Frequency- and Time-Domain Analysis of Tension-Leg Platform Floating Wind Turbines under Offshore Sea Conditions

Floating offshore wind turbines on tension-leg platforms (TLP-FOWTs) face critical design challenges due to complex coupled dynamic behavior, which, if inaccurately predicted, risks over-design or structural failure. This study aimed to develop and validate an integrated aero-hydro-servo-elastic OpenFAST model for TLP-FOWTs and establish a clear two-stage methodology linking efficient frequency-domain screening with detailed non-linear time-domain verification. The coupled model was developed using potential flow theory, Morison’s equation, Blade Element Momentum theory, and linear tendon stiffness, with frequency-domain analysis identifying natural modes and time-domain simulations performed under operational and 50-year extreme storm conditions. Frequency-domain results revealed a resonant platform pitch mode at 0.142 Hz, yielding a peak Pitch RAO of 1.65 deg/m at 0.143 Hz. Time-domain simulations under extreme storm conditions (Hs=14m, Tp=16s) produced a maximum platform pitch of 12.7°, a tower base bending moment of 245 MNm, and a critical minimum tendon tension of 5.1 MN—a 65% reduction from pretension indicating near-slack risk. Model validation against benchmark data showed excellent agreement, with deviations below 7% and only 3.1% difference at the resonant peak. This study concludes that resonant pitch motion is the primary driver of extreme loads and tendon slackness risk in TLP-FOWTs, and that the integrated two-stage framework enhances design reliability and cost-effectiveness. It is recommended that designers prioritize pitch resonance avoidance and tendon slackness checks, and consider increasing tendon pretension or modifying platform geometry for sites where 50-year minimum tensions approach critical limits.