Investigation of high order hydrodynamic load influence on a TLP FOWT

The influence of the second and third order wave loads on the TLP FOWT responses are investigated in this study. The second order wave loads are calculated by commercial wave diffraction/radiation analysis software tool. The third order wave loads calculation methods are developed in this study based on the well-known FNV formulation. The third order wave force is applied on the platform in dynamic simulation models as external force. It is observed that the third order wave force is cubic proportional to the wave heights. The sum frequency third order wave force has periods about 1/3 of the first order wave periods. With the same wave heights, the third order wave forces on the surface piercing structure are higher in shallower waters. The 2nd order wave loads have higher influences on the FOWT responses than the 3rd order wave loads in both fatigue and extreme load cases. The 3rd order wave force influence is negligible in fatigue load cases. The high order wave loads have more impact on the tower and mooring system responses than platform motion. On a TLP FOWT, the tower and mooring system usually feature high frequency resonance susceptible to excitation from sum frequency 2nd and 3rd order wave loads.