A review of coupling approaches for the dynamic analysis of bottom-supported offshore wind turbines

Offshore wind turbines are designed and analyzed using simulation tools (computer codes) capable of predicting the coupled dynamic loads and responses of integrated turbine and support structure systems. A partially coupled analysis using load exchange is often desirable to meet intellectual property rights interests of disparate design teams and developers of the various components. Of interest is the assessment of the validity of such partially coupled analysis approaches for offshore wind turbine systems with various types of foundations. Computer-aided engineering (CAE) tools-such as the National Renewable Energy Laboratory's (NREL's) open-source FAST software and commercial structural analysis and design software-can use partial or full coupling to simulate loads and motions for utility-scale offshore wind turbines with various support structures. In partially coupled analysis, computed tower-base loads from the aeroelastic simulation tool are fed to structural response simulation tool at a suitable interface; required hydrodynamic and wind loads are introduced as needed on the exposed substructure. In fully coupled analysis, the transfer is two-way-i.e., forces at the interface are intrinsically exchanged between the aeroelastic and structural tools at each time step. It is desirable to compare the loads between alternative partially coupled models that are calibrated against a reference fully coupled case. This paper proposes a framework for the assessment of the validity of partially coupled models in the prediction of short-term and long-term loads and motions associated with large utility-scale bottom-supported offshore wind turbines that might be deployed in offshore wind power development.