A frequency-domain optimization procedure for catenary and semi-taut mooring systems of floating wind turbines
Serag‐Eldin Abdelmoteleb, Erin E. Bachynski
Abstract
This work presents an efficient method for assessing mooring system designs for floating wind turbines (FWTs) based on frequency-domain analysis. The method is used to explore the design space and design-driving constraints for catenary and semi-taut mooring systems for semi-submersible FWTs with power ratings from 5 to 25 MW. The proposed method combines a previously presented model for low-frequency rotor-aero-servo dynamics in frequency-domain with a frequency-domain lumped mass model for estimating the wave-frequency dynamic tension, which has often been treated quasi-statically in previous studies. Multiple design constraints including ultimate limit state, fatigue limit state, and maximum allowable offset were considered in design space exploration and optimization. The main design-driving criteria were found to be the maximum offset and fatigue life. The resulting designs were tested using nonlinear coupled time-domain analysis and found to satisfy all the required design criteria. The frequency-domain model captures the main trends of the motion and tension statistics of the FWTs while providing conservative estimates for fatigue damage for most conditions. The discrepancies between the frequency- and time-domain results are mainly due to overestimation of the surge resonance response due to the linearization of aerodynamic damping in the frequency-domain model. • Frequency-domain model with rotor-aero-servo dynamics and mooring line dynamics. • Both low- and wave-frequency tension processes contribute significantly to fatigue. • FWT mooring design is mainly driven by fatigue and maximum offset constraints. • Larger maximum offsets result in significant reduction in FWT mooring material cost. • Chains that satisfy FLS for a 25 MW FWT are larger than current commercial sizes.