CFD Modelling of Chain Induced Sediment Escape Out of Seabed Trenches
Hydrodynamic Analysis on Trench Evolvement for Floating Wind Mooring Systems
T.M. Slob (TU Delft - Mechanical Engineering)
R.L.J. Helmons – Mentor (TU Delft - Mechanical Engineering)
G.H. Keetels – Graduation committee member (TU Delft - Mechanical Engineering)
P, Triantafyllos – Graduation committee member
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Abstract
Mooring chains of floating offshore wind turbines generate cyclic seabed loading that excavates trenches and may transport sediment out of the trench. This thesis is the first to investigate the physical conditions under which such sediment escape ceases, using a Computational Fluid Dynamics (CFD) model that couples a Boussinesq-type incompressible flow solver with an immersed boundary method and suspended-sediment transport. Forty simulations were conducted across four trench depths (3−9 m), five grain sizes (0.0625−1 mm), and two inflow regimes (0.05-0.36 m/s) representing calm and storm conditions in the North Sea.
A new Escape Potential Index (EPI) was introduced primarily as a simulation prioritisation tool to identify edge cases and minimise the number of non-informative runs. Classical predictors such as the Rouse number were insufficient for this purpose as they do not include confinement or chain-induced uplift. The EPI therefore combines settling behaviour, chain forcing, and trench geometry into a single heuristic ranking. Although not a universal threshold, EPI correctly orders cases by mobility and, when combined with the Rouse number, defines a separation curve that distinguishes escaping from stable configurations and explains why classical suspension theory under predicts escape under cyclic chain forcing.
The results show that trench depth is the dominant control on sediment mobility. Shallow trenches strongly couple chain motion to the flow, producing intense suspension even under weak currents, while deeper trenches reduce uplift through geometric confinement and increased travel distance. A stabilisation threshold emerges: beyond approximately 5−7 m depth, all but the finest sands remain trapped. Grain size dependence follows classical settling behaviour, with fine material escaping across all depths and coarse fractions remaining largely immobile. When the depth–grain size limits are compared with North Sea bathymetry, extensive central and northern deep water areas appear suitable for floating concepts.
The study provides a mechanistic basis for predicting when sediment escape ceases, though limitations such as the rigid seabed, simplified chain geometry, and steady-state inflow imply that results represent upper bound mobility estimates rather than full morphodynamic evolution.