Effect of members and local joints flexibilities on the dynamic and fatigue response of semi-submersible platforms for floating offshore wind turbines

This study investigates the effects of member and local joint flexibilities on the response of a semi-submersible floating offshore wind turbine (FOWT) by carrying out fully coupled nonlinear time-domain aero-hydro-servo-elastic dynamic analysis of the OC4 floater. Rigid body models and flexible models of the OC4 semi-submersible floater, supporting the NREL 5MW wind turbine, are used to perform this study. A total of four unique models of the FOWT system were developed and these include a model with a rigid body floater (M1), a flexible floater with each OC4 column modelled as separate rigid bodies connected by flexible braces (M2), and a flexible floater with local joint flexibilities explicitly introduced at inter-member connections to capture joint-level deformation (M3). The fourth is a parametric sensitivity model, derived from the previous one, where the rotational stiffnesses of the floater joints are systematically increased to assess their influence on the system dynamics as validation for M2. Decay tests were first carried out on all 4 models, to ascertain their characteristic modal behaviour, then fully coupled dynamic simulations were carried out for all the models. Analysis was performed for cases of regular wave only, random wave only, regular wave plus steady-state wind, and random wave plus turbulent wind. The global and local response of the FOWT system is then investigated. Results show that floater flexibility modifies the platform pitch natural period by about 10%. The surge and heave natural periods are, however, minimally affected. Platform flexibility also significantly affects the local response of floater braces (pontoons), the tower base loads and tower top kinematics, with implications for both design optimisation and performance forecasting in next-generation FOWT systems. Fatigue analysis of the tower base and selected brace connections reveal that fatigue damage is overpredicted at the tower base when the platform is modelled as a rigid floater, leading to a conservative (underestimated) fatigue life by approximately 50%, while fatigue damage is substantially underpredicted up to about 80% in the pontoon braces when the platform is modelled as rigid. • Global floater motions remain unchanged with flexibility, while local stresses change. • Rigid models overpredict tower fatigue by 50 % and underpredict brace fatigue by 80 % . • Local joint flexibility reduces brace fatigue damage by 30 % in flexible models. • Rigid joint assumptions may remain valid for stiffened FOWT columns and chords.