Empirical research for the hydraulic stability of rock-filled mesh bags in offshore application for the stabilisation of cable protection systems

Abstract

The expanding offshore wind industry is facing challenges with excessive movement of the power cables that connect offshore turbines to the grid. The casings that surround the exposed parts of cables near structures, known as cable protection systems (CPS), show damage before the end of their expected service life, requiring costly repair and maintenance. A potential solution is to stabilise the CPS by placing one or more rock-filled mesh bag (rock bag) on top of it. There are multiple rock bag manufacturers and distributors. The bags are made of polyester netting that is relatively flexible. The sizes considered for this study are 4 and 8 ton (4000 and 8000 kg), but smaller and larger sizes are available. Currently, little is known about the behaviour and stability of these rock bags in offshore environments. The goal of this study is to expand the knowledge through empirical model testing.
480 experiments were conducted in a flume in the hydraulic engineering laboratory of Delft University of Technology. Two set-ups were used, one with a flat rough bed, to test two or three rock bags side by side and grouped formations of bags. The other is a realistic halved model of a monopile, scour protection and CPS. Regular wave, irregular wave and combined wave-current conditions have been tested.
Analysis of before and after top images of the tests is used to establish a failure criterion. In this way, a stability limit applicable for irregular wave fields is found. The effects of grouping and near monopile flow amplification are quantified.