NONLINEAR AERO-MOORING EFFECTS ON FLOATING OFFSHORE WIND TURBINES UNDER RANDOM WIND AND WAVES: ENVIRONMENTAL DIRECTION EFFECTS

Floating offshore wind turbines (FOWTs) have been extensively investigated to harvest the vast offshore wind potential at transitional to deepwater locations. Among the main FOWT components, mooring systems play an important role in the platform dynamics, being the primary source of restoring force in the surge and sway directions. For such degrees, the platform motion can be highly affected by turbulent wind loads due to the platform's low natural frequencies. Catenary mooring systems are generally applied to FOWTs, which are characterised by nonlinear loads (asymmetrical). In such a system, when the mean aerodynamic thrust force acts on the platform, it leads to an offset position that modifies the mooring restoring force, changing the platform's natural frequency. In this context, this work investigates the aero-mooring nonlinear effects on the platform dynamics under environmental conditions coming from different directions. The OC4 semi-submersible platform supporting the NREL 5-MW baseline offshore wind turbine with three catenary lines is chosen as a base case. The FOWT is subject to random loads from the wave and wind (temporal and spatial effects). Since efficient tools are favourable during the initial stages of development and in assessing an extensive range of environmental conditions, the numerical model is built using frequency domain modelling and statistical linearisation to estimate the nonlinear effects. The results show that the nonlinear mooring loads affect the natural frequency, making the platform more susceptible to stochastic wind loads when the environmental direction comes with an angle of 60o with respect to the platform.