PartnerShip: Why are you involved in the development of floating offshore wind turbines?
Henrik Stiesdal: The International Energy Agency (IEA) recently declared that conventional offshore wind farms could be capable of delivering slightly more than the world’s present electricity load, which is of course good. But if transport and industry are to be made carbon-free, then we will have to produce very much more electricity than our present needs. According to the IEA, floating offshore wind is theoretically capable of delivering 10 times our current electricity load. There are a number of arguments in favour of floating wind. For one thing, conventional fixed-bottom offshore wind farms have to be installed in shallow waters. There are not so many suitable sites around the world. But for floating wind farms, you can go out to deeper water. Floating wind also has some less obvious advantages. The most important one is that one size fits all. Normally in fixed-bottom offshore wind, you have different foundations for different water depths. You often even need different foundations in a given project because on large wind farms the conditions are not the same for all the turbines. But with a floater, it’s always the same foundation, which is a big advantage for serial production.
PartnerSHIP: You are currently working on the TetraSpar demonstrator, in partnership with Bourbon Subsea Services. What are your objectives?
H.S.: I’m not trying to develop new wind turbines, because anything Vestas or Siemens can do is much better than what we could do. But they don’t do floaters. The problem is that construction of floaters still needs to be industrialised. So my idea was to develop a form of floater where modular components can be factory-produced, transported by road – one-by-one if necessary – to the quayside, where they can be assembled using an on-shore crane. Thanks to factory manufacture and on-shore assembly, significant cost savings are achieved. The whole process of manufacture and assembly of the TetraSpar closely mirrors that of the wind turbine itself, and this has never been done before. We are currently engaged on assembling a TetraSpar floater in Grenaa, Denmark, working closely with Bourbon Subsea Services, which has been working with us on the upstream engineering of the project. We are planning for it to be transported and installed off the coast of Norway during the summer, although the dates will depend on weather conditions and, of course, the additional problems for an international project like this posed by Covid-19 restrictions and quarantine regulations.
PartnerSHIP: Tell us about your collaboration with Bourbon Subsea Services.
H.S.: We knew that this project would involve doing complicated things that have never been done before, so we would need the most experienced player in transporting and installing floating offshore turbines. We issued a tender enquiry according to procurement rules, and BOURBON’s tender was the most attractive in every respect. The key to this cooperation really lies in the work done upstream, between us. BOURBON was involved since September 2019 to detail the impact of the installation phase on the design of the floater. They have given us excellent service in the fine-tuning of the concept so that the risks involved in using new processes are as small as we can make them. Many people might imagine that the main purpose of a demonstrator like this revolves around the weight of the structure, or the cost, or whatever, but matters most is ensuring that everything is done in a safe manner, and then, of course, at a low cost. BOURBON’s cooperation has been vital. It really has been a super, constructive collaboration, and we are extremely happy with it.
A pioneer in offshore wind technology
Henrik Stiesdal, 64, one of the world’s leading pioneers in offshore wind technology, has at least 650 patents relating to wind power technology to his name. He designed his first wind turbines as long ago as the mid-1970s, before going to university, and licensed them to Vestas. During his studies he served as a part-time consultant first for Vestas and later for Bonus. After completing his doctorate, he became full-time technical manager of Bonus. In 1991 he was responsible for the technical development of the first offshore wind turbines installed in Denmark, at Vindeby. Stiesdal later became Chief Technology Officer at Siemens Wind Power. In 2014 he set up his own company, researching and innovating in a number of areas that are key to the energy transition, such as floating offshore wind power and mid-term energy storage.
How the TetraSpar foundation works
The future of the offshore power sector lies in floating wind turbines, which can be sited in deep seas where wind speeds are greatest. But the production and installation of floaters have to be made cheaper for the energy source to be more economical. The TetraSpar Demonstrator, developed by Henrik Stiesdal, sets out to do just that by industrialising the process. The floater consists of modular tubular steel members that are factory-manufactured, and easily can be transported as individual elements to a large assembly zone at the quayside. The assembly itself is carried out using a special joining system developed for the project and a standard onshore crane. The finished triangular structure is about 65 metres long on the ground, and weighs just over 1,000 tonnes. It is associated with a much heavier keel, a ballasted structure weighing about 1,500 tonnes that is assembled separately, which ensures that the centre of gravity is lower than the centre of buoyancy. After the floater is lowered into the water, the fully assembled wind turbine in installed on it prior to being towed out to sea.