Experimental study on the local scour around flexible submarine cables exposed to three-dimensional current loading

Abstract

The flexible cables are prone to scour-induced damage, posing risks to the safe operation of underwater structures. To investigate these impacts, this study carries out three-dimensional laboratory experiments on local scour around a flexible cable. In the experiments, the flow is unidirectional, and only clear-water scour is considered under a constant water depth of 0.45m. By varying the flow intensity (0.7, 0.8, and 0.9), flow incident angle (α = 60°, 75°, 90°), and cable diameter (D = 5 cm and 6 cm), the paper systematically investigates the mechanisms of the local scour, cable motion, strain, lateral scour development velocity, and quasi-equilibrium scour state. Due to the flexible nature of the cable, its motion differs from that of a rigid pipeline. There is a significant sagging during the scour process, which is quantified by the vertical displacement of the cable. Cable vibration is also observed, which is closely related to its natural frequency and varying test conditions. The deflection of the cable depends on the cable flexibility, test parameters, and the scour process. The result shows that the strain profile along the cable length typically exhibits a parabolic distribution. A point of discontinuity can be observed in the strain time-history curve, which signifies the completion of the lateral expansion of the span shoulder. Visual observations in the texts are consistent with mechanisms previously reported for seepage-induced pipeline-scour initiation, further supporting the applicability of this mechanism for flexible cable. An asymmetric scour pattern is prominent when the flow is oblique to the cable. A large incident angle leads to a more pronounced speed difference between the lateral scour development in two directions. Higher flow intensity results in an increased scour rate and deeper scour depth, as compared to lower flow conditions. As expected, a larger cable diameter yields a larger scour depth.