Long-term performance estimation of the Spar–Torus-Combination (STC) system with different survival modes

This work addresses a combined wind and wave energy concept called the Spar–Torus-Combination (STC) system. Two additional survival modes for the original STC system (in a survival mode with the tours locked to the spar) have been proposed, namely the submerged mode and the low power take-off (PTO) damping mode. The performance of the STC system with different modes has been investigated based on a coupled analysis of wind–wave-induced stochastic response using the SIMO-TDHMILL code in the time domain. The energy production, structural fatigue damage and extreme responses of the STC system have been estimated based on the long-term wind–wave joint distribution at two selected sites in European waters. The hydrodynamic loads on the spar and torus were estimated using potential theory by accounting for the hydrodynamic interactions. The aerodynamic loads of the rotor were calculated by a validated simplified thrust model (TDHMILL). The annual wind and wave power production have been obtained using the hourly mean output power of each short-term condition and its occurrence probability. The annual fatigue damage of both mooring lines and the tower has been calculated based on the S–N curve approach and the Palmgren–Miner׳s linear damage hypothesis. The extreme responses of the STC system have been investigated by assuming that the largest values in each short-term case follow the Gumbel distribution. The results show that the new proposed survival strategy can significantly reduce the long-term fatigue damage and extreme responses of the original STC system without sacrificing power production by avoiding the possible heave resonance effect when the torus is locked to the spar at the mean water level (MWL). Furthermore, considering the fact that long-term analysis with all sea states is time consuming, the contour line method has also been applied to effectively estimate the extreme responses of the original STC system and has been validated by a comparison with the full long-term analysis method.