Assessment of the Importance of Mooring Dynamics on the Global Response of the DeepCwind Floating Semisubmersible Offshore Wind Turbine

Abstract

This paper studies the influence of mooring line dynamics on the response of a coupled floating offshore wind turbine against an equivalent uncoupled model. The semisubmersible modeled in this paper is based on a design developed by the DeepCwind program and uses the National Renewable Energy Laboratory’s (NREL’s) 5megawatt (MW) baseline wind turbine to represent the tower, nacelle, and blade properties. The uncoupled model was formed using FAST, an open-source program that models the wind turbine aerodynamics, control, motion, tower/blade flexure, and wave forces, but with the mooring line forces treated using a quasi-static approximation. In contrast, the coupled model was enabled by pairing FAST with OrcaFlex. OrcaFlex replaces FAST’s wave force and quasi-static cable model with an equivalent subsea fluid-structure representation and a lumped-mass cable system to capture the mooring line dynamics. This analysis revealed that an uncoupled model using the quasi-static mooring approximation can underestimate peak mooring line loads versus a coupled model using a dynamic mooring line.