Implementation of seismic soil-structure interaction in OpenFAST and application to an offshore wind turbine on jacket structure

One of the challenges in design of offshore wind turbines (OWTs) is that the analyses are performed using specialized software dedicated to hydro-aero-servo-elasto-dynamic analyses which often cannot rigorously perform seismic soil-structure interaction (SSI) analyses. This work presents a methodology to extend these tools to include seismic SSI analyses in these tools with a specific application in the open source OWT analysis tool OpenFAST. The developed method is applied to 10 MW offshore wind turbine on a jacket structure founded on piles. The SSI is implemented using a multi-step sub-structuring method. The method is based on the SSI stiffness and kinematic interaction. The jacket base is attached to pile foundation springs, and excited by forces calculated from the pile-head motions during the earthquake. The spring stiffness and pile-head motions can be determined using well-established methods. In this, they are obtained using the finite element program Abaqus. A complementary integrated Abaqus model of the jacket and tower is then used to verify the implementation of the multi-step method in OpenFAST. The IEA 10 MW reference OWT established in the European research project INNWIND is used in the verification. Using the developed model, the study then attempts to investigate some of the characteristic earthquake responses of the OWT structure. Simulations show how the top of tower displacements are dominated by the wind-induced forces during production form the rotor-nacelle-assembly, while the tower top accelerations and base overturning moments are dominated by the earthquake-induced loads.