A study of nonlinear hydrodynamic and mooring modelling for the Volturn floating wind platform in comparison with experiments

For coupled hydrodynamic-aerodynamic-mooring models for floating offshore wind platforms, there has been little time domain validation against experimental data which can reveal the origin of uncertainties for this complex problem. Here we use data for focussed waves incident on the 15 MW Volturn semi-submersible platform at 1:70 scale and free decay tests where fairlead tensions and instantaneous platform position have been measured. This enables mooring models to be assessed independently with known fairlead motions. The catenary chain moorings are modelled by quasi-static theory and in lumped-mass form including dynamics and damping using the MoorDyn code. Tension predictions by MoorDyn with fairlead motion input were always highly accurate in both free decay and focussed waves while quasi-static theory only gave equivalent accuracy in surge decay tests. In relatively large focussed waves, motion prediction by coupled modelling was substantially improved by including the instantaneous platform position in the excitation force formulae, giving a simple nonlinear extension within an otherwise linear model (apart from the drag formulation). Both mooring models gave similar predictions of platform motion in free decay and focussed waves generally approximating experimental results. While mooring tension was well predicted by MoorDyn in all cases, quasi-static modelling again only gave good prediction for surge free decay. Irregular wave interactions at full-scale have also been simulated with wind thrust effect and the statistics of the platform motions and fairlead tensions are analysed, showing the importance of dynamic mooring effects on mooring fatigue but with little effect on maximum tension or platform motion.