Abstract

A simultaneous Distributed Acoustic Sensing (DAS) and Distributed Temperature Sensing (DTS) system is presented. In DAS, perturbations across hundreds of metres of fibre are detected by the use of phase-sensitive Optical Time-Domain Reflectometry (OTDR) from Rayleigh backscattering caused by these perturbations. DTS relies on Raman OTDR in a similar fashion. The capabilities of the DAS system are presented, showing detection limits on the order of sub Hz to kHz while achieving spatial resolution on the order of metres. The ability to differentiate between different types of perturbation including tapping, transient signals and smooth, strumming motion is demonstrated.

Introduction

With continued growth of the offshore renewable energy sector, protection of the offshore infrastructure becomes increasingly important. A key area for this is cable monitoring, whereby damage can cost operators millions of pounds in downtime, fault location and repair.

Recent advances in floating platforms mean more dynamic cables are being deployed. It is costly to carry out cable surveys using remotely operated vehicles (ROVs)'s and even more costly to locate and repair subsea power cables (Wang, Er and Zhu, 2020; Li et al., 2020; Rao et al., 2021). The optical communications fibre commonly incorporated into power cables can be used to monitor temperature, acoustic signatures or even strain and there are commercial systems available to do this over ranges of up to 100km (Sinnett et al., 2020; Malakzadeh, Pashaie and Mansoursamaei, 2020). In the future, a larger scale project with appropriate hardware could extend the system we describe here to such ranges.

Distributed fiber optic sensing is attractive as it can, in some cases, be retrofitted to already installed fiber, and therefore can be lower cost than installing dedicated in-situ monitoring. A further benefit is that the interrogator is easily maintained at its on-shore location. The technique also offers the ability to monitor the entire fiber length at once for continuous monitoring (Williams et al., 2019). To date there has been little quantitative work on distributed sensing for cable motion sensing. We report results of an initial investigation into the use of distributed acoustic sensing (DAS) to monitor a dynamic cable with potential applications for marine power cables, mooring lines, ROV tethers and umbilicals. Using a controlled frequency generation system, the response to different types of cable perturbation is explored. The novel demonstrator is a multifunction distributed temperature and acoustic sensor and works by detecting the Raman and Rayleigh scattering components of incident light inside the fiber, which are influenced by environmental disturbances. Using time-of-flight, the sensing system can infer where the disturbance occurred along the fiber to within 10m accuracy. DAS measurements alone are capable of providing data on movements of the fiber under test and so can be useful in detecting different types of motion of installed cabling e.g., that induced by offshore currents and wave motion, cable strumming or external interactions. High sensitivity to vibrations has also been observed, presenting possibilities of frequency monitoring on the order of sub Hz to kHz. This frequency range can be used to pick out specific disturbances, including different types of boat engines. With a quantitative characterisation of DAS response to a wide range of mechanical perturbations we can provide a sensing system that is capable of classifying a number of cable motion types and provide insight into cable protection applications in increasingly active subsea environments (Wang, Lu, Ye and Cai, 2020).

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