Daisy-chained floating wind turbines

Abstract

The offshore wind industry has grown significantly since the installation of the first offshore wind farm in Vindeby, Denmark, in 1991. In recent years, the need to expand offshore wind capacity has led to an increased interest in floating offshore wind turbines (FOWTs), especially for deeper waters where bottom-fixed structures are impractical. This research explores the feasibility and behavior of daisy-chaining FOWTs to reduce mooring costs and anchor points, which currently account for a significant portion of installation expenses and are challenging to install.

The study focuses on spar-type FOWTs, investigating how daisy-chaining impacts the motion, stability, and mooring line forces in different wave and wind conditions. By conducting dynamic simulations with OrcaFlex software, two configurations, triangular and hexagonal, were analyzed to assess the effects of connecting multiple FOWTs. Results indicate that the daisy-chaining doesn’t increase the amplitude of roll and pitch motions compared to a single FOWT, but the system is susceptible to resonance in high sea states, causing excessive yaw motions around the z-axis. These resonances occur due to certain modes in the system being excited by peak wave frequencies, leading to amplified loads.

Despite these challenges, the study demonstrates that daisy-chaining can be feasible if the mooring and connection line design parameters are optimized to avoid resonance issues. The forces experienced in the mooring lines were found to be well within the available safe working load of industry-standard steel wire ropes, but careful design considerations are necessary to mitigate fatigue and ensure stability. This research contributes to the limited literature on shared mooring systems for FOWTs and offers insights into potential cost saving and design strategies for future offshore wind farm projects.