
  1. Table 5.1   Open ▸ summarises the relevant material emission intensities sourced from the ICE database (Jones and Hammond, 2019), and corresponding emissions values.

 

Table 5.1:
Emission Factors and Total Emissions for Embodied Carbon of Material Use

Table 5.1: Emission Factors and Total Emissions for Embodied Carbon of Material Use

 

5.2.2. Offshore Substation Plant

  1. There is limited design information concerning the offshore substation plant, and there are few published LCAs from which to calculate embodied carbon emissions associated with offshore substation equipment. Data from an Environmental Product Declaration (EPD) for a 16 kVA–1,000 MVA transformer (ABB, 2003) has therefore been used to provide an approximation of the potential order of magnitude of emissions from the offshore substation plant, as transformers are among the major substation plant components and have a relatively high materials and carbon intensity.
  2. The LCA (ABB, 2003) listed a manufacturing GWP of 2,190 kgCO2e per MW. This was scaled by the current estimated Array output capacity of 3,600 MW to give an estimated embodied emission value of 7,884 tCO2e. This value includes lifecycle stages A1-A3.

5.2.3. Vessel and Helicopter movements

  1. Indicative vessel and helicopter movements were used to calculate emissions associated with their activities during the construction phase. Emissions associated with vessel movements were calculated using approximate fuel consumption rates alongside indicative vessel movements and typical activity timescales, using data provided by the project team and data contained within the Project Description (volume 1, chapter 3).
  2. Where approximate fuel consumption rates were not available, emissions were calculated by estimating total main engine capacity requirements, vessel speed and distance from port, based on indicative vessel information and likely base ports provided by the project team.
  3. These variables were used to calculate total fuel use for vessel movements during the construction phase of the Array. This value was then scaled by the emission factor for marine gas oil (0.258 kgCO2e/kWh, or 3,245 kgCO2e/tonne) (DESNZ and DEFRA, 2023), totalling 376,627 tCO2e.
  4. Helicopter movements and their associated emissions were calculated by determining the anticipated fuel consumption, informed by their predicted movements. An indicative number of return trips and assumed distance (280 km) from a potential helicopter base, alongside average fuel consumption (430 kg/hr) and fuel economy data (145 kn/hr) (obtained from manufacturers specifications) were used to estimate fuel consumption. Emission factors for aviation turbine fuel (2.54 kgCO2e/l) (DESNZ and DEFRA, 2023) were then scaled by the fuel consumption to give associated emissions, totalling 8,988 tCO2e for the Array.

5.3. Summary

  1. Table 5.2   Open ▸ summarises the calculated construction phase emissions associated with the Array, which totals 9,539,061 tCO2e.

 

Table 5.2:
Construction Phase Embodied Carbon Emissions Summary

Table 5.2: Construction Phase Embodied Carbon Emissions Summary