Influence of Bag Shapes on the Hydrodynamics of Subsea Filter Unit in Power Cable Protection

Filter units (FUs), which resemble cylindrical mesh bags filled with rock aggregates, are commonly used for scour protection and secondary stabilization of subsea infrastructure, such as subsea power cables in offshore wind farms. A key consideration in the stability design of the FUs is the assessment of hydrodynamic drag, inertia, and lift forces acting on the FUs. These forces’ magnitude is expected to be influenced by the as-laid geometric configuration of the FUs. This study explores a range of bag configurations to gain insights into the hydrodynamics of FU bags, which include (i) a configuration derived from field observations, (ii) a configuration generated from a 3D scan of a model-scale FU used in laboratory testing, an idealized hemisphere configuration and (iv) an idealized configuration of an ice hockey puck. The study also examines various flow conditions, including steady flow and oscillatory flow induced by surface waves. To enable a meaningful comparison, the same volume of the FUs with different configurations was used in the analysis. The findings indicate that the hemispherical FU experiences the least drag forces and is much less likely to roll under steady flow conditions. These results contribute novel insights into the understanding of hydrodynamic forces acting on FUs. It is expected that this understanding will enhance the stability design of FUs for various metocean conditions, consequently reducing the risks of subsea power cable damage resulting from cable-FU interactions.