Offshore wind turbine jacket substructure : a comparison study between four-legged and three-legged designs

Abstract

A comparison study was conducted between a conventional four-legged and a newly-developed three-legged bottom fixed jacket substructure for offshore wind applications. Fatigue (FLS) and ultimate limit state (ULS) analyses were performed, and results show that the three-legged concept is feasible as an interesting alternative to the four-legged design, while potentially more cost-efficient, with a 17-percent reduction of structural mass and a 25-percent reduction in the number of welded joints. Further analyses were carried out to evaluate the sensitivity of the dynamic performance with respect to different load cases, loading directionality, and wind-wave misalignment effects. Results show that both designs are highly susceptible to the change-of-load direction, therefore recommending a finer incident angle resolution (a gap of 15 degrees or less) to be used in the analysis. The overall wind-wave misalignment effect is comparably smaller, but could contribute to a significant impact if the joints are close to being critical.