Session: 09-02-02 Wind Energy: Mooring System II

Submission Number: 175535

Low Frequency Dynamic Response of Floating Offshore Wind Farms With Shared Mooring Systems 

Shared mooring systems are a promising pathway to reduce capital and footprint costs for floating offshore wind farms (FOWFs). This work investigates the low-frequency (LF) surge-sway responses of multi-turbine arrays with shared moorings versus individually moored turbines, addressing a noted gap between previous frequency-domain predictions and time-domain findings. The former concluded that there was a significant difference in the dynamic response of the shared mooring system compared to the traditional individual mooring system, and the latter concluded that there was no significant difference.

We examine a single-turbine baseline and a 2×2 lattice with shared mooring lines using a dual-track methodology. In the frequency domain, the coupled multi-body equations are assembled first. The added mass matrix is estimated via strip theory. The stiffness matrix, obtained from MoorPy quasi-statics, is non-diagonal due to the shared mooring lines and represents mechanical coupling between the turbines. Damping is dominated by spar-buoy viscous drag and constructed through stochastic linearization, with drag coefficients selected empirically following DNV-RP-C205 guidance. Second-order difference-frequency wave loads are represented with Quadratic Transfer Functions (QTFs) computed in HydroD (Sesam) for a single spar at 45° heading. Given large spacing between turbines for energy yield, inter-body hydrodynamic coupling is neglected and the single-body QTFs are applied to all floaters. In parallel, we perform time-domain simulations in SIMA (SIMO-RIFLEX) of the same configurations to obtain response statistics and power spectral densities (PSDs) for cross-validation.

Modal and nodal analyses reveal that shared mooring introduces distinct array modes and multiple LF PSD peaks, whereas the individual-mooring baseline exhibits a single, narrow-band peak near its natural frequency. The in-line modal shapes are found to be associated with higher response and excitation loads. The causes of mooring line failure or anchor line failure are also identified from the mode shapes. Time-domain simulations corroborate frequency-domain predictions, confirming multiple LF peaks at eigenfrequencies identified by modal analysis and a single dominant peak for the individual system.

Sensitivity studies indicate that frequency-domain results depend on sea state (significant wave height and peak period) and the selection of drag coefficient. The stochastic linearization of quadratic drag introduces a higher-order effect into the solution; therefore, the solution is amplitude dependent. Higher-fidelity CFD or model tests are recommended to obtain an accurate drag coefficient and other damping effects. Comparing Newman’s approximation to full QTFs shows that while Newman’s approximation tends to underestimate loads, it also predicts lower damping characteristics. Consequently, using Newman’s approximation leads to higher responses at lower frequencies but a lower response at higher frequencies.

The fact that the dynamic response of the shared system is significantly different from the individual mooring system highlights the need for a more robust design consideration for shared mooring systems of FOWFs.

Presenting Author: Zhilong Wei Technical University of Denmark

Presenting Author Biography: Zhilong Wei is a postdoctoral researcher at the Technical University of Denmark (DTU). He received a joint PhD from DTU and the Norwegian University of Science and Technology (NTNU) in 2025. His current research focuses on shared mooring systems for floating offshore wind farms, particularly the use of synthetic fiber ropes such as polyester and their nonlinear tension-stretch behavior in design and analysis.

Authors:

Rajendran Arun Ingersol Technology Centre for Offshore and Marine, Singapore (TCOMS); National University of Singapore
Yanlin Shao Technical University of Denmark
Zhilong Wei Technical University of Denmark