Assessing weather windows for the offshore wind development using combined meteorological and oceanographic reanalysis data

Germany aims to substantially expand its offshore wind energy by 2045, increasing the installed capacity from about 10 GW today to almost 70 GW, with offshore wind expected to supply up to 25 % of the national electricity demand. Achieving this target requires the development of offshore wind in increasingly remote areas, where long-term observational reference data are scarce and meteorological and oceanographic conditions are less well understood. However, a key factor for the safe and cost-effective installation, operation, and maintenance of the offshore wind farms is the assessment of “weather windows”, defined as periods during which meteorological and oceanographic conditions like wind and waves are below the operational limits of the vessels used. The frequency and duration of such weather windows directly affect installation schedules, turbine accessibility during operation, as well as the vessel requirements, and thus the financial viability of offshore wind projects. At the same time, this has a major impact on the corresponding bids submitted in tenders of new offshore wind sites.To achieve Germany’s offshore targets, new offshore wind sites have been tendered annually since 2021 by the Federal Network Agency, in cooperation with the Federal Maritime and Hydrographic Agency (BSH), according to the Offshore Wind Energy Act (WindSeeG). The German Weather Service (DWD) supports the BSH in compiling detailed information on the prevailing meteorological conditions at the tendered sites and in continuously providing new and improved products. The meteorological dataset for each site typically combines one year of site-specific in-situ measurements obtained with floating LiDARs and several long-term reanalysis datasets. Both provide the basis for the comprehensive report on the expected conditions at an offshore wind site. All data and reports are publicly available via the BSH’s PINTA portal – https://pinta.bsh.de.This study presents a new comprehensive assessment of combined wind and wave conditions for selected offshore wind sites, using multi-decadal atmospheric and oceanographic reanalysis data. For the first time, the new regional reanalysis product ICON-DREAM-EU from DWD is included alongside well-established reanalysis datasets. The resulting weather windows are evaluated in terms of their frequency, duration, and seasonal variability, considering both average and extreme cases. Generic thresholds relevant to the offshore wind industry are used with a focus on near-surface wind speed and sea state.The results show distinct patterns of favourable conditions and reveal substantial differences between the reanalysis datasets. These differences highlight uncertainties inherent in assessments based solely on reanalyses and underscore the importance for high-quality, site-specific in-situ measurements. The study supports improved planning and risk assessment for the offshore wind development and emphasizes the value of the in-situ and reanalysis data provided year after year via the PINTA portal for the energy transition.