F.P. de Wit1*, E. Bijl2, S. Bom1, Y.B. Steenman1, L. Baelus3, K. Van Doorslaer4, P.Y. Guillermin5
1 Svašek Hydraulics, The Netherlands; 2 CDR International, The Netherlands; 3 IMDC, Belgium; 4 DEME nv, Belgium; 5 Elia, Belgium
* Corresponding author: wit@svasek.com
Introduction
The production of wind energy necessitates the transportation of energy to the mainland via cables. The power generated by the wind turbines is commonly converted at nearby offshore platforms before being transported ashore. However, due to the recent growth of wind farms, the construction of an energy island serves an economically viable alternative, providing a much larger capacity. In 2024, the construction of the world’s first energy island commenced: the Princess Elisabeth Island in the Belgian coastal zone.
Even though 40 kilometers offshore, wave and current conditions can be classified as coastal, given the strong tidal currents and water depth of 20 meters. One of the challenges is the large structure being fully exposed to waves and currents. To ensure the island’s stability, proper scour protection is required. Because of the novelty of this project, no standard formula for scour protection rock is available. Therefore, a combination of numerical models, a stability formula and laboratory testing is applied.
Objective and Methods
This study aims to spatially predict the required rock diameter and stability of the scour protection. This is done by applying a rock stability formula (van Rijn, 2019) in which wave- and current-induced bed-shear stresses are provided by two state-of-the-art numerical models.
Near-bed wave-induced bed shear stresses are predicted using the nonhydrostatic wave model SWASH (Zijlema et al,. 2011). This model is capable of simulating highly nonlinear storm waves, standing wave patterns and diffraction around the sharp island edges. The flow induced bed shear stresses are predicted using the fully 3D CFD model TUDflow3d (de Wit, 2015). This model resolves the turbulent eddies and wake zones that arise when the undisturbed tidal flow is obstructed by the energy island. A range of flow and wave conditions are considered to obtain governing bed shear stresses around the island.
In addition, the stability of the rock scour protection has been tested with 3D laboratory wave and current experiments. These have been conducted by TM Edison, a joint venture from the Belgian marine contractors DEME and Jan De Nul, at the laboratory facilities of DHI in Denmark (van Doorslaer et al., 2024).
Results
The rock stability formula with substitution of model results is applied in two stages. In the first stage, an estimate of the required rock size diameter is determined. In the second stage, the formula is applied inversely which gives the dimensionless movement factor r as a function of the rock size diameter for a certain rock class (r = 0.4 for occasional movement at some locations and r = 1.0 for frequent movement at nearly all locations). For conditions with a return period of 100 years, the left panel of the figure visualizes the movement factor r. The right panel of the figure shows a picture of the laboratory experiments. For a range of return periods and storm directions, the laboratory experiments were conducted. After each of these tests, the movement was quantified. At the NCK days the numerically predicted movement will be compared against the observations from the laboratory experiments. It should be noted that both numerical and laboratory results are from a preliminary phase of the project.
Numerically predicted movement factor r under combined wave and current loads (left). Picture of the 3D laboratory experiments used to validate the numerical predictions (right).
References
de Wit, L. (2015). 3D CFD modelling of overflow dredging plumes. Phd thesis, Delft University of Technology, Delft, The Netherlands.
van Doorslaer, K., Vasarmidis, P., van Olst, B., van Koelen, L., Bijl, E., Weygers, M., Guillermin, P.-Y., Streicher, M., and Dixen, M. (2024). Physical modelling for the princess elisabeth island, overtopping design for a caisson with a double wave wall geometry. In Coastal Engineering Proceedings.
Van Rijn, L. (2019). Critical movement of large rocks in currents and waves. International Journal of Sediment Research, 34(4):387–398.
Zijlema, M., Stelling, G., and Smit, P. (2011). Swash: An operational public domain code for simulating wave fields and rapidly varied flows in coastal waters. Coastal Engineering, 58(10):992–1012.
