Multi-Objective Optimisation of Cost and Carbon of Offshore Wind Foundations

Abstract Offshore wind allows the UK to harness its naturally strong winds and provides a sustainable alternative to the importing and burning of fossil fuels for energy. One of the latest developments for offshore wind is ScotWind, which will bring an estimated 30GW of additional wind power to the UK, 19GW (64%) of which is expected to be floating wind. Floating windfarms can occupy much deeper waters than traditional bottom-fixed structures, but are a very new technology for the UK, with only the Hywind and Kincardine pilot sites using floating foundations. Another developing technology is the increasing wind turbine sizes, with predictions that ScotWind farms could use turbines as large as 20MW. These recent advancements mean an optimisation study is a useful method for answering key design questions, such as optimal foundation choice and manufacture location when considering both cost and embodied carbon. The cost and embodied carbon from over 90 offshore wind foundation design scenarios were analysed to determine the most optimal design choices for future ScotWind sites. Support Vector Regression and non-linear relationships were used to determine the mass of steel and grout for pin-piled jackets in water depths between 40-80m and turbine ratings 12-20MW, and for steel semi-submersibles (semisubs) from 10-20MW. The embodied carbon from the materials was calculated using data from ecoinvent, the Inventory of Carbon and Energy and worldwide steel fabricators, and the costs were calculated using steel production costs from World Steel, labour costs and industry rates. The cost and carbon from shipping the foundations from China, Dubai, Spain and the North of England to Scotland's Port of Cromarty was calculated to compare fabrication location effects. A theoretical scenario of local manufacture using green steel in Scotland was also considered, using projected costs and embodied carbon. The embodied carbon (gCO2eq/kWh) of each scenario was plotted against its relative cost (£2023/kW), and the multi-objective optimisation was performed using Pareto frontiers. Spain was found to be the optimal fabrication location for jackets, producing 28% less embodied carbon than fabrication in China, and 10% lower cost than manufacture in the UK. For semisubs, partial manufacture in Spain, followed by assembly and finishing in the UK, was found to be the most optimal in terms of carbon and cost, producing 13% lower carbon emissions than partial manufacture in Dubai, and costing 5% less than manufacture in the UK. Furthermore, it was shown that as offshore wind foundations increase in size, manufacture in China and Dubai becomes less optimal in both cost and carbon due to requiring more return ship trips. This shows Europe's advantage of closer manufacture and assembly of offshore wind foundations, which could be harnessed as the industry evolves larger turbine sizes and semisub foundations become more prevalent.