Coupled response analysis of a floating wind turbine with free hanging solid ballast

The productivity of a Floating Offshore Wind Turbine (FOWT) primarily depends on the platform's stability. In the present work, we focus on improving the coupled responses of a novel FOWT for better functionality. In its pursuit, we adopt the concept of a Free Hanging Solid Ballast (FHSB) wherein a solid ballast with the equivalent percentage of the weight is suspended from the floater's keel using tendons. The basic idea is to lower the overall centre of gravity of the integrated system. The study is devoted to site conditions of the Indian sub-continent waters, as nearly 99% of offshore wind energy remains untapped. Numerical results are obtained by coupling the National Renewable Energy Laboratory's (NREL) openFAST tool with Orcaflex and validated with data available in the literature. Subsequently, comparison and quantification are made between two platform concepts supporting wind turbines: a conventional semisubmersible (FOWT) and a semisubmersible with an FHSB. The results show a shift in the natural pitch frequency inward (lower frequencies) for the semisubmersible having the FHSB. Moreover, a reduction of 14.42% in heave and 14.32% in pitch responses are observed for the semisubmersible FOWT-FHSB design, albeit this has the same displacement volume as that of the conventional semisubmersible FOWT. Besides, as the FHSB suspended length increases, the floating structure is more restrained in the heave mode but susceptible to changes in pitch mode. The trade-off between the heave and pitch modes is explained and discussed in detail.