With rows of wind turbines following the curvy shapes of the mountains that hold them in many parts of the world, today wind energy has become an integral part of our global environment. While most people are familiar with onshore wind generation, its offshore variant is often less known. This is especially true for floating offshore wind energy, but this is about to change. Floating offshore wind energy will soon be making its entry into our energy mix and it is here to stay.
Despite its success as a well-proven concept, onshore wind energy has received some criticism. Even while some people enjoy the view of the blades sliding gracefully through the sky, many label it as visual pollution. Onshore wind farms also claim large parcels of land, which is particularly a concern to regions with limited land availability. In contrast, there is much less intense use of area in the sea, which provides interesting opportunities for further expansion of wind capacity. Additionally, due the absence of geography that works as a restrictive factor on land, much higher winds speeds can be found at sea. However, fixed offshore wind structures are limited to water depths of about 30 meters or less. Floating structures offer the promise to catch the stronger winds beyond that threshold.
Placing a wind turbine on a floating structure is anything but straightforward. From an engineering point of view there is a major difference between offshore fixed structures and offshore floating structures. The motions of the latter will introduce additional dynamic loading on the turbine and its tower. As a consequence some structural elements need reinforcement and new phenomena can surge, such as resonance between the floating structure and the turbine’s blades. In order to address these issues, the industry will need to develop adequate analytic tools and control systems, as well as improve its wind prediction capabilities. Managing the interaction between the floating structure and the blades can prove to be key to reach the desired production levels. As a matter of fact, the portfolio of several leading research institutes projects include projects related to these topics, such as the NREL. Improving the efficiency of the transmission to shore is another crucial link, as deeper waters are generally found at larger distances from the coast.
However, the rising interest in floating wind generation is also bringing about a more fundamental shift. In an industry where turbines long dictated the pace of innovation, structures have entered the picture. It has become a two-man show, as the creation of consortia between turbine manufacturers with companies with offshore experience proves, which also tend to include electric companies. This is also evidenced by the creation of new wind groups in various established offshore engineering companies. However, in order to be successful, the rather conservative offshore industry will need to be able to reinvent itself. Merely transferring the oil & gas standards to the wind industry can lead to overly restrictive norms that will not allow offshore wind structures to keep afloat in an economic sense. There is also an enormous logistical challenge attached to the operation and maintenance of floating offshore wind units. The industry will need to address these timely and, more importantly, at a reasonable cost.
Two structural concepts are currently dominating the technological landscape: one based on conventional Tension Leg Platforms and the other on Semi-Submersibles. Since the motions of the first platform are significantly smaller, it offers the promise of more “turbine-friendly” design conditions. However, its anchoring system is generally thought to add complexity to the design and to the operation and maintenance. The industry proposes less variants of a third concept, which is based on a typical spar design. Although it offers good motion features, this concept requires water depths of over 50 meter (5MW turbines), which makes it less versatile than the other options that can be applied in water depths from 7 meters on. We can also see a clear divide between projects that offer a single turbine versus multiple turbines on a single structure. Within the industry there is some uncertainty concerning the impact that multiple turbine concepts could have on their mutual efficiency. Consequently single turbine concepts seem to be leading the way. For now, vertical turbines, some of which are remarkably innovative, seem to be a step too far for the industry. Although there are several concepts available with vertical wind turbines, the industry generally focuses on horizontal turbines that are the basis of the most advanced proposals.
Although the local conditions will ultimately decide what concept is most suitable for a specific site, their differences will be better outlined as more experience is gained through pilot projects. Hywind or Windfloat are two examples of the various demonstration projects that have been set-up. The knowledge extracted from such undertakings will be used to advance designs, drive down the costs, improve economic evaluations, assess its environmental impact and optimize logistics. It is a crucial step that has to be passed satisfactorily, before considering large-scale deployment of floating offshore wind farms.
Wind in National Energy Mix
Even if the winds are more constant offshore than onshore, its intermittent character leads to uncertainty in the delivery of power. Governments should be aware that this impedes wind energy to qualify as a base load for energy provision and make sure to develop sufficient alternatives to achieve a well-balanced energy mix for their country. Also, floating offshore wind energy still has to improve its cost competitiveness. Nevertheless, floating offshore wind energy can be an important part of a nation’s energy mix as the impressive expansion plans of Japan show. Policies can help the industry to overcome the hurdles on the way to large-scale deployment. This does not only refer to financial support for pilot projects, but also relates to regulatory flexibility, permitting and stimulate knowledge sharing.
Although floating offshore wind energy still has several critical hurdles to overcome, it is a promising technology. It is to be hoped that the pilot projects will help to drive costs down and enable it to make its leap to large-scale deployment.
Image: Offshore Wind Energy via Shutterstock