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 ... Read more

The European offshore wind industry has experienced significant growth in the past decade, mainly focusing on shallow areas in the North Sea to reduce the Levelised Cost of Electricity (LCoE) and compete with fossil fuels. However, as shallow areas become scarcer and the industry seeks greater independence from government subsidies, a shift towards deeper waters is anticipated, and already observed in Europe. In the northern part of the North Sea (60-120 meters deep), jacket foundations are currently favoured, despite drawbacks such as extensive engineering efforts, weld requirements, challenging series production, and high costs. This misalignment with the industry's LCoE reduction goal highlights the need for a technologically viable and economically attractive foundation concept for waters in the 60-120-meter range.

To combat this challenge, perforated monopiles are being developed. The perforated monopile consists of a monopile with perforations, either circular or elliptical, around the splash zone, with the goal of reducing the frontal area, and thus reducing the hydrodynamic loads on the structure. These concepts aim to combine the ease of manufacturing of a monopile, with the reduced area affected by hydrodynamic loads that are common for jacket structures. The research done so far on these perforated monopiles has only looked at the reduction in hydrodynamic loads, which have proven significant. These reductions in hydrodynamic loads should enable the perforated monopiles to be used in deeper waters compared to regular, non-perforated, monopiles. They could provide a tempting alternative for the more expensive jacket structures, but more research is necessary, especially in analyzing other loads that the perforated monopile may be subject to.

This thesis aims to look at one such different load that affects this perforated monopile, namely the installation loads induced by hammering. The first part of this thesis will look at stresses and fatigue damage during the installation of non-perforated monopiles. The second part will analyze the increased stresses, possible losses in hammer energy, and increased fatigue damage, all due to the presence of perforations. Finally, several alternatives, such as different geometries of perforations and different hammer loads will be analyzed with regard to their effect on fatigue damage.

The fatigue damage due to installation is found to increase significantly due to the presence of perforations, increasing from 5% for non-perforated monopiles, to up to 118% and 112% for the two most promising geometries analyzed, thus proving a show-stopper for installation via impact hammer, if no measures are taken.

Changing certain parameters, however, either the geometries of the perforations, or the characteristics of the hammer used, shows that installation is indeed possible. Using different geometries of perforations, that maintain a significant reduction in area, shows installation is possible, whilst limiting the fatigue damage to 53%. A reduction in hammer force by a factor of 2, also decreases the fatigue damage by 34% on average. The use of a so-called vibro-hammer also shows promising, resulting in a halving of the fatigue damage compared to the use of an impact hammer, but more research needs to be done to confirm this final finding.

To conclude, this research shows that installation of a perforated monopile is possible, although most, if not all of the reduction in fatigue damage due to hydrodynamic loading is cancelled out by the increase in fatigue damage due to installation. Geometries and installation methods may exist that improve the fatigue life of the structure, but this research was unable to find them. Future research may be able to find geometries and installation loads that do reduce overall fatigue damage.

Further research is also necessary before perforated monopiles can be taken into service, such as the confirmation of the energy losses in installation due to perforations. Also, several other load cases need to be analyzed, to ensure the perforated monopile survives its designed lifetime. ... Read more

To meet global green energy targets, the bottom founded offshore wind industry is looking for ways to economically expand markets to deeper waters. A reduction of the hydrodynamic load is necessary to achieve this. One option is to perforate the monopile around the splash zone. Here the related work of Q. Star is continued through the implementation of a 2D Navier-Stokes based LES CFD model with VMS closure in Gridap.jl. The numerical simulations are gathered to create a dataset on which a machine learning surrogate model is trained. The analysis does not include secondary design aspects such as manufacturing, installation, noise mitigation, scour, corrosion, acidification, or marine growth. Neither does it require secondary fluid phenomena such as 3D simulations, FSI, VIV, and fatigue.
Analysis of 2D perforated cylinders, built up parametrically using the wall thickness, angle of attack, diameter, inflow velocity, number of perforations and porosity as input parameters shows that the first two are of negligible influence, and number three and four can, at least for their mean values, be modelled accurately using Morison equations. A non-scaled diameter does prove important in reducing the random nature of frequency-based effects. The number of perforations and the porosity provide complex interactions both from a design-force mean and variability perspective, as well as for the frequencies and vibrations they generate. A 2D LES-VMS model gives the perfect trade-off between cost and accuracy, predicting a possible drag reduction of more than 50% compared to a closed cylinder with large diameter.
Different machine learning surrogate models are analysed with the goal of massively speeding up the analysis in the future. This is achieved by a factor of 350 thousand using Random Forests, Gaussian Processes and Neural Networks. Although the Gaussian Processes preliminary show the best accuracy, below 6% error for millisecond predictions, further work on Neural Networks could give them the advantage in future analyses. ... Read more