Investigation of the dynamic response of a spar FOWT, and its sensitivity to variations in draft

Abstract

The objective of this thesis is to investigate how the dynamic properties of a spar supported FOWT change when the draft is reduced, and what draft is needed in order to support a 10 MW wind turbine. Design solutions of four spar-buoy concepts with drafts ranging from 60 to 120 m are identified in order to support the DTU 10 MW reference wind turbine. They are based on the "OC3-Hywind" spar, developed by the National Renewable Energy Laboratory. Modifications have been made to the wind turbine tower to make it more suitable for an offshore location. The diameter of the main hull of the spar has been adjusted to achieve the necessary stability. The designs have been modelled using a geometry modelling tool, and hydrostatic and hydrodynamic analyses have been performed using 3D potential theory. The mooring system was simplified to a horizontal stiffness component included in the restoring matrix. Analyses of the natural periods and dynamic response in surge, heave and pitch have been performed for varying environmental loads. The dynamic response was found to increase when the draft was reduced, however, for 60 m draft it was still considered to be within acceptable limits. It is concluded that it is possible to design a shallow draft spar-buoy with the necessary stability to support a 10 MW wind turbine. This will however lead to an increased volume which would mean higher material expenses. Furthermore, a more detailed analysis of the mooring system is needed as this has a significant impact on the dynamic response.