Offshore Wind Farms Innovative Laying of Power Cables

W. Griffioen, V. Goncalves (Plumettaz, Switzerland)

In the Nissum Bredning (Limfjord) at Thyborøn, Denmark, 12 medium voltage cables were installed into pre-installed (jetted into the seabed) HDPE pipes to connect 4 offshore wind turbines, each 7 MW. The cable in pipe solution was one of the 8 innovations in this pilot project. The 72 kV cables contained a single core with 630 mm2 solid Al, had a diameter of 68.1 mm and a linear mass density of 4.6 kg/m and a maximum length of 1600 m. Two HDPE pipes were used, 110/90 mm SDR 11 and 125/90 mm SDR 7.4, the latter used in a HDD drill to cross a fairway. Installation of the cable was done by a technique where water under pressure was used to move the cable forward. With this technique, called WaterPushPulling, the cable is pushed into the pipe by a machine with caterpillar belts while at the same time water under pressure is injected into the pipe. With a suction sealing pig at the front end of the cable this water pressure is creating a pulling force. The combination of pushing and pulling, combined with buoyancy on the cable in the water, makes it possible to install long lengths of cable while axial and radial forces in the cable remain low. Once the cable has been fully installed into a pipe section it can be transported further to a next pipe section, loose from the machine. To enable this another pig is placed behind the rear end of the cable and the pipe is closed around it. Then water under pressure is applied again and the cable moves by the sole action of the water flow, with the rear and front pig sharing the pressure difference. With this technique, called FreeFloating, the cable can be launched from any convenient location to any desired destination, like tube post, and this can be done over multiple pipe sections. With on land projects the launch location could be an easy accessible location in a suburb, with the desired destination the city centre, as is done on a regular basis in Copenhagen (until 4 subsequent sections, with 145 kV cables, 111 mm diameter, with 2000 mm2 Milliken Al conductor). Or from the entrance of a long tunnel to multiple locations inside the tunnel, now under preparation in Switzerland. With offshore projects the launch location is still on shore while the destination is between offshore locations, like offshore wind turbines. Here the HDPE pipes enter and leave the turbine feet by J-tubes. In order to enable a cable to pass by FreeFloating temporary pipe loops are made. In the project all links between the turbines, including the connection to shore, were made with a bundle of 4 pipes 110/90 mm (one spare) and a smaller pipe in the center. First the 3 (export) cables for connecting the first turbine were installed, by WaterPushPulling only. Next 7 (array) cables were installed by first WaterPushPulling them (in the 4th spare pipe) and then FreeFloating them to their final position. First, the turbines directly adjacent were connected (FreeFloating over 2 sections). Then one cable was also FreeFloated over 3 sections, passing 2 turbines. The weather conditions were severe (even Beaufort windforce 8), but the cable installation could be done all days, showing how much the working window can be enlarged for offshore in the “bad season”. The remaining 2 links to connect the last turbine were done from a vessel, on a calmer day, not because it was needed to do that, but because we had planned on beforehand to show also this possibility in the pilot. Understandable that there were some discussions to also install the last 2 cables by FreeFloating from shore. The severe weather conditions became also visible when record electricity production was reported from the Nissum Bredning windfarm. For monitoring and communication optical cables were installed into upjacketed 40/29 mm HDPE pipes (central in the bundle) using the Floating technique, this time all the way from the substation. The Floating technique is like WaterPushPulling, but without a pig at the end of the cable. This means that the water can flow faster than the cable, creating a uniform propelling force on the cable, so the friction experienced can be locally compensated (for the right conditions) and building up of axial forces is limited. The forces here are much lower than with WaterPushPulling, so the route shall not have too sharp bends where excessive local friction could be generated. But, when the route is okay extremely long cable lengths can be installed (record today 12 km, but this will be more in the future, in principle also possible with energy cables). A MiniJet with sonic head was used, with longest distance reached of 3.8 km. There is in fact no limit how far the cable can be FreeFloated, as the water pressure difference is mainly effective at the pigs. There might be some viscous pressure loss over the feed length of pipe, but this can be reduced at wish by reducing the cable (and water) speed. In fact, higher cable speeds are reached with FreeFloating (in the Copenhagen and Nissum Bredning projects 25 m/min and 28 m/min were reached, respectively) than with WaterPushPulling, for ducts not too narrow and long. In theory FreeFloating is still possible over long distances (40 km!) for remote offshore windfarms while at the same time a high speed (40 m/min!) can be reached. This high speed is still safe, as at a sudden stop a compression wave in the cable is traveling upstream, like a water hammer, usually at 40 m/min not causing more force than the cable’s max pushing force. This means that for 40 km from shore in 24 hours a cable can be installed, including preparation. And with a second or third feeder duct the daily production can be enlarged (still with one machine, with simple water feeding units for the cables underway with FreeFloating), making installation economical also for such long distances.