Comparative study on coupled dynamic characteristics of four-rotor floating offshore wind turbines under operating and fault conditions

Upscaling floating offshore wind turbines (FOWTs) aggravates the weight, blade flutter and installation limitations of conventional single-rotor schemes, making multi-rotor wind turbines (MRWTs) a promising alternative. However, the dynamic characteristics and fault response mechanisms of four-rotor FOWTs remain insufficiently understood. This study establishes a fully coupled numerical model of a 20 MW four-rotor FOWT and compares its dynamic performance with an equivalent single-rotor FOWT. The results show that rotor phase differences eliminate 1P/2P periodic loads, providing stable dynamic responses and shortening the startup time by 80 s for the four-rotor FOWT. Under coupled wind-wave conditions, the four-rotor FOWT exhibits larger platform motion and mooring line tension fluctuations due to frequent blade pitching. Faults trigger remarkable asymmetric structural responses: single-rotor fault yields 2.3 m sway, 6° yaw and tower-base torsional moment 18 times the baseline; diagonal dual-rotor fault results in 12° yaw with torsional moment amplified to 37 times the baseline. Combined fault condition of mooring line 3 and rotors further induces large platform horizontal excursion and nearly 30° yaw deflection. This study verifies the load-mitigation and fault-tolerance merits of four-rotor FOWTs from rotor redundancy and highlights critical torsional risks under asymmetric faults, supporting the design and operation of large-scale MRWTs.