Senior Data Scientist, Chris Nelson PhD takes us through our approach to identifying and measuring exposure.
The sight of offshore wind farms off the coast of the UK has become increasingly common over the past decade. These towering turbines produce huge amounts of energy, with current estimates suggesting they power more than 15 million UK homes. The generation of such massive amounts of energy creates a range of complex engineering challenges. Firstly, the energy must be transported to the shore under treacherous seas, with this job being completed by highly specialised export cables. Failure of these cables can lead to economic losses in the range of tens of millions, meaning they must be protected against all possible threats. Export cables often have to pass through delicate ecosystems as well, meaning efforts need to be made to protect the nature around them from harm.
The first and foremost method of protecting both cables and the environment is burial. Indeed, for many years the Carbon Trust has advocated cable burial risk assessment as the go-to approach for reducing cable failures in offshore wind farms. Of course, developments in distributed optical-fibre sensing have given asset owners and managers more options to reduce cable failures, but burial still remains essential.
A buried cable is less at risk from anchor strikes or trawling, it is better insulated and so does not leak as much electromagnetic field into the surrounding water body, limiting distress to wildlife, and it is better shielded against the damaging effects of strong tides and extreme weather. During installation, therefore, efforts are made to lower the cables into trenches dug out from the seabed, often to depths of more than 2 m. But how do cable owners know how successful these efforts have been? And how do they keep track of cable burial in the highly dynamic, but also highly inaccessible, undersea environment?
Real-time Exposure Monitoring
The conventional way to test for exposure along an export cable is to conduct a visual survey. This may involve sending down a remote operated vehicle (ROV) to travel along the length of the cable looking for regions that are directly visible. This is an effective method, but it is expensive, time-consuming, and provides only a snapshot of current conditions with years often passing between surveys. This means cable owners are in the dark about whether their cables are becoming more exposed or not through time.
This is why Indeximate have invested significant efforts in understanding how to extract the signatures of exposure from Distributed Acoustic Sensing (DAS) raw data. DAS data is relatively cheap to acquire, it can be collected using redundant optical fibres that already span the length of the export cable, and, importantly, it allows for continuous monitoring. Alone, but particularly when benchmarked from survey results, Indeximation® offers real-time insights into how cable burial conditions are changing along the entire length of an export cable. Conveniently, Indeximate’s Scattersphere® provides cloud-stored client access to map-based records of these changes, enabling real-time views on cable burial, and an ability to perform forensic analysis for cable sections of concern.
Building up a Picture of Exposure
Exposure and burial can occur over short and long time-scales. On short time-scales, burial status can be changed, for example, by human activities such as Controlled Flow Excavation during the initial installation of the cable. On long time-scales, like slow drifting of sand dunes on the seafloor, or unfilled trenches, gradual exposure of cable sections may occur that increase threat of damage
Figure 1: Two images showing the acoustic response of a section of cable from before and after Controlled Flow Excavator burial. The background dB level experienced by the cable drops by around 20 dB after burial. This is real data plotted on an artificial cable. A movie of the burial itself is included below.
With all of this in mind, Indeximate have developed algorithms that use the temperature and stress sensitivity of DAS to extract signatures of exposure from data covering a range of time-scales, depending on customer need. If a customer is looking to assess the effectiveness of active burial efforts in the short-term, for example, then clear changes in the response of the cable to pressure as several tonnes of seabed material are piled on top of it can be utilised. Spatial variations in the change of these metrics can reveal areas that may have been missed, or shallow buried. An example of the acoustic response of a cable from before and after burial can be seen in Figure 1, with a movie of the burial itself being included below.
If, on the other hand, a customer is worried that the slow drifting of undersea sand dunes may have unburied a section of cable, then exposure can be identified by measuring the temperature response of the cable to tidal cycles and current loading. An exposed region of cable that loses thermal energy to surrounding water by convection displays a distinct thermal evolution compared to a buried region of cable which heats its local environment by convection. These subtle changes allow regions of cable that are either newly or consistently exposed to be identified in long datasets where the signal can reveal itself above the noise.
In Figure 2, we plot an example of temperature variation on tidal cycles measured directly Indeximated DAS information. The blue line plots an exposed region of cable, whilst the orange line plots a buried section of cable. The clear signatures of tidal heating and cooling are immediately evident in the exposed region of cable. These insights, obtained through unobtrusive and cost-effective methods, can prove essential to cable owners trying to protect their nationally critical assets.
Figure 2: A plot of temperature through time for an exposed (blue) and a buried (orange) section of an export cable. Tidal heating and cooling cycles are clear on the exposed section of cable.
If you are interested in finding out more, feel free to reach out to the Indeximate team via our website: https://indeximate.com/contact-us
