6. Assessment of Operation and Maintenance Effects

6.1. Blue Carbon

  1. The long term temporary (35 year) footprint of the Array, including from anchors and mooring lines, OSP foundations, cables and associated scour protection, has the potential to affect the sequestration of blue carbon over its 35 year operation and maintenance lifetime. However, as mentioned in section 3.3, evidence on sequestration rates in offshore sediments indicates little to no sediment accumulation in some areas of the North Sea (Cunningham and Hunt, 2023). Therefore, it is assumed that there will be negligible effects of loss of sequestration capacity within blue carbon habitats over the Array’s 35 year lifetime.
  2. During storms and rough weather, mooring lines may cause disturbance additional to the permanent footprint of the Array during the operation and maintenance phase, owing to dragging along the seabed. The additional areas of disturbance during this phase of the Array have been calculated to be 122 ha, based on values provided in volume 1, chapter 3.
  3. Applying the same methodology detailed in section 5.1, the total emissions from disturbance to blue carbon habitats during the operation and maintenance phase of the Array lies between 447 tCO2 and 2,233 tCO2.
  4. To provide a conservative assessment, the greatest emissions value of blue carbon has been reported below.

6.2. Avoided Emissions

  1. The magnitude of impact of the Array is determined by the quantity of marginal electricity generation sources it displaces and associated GHG impacts. The marginal energy generation displaced is determined by the total annual energy output values for the Array (see Table 6.1   Open ▸ ). The associated GHG emissions are determined by the GHG intensity of the displaced sources of generation.
  2. Table 6.1   Open ▸ sets out the parameters for the Array and the associated annual energy output.

 

Table 6.1:
Energy Flows from the Array

Table 6.1: Energy Flows from the Array

 

  1. The input and output figures for the operation and maintenance phase of the Array are then calculated against the assumptions stated within the DESNZ long-run marginal (DESNZ, 2023a). This allows for a direct presentation of the cumulative GHG emissions avoided throughout the operational lifetime of the Array and therefore, how the Array contributes towards reaching net zero targets in the UK and Scotland.
  2. The marginal source displaced may in practice vary from moment to moment depending on the operation of the capacity market, i.e. led by commercial considerations and National Grid’s needs at any given time. For the purpose of this assessment, longer-term trends (annual averages) have been used as it is not possible to predict shorter-term variations with confidence. It should be noted that as the UK and Scotland move towards their 2050 and 2045 net zero carbon targets, respectively, the marginal source of electricity generation will likely become a combination of renewables (predominately solar and wind) and storage. It is important to note therefore that from circa 2035 onwards, long-run marginal projections assume that there is no unabated fossil fuel generation, in line with UK Government policy. As such, comparing the Array’s GHG impacts with the marginal source of generation is likely to represent an underestimation of its true avoided emissions.
  3. The DESNZ long-run marginal grid carbon intensity factors do not properly consider the embedded construction phase GHG impacts of the sources of generation. It is therefore not a like-for-like comparison to compare the lifetime carbon impacts of the Array with the DESNZ long-run marginal or grid-average source.
  4. Table 6.2   Open ▸ displays the annual power output and emissions avoidance of the Array when comparing the abated fossil fuel generation using the DESNZ (2023a) long-run marginal carbon intensity for the future UK Grid.

 

Table 6.2:
Operational GHG Impacts

Table 6.2: Operational GHG Impacts

 

6.2.1. Sensitivity Analysis

  1. The long-run marginal figures, which have been used in Table 6.2   Open ▸ , are dynamic and show year-on-year decarbonisation of the UK electricity Grid towards the UK’s committed net zero 2050 pledge. The long-run marginal carbon intensity figures account for variations over time for both generation and consumption activity reflecting the different types of power plants generating electricity across the day and over time, each with different emissions factors.
  2. However, as discussed in paragraphs 36 and 66, long-run marginal figures should be treated as indicative projections rather than prescriptive targets, and the projections assume abatement of fossil fuel generation sources within the UK electricity grid. By the time the Array is anticipated to be operational (2039), the UK is expected to have made significant progress towards a low-carbon electricity grid, with the current Government policy target year of 2035. Nevertheless, the UK Government has highlighted that some ‘transition’ fossil fuels will continue to play a part in the UK’s energy supply. Therefore, it is likely that the true value of the avoided emissions displaced as a result of the Array’s contribution to the UK electricity grid would be higher than that of avoided emissions detailed above.
  3. As such, a sensitivity analysis has been carried out using the current UK electricity grid carbon intensity (252.974 kgCO2e/MWh) and current estimated intensity from electricity supplied for ‘all non-renewable fuels’ (424 kgCO2e/MWh) as detailed in section 4.1.
  4. Although the use of the current UK electricity Grid average and DESNZ ‘non-renewable fuels’ carbon intensities would conclude greater avoided emissions and an ultimate reduction in carbon payback period, these are static baselines and do not account for future UK electricity grid decarbonisation. Further, as the Array’s generation output would be dictated by day-to-day demand alongside commercial factors and the National Grid’s needs, the benefit of provision of additional low carbon electricity capacity cannot be used to quantify avoided emissions. As such, the long-run marginal provides a conservative quantification of avoided emissions for the purpose of this assessment.
  5. Table 6.3   Open ▸ details the potential avoided emissions for the assessment scenario (DESNZ long-run marginal) and two alternative scenarios as part of the sensitivity analysis (current UK electricity grid average as of 2023, and DESNZ ‘non-renewable fuels’ intensity as of 2023). These are presented for the entire assumed lifetime of 35 years for the purpose of the GHG calculations (whole life). The true avoided emissions value for the Array is likely to lie between the upper and lower limits shown in Table 6.3   Open ▸ .

 

Table 6.3:
Avoided Emissions Sensitivity Test

Table 6.3: Avoided Emissions Sensitivity Test

 

  1. Additionally, variations in load factors could have a similar effect on the avoided emissions in addition to other quantifications of emissions. Any change in the load factors would vary the MWh output accordingly. As the MWh output has been used as the base for the calculation of avoided emissions, any increase in emissions or avoided emissions would be proportionately similar to that of the above.

6.3. Fuel, Energy and Material Use During Operations and Maintenance

  1. The primary purpose of an offshore wind farm is to avoid the need for fossil fuel generation assets, reduce the national grid carbon intensity and provide additional electricity generation capacity. Emissions during the operation and maintenance phase of the Array refers to activities contributing to the high level management of the asset such as remote monitoring, maintenance activities, environmental monitoring, electricity sales, etc. Maintenance accounts for by far the largest portion and can be divided into preventative maintenance and corrective maintenance:
  • Preventative maintenance: proactive repair to, or replacement of, known wear components based on routine inspections or monitoring systems.
  • Corrective maintenance: includes the reactive repair or replacement of failed or damaged components. It may also be performed batch-wise when serial defects or other problems occur.
  1. The Array’s maintenance activities largely involve inspection, repainting, minor item repair and replacement, removal of marine growth, reburial of cables, and geophysical surveys. Emissions associated with such activities are largely captured with vessel or helicopter movements. Where materials are used (i.e. new paint and coatings, fuses, access ladders, etc.), associated emissions are negligible and immaterial, and as such have not been assessed further.
  2. Emissions associated with the proposed maintenance vessels and helicopter movements (refer to volume 1, chapter 3) follow the methodology detailed in section 5.2.3. Such emissions total 808,057 tCO2e over the lifetime of the Array.
  3. Of greater magnitude are emissions associated with material replacement of electrical plant (replacement of transformers and switchgear), cables and scour protection.
  4. Throughout the Array’s lifetime major plant equipment, such as transformers, will be replaced no more than once every five years for each OSP as informed by the project description (volume 1, chapter 3). This corresponds to a maximum of seven replacements per OSP over the lifetime of the Array, given its 35 year lifetime. As such, the embodied carbon emissions associated with the OSP plant, detailed in paragraph 53 have been scaled up by a factor of seven to provide the operation and maintenance emissions for the OSP plant. Though it is unlikely that all electric plant will need to be replaced once every five years per OSP, this is treated as the most conservative (worst-case) scenario. Total emissions from major OSPs (transformers) over the project lifetime were calculated to be 55,189 tCO2e.
  5. It is anticipated that approximately 5% of the total length of the inter-array cable will be required to be repaired or replaced per year. It is anticipated that the interconnector cables will undergo one repair or replacement event every five years, with the longest length of cable to be replaced 20 km long. Emissions associated with the replacement of such cables were calculated using the methodology detailed at paragraph 50. Total emissions from cable replacement over the project lifetime were calculated to be 509,630 tCO2e.
  6. Finally, it is anticipated that scour protection will be required to be replaced twice over the lifetime of the Array. Emissions associated with the replacement of scour protection were calculated using the methodology detailed at paragraph 50. Total emissions from scour protection replacement over the project lifetime were calculated to be 115,296 tCO2e.