Application of simple harmonic motion theory to the prediction of floating wind turbine first order motions

Abstract

An alternative solution for achieving a high stability floating foundation for supporting large horizontal axis wind turbines is compared against current floating offshore wind solutions. The testing of a 1:36th scale model of the solution, called Starfloat, in the wind/wave basin of the Cantabria Coastal and Ocean Basin, Santander and the design of this physical model, including scaling issues, is discussed. A method for calculating the added mass of entrained water in heave and surge and the added mass moment of inertia in pitch for semi-submerged cylindrical columns is presented and the computed natural periods of motion are compared against physical trial measurements obtained from motion decay tests. The wave forces exciting the simple harmonic motion are calculated and the resultant approximate solution for the uncoupled floating platform motions in heave, pitch and surge are compared against response amplitude operator curves obtained from the physical model trials, showing reasonable correlation. The importance of depth correction for wave particle velocities and accelerations is very evident and emphasises the need for physical models to be exposed to the correct orbital particle motions over the water column if measured pitch and surge motions are to be a realistic representation of full scale performance.