Jacobsen, N. G., Hoballah Jalloul, M., Welzel, M., Schendel, A., & Carstensen, S. (2025). Jet velocities in water replenishment holes: A large-scale experiment. Ocean Engineering, 342(122867), 122867. doi:10.1016/j.oceaneng.2025.122867
Water replenishment holes are a feature in monopiles that help maintain the internal water pH value close to ambient conditions, when an internal cathodic protection system is used. The presence of such holes lead to an oscillation in the internal water surface, warranting investigation to understand the potential impact on structural integrity. In fact, such oscillations lead to significant air compression, possibly leading to structural damage if no adequate ventilation system is in place. Another aspect that hasn’t been well investigated is the water jet that arises from water entering the monopile through the replenishment hole, possibly leading to structural damage if the water jet velocity is high enough.
In the paper entitled “Jet velocities in water replenishment holes: A large-scale experiment”, the authors leverage large-scale experiments conducted in the Large Wave Current Flume (GWK+)of the Coastal Research Center in Hannover, Germany. The 0.57m monopile at a scale of 1/15.8 was subject to both regular as well as irregular waves with the aim of understanding how the water level inside the pile responds to the incident waves, and what the magnitude of the arising jet velocities are.
The study finds that the response amplitude operator (RAOH=HMP/HI), which describes the ratio of the wave height within the monopile relative to the incident wave, behaves similarly in the 1:15.8 and the 1:85 scale tests for both regular and irregular waves. In fact, it was demonstrated that the internal response is a function of the ratio of the monopile diameter to the hole diameter as well as the wave characteristics (i.e. the wave period and the wave height), without any measurable changes between the two scales. Additionally, it was shown that the water jet velocity follows a similar response amplitude to that of the internal wave height. Furthermore, the experimental results were used to verify a numerical model, showing errors of a maximum of 7.7% when compared to the measurements. Applying the findings to field scale leads to maximum peak jet velocities exceeding 9 m/s depending on wave conditions. Such high velocities need to be accounted for and require additional investigations on the internal structure of the monopile when repeated over the entire lifetime of the structure. In conclusion, this paper again shows the importance of conducting experiments on different length-scales to investigate the dependence of physical processes. Furthermore, it shows that numerical models, calibrated with experimental results, are a powerful tool that allows engineers to assess expected structural loads with high accuracy.
