Transient tower and blade deformations of a Spar-type floating wind turbine in freak waves

During its lifetime, a floating offshore wind turbine (FOWT) encounters various normal and abnormal waves. Under extreme sea conditions, freak waves may suddenly enlarge the stress and deformation of the upper flexible structures and threaten the structural integrity and operational safety. To understand this effect, a Spar-type floating wind turbine (SFWT) was chosen and its dynamic model was numerically established. Four types of modulated freak waves (MFWs) with different properties were obtained by quantitatively modulating the freak wave profile via the corresponding decision parameters based on the Klinting and Sand definition criterion of freak waves. The dynamic performances of the SFWT under the MFWs were simulated in the time domain, and the internal loads and deformations of the SFWT blade and tower were analyzed. According to the results, the transient structural responses of the blade and tower have different characteristics in low frequency. The blade edgewise deformations are significant while the tower resonates with the floating foundation. The results on the MFW effect indicate that the deformation and bending moment of the tower and blade increase significantly with the fluctuation degree (FD) enlargement (the effect on bending moment is more pronounced). The previous height and impact height (PH and IH) have slight effects on structural deformations. The next height (NH) has little amplification effect on any load responses. Furthermore, the influence of wave properties on the blade tip deformation is almost instantaneous, while the effect on the bending moment has a lag in time.