Ringing the changes on cable installation
A new kind of real-time, 3D imaging is helping power cable installation overcome the constraints posed by the difficult conditions typical around wind farms.
“It used to be a case of survey before doing anything, then lay – then come back and see how you did. That’s been the standard practice for years,” Blair Cunningham of Coda Octopus told MJ.
While installation contractors do their best to keep subsea cable spooling evenly from the ship, a detailed check on what’s happening to it under the surface has been missing: waters can be energetic or very turbid so standard feeds from ROVs quite literally draw a blank. “The problem is, if either too much slack or too much tension is put on the cable, it will cause enough stress or fatigue to damage it,” said Cunningham.
However, Coda Octopus’ Echoscope has been ringing the changes. This returns high definition, volumetric images but as the system is sonar based, it can ‘see’ in low visibility, picking up even partially buried cables. While sonar isn’t usually that descriptive, this software cleverly ties a lot of signals together to give a continuous 3D picture: “You aren’t trying to decipher a blip on the screen,” said Cunningham. Usefully, it also inserts the present operation into a geo-referenced model of the lay corridor, so the installer can check the cable is being placed in the agreed location.
The Echoscope gives everyone a live view of cable touch-down as the images generated by the software are instantly available to the installation teams. It’s recently been used by LD Travocean’s installation of a hybrid, 150mm diameter electrical/fibre-optic cable between Quiberon and Belle Isle in water depths up to 35m: “This system gives a much better idea of the cable’s catenary, that is, what arc its making as it lands,” said Cunningham. “The image assists with the speed of the ship, how much cable to pay out and as the owner or developer is often onboard these days, if something unexpected crops up, it allows them to make a joint decision about adjustment on the spot.” That’s not unknown: “You can survey, but in some environments by the time you come back, boulders or debris have moved into your chosen path. This will show you what’s ahead, real-time.”
Impressively, dual units – complete with pan and tilt - were also used on a tricky operation on the twin HVDC cables for Dong’s West of Duddon Sands windfarm. After laying, the ends had to be located and brought to the surface for joining, but there were problems: the large tidal range meant the suspended cables needed continuous monitoring to ensure they weren’t excessively stressed or exceeding the designed bend radius – all without divers or ROVs as the conditions are simply too challenging. So, the Echoscopes kept an eye on things right until the cables was put back onto the seafloor. Interestingly, the system’s output was integrated into the Tug and Anchor Management System onboard the operations vessel, allowing automatic adjustment of its position relative to the cable touch-down point.
The technology is rapidly gaining acceptance. Mounted on subsea support vessel Deep Helder for the Race Bank project, the Echoscope’s primary task was monitoring pull-in operations, carrying out live measurements between sea-bed, monopile and cable. However, once the team saw what it could do, the range of additional tasks broadened to include cut-marker positions, structure and scour protection inspection, as well as monitoring the quadrant during laydown.
Finally, as the Echoscope can produce 3D, volume-rendered mapping of the as-laid cable, there’s a growing trend to use instead of post-installation surveys, saving vessel time and cost.
By Stevie Knight
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