Offshore wind electricity generation will play a key role in the transition to a sustainable energy sector. In 2020, the European Commission presented their Offshore Renewable Energy Strategy which includes 300 GW of offshore wind capacity by 2050 (European Commission, 2020). This research focusses on the steel mass development of OWT structural support components like the tower, substructure and foundation considering a trend of higher power turbines and exploitation at increasing water depth. The research follows an initial design approach, to estimate initial dimensions of monopile and jacket support structures for 5 reference offshore wind turbines. For simplicity, initial dimensions are based on design rules-of-thumb hence no ultimate, serviceable or fatigue limit states are considered. Reference turbines of 5, 8, 10, 15 and 20 MW rated power are included in the model, as these can be expected to be commercially available up to 2050. A water depth range of 20-80 meters is taken into account. Considering available development zones and environmental conditions, the study focusses on the North Sea region. For monopiles, the location of the natural frequency is used as a design driver to determine the tower and monopile diameter and plate thickness. For jacket structures, member slenderness and constant bay geometry are used as design drivers to determine member length, diameter and plate thickness. Overall, jacket structures show to be less steel intensive than monopile support structures considering their upfront steel requirements and have a higher steel recovery potential at end-of-life. Considering the climate change potential of OWT support structures, GHG emissions that emerge during steelmaking and recycling are taken into account. A GHG emission indicator of 1.22 tCO2-eq/t steel is derived with the aim to reflect European steel production processes. GHG emissions that emerge during transport, manufacturing and assembly or during the making of other materials than steel are not assessed in this research. A GHG emission payback time indicator is developed to provide insight into how the GHG emissions invested in support structure steelmaking compare to the GHG emissions avoided through the generation of renewable electricity. It is found that, depending on the combination of turbine rated power and water depth, for monopiles the emission payback time lies in a range of 4-9 months. For jackets this is 2-10 months.
In order to reduce the emission intensity of the offshore wind industry, this research recommends commissioners and designers to favour jacket over monopile support structures due to (i) reduced steel intensity for initial support structure manufacturing, and (ii) a higher recovery potential of support structure steel and its rare alloying elements at end-of-life. For 5 MW OWTs it is found that the application of jacket structures reduces an OWT’s total climate change potential with 15-20% compared to monopile structures. This research provides insight into to the contribution of bottom-founded support structures to the climate change potential of OWTs and shows that the combination of turbine rated power, water depth and support structure typology significantly influence the steel and GHG emission intensity of an offshore wind turbine.