Experimental study on water replenishment holes in offshore monopiles

Adequate replenishment of water within the monopile foundations of offshore wind turbines is essential to ensure the effective performance of cathodic protection measures applied inside the monopile to combat internal corrosion. This study presents experimental results for the process of replenishing water within a monopile with one opening below the still water level. The results are presented as (1) the relative wave height and set-down inside the monopile under various regular wave conditions and (2) head loss coefficient for the flow in and out of the water replenishment opening in the monopile. The study demonstrates a clear link between the ratio of the wave height of the internal and external surface elevations, the relative wave height, and the dimensions of the opening in the monopile as well as the wave period. Furthermore, a simple predictive model is proposed based on linear wave theory and static energy loss considerations. The orientation of the opening with respect to the wave direction has limited effect on the relative wave height, however it has a distinct effect on the mean surface level (MSL) inside the monopile. The lowest MSL occurred when the opening hole was positioned perpendicular (at a 90-degree angle) to the direction of wave propagation. Geometric considerations suggest that the flow in and out of the cylinder is slightly asymmetric and the results also show a small difference in the head loss coefficient for the flow in and out of the monopile, respectively. On average, however, the head loss coefficient was found to be ζ=2.0. •Performance of a single replenishment hole in an offshore monopile foundations.•Internal wave height as function of opening size, and incident wave height and frequency.•Mean surface level inside the monopile as function of the angle of attack.