Analysis of extreme internal load effects in columns in a semi-submersible support structure for large floating wind turbines

Abstract

Most studies on floating wind turbine floaters focus on rigid-body motion analysis, while analysis of structural load effects of the floaters is lacked. However, load effect analysis of hull structures is essential for the safety design checks throughout the service lifetime. Therefore, this study carries out the analysis of the extreme internal stress of columns in a 10-MW semi-submersible hull. A novel multibody approach is used for the numerical modelling of the hull structure, which can capture the internal forces and moments of the columns in the time domain. The environmental contour approach is employed to conduct the short-term analysis of the column extreme stresses. Important load cases are determined based on the sensitivity study of combined critical wave periods, important mean wind speeds, and the largest significant wave height with respect to column load effects. Long-term extreme values of the column stresses are predicted by the extrapolated method based on the short-term extreme analysis. Relative importance of internal load components on extreme value estimation of columns is studied. Simplified approaches are proposed to more efficiently predict the long-term extreme values of the column load effects, and applied in a case study. The analysis provides a good insight into the important global forces and moments as well as the direct wave load effect on the column stress analysis, which contributes to improve the design efficiency in engineering practice.