To build wind power farms offshore is challenging, but can be tremendously rewarding.
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Wind gets new sails - floating wind farms
- The story: In July, Royal Dutch Shell and Scottish Power announced they were jointly submitting proposals to the British authorities to build, off the coast of Scotland, the first large-scale set of floating wind farms in the world. Today the largest floating farm is a six-turbine, 50mw array due for completion in the North Sea. The new plan is in gigawatts (GW)!
- Offshore: Such offshore wind farms with foundations in the seabed are now part of the energy mix in several places. In the past four years their capacity has doubled, from 19 GW to 35 GW, and amortised costs have dropped by a third, from $120 per MW-hour to $80. They are, however, of limited deployability, being restricted to waters shallower than about 60 metres.
- 80% of the world’s offshore wind blows over places deeper than that. Making these accessible will unlock enough power to meet the world’s probable electrical needs in 2040 many times over.
- The trick is to build turbines which, though moored to the seabed, will float. If Shell and Scottish Power can pull this trick off, it will be a big step towards tapping that potential.
- From fringe to mainstream: In 2010, floating-turbine technology was a fringe affair. The difficulty was not the turbines themselves, but making them float. The oil and gas industry had developed a range of floating foundations that could keep titanic objects like drilling rigs stable at sea. But transferring that know-how to wind power was not easy. First, unlike an oil rig, a wind turbine is lanky and top-heavy, making it prone to tip over. Second, turbines generate powerful gyroscopic forces that would further destabilise a floating machine.
- Solved: A decade of development gave two things: proof that turbines can float and clarity as to how these floating units might look. Engineers achieved this through patient prototyping. They took designs previously tested in university wave pools and scaled them up into small demonstration units off the coasts of Norway, Portugal and Japan.
- Each unit, bedecked with sensors, gathered data on variables such as pitch, wind speed and wave height. These data were then folded into designs, for bigger, more stable units. The results, visible today in newer models off the Norwegian and Portuguese coasts, can safely float turbines four times as powerful as their predecessors. Engineers therefore consider the flotation problem solved.
- Four approaches to flotation have emerged. The commonest is a semisubmersible, that uses buoyant steel triangle that has water-filled cans at two of the vertices. These ballast tanks balance the weight of a turbine at the third vertex, with water pumped around inside the triangle to trim its stability.
- Another way is to stick a turbine on a bottle called a spar that is filled with heavy ballast, to make it float upright. We can place the turbine on top of an 80-metre-high concrete tube containing water, rocks or some other cheap and heavy material.
- Two other approaches are less developed, but may prove useful. There can be a tension-leg platform, a starfish-shaped steel structure with a turbine at its hub. The starfish is submerged and yoked to the ocean floor with cables, holding the turbine upright. We can also erect the turbine on a flat concrete or steel barge that resembles an empty picture frame. When the turbine sways, water sloshes within the frame, dampening its movement.
- The engineering problem: Bigger farms obviously require more turbines and bigger turbines. The bigger a turbine is, the harder it is to maintain. Wind turbines occasionally need big parts, like blades or generators, replaced. That is challenging on terra firma. But on land, a crane can brace itself against the earth. At sea, “jackup” vessels achieve similar stability by dropping steel legs to the seabed. Floating turbines will operate in waters too deep for jackup vessels to work, so any vessel servicing one will have, itself, to remain floating.
- Engineering problem: For floating turbines, an alternative may exist. Unlike fixed turbines, they can be unplugged and dragged to shore. If a floating turbine is near the shore, it may be easiest to tow it back to port for repair. If far away, exotic gadgets like the climbing cranes may work better.
- Summary: It may soon be possible to extract a lot more electrical power from the wind, to do so without covering hillsides with turbines, and to make a profit while doing it. And that is a salivating prospect waiting to take wings.
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