Anyone who has spent time close (at a safe distance, of course) to active power lines will recognise the characteristic hum of energy being distributed to local homes and businesses. This continuous audible electromagnetic signal, with a frequency of 50 Hz in Europe and 60 Hz in the US, is caused by the periodic reversal of the alternating current (AC) when the cable is carrying load. Although you are less likely to hear it directly, sub-sea power cables also produce these strong electromagnetic signals along their lengths which can spread out into the surrounding water. This may be irritating and disorientating to certain marine creatures and, as such, should be limited as much as possible. But these electromagnetic signals do have certain benefits for those of us that are interested in cable health monitoring. Specifically, thanks to one of our innovative patents and the sensitivity of modern Distributed Acoustic Sensing (DAS) Interrogator Units, Indeximate is able to detect these electromagnetic signals and use them to determine changes in the integrity of the cable armour that protects these vital assets from the hostile environment in which they lie.
Our Patented Method
When a cable is in good condition, the electromagnetic forces within its core are perfectly balanced by the constrictive effects of the helical armour, keeping everything nicely cylindrically symmetrical and contained. The forces are understood and possible to calculate, so the strains measured on the fibre can be related to the stiffness of the cable structure. With a perfect cable, homogenous ground conditions and cable tension, this response should be consistent both along the cable and through time. In reality we see variations that are due to a range of factors (including changes in AC current and burial depth), which need to be understood and interpreted to assess the health of the cable. Overall a very complicated picture.
Thankfully, though, using our proprietary algorithms, Indeximate are able to work with these inherently complex long-term datasets to extract valuable signatures to understand the cable armour integrity every 10 m through time. Crucially, this allows us to identify regions of cable that may need remedial actions before these areas become critically damaged. This helps cable owners to optimise their uptime, reduce repair costs, and lower the levelised cost of energy from their generation equipment.
Figure 1: Indeximate patent for delivering cable integrity from AC current flow fibre response
To illustrate some evidence of the behaviour of the cable as a result of applied electrical signal, we plot an example of a 50 Hz electromagnetic signal through time along an export cable in Figure 2. This cable contains two distinct cable types along its length, which we here refer to as Cable 1 and Cable 2, connected by a join running down the centre of the image. Clearly the base signal in the two cable types (the left and right portions of the image) is different (denoted by the different background colours, red is a bigger signal than teal). However, the overall trends are the same and the dominant temporal behaviour is the electromagnetic signal increasing and decreasing with the current running along the cable.
Figure 2: 50 Hz signal measured using DAS along two distinct sections of cable connected by a joint half way along the x-axis. The two cable designs are easily differentiated at the join. The horizontal stripes correspond to increasing and decreasing electromagnetic signatures correlated to the increasing and decreasing current load. The locations of these are consistent in both cable designs.
Joining the dots – A window into join health
In addition to their ability to reveal insights about the integrity of the armour of power cables themselves, these electromagnetic signals are also a powerful tool for examining one of the main problem areas of any sub-sea asset – the joints. At these locations, two distinct cables with different magnetic properties are connected together, whether out of design, due to differing load capacity requirements in different locations for example, or due to a repair that has been forced upon the cable operators after a failure. In both of these cases, the power lines and optical fibres contained within the overall cable structure are no longer enclosed within the armour that runs along the length of the rest of the cable at the joint location, meaning a different way to protect them is required. The current industry standard solution is that cable joints are enclosed within a large metal structure, that requires incredible skills to engineer and which can reach sizes comparable to a military torpedo.
Whilst this heavy submerged tube protects the essential internal cable infrastructure, its metallic nature also acts to enhance the electromagnetic signals that Indeximate uses to investigate cable integrity. What this means is that joint locations become immediately evident at electrically related frequencies measured along the cables. Measuring how the 50 Hz and related harmonics change through time around joints allows us to investigate the stability of these features in the unforgiving underwater domain. In Figure 3, we plot an example of the 50 Hz signal around a joint at two different times separated by around one year. The joint itself is clearly visible in the centre of the x-axis, where increased electromagnetic signals are evident. The strength and shape of these signals increases between the earlier and later measurement indicating the condition of the joint is changing through time.
Figure 3: Averaged 50 Hz signal measured along 4 km of cable around a joint at two times as observed using DAS data. The joint itself is clearly identified by the region of increased 50 Hz signal. The differences between the two profiles (in both amplitude and shape) suggest that the location is not entirely stable and should be monitored more regularly in the future to check for signs of degradation
In terms of stability, the weight and size of the metal casings at joint locations means they are particularly susceptible to submergence into the seabed, especially if scour is present, which can happen unevenly along the length of the join. If this is the case, the angle of the cables at the exit from the join may change leading to increased bending and a lowering of the overall cable armour integrity at those locations, which in turn can be highlighted by Indeximate through our analysis of DAS data.
For stakeholders that may be concerned about a cable or a joint, it is important to keep in mind that getting measurements of these electromagnetic signals as early as possible is highly beneficial. This provides a benchmark against which changes through time can be assessed. Continuous monitoring is also preferred as this allows us to differentiate between sudden changes (e.g. a sinking of a join during a storm surge) and gradual degredation (e.g. a gentle scour occurring each tidal cycle).
If you want to learn more, feel free to reach out at: https://indeximate.com/contact-us/
