Second and third order sub-harmonic wave responses of a floating wind turbine

The expansion of offshore wind farms into deeper water requires floating concepts, as bottom-fixed monopile turbines become prohibitively expensive. In this work, a catenary moored floater system is investigated. Such systems typically have large natural periods in those modes where the hydrostatic stiffness is small (or zero), as the restoring force provided by the mooring is low. Since the hydrodynamic damping is weak, the systems are highly resonant. It is well known that low-frequency resonant motions of soft moored structures can be excited by second-order hydrodynamic loads. Here, we show that, for the floater considered, third-order difference interactions can also give rise to considerable resonant pitch motions.