Dynamic Response Analysis of Spar-Type Floating Wind Turbines and Mooring Lines With Uncoupled vs Coupled Models

Abstract

Offshore floating wind turbines (OFWT) are supported by the flexible mooring systems subjected to nonlinear hydrodynamic wave and current forces. Depending on the floater type and environmental condition, the mooring responses can have a significant impact on the overall dynamic performance of OFWT. To evaluate the dynamic responses of OFWT, both uncoupled (quasi-static) and coupled (dynamic) mooring models have been proposed in the literature and in practice based on the use of the well-known FAST software and the FAST-Orcaflex package, respectively. This paper will investigate and compare the dynamics of the OFWT and the mooring lines using uncoupled vs coupled models, based on the OC3-Hywind Spar platform supporting the 5MW wind turbines developed by the National Renewable Energy Laboratory. Preliminary numerical studies in several load cases reveal substantial differences in the OFWT and mooring dynamics obtained by the two approaches, e.g. under regular and irregular waves. The levels of differences are reported, and the comparisons with available experimental results are also made to validate the model analyses and outcomes. The importance of mooring line dynamics and their contributions to the overall 6-DOF responses of OFWT are highlighted which should be recognised in the analysis and optimization design.