Experimental study of non-linear wave impact on offshore wind turbine flundations

A novel analysis method is proposed for the fluid loading on a surface piercing cylinder in steep waves. This is based on experiments using compact focussed wave groups where phase can be explicitly controlled. By combining measured force time histories for a steep wave group and that from the same wave group but inverted, the complete harmonic structure of the total hydrodynamic loading can be extracted. The 2nd-5th harmonic load components scale with the appropriate power of the envelope of the linear force component in both time variation and amplitude. This justifies the use of a Stokes-like decomposition of hydrodynamic load comparable to the classical treatment of undisturbed free wave elevation.