Online acoustic monitoring and localisation of partial discharges within subsea umbilicals

Abstract

There has been an increasing requirement in the quantity of subsea power cables within both the oil and gas and the energy transmission markets. Both sectors are pushing their current operating boundaries, putting more emphasis on the criticality of the components integrity to ensure they are able to meet their lifetime requirements, or in some cases allow for lifetime extension. Failures within subsea power cables are uncommon, however when they do occur, they lead to excessive production downtime and costs. Having the ability to monitor the condition of the cables for potential failures, whilst the system remains online, would be highly beneficial as these failure costs could be eliminated. The condition of a cables insulating layer is key to its operational lifetime and levels of partial discharge (PD) occurrence can provide an indication of its integrity and of any likely upcoming failures. A system which would allow for detection and localisation of PD occurrence would enable potential failures to be reviewed and an assessment made prior to complete breakdown, thus allowing for changes in operation or planned maintenance to remove the large risks and costs associated with unplanned production downtime. This thesis examines methods for PD detection and localisation within a subsea power cable, whilst considering the many challenges associated with the environment and application of the technology. State-of-the-art technology is reviewed along with an assessment of the market requirements and current gaps in PD evaluation systems. Existing technologies focus on offline testing techniques whereby testing is only performed once a failure has occurred or on occasions during planned maintenance, however, having a system which can operate whilst the cable remains online is vital in order to have an understanding of the condition of the cables and in turn reduce or remove downtime costs. When PD occurs, a number of emissions are produced such as electromagnetic, optical, acoustic, thermal, and chemical, meaning a variety of sensing methodologies could be used. Acoustic sensing techniques are the focus of this work as this allows most major challenges to be addressed, with particular emphasis on the ability for online monitoring as well as a retrofit design. Acoustic techniques offer a greater potential for accurate localisation of PD in comparison to electromagnetic techniques due to the low velocity of sound wave propagation. Acoustic methods also enable both through-cable and through-water sensing of the discharges, thus both areas are explored. Experimental through-cable testing validates the methodology for both detection and localisation of the PD occurrence. Signal characteristics are explored, determining a broadband result, with the main signal energy being below 5kHz and a signal velocity of around 2442m/s. Time difference of arrival (TDoA) techniques are evaluated, with cross correlation methods showing favorable results in low signal to noise ratio (SNR) environments, providing a position estimation within 2m of the PD location. Comparative testing with existing electrical detection technology provides an assessment of the severity of PD which can be detected acoustically, with average discharges measured at 25.5pC peak PD and an average of 837pC/cycle for PD activity. Through-water experimental testing confirms the ability for PD localisation in an underwater environment. The acoustic signal characteristics are broadband, spanning the measured 22kHz frequency spectrum and are of short duration. The source level of the PD acoustic emission is established to assess the detectability range in varying subsea environments, with a maximum range of 1000m when considering a low noise spectral density level of 45dB. As the time of PD occurrence is unknown, standard time of arrival techniques cannot be used and thus hyperbolic positioning is implemented within the developed localisation algorithm to estimate the position of the PD source. With consideration of the receiver geometry, a highly accurate PD location can be established enabling planned intervention of the fault region and therefore preventing retrieval and replacement of the complete power cable length.