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The case for steel floating wind substructures

OPINION | Steel foundations are lighter, cost-efficient and more bankable, writes Principle Power's Dominique Roddier

Recently, there has been renewed interest in the choice of construction material for floating offshore wind hulls. Specifically, the industry has been considering whether steel or concrete hulls will lead to the lowest possible levelised cost of energy (LCOE) for a floating wind project.

Principle Power is not fundamentally against concrete hulls; actually, we have been doing quite a bit of work over the last five years on alternative materials and are finding out that the challenges are significant. Because steel is the material of choice in oil and gas offshore platforms, we are staying with our philosophy today to minimise risks (bankable turbines and significant experience with material), until we have fully financed projects as an industry. However, no matter the choice of construction material, the ultimate goal of the designer should be to reduce costs and risks evenly such that non-recourse financing can be secured.  

The choice of materials has significant impact on design decisions because steel and concrete are not directly interchangeable. The engineering team must optimize the shape of the hull based on the unique properties and fabrication techniques of each material. The most attractive aspect of concrete is that it is cheaper than steel on a per-weight basis, however, holding design criteria constant, more concrete is needed, meaning that the hull will be much heavier than if it was solely made out of steel. For example, the WindFloat 1 prototype, designed to support a Vestas V80 2MW turbine, had a steel hull weight of 1,200 tons, while the concrete hull of an Ideol full-scale demonstrator currently in fabrication — due to support the same turbine — is reported to weigh 5,000 tons.

The case for concrete floating wind substructures

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The delivered cost of the hull is influenced by a number of elements, including material, fabrication, load-out, and transportation. At face value, a 5000-ton concrete hull would have to be four times less expensive to be competitive with steel. These levels are difficult to achieve, given the need to use offshore grades of concrete that must be pre-stressed and reinforced to deliver suitable performance. Further, the advantages of lower fabrication costs are offset by increased logistical difficulties and facility limitations (load out, transportation), where the cost is primarily driven by gross tonnage and global dimensions.

It is also unclear whether concrete will prove to be as suitable for commercial-scale projects. Concrete structures are generally produced by “slip-forming,” a process that uses continuous pouring of concrete to produce structures in one piece. The delivery schedules associated with a commercial project demand production rates of one unit per week. This, coupled with the long cure-time of concrete, means that many units must be under construction at once. The result is a high demand for reinforced production space, which is rare in most ports and suggests that concrete hulls would require specialist facilities.

Steel hulls such as our WindFloat are designed in modules so that subcomponents can be produced in yards with cheaper fabrication costs and then assembled at a port near the project site. This hybrid strategy is highly flexible, taking advantages of economies of scale, minimising the need for local infrastructure improvements, and striking an appropriate balance between local economic development and cost efficiency. Principle Power expects cost reductions of up to 30% to be achievable relative to current methods.

Mooring hardware and installation costs scale with gross tonnage. For reference, the 1,200-ton WindFloat 1 was moored with four anchors, each 9.5 tons and is by far the smallest mooring system of all multi-megawatt prototypes installed or in fabrication. In similar conditions, designers of heavier hulls will have greater challenges in reducing their anchor weights, mooring tensions, and the size of the installation/O&M vessels to competitive levels.

Many in the industry point to the absence of corrosion as a strong argument for a concrete hull, however corrosion is not a significant problem. Over the past few decades, many large, steel tankers have been fully refurbished and turned into FPSOs [Floating Production, Storage and Offloading units for the oil & gas industry] after a long life at sea. Often, these hulls have a mixture of various coatings, anodes and/or impressed current cathodic protection systems. Furthermore, recent innovations in steel coatings have increased the range of solutions available, minimising further the cost of corrosion protection and improving its effectiveness.

After many detailed internal studies, Principle Power has concluded that today, steel still offers the best balance of cost and risk, when considering the implications of this material choice across the full life-cycle of a floating offshore wind project. Further, the outlook for steel fabrication suggest that costs can be reduced dramatically as larger orders enable the serialisation of production processes. That said, we continue to look for ways to optimize the WindFloat and after de-risking the technology with steel, will continue to evaluate alternative materials that show potential for additional cost reduction.

Dominique Roddier is chief technology officer at US-based floating wind technology specialist Principle Power

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