Wave Run-Up on Monopiles

Abstract

The rapid expansion of offshore wind energy, especially into intermediate depths and more non-linear wave environments, has increased the need for accurate prediction of wave-structure interactions, particularly wave run-up and the associated hydrodynamic forces on monopile foundations. These predictions are essential for ensuring the structural integrity, safety, and cost-efficiency of offshore wind turbines. Existing semi-empirical formulas are limited in their applicability, often overestimating run-up heights and failing to capture the complex non-linear interactions that occur under steep (So > 0.03) or near-breaking wave conditions typical of intermediate water depths.

This thesis presents a numerical model based on Computational Fluid Dynamics (CFD), developed using OpenFOAM and the waves2Foam toolbox, to predict wave run-up and the associated hydrodynamic forces on monopiles under varying hydrodynamic conditions. The model was validated against experimental data from de Vos et al. (2007), achieving excellent agreement with less than 5% error in peak wave run-up predictions. Following validation, the model was applied to assess the influence of scale effects, turbulence, monopile geometry, and key wave characteristics on run-up behavior and force magnitude.

The results confirm that scale effects on surface elevation and wave run-up are limited, while hydrodynamic forces show some sensitivity, particularly viscous forces in the vertical direction. Steep, high-energy waves near the breaking thresholds (So ≈ 0.04 to 0.05) produce significantly larger run-up heights and slamming forces, underlining their importance in design considerations. The geometry of the monopile also impacts localized flow patterns and wave wrapping, while turbulence contributes to increased lateral and viscous forces. However, its effect on the critical run-up at the front of the structure remains minimal.

By integrating turbulence modeling, scaling, and mesh optimization, this study establishes a robust, scalable, and accurate numerical framework for simulating complex wave-structure interactions. The model demonstrates significant improvements over existing empirical approaches, especially in intermediate water depths where non-linearity, shoaling, and breaking are present.

While the model shows excellent predictive capabilities, areas for further improvement remain. These include local mesh refinement around the monopile, higher-order discretization schemes to better capture breaking waves, and more extensive validation with recent experimental datasets. However, these enhancements would come at significant computational cost. Given the already high level of accuracy (>95%) in capturing peak wave run-up, the current model offers a practical balance between precision and efficiency. Future efforts should focus more on applying the model to irregular wave conditions and additional hydrodynamic parameters rather than fine-tuning. Researchers with greater computational resources may explore these improvements but should weigh the trade-off between added accuracy and computational expense.

The model configurations are publicly available to support future research and industry adoption: https://github.com/hiddded/MSc-thesis-wave-run-up-on-monopiles