Dashed Against the Rocks - Comparison of Distributed Acoustic Sensing (DAS) Data and Physical Response Models of Subsea Cables

Abstract Distributed Acoustic Sensing (DAS) is a novel technique for measuring strain in a Fibre Optic Cable (FOC) using laser light that is finding new applications in the subsea cable industry. Subsea power cables are multi-component unbonded structures where the components are wound and bound together mechanically. They frequently contain a steel tube containing fibre optic cores, which are able to be used for DAS measurements. DAS may be able to provide real-time monitoring of subsea cable motion to enable improved cable reliability and estimates of remaining lifetime. For DAS to be used to quantitatively determine cable movement, it is critical for the measured signals to accurately reflect the physics of subsea cable motion. DAS data was collected during a one-hour Remotely Operated Vehicle (ROV) survey of a subsea cable in the MeyGen Tidal Stream Energy Project, Scotland in 2022. During the survey the ROV was made to bump into the cable around 100 times to investigate the resulting DAS strain response. The analysis reported in this paper aimed to find relationship(s) between the physical properties of the cable and the DAS strain response after each ROV bump. The analysis has found evidence of sound propagating through the copper power cores of the cable in the DAS strain data. Physical models of cable vibration were also considered, with evidence suggesting that the measured DAS response is more likely to be axial vibration of a helically-wound conductor (like a coiled spring) rather than a direct measure of the overall cable bundle response. The conclusion is that the DAS signal cannot be reliably treated as a direct strain measurement of the overall cable response – instead it is critical to consider the cable as a composite complex structure where the measured response could be from multiple elements within the bundle. The analysis suggests that cable axial strain responses are able to be identified, whilst responses attributable to cable bending have not been identified in the data. These outcomes have implications for the relevance and benefits of DAS in real-time monitoring of cable integrity, especially where the primary response mode of concern is cable bending under externally-applied loads.