This thesis investigates local and global scour processes around four-legged jacket (4LJ) foundations
for offshore wind turbines under current-only conditions using the 3D CFD Large Eddy Simulation (LES) model TUDflow3D. Jacket foundations, while increasingly deployed in deeper offshore environments, remain less studied than monopiles on local scour and scale effects and introduce new scour patterns like global scour. The research addresses this gap by evaluating the predictive performance of TUDflow3D against laboratory experiments, extending simulations to field scale, and assessing the applicability of monopile-based empirical scour relations to jacket configurations.

Laboratory-scale numerical simulations replicate the experimental study of Welzel et al. (2023) for both clear-water and live-bed regimes, validating the model for a 4LJ foundation. Model performance is assessed through morphodynamic comparison and time-averaged observation of the hydrodynamics, supplemented by a sensitivity analysis on numerical parameters: morphological acceleration factor, relaxation factors, numerical domain and grid resolution.

The validated model is then scaled to field conditions using a mobility similarity approach, enabling
investigation of scale effects on scour magnitude, spatial extent, and equilibrium timescales.
After the scaling up, both models are analyzed in order to enhance the knowledge that is known until now from 4LJ foundations.

For local scour, the spatial extent of scour was determined by adapting a formulation for monopiles and quantified throughout the entire scour evolution, revealing a consistent footprint over time. Scaling to field conditions demonstrated that the upstream–downstream scour pattern persists, but with reduced magnitudes in local scour depth due to scale effects, observed by the reduction of time-averaged bed shear stresses around the legs. This reduction in magnitude was approximately 30%–35% when compared in terms of dimensionless scour for both regimes and for both upstream and downstream piles. Timescale analysis showed that scour equilibrium occurs later at locations farther from the piles. When scaling, it was also found that scour on clear-water regimes
require months while live-bed regimes is in the order of magnitude of days to reach local scour equilibrium.

For global scour, live-bed conditions produced a footprint extending up to twice the jacket footprint
radius with rapid initial development, while clear-water conditions yielded slower, more confined scour. The order of magnitude for the global scour is variable depending on the location, but it reached values around 0.9 - 1 D on the center of the jacket and downstream of it.

Finally, empirical monopile scour formulas were evaluated, showing that they still provide reasonable order-of-magnitude estimates for upstream piles and capturing scale effects, as well it was proved that the scaling up from laboratory scale to field scale developed for monopiles can be useful also for 4LJ foundations.

The study demonstrates that CFD LES modeling is a robust and flexible tool for detailed, process based scour prediction around complex foundation geometries. Beyond validation, its ability to simulate field-scale morphodynamics and provide high-resolution temporal and spatial data makes it a valuable complement to laboratory testing, ... Read more

The Eastern Scheldt storm surge barrier (ES-SSB) is the largest hydraulic structure in the Netherlands. Its semi-open inlets allow for North Sea waters to enter and leave the Eastern Scheldt estuary with each tidal cycle, and can be closed during extreme storm events. The flow through the barrier is strongly contracted, and complex flow patterns emerge. Among characteristic flow features are the shallow jet and shallow mixing layer, generated as a result of large transverse shear stresses with horizontal lengths scales greatly exceeding the water depth. Large mean velocities in combination with the developing lateral non-uniformity of the flow between slack water and maximum flood gives rise to higher bed shear stresses. A bed protection up to a distance of about 600 m from the barrier is applied to stabilize the bed against increased hydraulic loading. Scour hole development adjacent to the applied bed protection was anticipated for, but expected equilibrium depths have not yet been reached. Reaching local depths of 60 m with respect to the water surface, these scour holes may on the long term be a threat to the stability of the barrier. Broekema (2020) concluded that during flow contraction, separation of flow near the bed of the scour hole is suppressed and high flow velocities in streamwise direction are found near the bed. Cyclic variations in lateral non-uniformity affect turbulence intensities and subsequent mixing of mass and momentum. Scour growth is therefore enhanced in two ways: i) velocities in the main flow remain high due to horizontal flow convergence, and ii) lateral velocity gradients are associated with larger turbulence intensities that are likely leading to larger bed shear stresses... ... Read more