What affects the damped frequency result for floating wind turbine
Category: Linearisation & Campbell diagram
Problem
What affects the damped frequency result
Solution
The Wamit result, used for the BEM hydrodynamics definition, is a frequency domain dependent definition. The hydrodynamics modelled from the Wamit results are only included in time domain simulations in Bladed where a time history is used to derive the frequency dependent hydrodynamic contributions at each point in time. The model linearisation is a “timeless” calculation so there is no contribution from any of the frequency dependent Wamit results in the linear model. Therefore, changing the Wammit result will not change the damped frequency as the damping is not effected. In general, the quadratic damping and the radiation damping is not captured by the linear model. Only linear damping is present. As a workaround it is possible to tune a linear damping coefficient based on free decay simulations in Bladed (parked time domain simulations where initial platform position is defined not in equilibrium, the platform then moves to the equilibrium position during the simulation). From this, you are able to estimate the amount of damping that should exist. This can then be added to the model pre-linearisation as a global linear damping term to compensate for the lost damping in the linearisation process. This makes the linear model more representative of the actual system which is helpful for control design. After control design, for the time domain simulations, you would remove this added linear damping otherwise you will be double accounting for the frequency dependent damping.
Keywords floting; linearisation