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In frequency-domain (FD) models of floating wind turbines (FWT), it is common to regard the interaction between nacelle motions and thrust by means of a constant aerodynamic damping coefficient. This approach is effective at higher motions frequencies, but does not consider interactions between nacelle motions and the blade pitch control system. As a result, the motions and loads at frequencies closer to the controller bandwidth may be underpredicted. A remedy for this problem is to include the linearized thrust expression in the FD model, such that the dynamic effects related to control are considered. In this paper, these dynamic effects are related to frequency-dependent damping and inertia terms. Expressions for damping and inertia coefficients are obtained with two different methods, and then included in the FD model. The resulting responses are compared to those obtained with the constant damping coefficient method, and also with coupled time-domain simulations of a semi-submersible 10 MW FWT. The better performance of the FD model with frequency-dependent inertia and damping coefficients encourages the adoption of the linearized thrust approach for representing the interaction between nacelle motions, thrust and control system.

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Design optimization of mooring systems is an important step towards the reduction of costs for the floating wind turbine (FWT) industry. Accurate prediction of slowly-varying horizontal motions is needed, but there are still questions regarding the most adequate models for low-frequency wave excitation, and damping, for typical FWT concepts. To fill this gap, it is fundamental to compare existing load models against model tests results. This paper describes a calibration procedure for a three-columns semi-submersible FWT, based on adjustment of a time-domain numerical model to experimental results in decay tests, and tests in waves. First, the numerical model and underlying assumptions are introduced. The model is then validated against experimental data, such that the adequate load models are chosen and adjusted. In this step, Newman’s approximation is adopted for the second-order wave loads, using wave drift coefficients obtained from the experiments. Calm-water viscous damping is represented as a linear and quadratic model, and adjusted based on decay tests. Additional damping from waves is then adjusted for each sea state, consisting of a combination of a wave drift damping component, and one component with viscous nature. Finally, a parameterization procedure is proposed for generalizing the results to sea states not considered in the tests.