The Dynamic Coupling Effects of a MUFOWT (Multiple Unit Floating Offshore Wind Turbine) with Partially Broken Blade

Recently, several numerical simulation tools for the analysis of a Floating Offshore Wind Turbines (FOWT) have been developed by many research institutes. Most of the existing numerical tools can analyze only a single turbine with a floating platform. In this study, an advanced numerical simulation tool has been developed for the rotor-floater-tether coupled dynamic analysis of Multiple Unit Floating Offshore Wind Turbine (MUFOWT) in the time domain including aero-blade-multiple tower dynamics and control, mooring dynamics and platform motion. In particular, the developed numerical tool is designed based on the single turbine analysis tool FAST, which has been developed by National Renewable Energy Laboratory (NREL) for years. For linear or nonlinear hydrodynamics of floating platform and generalized-coordinate-based FEM mooring line dynamics, CHARM3D is incorporated. So, the entire dynamic behavior of a floating offshore wind turbine can be obtained by coupled FAST-CHARM3D in the time domain. To analyze the dynamics of multiple turbines on a single platform, the coupled FAST-CHARM3D is further expanded and re-constructed. The global coefficient matrix that includes one floating platform and a number of turbines is built at each time step of the simulation, and solved simultaneously to obtain the responses of the entire degrees of freedom of the MUFOWT system. To investigate the dynamic coupling effect between platform and turbines, a five-turbine semisubmersible is modeled and simulated. To check the dynamic coupling effect, extra 1P excitation from one turbine by partially broken blade is intentionally generated and the transferred loads to the platform and the other turbines are measured. The analysis shows that the dynamic load imbalance of one turbine in MUFOWT may induce significant changes in the performance of the other turbines or a floating platform.