3.4. Operational Avoided Emissions

20. The assessment also considers the GHG emissions that would not be generated (i.e. avoided) during the operation of the Array during the future baseline, using a variety of scenarios to characterise the future baseline (see section 6.2).

3.5. Assumptions and Limitations

21. Most of the construction phase GHG emissions associated with the manufacturing of components are likely to occur outside the territorial boundary of the UK and hence outside the scope of the UK’s national carbon budget, policy and governance. However, in recognition of the climate change effect of GHG emissions (wherever occurring), and the need to avoid ‘carbon leakage’ overseas when reducing UK emissions, emissions associated with the construction phase have been presented within the assessment and quantification of GHG emissions as part of the Array.
22. There is uncertainty about future climate and energy policy and market responses, which affect the likely future carbon intensity of energy supplies, and thereby the future carbon intensity of the electricity generation being displaced by the Array. Government projections consistent with national carbon budget commitments have been used in the assessment (‘long-run marginal’ projections). It should be noted that latest Government projections include an increase in renewable energy generation, like the Array (DESNZ, 2023a), consistent with the Government’s current policy of a low-carbon electricity grid by 2035, with no unabated fossil fuel generation (BEIS, 2021). As such, for the Array’s operation and maintenance lifetime, the long-run marginal projections presented assume that the Array will displace low-carbon sources of electricity, essentially comparing the Array to projects similar to itself. Therefore, multiple scenarios have been considered to present a likely range of avoided emissions, including displacement of non-renewable fuels as an upper estimate for the likely avoided emissions, and comparison to the long-run marginal projections as a lower estimate.
23. The specific wind turbine technology and design of associated infrastructure that would be used by the Array have not yet been specified. Thus, there is a degree of uncertainty regarding GHG emissions at all project phases resulting from the manufacturing and construction of wind turbines and infrastructure. This assessment seeks to limit the impact this might have by using Maximum Design Scenario (MDS) material quantities and material types (i.e. those with the greatest carbon impact), as informed by engineering input, in the calculation of construction phase emissions and emissions resulting from repair and maintenance activities. It is unlikely that these MDS material quantities will be used in the final design of the Array, owing to improvements in wind turbine and associated infrastructure design and refinements to design assumptions. As such, calculated emissions represent a conservative MDS.
24. Blue carbon that is released as a result of marine habitat disturbance dissolves into coastal and marine ecosystems, such as the ocean. As such, this impact does not directly contribute to the global atmospheric mass of CO2 (the receptor). However, it is likely to indirectly impact atmospheric CO2 concentrations, as an increased concentration of dissolved CO2 alters ocean and calcium carbonate (CaCO3) chemistry. Though interactions between different states of carbon in the oceans is complex, it is likely that increased concentrations of ocean CO2 will overall reduce the capacity of oceans to absorb CO2 and cause a greater potential for the ocean to release CO2 to the atmosphere under certain conditions (IPCC, 2021). As such, for the purposes of this assessment, remineralisation of blue carbon stocks has been assumed to have the same impact as the release of an equivalent mass of CO2 to the global atmosphere.
25. The benthic survey data used in the assessment of impacts to blue carbon stocks (see volume 3, appendix 8.1, annex A) was collected based on industry standard sampling methodologies. It should therefore be noted that there is a natural limitation in the accuracy of interpretations from extrapolating habitat information from samples. However, the samples taken have been deemed sufficient to accurately characterise the habitats present within the site boundary.
26. Literature used to calculate the blue carbon value of habitats within the Array provide stored blue carbon factors for the top 10 cm of sediment only, as there are limited data available on sediment thickness and organic carbon content of deeper sediments (Cunningham and Hunt, 2023). As organic carbon contained within deeper sediments is likely to be more stable than that found in the top sediment layer, and less vulnerable to remineralisation following disturbance (Smeaton et al., 2020), the baseline characterisation of the blue carbon value of the habitats within the Array is deemed to be sufficient.
27. It is important to note that the Array would not operate in isolation, as the Proposed offshore export cable corridor(s) and Proposed onshore transmission infrastructure are required to connect the Array to the grid. Without this transmission infrastructure (subject to separate consent applications), avoided emissions presented as part of this assessment would not be realised. As such, it is necessary to consider the embodied emissions of the transmission infrastructure within the cumulative assessment, so as to understand the whole life net effects of Ossian.
28. The design parameters of the Proposed offshore export cable corridor(s) and Proposed onshore transmission infrastructure required to connect the Array to the grid are not yet available. As such, calculations for the assessment of cumulative effects, in order to quantify whole life GHG emissions for both the Array and associated transmission infrastructure, have been based on high level, indicative parameters. These parameters will be refined in subsequent applications for the transmission infrastructure, alongside the associated calculations as more information becomes available.

4. Baseline

4.1. Current Baseline

29. With regard to GHG emissions, the current baseline is the offshore sea surface, water column and seabed use for the Array, which will be impacted either through permanent seabed take (e.g. laying of interconnector and inter-array cables, installation of OSP foundations and wind turbine anchors), or through temporary seabed take and other disturbance (e.g. additional areas required during construction and the dragging of mooring lines across the seabed, dependent on technology type). Benthic surveys classified eight different sediment types within the climatic effects study area. Muddy sand comprises approximately 31% of the study area, sand comprises approximately 24% of the study area, slightly gravelly sand comprises approximately 18% of the study area and the remaining sediment types (gravelly sand, sandy gravel, muddy sandy gravel, slightly gravelly muddy sand and gravelly muddy sand) comprise the remaining 28% of the study area (refer to volume 3, appendix 8.1, annex A for more information).
30. The standing blue carbon stock in the subtidal sediments present within the Array is presented in section 5.1.
31. With regards to the current baseline concerning the UK electricity grid at the time of writing, the conversion factor for companies reporting UK electricity generation carbon intensity resides at 252.97 kgCO2e/MWh (including scope 3 but as generated, i.e. excluding transmission and distribution losses) (DESNZ and DEFRA, 2023).

4.2. Future Baseline

32. The future baseline GHG emissions for existing land use (seabed) without the Array are expected to remain similar to the current baseline identified in section 4.1. There are limited published data available regarding blue carbon sequestration rates for sedimentary marine habitat types. However, it is acknowledged that blue carbon sequestration rates in marine habitats are lower than those of terrestrial habitats, in particular sediment-based habitats. Some sediment areas of the North Sea, for instance, experience almost no sediment accumulation and associated carbon sequestration (Cunningham and Hunt, 2023). As such, no material change to the blue carbon stocks currently present within the Array is anticipated in the future baseline.
33. The future baseline for electricity generation that would be displaced by the Array depends broadly on future energy and climate policy in the UK, and more specifically (with regards to day-to-day emissions) on the demand for the operation of the Array, compared to other generation sources available; this will be influenced by commercial factors and National Grid’s needs.
34. The carbon intensity of baseline electricity generation is projected to reduce over time and so too would the intensity of the marginal generation source, displaced at a given time.
35. DESNZ publishes projections of the carbon intensity of long-run marginal electricity generation and supply that would be affected by small (on a national scale) sustained changes in generation or demand (DESNZ, 2023a). DESNZ projections over the operation and maintenance phase of the Array’s lifetime (2039 to 2073) are used to estimate the potential emissions as a result of the Array.
36. Historically, Combined Cycle Gas Turbine (CCGT) plants have been the long-run marginal electricity generators, and previous marginal emissions factors reflect the as-generated emissions of a typical CCGT plant. However, as the power sector decarbonises in line with UK policy, low carbon generation will increase significantly both as a proportion of total and marginal generation. Long-run marginal projections reflect these anticipated changes, and as there remains much uncertainty in the pace of innovation, demand and technical feasibility, are indicative projections rather than prescriptive forecasts (DESNZ, 2023a).
37. DESNZ also publishes projections of the grid-average emissions factor, which is the carbon intensity of all sources of electricity generation, at the point of generation (i.e. excluding transmission and distribution losses). The marginal factor is assumed to converge to the grid average emissions factor by 2050 and both projections are assumed to be constant after that point.
38. National Grid publishes ‘Future Energy Scenario’ (FES) projections (National Grid Electricity System Operator (ESO), 2023) of grid-average carbon intensity under several possible evolutions of the UK energy market. The DESNZ grid-average projection sits generally above all the National Grid ranges, and as stated above, the marginal factor is assumed by DESNZ to converge with it (and hence with National Grid’s scenarios) by 2040.
39. As illustrated in Figure 4.1   Open ▸ , all of the FES grid-average carbon intensity projections achieve net negative values due to the sequestration of biogenic CO2, via Bioenergy with Carbon Capture and Storage (BECCS). It has been assumed that the Array would not displace other forms of electricity generation with net adverse GHG effects. Figure 4.1   Open ▸ illustrates both the DESNZ and National Grid projected carbon intensity factors for displaced electricity generation.

Figure 4.1: DESNZ and FES Future Grid Carbon Intensities

40. Table 4.1   Open ▸ lists the DESNZ grid-average and marginal factors for the 35 years of the Array operation and maintenance phase.

Table 4.1: DESNZ Grid Average and Long-Run Marginal Grid Intensities