The case for concrete floating wind substructures

OPINION | Why not build a reusable foundation that can last for more than 100 years? asks Trond Landbø of Dr.techn.Olav Olsen

Although almost ten years have passed since the first full-scale floating wind turbine was installed (Hywind, off Norway), no floating wind farms have yet been constructed and no large-scale floating wind farms are under development. Floating wind is still dominated by technology development and testing of concepts in order to bring the cost of energy down to a level comparable with bottom-fixed offshore wind. And there is still some way to go to achieve this.

But there are good reasons to believe that floating offshore wind will be able to compete with bottom-fixed offshore and onshore wind in the future. The obvious reasons are the increased energy potential far offshore and the fact that most offshore areas close to energy consumers have water depths of 60 metres or more. Such locations will be suitable for floating support structures, with mooring lines and anchors tailored for water depth and seabed soil conditions. The main support structure, the floater, can to a large degree be standardised for the turbine it is supporting, which is an effective way to reduce the cost of energy.

Substructures for bottom-fixed wind turbines, however, must be tailor-made for the exact water depth and soil conditions at a site. Within a wind farm, both water depth and soil may vary significantly, which means there will be a degree of individual design within the wind farm, and the transfer of experience from other wind farms may also be limited. In the long run, this will put a restriction on possible cost reductions.

The case for steel floating wind substructures

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There are currently two main materials used for offshore wind support structures: steel and concrete. So far, steel has clearly been dominant, due to the preferred use of steel monopiles. Concrete has so far been used typically for gravity-base structures where weight is essential.

For floating substructures, which have been used in the oil & gas industry since the 1990s, the dominant material has also been steel. The reason being that steel floaters can be made smaller and lighter than concrete ones. This is, to some degree, also relevant for floating wind units. But for soft-moored (catenary) floaters, the requirement for high masses to achieve low motions, plus the need for fatigue resistance, makes concrete floaters very attractive.

Oil and gas structures are typically designed for a lifetime of 20-30 years, due to the finite resources in a particular reservoir. Offshore wind, however, does not have such a limit and is more heavily exposed to fatigue loading. Concrete can, without much extra cost, be designed for a very long design life (100+ years) and it has excellent fatigue resistance. The concrete floater can therefore be used for three or four generations of wind turbines, which makes sense since a floater can easily be detached from the mooring system, towed to shore for replacement of turbine and reinstalled at the same location or somewhere else entirely.

This should make concrete floaters very attractive in the future when methods for mass fabrication are further developed. In comparison with today’s steel fabrication yards, there seems to be a much higher potential for optimising cost related to mass fabrication of concrete floaters.

Some argue that the extra design life for concrete compared to steel will not contribute much to the overall cost reduction. But if we want offshore wind to produce energy for future generations, we should have a longer perspective. It seems absurd that a wind farm’s steel substructures will have to be completely replaced around 25 years after start-up — particularly if concrete floaters can be reused.

An additional argument for using concrete is that it has proven to be particularly robust for large structures — and one way to reduce the cost of offshore wind is to increase turbine size, which will require extra strong support structures.

Olav Olsen has developed its own concrete floater design, OO-Star Wind Floater, which can be constructed and fully assembled, including the wind turbine, without the requirement for deep-water facilities.

Trond Landbø is manager of the renewable energy business area at Norwegian engineering consultancy Dr.techn.Olav Olsen

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