A new FounDyn module in OpenFAST to consider foundation dynamics of monopile supported wind turbines using a site-specific soil reaction framework

Abstract

A FounDyn module is created in OpenFAST to consider foundation dynamics, which is an appealing supplement to the current version of OpenFAST. The FounDyn module receives the motions from the SubDyn module and sends the forces back to the SubDyn module. In FounDyn, the soil-monopile interaction is captured using a site-specific soil reaction framework. The soil reaction framework possesses the same configuration of the semi-analytical 1D model to consider effects of the large plie diameter and the small pile aspect ratio but uses new site-specific soil reaction models. The soil reaction models are nonlinear and hysteretic, which match the desired modulus reduction curve by identifying three parameters in a hyperbolic function and a linear function using genetic algorithm (GA) and manual parameter tuning, and the desired damping curve by applying the Ishihara-Yoshida rule that controls the unloading-reloading curves iteratively through three parameters. The FounDyn module is verified by the well-confined OC3 project in terms of modal frequencies, tower top displacement and shear force and moment at the mudline, and reasonable agreements are achieved between them. A series of emergency shutdown analyses of the NREL 5 MW wind turbine are performed using OpenFAST plus FounDyn. The results show that the misalignment of wind and earthquake affects the tower bending moments largely. The earthquake excitation is found to be the design driving load, prevailing over the wind excitation for the design of wind turbine supporting structures.