2.1 – The response of cables to the pressure of tidal flow
Exposure is a big topic for subsea cables, and one of the mechanisms which can de-bury cables is erosion of the sediment by tidal streams. The streams tend to wander in an unpredictable way, and it is important to track this movement over time. We know that the fibres within a power cable are incredibly sensitive to strain, and the pressure that the cable experiences due to changes in the surrounding environment cause changes in strain on the fibre which can be measured. This can still be done on a buried cable which provides an indicator of things to come.
Identifying a tidal stream in an exposed cable is easy – look during the flow and you’ll see noise – job done. In buried sections it’s harder. Careful analysis of DAS data from long recordings from fully buried cables reveals low frequency signals showing tidal related patterns on subsea sections emerging from the noise. DTS analysis shows no equivalent temperature changes in these regions, and we can therefore conclude the cause to be strain. The origin of this strain is from the changes in water pressure as water flows around bathymetric features in the seascape. The image is showing the resultant strain along 2km of fibre where one clear location shows a positive on the ebb tide and negative on the flood tide (the fibre’s core response to increasing pressure is negative). These measurable signals allow tracking of the streams over time, including the sudden changes that may occur in the aftermath of a storm. Not only can the potential erosion location be tracked, but the DAS data can reveal if and when the cable actually becomes exposed. The second image shows an explanation of the bathymetry and what is being affected by the tidal stream.
2.2 – Wind Turbine Structural Health Monitoring
Although we are focussed on cable health monitoring, our Indeximation approach allows us to retain long term data, without losing useful information. This presents opportunities for further analysis and data re-use for lots of additional purposes. For example, Wave height measurement or Marine accident investigation.
Where the fibre runs through a wind farm, either via the export or inter-array cables, and passes close to or through wind turbines we can take a detailed look at their acoustic properties. The spectrograms below show frequencies below 25 Hz and are taken from an export cable where the fibre passes close to two different wind turbines. The blue line shows the measured power over the period. We expect to see frequencies related to the vibrational modes of the wind turbine towers, which should be relatively stable with time; the strong signals at around 4 Hz in the images below are probably examples of this. Changes or anomalies in these signals could be indicators of structural problems or loose bolts (as explored in this paper – https://link.springer.com/article/10.1007/s13349-021-00483-y). In addition, we can see frequencies that vary with the measured power (image 3), which are probably related to the gearboxes in the turbines.
As we are recording continuously, we build up a huge data store that can either inform on any potential developing problems or allow retrospective analysis after problems occur to give further insight into the causes of failure.
2.2 – Should I choose Quant or Qualitative DAS?
In our work we often get asked by customers about the two different types of DAS unit commonly found and which they should use. As it’s our tool of choice for extracting info on the condition of cables, we thought it was time to put together some of our thoughts on the topic.
You can read the complete long-form article here.
