Summer, 2012: A heavy-load vessel approaches the German Bight. 45 km off Borkum’s northern coastline, the gigantic barge stops and drops 800 t of steel with millimetre precision onto the bottom of the North Sea. Over a ten-month period, this spectacular process was repeated a total of 40 times. But what was being dumped time and again into the depths? 30 m below the surface, 40 tripods have been erected since the summer of 2013, forming the foundations of the Borkum offshore wind farm –an impressive feat of engineering.
Another 40 tripods are destined for installation during the next phase of construction. Over an area of 56 km2, 80 wind turbines will eventually generate a total of 400 MW of power – without emitting a single tonne of CO2. At the time of writing, the first phase of construction has been completed and the wind farm is already delivering 200 MW, supplying 200,000 households with electricity. By the final phase of construction the Owner Operator, Trianel Windkraftwerk Borkum GmbH & Co. KG, expects to have made an investment of € 1.6 billion. After all, the wind in the North Sea is something you can count on; with no obstacles such as mountains or buildings, it blows at speeds of around ten metres per second. [sam_block id=”10″ name=”Banner central 728x90px”]
Complex inner workings
The harsh environmental conditions mean that the foundations of the turbine must withstand extreme loads. The 40 tripod-style steel foundations, weighing 36,000 t (around three-and-a-half times the weight of the Eiffel Tower), are now embedded in the sea floor. A single tripod is 30 m tall, of which only a couple of metres are visible above the surface. Together with a central tube mounted on top, the complete structure stands 50 m tall.
While the dimensions and the tremendous weight of the tripods are impressive, the interior detail is even more amazing. What looks like a compact steel giant is really a high-tech structure with numerous pipes, connections, lines, platforms and individual parts. Ingenieurbüro Schlattner GbR from Osnabrück, Germany, was hired by Offshore Wind Technology (OWT) GmbH to participate in the detailed engineering and planning of the gigantic tripod-style foundations. Two variants had to be prepared for different sea locations, each with three sub-variants depending on their positioning with reference to the transformer platform. In each case, AVEVA Bocad Steel™ was an indispensable planning tool for Marit Bachmann, engineer at Schlattner, who has been working with Bocad software since 2000.