The importance of mooring line model fidelity in floating wind turbine simulations

Accurate computer modelling is critical in achieving cost effective floating offshore wind turbine designs. In floating wind turbine simulation codes, mooring line models often employ a quasi-static approximation that neglects mooring line inertia and hydrodynamics. The loss of accuracy from using this approach has not been thoroughly quantified. To test whether this widely-used simplified mooring line modelling approach is adequate, the open-source floating wind turbine simulator FAST was modified to allow the use of an alternative, fully dynamic, mooring model based on the hydrodynamics simulator ProteusDS. The OC3-Hywind floating wind turbine design was implemented in this newly-coupled simulator arrangement and tested using a variety of regular wave conditions. The static equivalence between the built-in quasi-static mooring model and the newly-coupled dynamic mooring model is very good. Tests using both models were performed looking at scenarios of the response of the system in still water and the response to regular waves and steady winds. The dynamic mooring model significantly increased the overall platform damping in translational DOFs during motion decay tests in still water. There was very little difference between the models in coupled tests where regular wave excitation was the primary driver of platform motions, except for the addition of small levels of power in the higher frequencies of the platform motion spectrum. The nature of the different tests suggests that it is only in situations where the platform motions and wave velocities are not synchronized that the damping from the dynamic mooring model makes a large difference. This points to irregular wave conditions as providing a better test of the differences between mooring models.