A comparison Study of the Mooring Simulation in the Design and Analysis of Floating Offshore Wind

Abstract

Abstract In wind turbine design, various critical aspects demand attention, including wind turbine design, assessment of floater structure, and optimization of the mooring system. Turbine designers prioritize control system optimization, relying on predicting responses like power generation and nacelle accelerations. Evaluating floater structure involves motion responses and loads on interface positions, like the tower base and mooring fairleads. The latter is crucial for positioning system optimization. These considerations often necessitate a comprehensive simulation of the entire system. This paper conducts a comparative study of outputs of wind turbine, motion performances of the floater, and tensions of the mooring lines, involving various simulations using different mooring models, exploring both frequency and time domains. In the frequency domain, it begins with hydrodynamic analysis using equivalent springs and proceeds to a statistical analysis with a catenary mooring line model. Emphasizing time domain analysis, the paper extensively compares results from coupled analysis with a quasi-static catenary mooring model and a dynamic finite element mooring model. The wind turbine and floater models are kept identical, encompassing three power generation design load cases and two parked cases. The assumption is that the quasi-static approach can significantly reduce coupled analysis time, prompting an examination of its accuracy for the mentioned design focuses. The conclusion reveals that, overall, the results for wind turbines and the tower—such as power generation, nacelle accelerations, and tower base moments—are very similar using two different mooring models in time domain analysis. Regarding floater motions, both approaches yield comparable results in the wave frequency range, but the quasi-static model tends to overpredict low-frequency responses. This conservative tendency affects global performance prediction more than structure assessment, as the latter mainly relies on wave frequency responses. Fairlead tension statistics show similar outcomes, but the power spectrum density indicates that the quasi-static mooring line may miss dynamic aspects. For capacity checks, the quasi-static approach remains valid, while it's recommended to use the dynamic FEM mooring line model for fatigue checks. A simplified approach may still be practical to streamline analysis for design iterations.