Optimized retrofit design of in-service monopile foundation offshore wind turbine transition zone

This study proposed a new retrofit solution using bolted connections for in-service offshore wind turbines (OWTs) with monopile foundation via a finite element analysis-based optimization approach. The monopile foundation 5 MW OWT provided by National Renewable Energy Laboratory (NREL) was selected for this study. The transition zone of the monopile OWT subjected to wind and wave loadings, the grout connection of the tower to the monopile, was specifically simulated with three different models using the ANSYS software. The three models included a plain model resembling the installed monopile, a shear key model considering the Det Norske Veritas (DNV) updated design, and a retrofit model incorporating the plain model with the bolted connection installed. Numerical results of three different transition models were compared to determine the efficacy of the proposed retrofit solution. Along with the numerical results, the proposed retrofit bolted connection was optimized through implementation of response surface methodology (RSM) with six input parameters (e.g., bolt diameter) and three outputs (e.g., fatigue life). With the RSM data, the desirability function was employed on three different runs, in an attempt to obtain the most optimal retrofit bolted connection for the monopile OWT. A key finding indicated the optimized configuration of the bolted connection with high strength bolts of 0.036 m diameter to be installed in eight columns and five rows exceeded the required fatigue life.