Improving subsea cable detection

One of the major cost elements in developing an offshore wind farm is to be found in the area of cabling and grid connection. There are significant costs incurred with the production of the inter array and export cables, their installation and the related connection activities ensuring the transfer of the energy produced to the national grid. It is at the same time also one of the most vulnerable elements.

Faults in an inter array cable can shut down a single turbine but when the fault occurs on the export cable it can shut down a complete wind farm leading to not only high costs for the repairing activities but also the significant loss of revenue each day.

With onshore grids remote monitoring is possible and when a problem occurs in most cases the problem area is identified and the operator can send off a technician who can reach the site and solve the problem in a few hours in the best case.

Offshore, it is a different story. It is much more difficult to monitor the subsea cables. The seabed is not static; buried cables have been known to become exposed due to scouring or movement of sediments, and sometimes they have been pulled from their original path by fishing vessels or anchoring. Many wind farms have experienced difficulties with, or damage to their subsea cables.

There have been initiatives set up to look into those risks such as the Joint Industry Project (JIP) CableRisk by DNV GL together with industry stakeholders. However to prevent problems in the future it is necessary to plan surveying activities to check if the cables still lie on the correct path and to determine the condition of the cables. At the moment there are no strict rules on European level that outline the frequency for these check-ups. In Germany annual surveys to re-calculate the route of the cable are required by law but in other countries it is up to the operator to decide if and when to make these checks.

Operators usually deploy ROVs for this purpose. These operations are time intensive, which, in turn means high costs. Even when scanning activities have been planned it all still depends on the right weather window. Although it is possible to calculate the exact location of cable damage on a chart using specific programmes the ROV will still have to find the exact position of the damage on the seabed.

Cable survey system

Now there have been improvements in the surveying methods that can help in the move towards the overall cost reduction of offshore wind. Offshore WIND spoke at the WindEnergy exhibition in Hamburg to Kris van de Voorde, Sales Manager at DOBstar, a joint venture between US company Optimal Ranging, Inc., provider of equipment and services for underwater cable and pipeline surveys, and Dutch engineering and survey company Deep BV.

For the past two years DOBstar have worked on ‘fine tuning’ the design of Optimal Ranging’s ORION patented underwater cable survey system so that it can be used for wind farm cable surveys in northern European waters.

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A prototype consisting of six 3-D magnetic field sensors, a triaxial accelerometer and digital compass, was developed. These sensors are calibrated against a model of the magnetic field that is expected from a cable carrying an active AC wave current. The depth of a cable can be measured in real time with an accuracy of 95% under ideal conditions. It has also been integrated with leading hydrographic survey software.

Precise and fast measuring

In cooperation with Swedish marine surveying company MMT the prototype was tested on survey projects for the projects NorNed and Sk4 that were contracted to MMT by Statnett. With the tool mounted on a ROV the outcome of the tests indicated that more precise measuring was possible than with the tools used currently in the offshore wind industry.

Mr van de Voorde: “Most of the current surveying systems used are able to detect a cable within a radius of 2 metres on each side of the sensor. If the cable is just out of this reach the ROV will have to go up and down in one area to track its location again. During recent surveys we found that we were able to track cables at a distance of up to 10 metres, which improved efficiency substantially.”

Beside this advantage the tool will soon be able to also detect more than one cable in the same area. This is enabled by a narrow band filtering engine that provides quadrature demodulated magnetic field strengths which identifies the individual cables in the magnetic area.

But, as he adds: “What actually surprised us even more is that the tests not only proved its efficiency and accuracy, as we were expecting beforehand, but that it turned out also to perform faster than current systems used.” Using an operational speed of 2 knots, the operation was done in 7 days, a day less than when using existing tools in the market.

The tool can be mounted on the vessel itself, a ROV or sledge, depending on water depths, up to 600m. Deep BV is currently looking to test it also on a depressor wing behind a vessel. In the meantime the company continues brainstorming on further improvements such as a new design for the housing and the possibility to increase operational range at even deeper levels. Another next step is passive measuring.

Mr van de Voorde: “The criteria required for an ROV to be able to detect and follow the route of a subsea cable is that the cable has a magnetic field from either transferring electricity or from materials used in the cable. It would be great if all subsea cables, whether inter array or export cables, would include a sensor strip in the cable. However more tests will need to be made to make sure that this does not interfere with the current flow or other related areas.”

Sabine Lankhorst