Filter Units for Cable Stabilisation: The Effect of Fill Material and Volumetric Filling Ratio on Performance

Fiter units (sometimes called rock bags) are a very popular solution to improve the stability of subsea cables and cable protection systems (CPS) in the offshore wind industry. They are easily filled quay-side from local rock quarries and installed from vessels that are already mobilized as part of a windfarm construction spread. However, very limited data and research has been presented on the hydrodynamic properties and stability limit states for these mesh bags. No prior known literature describes the influence of filter material properties on their lateral resistance and stability, especially in the typical case where they are placed over a cable and are subjected to external loading from the cable or CPS. This paper presents results from a parametric study involving scaled physical model tests at scale factor of around 1:10. The tests explore the performance of the bags when filled with different materials to different levels of filling. The results show that bag material and filling level have strong influence on the filter unit and cable stability due to their strong influence on the filter bag weight distribution between the cable and the adjacent seabed. Empirical models for use in engineering design are proposed based on the experimental data.