Effectiveness of combination rule at predicting the elastic seismic demand of jacket-supported offshore wind turbine subjected to bi-directional motions

Abstract

This study investigates the effectiveness of the percentage combination rule given in building regulations for predicting the maximal dynamic responses of OWT under the application of two horizontal seismic components. Time-history analyses were performed to determine the critical responses of OWT under contemporaneous action of both longitudinal and transverse components of the earthquake loadings, considering the ground motion directionality. Subsequently, response spectrum analyses were also performed to obtain the peak responses of the structure using the percentage (30 % and 40 %) combination rule, resulting in the following observations. The results reveal that the 30 % combination rule as currently mandated in building regulations leads to an adequate calculation of the tower top displacement and tower top rotation of OWR, with a relative difference range between 3 - 25 % percent in most of the examined scenarios. The 30 % combination rule outperforms the 40 % combination rule in predicting responses to bi-directional shaking. Results also show that the responses under uni-directional excitation underestimated up to 96 % when compared to bi-directional excitation of the OWT structure. Finally, a performance-based analysis was conducted against different degrees of orientation to quantify the various engineering demand parameters of the OWT. � 2024 Institution of Structural Engineers