DNV Comparative Study Concrete vs Steel Substructure
Over the last years, the floating wind industry has been fed with various comparative reports, demonstrating that concrete floaters had a significantly lower carbon footprint than their steel counterparts.
The main reference in that respect was published in 2022 by DNV. It is still considered as a reference, even though the study was commissioned by WindWorks Jelsa AS, a Norwegian group aiming at developing a concrete floater fabrication yard in Norway.
For the credibility of the floating wind industry, it is essential that a more realistic representation of the potential execution schemes for steel floaters is considered, enabling a fairer comparison between the two materials. In this respect, the assumptions of the DNV report must be re-evaluated.
At Ocergy, we are developing a steel semi-submersible floater, the OCG-WindTM, with a demonstrator of 3MW to be commissioned by summer 2026 and have developed a technology specifically for ease of industrialization.
We have reviewed the DNV report methodology and results. Some key assumptions on steel floaters must be challenged and adjusted:
- The mass of the steel floater considered to reflect OCG-Wind properties for a 15MW turbine in similar conditions (North Sea environment) is significantly lower.
- In the case of Asian production, the transportation emissions should be adjusted to reflect a modular approach like OCG-Wind, which will be transported by sub-components rather than fully assembled floaters, enabling optimized deck space use.
- Sensitivities on recycled steel rates must also be considered, as major global steel manufacturers can today provide commercial volumes of required recycled steel plates thickness and grade for OCG-Wind design.
Results obtained from these sensitivities are diverging very significantly from the DNV conclusions and offer a new perspective for steel floaters:
- With rather unfavorable assumptions (full supply of steel and fabrication in China), the OCG-Wind floater has the same carbon footprint as the concrete semi-sub from DNV report
- At a time of strategic European re-industrialization focus, assuming a European supply and fabrication of the OCG-Wind floater results in a carbon footprint 60% lower than a concrete semi-sub
Our full report can be found here : Comment paper – DNV comparative study concrete vs steel substructure
These considerations have further implication: should WTG OEMs also use green steel for their towers and nacelle frames, floating wind farms lifecycle CO2 based on the OCG-Wind technology could approach the lowest CO2 emission energy, such as hydropower.
And a low carbon footprint is only one of the benefits of the low-weight steel modular approach.
There are two requirements for foundations to enable the multiple gigawatts of new power every year expected to be delivered by floating wind farms:
- Rapidity of deployment
- Low cost
In both aspects, a modular steel semi-submersible, such as OCG-Wind, is significantly better than concrete floaters. Steel modules are efficient to build by an existing supply chain, with no expensive new facilities needed. Transporting modules instead of fully assembled units allows the use of a variety of available deck carriers, smaller and more cost effective than large semi-submersible ships. At the marshalling harbor, one floater assembly station is sufficient to deliver over 50 floaters per year.
This contrasts strongly with concrete floaters which will require massive investments in new factories and construction quays large enough for multiple assemblies due to the tremendous weight of such structures and the difficulty to move them on land. Many fabrication stations will be needed to achieve the same throughput, because of the duration necessary to fully complete a concrete floater. Given that only a few years of operation for such plant could be guaranteed, such infrastructure will lead to a very high unit cost.
Further driving up the cost difference is the necessity for the project to plan decommissioning and dismantling of the units at the end of the project life. A dedicated facility will have to be created to safely breakdown the massive concrete structures, with very limited use and resale value for the raw material that will be retrieved. The cost of such endeavor could be similar to initial fabrication cost, due to the time, energy, and specialized machinery and labor required.
By contrast, a modular steel semisubmersible floater will be far easier to dismantle due to the weight difference and the ability to disassemble the main components. Additionally, scrap steel will be entirely recycled and have good resale value.
Several technical factors further differentiate these approaches. A follow-on article will compare the fully industrialized fabrication of modular steel components versus that of reinforced concrete floating structures.
