Marine renewable energy (MRE), such as offshore wind (fixed and floating), wave, tidal and floating solar photovoltaic, can play a major role in decarbonising the global energy supply. For example, the EU Offshore Renewable Energy Strategy aims to achieve 300 GW of offshore wind by 2050, complemented by 40 GW of wave/tidal stream (European Commission, 2020). In 2022, global installed capacity of offshore wind was 66 GW, representing 8% of total global wind capacity, and a year-on-year increase in offshore capacity of 22% (IEA, 2023). Installed capacities of wave and tidal stream, although lower (29.4 MW and 41.2 MW, respectively (European Commission, 2023), are also expected to grow strongly.
Installation and operational maintenance costs represent a large portion of the total cost of MRE projects; for offshore wind turbines, operation and maintenance costs have been estimated at 25–30% of total life cycle cost (Röckmann et al., 2017). However, installation and maintenance operations are intricately linked to met-ocean conditions, with the type of work and a vessel’s met-ocean operating constraints determining whether operations can be safely completed. Weather windows are periods of time where specific operations can be started and completed without any weather-induced delays or potential safety risks (Dowell et al., 2013). Extended gaps in weather windows can lead to increased costs and reduced energy outputs, thus affecting the LCOE at a given location.
Maintenance of MRE devices presents unique challenges compared to onshore renewable technologies. In circumstances where a device malfunctions, immediate interventions might be impeded by adverse met-ocean conditions, necessitating an indefinite waiting period for a suitable weather window and resulting in reduced availability. Accessibility of offshore wind turbines, for example, can range from 60–87% compared to 100% for onshore turbines while their availability can be as low as 80%, compared to 98% for onshore turbines (Cevasco et al., 2021; Salzmann, D.J., 2010; van Bussel, 2002). Availability of MRE devices is strongly determined by the maintenance strategy (Dowell et al., 2013). Periodic scheduled maintenance is preferable to the corrective maintenance required for unforeseen failures as the latter usually costs more and results in reduced availability. Salzmann notes that 60% of O&M costs in offshore wind farms are for corrective maintenance (Salzmann, D.J., 2010). A knowledge of the frequency and length of weather windows can play a significant role in the planning and scheduling of installation and maintenance of MRE devices resulting in reduced costs and higher revenues.
Recent examples of published weather window analyses include sites in the North Atlantic Ocean near Portugal (Martins et al., 2014), the North Sea and Southern Baltic region (Gintautas & Sørensen, 2017), Japan (Japan Sea and Pacific Ocean) (Taniguchi et al., 2016), China (Yang et al., 2022), and the Black Sea (Onea & Rusu, 2019). Significant wave height (Hs) is the primary parameter used in these studies (O’Connor et al., 2013b, 2013c, 2013a, d). Wave conditions affect crew transfers, but other operations also rely on wind conditions (Dowell et al., 2013) and incorporating wind speed with Hs can change accessibility evaluations (Martins & Muraleedharan, 2015). Tidal currents and wave period, though less important, may also be included (O’Connor et al., 2012). Weather window analyses may be conducted using either statistical analyses or timeseries analysis (O’Connor et al., 2013b). The former is used where there is a lack of continuous data but timeseries analysis is preferable as it is more comprehensive; however, it requires long, continuous wave/wind datasets which may not always be available.
Ireland’s Offshore Renewable Energy Development Plan highlights substantial potential for offshore wind and wave energy development (Department of Communications, Energy and Natural Resources, 2014), particularly on Ireland’s west coast where wind and wave resources are some of the largest in the world (Atan, 2017). While the wave resource has been assessed in detail (Atan et al., 2018; Atan, Goggins, Harnett, et al., 2016; Atan, Goggins, & Nash, 2016), there has been limited analysis of weather windows. O'Connor et al. used historical timeseries of significant wave height from the M1 and M3 wave-buoys off the west and southwest coasts to determine frequency of occurrence of weather windows of different lengths (6, 12, 24 and 48 hours) and lengths of waiting periods between windows (O’Connor et al., 2013d). In a follow-up study, the same authors evaluated weather windows using both wave height and wind speed at the Atlantic Marine Energy Test Site (AMETS), also on the west coast, and the M2 wave-buoy in the Irish Sea (O’Connor et al., 2013a). Significantly, a main deficiency in both studies was the short period of data analysis, 3 years and 1 year respectively, meaning neither study accounts for the strong inter-annual variation in wind and wave climate that occurs in Irish marine waters (Atan, 2017) and so may be misrepresentative depending on the chosen years.
This paper presents the results of an extensive weather window analysis of Irish waters, including the Atlantic Ocean and the Irish sea which are both set for offshore wind farm installations in the near future. Continuous, 12-year hourly datasets of wave heights and wind speeds were used to account for inter-annual variations in wave/wind climate (IEC, 2015). Wave data were produced by a validated SWAN wave model while ERA5 reanalysis datasets were used for wind. The paper presents analyses at three key sites: the Atlantic Marine Energy Test Site (AMETS), the Galway Bay Test Site (GBTS), and the Westwave Demonstration Site, as well as producing spatial access maps for all of Ireland’s coastal waters designed to inform stakeholders in the MRE sector. Section 2 of the paper presents details of the met-ocean datasets, typical MRE vessel operational constraints and the approach used for the weather window analysis. Section 3 presents the results in the form of persistence tables and window counts for each of the three test sites and accessibility maps for the whole of Ireland’s marine area. Finally, a discussion of results and the key conclusions are presented in Section 4.