COMPARISON AND VALIDATION OF HYDRODYNAMIC LOAD MODELS FOR A SEMI-SUBMERSIBLE FLOATING WIND TURBINE

In global aero-hydro-servo-elastic analyses of floating windturbines (FWTs), the hydrodynamic loads are usually found frompotential flow theory and applied in a single point of a rigidhull. When the hull is relatively stiff, this approach ensures correctbehaviour for the six rigid body degrees-of-freedom (DOFs),but provides no information about the internal loads in the hull.The current work considers a simplified method to include distributed,large volume hydrodynamics in the global analysis,where frequency-dependent loads from potential theory are appliedon a finite element (FE) model of the hull in a strip-wisemanner. The method is compared to conventional load modelsfor a braceless 5MW semi-submersible FWT, and validatedagainst experimental results from model tests with focus on internalloads and rigid body motions in the main wave-frequencyrange. The global motions are accurately predicted by the distributedmodel for all investigated load cases. Good agreementwith experimental results is also seen for the column base bendingmoment in wave-only conditions, although extreme valuesare not captured correctly due to limitations in linear theory.In combined wave-wind conditions, the measured bending momentsare significantly increased because of the wind-inducedmean angle of the platform. This effect is not considered in thenumerical model, which therefore underestimates the moment response.However, an approach which calculates the loads in theactual mean configuration of the hull is found to give reasonablyaccurate results, at least in moderate wave conditions.