4.1 – Subsea power cable temperature from multiple sources
This week in our series of posts from the end of a cable, we want to highlight the importance and utility of temperature analysis in the case of a power cable crossing other cables & pipelines.
DTS (1) is well established as a tool for identifying hot spots and monitoring thermal ratings. Calibration and attenuation issues can obscure some detail, however when considered as a gradient rather than an absolute and compared with its DAS sibling (DTGS) the picture becomes richer and calibration issues can be resolved.
The same DTS plotted as a delta (2) from a one-week period after day zero shows a temperature rise in-line with wind related production and on a vertical scale of 1km shows a clear disturbed region around a cable & pipeline crossing –often treated with mattressing or rock dumping. Plotted as rate of change (3) it shows two strong heating periods with a peak heating rate of 0.1K per 30min and the crossings become very clear.
DAS (in its DTGS guise) adds further nuance (4) here the scale is in ±0.1mK/s! The pictures are broadly similar but if we zoom in time (5) we see very nuanced heating and cooling. Finally if we integrate the DTGS to pull out TOTAL change we recover a perspective similar to the DTS delta plot but with added fine detail of fine scale environment change on top of the forced heating (6).
4.2 – Exploring the response to power
We’ve previously shown how the thermal response of subsea cables reveals important information about the properties and condition of the cables. Due to its remarkable sensitivity to temperature, DAS provides valuable insight into heating from wind power generation. The Distributed Temperature Gradient Sensing (DTGS: heating rate – not absolute temperature) images (1 and 2) show how the thermal response to current varies in two different cable sections. Cable 2 shows a retarded heating cycle due to increased insulation around the fibre. The blue overlay shows the measured power generation.
Plotting each section as a time series (3) verifies that the output closely follows the measured active power from the wind farm when generating.
We also look at the electro-mechanical response at every point along the cable (4). As expected, we see a very clear relationship to generated power. This electro-mechanical response is however also affected by cable integrity, we use this to derive weakness metrics at every point along the cable, something we will explore further in the next post.
4.3 – Introducing the EM metrics
We introduced above how the electro-mechanical response of a cable to applied electrical power reveals key information about the condition and properties of the cable.
We derive two indicators from the acoustic response, each of which reveals different information about integrity. The first indicator (EM1) is related to the mechanical properties of the cable, such as weakness in the armour or deformation. The second indicator (EM2) is a direct result of the cable response to applied current. Image (1) illustrates EM1 from an export cable and shows how it matches over time with measured power (overlaid blue line). There are two cable types present and the difference between them is very clear.
From this response we can derive a metric to indicate areas of weakness or damage. In this case (a healthy cable) for EM1 we see that the performance is relatively consistent (2). EM2, however, shows some clear anomalous locations (3).
EM2 is sensitive to features such as cable and gas pipeline crossings and most of the features we see in the images below can be explained by benign factors. This isn’t always the case, however and we’ll see in our next post what can happen when deterioration arises and how we identify the location of poorly performing sections of cable.
4.4 – Identifying and monitoring suspect sections of cable
Back on the 8th August we introduced how the electro-mechanical response of a cable to applied electrical power reveals key information about the condition and properties of the cable.
We derive two indicators from the acoustic response, each of which reveals different information about integrity. The first indicator (EM1) is related to the mechanical properties of the cable, such as weakness in the armour or deformation. The second indicator (EM2) is a direct result of the cable response to applied current. Image (1) illustrates EM1 from an export cable and shows how it matches over time with measured power (overlaid blue line). There are two cable types present and the difference between them is very clear.
From this response we can derive a metric to indicate areas of weakness or damage. In this case (a healthy cable) for EM1 we see that the performance is relatively consistent (2). EM2, however, shows some clear anomalous locations (3).
EM2 is sensitive to features such as cable and gas pipeline crossings and most of the features we see in the images below can be explained by benign factors. This isn’t always the case, however and we’ll see in our next post what can happen when deterioration arises and how we identify the location of poorly performing sections of cable.
