Dynamic modelling of mooring system for integrated analysis of floating offshore wind turbines

This paper presents a theoretical model and numerical tool for the dynamic analysis for mooring system of floating offshore wind turbines. The theoretical framework was built based on the lumped-mass mathematical model, with the initial configuration determined based on multi-segment catenary mooring line theory. Morison's Equation was employed to calculate the drag component of fluid damping loads, and the seabed interaction was accounted by prescribing the upthrust load on the lay-down segment. Furthermore, the dynamic mooring line module was integrated into an in-house program, DARwind, an advanced aero-hydro-structural-servo solver designed for global dynamic analysis of floating offshore wind turbines. By incorporating the proposed mooring dynamic model, DARwind is enhanced using a multi-body dynamic framework, employing the Newton-Euler method alongside Kane's Dynamic Equations. Numerical simulations were conducted using DARwind incorporated with dynamic mooring line model, the simulation results were benchmarked against the those from OpenFAST. Comparative results between lumped mass and quasi-static catenary mooring line model are presented. This upgrade enables DARwind to capture the dynamic responses of floating offshore wind turbines with feasibility and accuracy. The analysis results demonstrate that the proposed mooring dynamic model integrates effectively with the DARwind program, the RMS deviations of fairlead tensions between DARwind and OpenFAST are less than 13 %. The underprediction of fairlead tension in quasi-static approach are highlighted. This research will upgrade DARwind to become a more reliable numerical tool for integrated analysis of floating offshore wind turbine.