Integrated analysis of fixed offshore wind turbine system with UHPC composite cap foundation

The high-rise pile cap foundation (HPCF) with a prefabricated ultra-high performance concrete (UHPC) composite cap represents a novel foundation design for fixed offshore wind turbines (OWTs). Coupling dynamic analysis is crucial for these OWT systems, yet existing research struggles to comprehensively model the cap constructions while maintaining computational efficiency. This paper proposes an innovative integrated analysis framework based on the OpenFAST code framework, the highlight of which lies in the establishment of an improved HPCF model to characterize the complex configurations of caps. Since tedious finite element analysis (FEA) through Abaqus is only performed once to calibrate the improved HPCF modeling, instead of being conducted for each of the thousands of load cases in the codes, the barriers of computational efficiency can be removed. Furthermore, the Det Norske Veritas (DNV) formulation is generalized to characterize the soil-structure interaction effect for inclined piles. Consequently, the relative errors of modeling primary stiffness components of an inclined pile-soil system can be reduced from up to 12 % to within 3 %. Moreover, both direct and indirect impacts of HPCF constructions are investigated through integration of coupling dynamic analysis with modal analysis. Case studies consider various load cases and cap configurations, together with a parametric analysis exploring parameterized HPCF equivalent stiffness due to construction variation and nonlinearity development of caps. A notable discrepancy of up to 13 % in the dynamic response amplitudes can be observed across different cap designs for one load case, which demonstrates the significance of comprehensive characterization of cap configurations throughout the aero-hydro-servo-elastic coupling dynamic analysis.