Water replenishment inside monopile subject to irregular waves

The internal water level response was experimentally investigated for a monopile with one opening below still water level and subject to irregular sea states. The experimental campaign included an in-depth analysis of a large number of repetitions to document the measurement uncertainty. Subsequently, the measurement uncertainty was reduced to the desired level by applying an adequate number of repetitions throughout the experimental campaign. In the end the non-dimensional internal water level response in irregular waves was shown, with appropriate characteristic wave parameters, to follow the same non-dimensional function as for the regular wave case. Continuity relates the internal water level response to the cross-sectional averaged jet velocity in the opening. Statistics of the peak values of the jet velocity between successive zero-crossings were derived from the jet velocity time-series. The exceedance probability of the peak values of the velocity in non-dimensional form was found to closely follow the square root of the Rayleigh distribution. In reverse, the function for the non-dimensional internal water level response and the probability of exceedance for the jet velocities provide design formulas for water replenishment processes in irregular sea states. Finally, the mean set-down of internal surface water level was studied as function of the orientation of the opening in the monopile relative to the propagation direction of the incoming irregular waves.