From the offshore platform to the onshore substation
The submarine cable route starts at the offshore substation: the platform on the high seas acts as the connection point for the wind farm. The wind turbines are usually connected to the platform with 33-kV lines. A grid transformer usually increases this voltage from 150 to a level of 220 kV. This voltage level enables the low-loss transport of electricity to land.
Submarine cable production and laying
The cables are so-called three-conductor cables. Each single conductor consists of the actual copper conductor, the conductor screen and insulation screen to control the fields and a high voltage insulation made of cross-linked polyethylene (XLPE). A fibre-optic cable is integrated in the submarine cable for the data transmission of measurement and control signals. The cable core is coated with galvanised steel wires protecting it from mechanical damages, such as ship anchors. The submarine cables are calculated and adapted individually for each connection. They have a diameter of up to 27 cm and weigh approximately 100 kg per metre.
Wind, weather and waves - the challenges on the high seas
Laying the cables on the sea floor is a major challenge. The cables are first rolled up on big cable drums by the producer, then loaded onto a cable ship and brought to the high seas. Near the coast, the cable is pulled from the land by the ship. During the laying process, floats keep the cable at the water surface, so that it is not damaged by rocks and uneven terrain on the sea floor.
When the cable is linked to the onshore connection point, the floats are removed: the cable slowly sinks to the sea floor. The ship then goes further out, the cable unrolls from the drum and falls to the sea floor. The cable end is connected to the end of the next cable section with a sleeve.
There are numerous different laying methods and tools adapted to the characteristics of the sea floor. While tool carriages similar to ploughs are for example used for hard and stony sea floor, the conditions are much easier if the soil is sandy. In this case, an underwater spooling sled goes over the whole cable and creates a one-metre deep channel. The cable sinks into the channel and is embedded in the sea floor by the effect of the current.
In the last phase, the cable end is connected to the transformer on the platform. The cable is now ready for the transmission of the electricity generated by the wind turbines.