3.1 – Forensics for Marine Accident Investigation
When working on offshore power cables we may be spending our time looking inwards at the cable and the environment but we also by circumstance look outwards at the same time, capturing everything that is taking place in the locality. Ships are a regular occurrence of course and can be of interest to the cable owner due to the potential for damage from bottom trawl or anchoring. We profile the pattern of life and advise on risk.
We also capture information about benign movement on the surface and here there is a great forensic utility for marine accident investigation.
We are used to seeing vessels passing and their wakes incoming, but this pattern (1) caught our eye – a massive pulse in acoustic energy at 4AM that spanned 15km. Looking at our parallel AIS data (2) we can see there are two ships on a crossing trajectory. When we zoom (3) we see the characteristic wake signature with broadside null that cables generally experience, zooming in further (4) we can see the evidence of the wakes interacting over a few minutes.
DAS analysis places the vessels within 90m of each other, AIS tells us the ships are 4kT and 15kT – which should by maritime law keep apart 0.5 nautical miles – a reportable event.
DAS can fill in the gaps in the AIS record, giving a second by second plot of trajectory and position as well as providing evidence on the scale of any event potentially extending the utility of measurement and bringing benefit to asset owners.
3.2 – Identification of subsea cable repairs
When detecting and measuring electrical and mechanical anomalies it’s good practice to tie up measurements with what you expect to find on the seabed. In the case where you find two points of electrical degradation separated by a few hundred metres of quite differently behaving cable, it’s quite often the case that you are looking at a spliced in additional section of cable – but how to verify?
The same instruments we use to measure degradation will also give us the answers. Look at the example below – our EM Risk metric shows two peaks separated by about 250m – classic splice in repair – with one peak here behaving rather poorly compared to the other.
Lots of sound sources exist to help us here – passing boats or the waves themselves – our cables are acoustically directional with sensitivity perpendicular to the cable being much less than along the cable. As the cable turns in a classic “Omega” loop it changes direction by 90° twice – we expect to see a null, a peak and a null before normal service is resumed. As well as exhibiting very nice Lloyd’s mirror effects from the underwater acoustics, the traces can tell us a lot about the routing of cable on the seabed.
3.3 – Measurement of Wave Height
We posted last week on forensic analysis of ship near misses, this week we continue with a nautical theme and explore the Measurement of Wave Height
As we’ve shown clearly before, not only are the cables sensitive to sound propagating through the water, but they are sensitive to gravity waves on the sea surface. This is shown by the visibility of ships wakes travelling for 10’s of kilometers along the cable. At the same time, waves generated by wind are clear, and their propagation speed is seen to decrease with depth exactly as expected. What is the mechanism for DAS to be able to detect these waves, and how can this information be used?
The hydrostatic water pressure at the seabed is dependent on the depth of water at that location, and the pressure below the crest of a wave is therefore higher than below the trough. So, as waves pass over the cable, we expect an oscillation in the hydrostatic pressure around the cable. The magnitude of this pressure change depends on a number of well understood factors. The fibre changes length with change in pressure, so this pressure oscillation becomes visible as a signal using DAS. Because the sensitivity of the cable to pressure can be independently measured, the height of the waves can be calculated.
So now, we have a distributed wave height sensor along the whole length of the asset. This array is active all this time, measuring and recording the height and period of every wave along its whole length in real time and with great sensitivity. So, as well as identifying ships from their wake, this information can be used during operations work to monitor for deteriorating weather conditions, as well as studying the environmental conditions along vast tracts of the sea.
Imagine having a wave measuring device at every point in your wind farm.
