Denmark might repeat its wind turbines success by being home to the development and production of future floating offshore foundations. These can open the market for offshore wind power at depths over 100 meters.
The former chief technology officer of Siemens, Henrik Stiesdal, points out that such an accomplishment requires an ‘open source’ approach to the task. Therefore, he has developed a new, inexpensive design for a floating wind turbines platform, unveiled at the recent Wind Industry Annual Meeting in Herning, Denmark.
The idea is to make the design freely available to businesses, educational institutions, and students who might want to develop further on the foundation; either to make a business out of it, or to improve their skills in a future energy form.
“Today’s cylindrical steel turbine towers has helped to ensure that onshore wind turbines are the cheapest electricity generation technology. The towers have become so cheap because all suppliers have been free to work to streamline the concept, and that is what I try to emulate,” he said.
Stiesdal also hopes that more young people could feel attracted to develop the relatively unexplored area such as floating offshore foundations, which includes many different disciplines. “This is just a suggestion for a solution. I hope that someone comes and says that they have found something that is even better, “he says.
Half-submerged in the sea
Stiesdal’s floating liquid foundation consists mostly of well-known and well-proven components from the industry that can be produced anywhere and transported to the port where the foundation must be assembled.
The foundation is of this type, a Tension Leg Platform (TLP), is used in the oil and gas industry, in which floats a partially submerged foundation. TLPs are stable because they are held in tension by the opposing forces of buoyancy and anchor lines. The lines are attached to anchors on the seabed by a variety of methods.
The base is dimensioned for a 6 to 7-MW wind turbine, and is a lightweight design in which a three-legged steel structure transforms bending moments to the pull-push forces. It’s all kept afloat by six relatively thin-walled tanks at each corner. The tanks are filled with air to a certain pressure, which is one of the innovative principles.
Stiesdal explains that if one makes such a structure, they must normally be sized for a relatively large hydrostatic pressure on the outside, which makes it necessary to stiffen them inside, typically with so-called stringers, which are welded inside. The welds, usually fillet welds, reduce the steel’s fatigue resistance.
Filling the tanks with air, however, equalizes the water pressure, and can thus save stiffeners and manage with a thickness of only 10 mm on the tanks.
Of course, pressurized tanks might leak. Stiesdal solves that problem with a valve arrangement at the bottom of each tank, so that – if anything happens – air can be pumped into the tank, thus pushing the water out until the leak is repaired. Each pressure tank can be replaced independently, and the foundation is almost unaffected even if one or more of the 18 tanks leak.
An inexpensive anchor
Also, a new and inexpensive gravitation anchor was presented. It consists of a base plate of concrete (corresponding to a wind turbine foundation) combined with a skirt of cheap, bolted sheet piles.
The wind turbine can be mounted to the foundation portside, after which the whole show can be towed out to a location with three tugs. Tugboats would have three temporary holds of liquid to press the foundation into position when reaching the correct location.
Furthermore, the TLP concept is a stable structure that makes it possible to create even quite large repairs at sea. When that is not possible, the platform can be released and pull it into a port of repair.
Stiesdal estimates the price of the new foundation for 100 to 200 meters of water will be about like the price of a normal offshore foundation for 50-m water, perhaps with a little luck even slightly lower. Published prices of such foundations is often in the range of 30 to 40 million Krones ($4.5 million to $6 million) including installation. On the other hand it is more expensive to install the turbine at sea on a normal foundation than in a port on a floating foundation.
How do you expect your colleagues in the industry will receive a new concept and your ‘open source’ initiative? “People with an open mind will think it’s fun, and it opens up new opportunities. Others, old friends in the industry, may see it as a torpedo in the water in terms of what they are working on,” he said.
DNV GL to evaluate the concept
To underline the seriousness of the new concept, Stiesdal has teamed up with DNV GL, a leading provider of certifications and approvals for offshore wind turbines. DNV GL has offered to perform an independent evaluation of the new concept.
“It is my impression that DNV GL’s generous offer of an independent assessment is driven by the ‘open source’-idea, so the results will be widely available. The work is carried out under the auspices of DNV GL’s internal ‘Extraordinary Innovation’ program. The plan is that the results of the assessment will be included in the open-source concept,” he said.
Stiesdal stresses that the project is still a work-in-progress and that despite assistance from a strong partner as DNV GL, there is no guarantee that the project will succeed. But he really hopes that it will inspire young people to become engineers and engineering students to work with wind power.
He adds that in many other technologies limited project opportunities for students to niche clean-energy projects. Here, with a little luck, may be the start.
Stiesdal is in the process of seeking patent protection for some of the new details of the project, mostly to ensure that others can not block design. He, in turn, can not guarantee that there exist other similar ideas already available.
“I have not spent money and effort on just a novelty. But if I can actually obtain patent protection, anyone who is interested, will be allowed to use the patented technology for free,” he said.
Filed Under: News, Offshore wind, Towers