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Offshore Wind Journal

Fixed versus floating offshore foundations – a false dichotomy?

Mon 25 Feb 2019 by David Foxwell

Fixed versus floating offshore foundations – a false dichotomy?
Equinor’s Hywind floating wind concept is being proposed for a growing range of potential applications

Floating wind has long been seen as the next generation of offshore wind and as a competitor to bottom-fixed concepts but cannot yet compete with it on cost. But that is likely to change

Floating foundations for offshore wind continue to be a hot topic within the offshore wind sector. In the drive to reduce the cost of offshore wind, particularly for floating foundations, attention is often placed on choosing the ‘best’ technology and processes for projects.

As BVG Associates senior consultant Andy Logan noted recently, floating foundation technologies are competing against one another as well as being collectively compared to their established fixed foundation cousins. But as he pointed out, although the technology is fundamentally important, it is not the only factor having a significant influence on project costs. Have we lost sight of this as we debate fixed versus floating foundations? he asked.

Mr Logan said that, imagining how the levelised cost of energy (LCOE) of fixed and floating foundation projects vary with increasing water depth, we might determine the water depth at which it makes commercial sense to use floating rather than fixed foundations.

Of course, the LCOE of a project is not perfectly correlated with water depths. Different projects using fixed or floating foundations will have their own capital and operational costs specific to the chosen foundation technology and these costs may themselves also vary with water depth. “When we analyse how sensitive LCOE is to these technology specific costs we see that the crossover point becomes a range rather than a single point,” he explained.

However, technology specific capital and operating costs are not the only factors that will determine the size of the LCOE crossover range either. There are also factors that contribute to LCOE applied to the project more generally. These include annual energy production, the cost of financing and project risk. When we consider how these influence fixed and floating projects' LCOE, there is an even wide range than before.

“What this tells us is that deeper water doesn’t necessarily mean a higher cost of energy,” Mr Logan explained. “In the hands of a capable developer, site selection and project design can make up for potentially higher capital and operating costs to achieve a lower project LCOE. As both technologies continue to mature, they each gain improved viability at intermediate water depths. The choice of foundation type then becomes an organisational or regional preference.”

Of course, floating offshore wind still has some catching up to do. Even if the rumours of a commercial-scale project on the horizon are true, the number of floating turbines will still be a very small fraction of fixed installations for a significant time to come. “Fortunately, floating wind can benefit from the wealth of learning we already have, and will have, in fixed offshore wind,” Mr Logan said, “not to mention that the rate of technology advancement and cost reduction have also surpassed industry expectations before.”

In Mr Logan’s view, the fixed or floating debate is a false dichotomy. While some sites may be viable for both fixed and floating foundations, floating does not exist to replace fixed. Floating foundations enable the offshore wind industry to expand the global reach of offshore wind. Equinor’s future Hywind Tampen project in Norway – that would not be economically viable on fixed foundations – is evidence of that.

“The size of the prize is big enough for all and if floating can mature in the way we expect then, like bottom-fixed offshore wind, it will support further cost reduction and drive down LCOE across the industry. There will always be a choice between fixed or floating at the project level, but even now we can see an LCOE competition between the two is not relevant. We know it can produce two winners,” Mr Logan said.

There are a growing number of floating wind foundations, but central to their likely success are cost, standardisation and industrialisation

Speaking at a recent conference, Lloyd’s Register renewables O&M principle engineer Mark Spring noted that compared with bottom-fixed offshore wind, the cost of floating wind is high – at least for the time being – but that this is expected to change.

Mr Spring noted that costs for bottom-fixed offshore wind have fallen steeply and that such is the great range and number of floating offshore wind foundations proposed, that the industry needs to identify which of these many concepts offers greatest cost-reduction potential.

He told the conference that apart from cost reduction, floating wind faces a number of other challenges. These include the need to drill into hard rock at depth in remote locations to install mooring systems. Others include the need to develop floating electrical substations, dynamic, high-voltage electrical cables for use with floating wind and the need, where possible to provide local content. All floating wind concepts must be capable of being industrialised and mass-produced.

EDF Energies Nouvelles project manager Christine de Jouëtte highlighted the need for design methodologies that could capture the effects hydrodynamic and aerodynamic forces on floating wind turbines, and for the industry to develop standards and rules for floating wind. She also highlighted the need for infrastructure in harbours capable of handling construction and assembly of floaters on a larger scale. This would require improvements to quayside infrastructure, cranes and large areas where assembly could take place before floaters were towed into place.

As highlighted in the Fourth Quarter 2018 issue of OWJ, wind energy veteran Henrik Stiesdal believes that standardisation will be one of the most important issues influencing the success or otherwise of the nascent floating offshore wind sector. “Windfarms in shallow water are an anomaly,” said Mr Stiesdal, a keen proponent of floating wind energy. “Most of the future demand for offshore wind will come from deepwater, which means floating wind, “ he said.

However, in Mr Stiesdal’s view, there are too many floating offshore wind concepts that are technically interesting but which fail the test of whether they could be standardised and manufactured in large quantities. The market will need a technically advantageous concept that can also be industrialised, he told delegates. “Because they cannot easily be industrialised,” he said, “some will inevitably fail the economic viability test.

“For deepwater floating windfarms the yardstick will not be costs compared with bottom-fixed offshore wind,” he said. “The yardstick against which floating wind will have to be able to compete is solar PV and storage.” Mr Stiesdal said that when the floating offshore wind market takes off, the ability to industrialise concepts successfully will be a “super-strong lever.”

In February 2019, the companies behind the TetraSpar floating offshore wind foundation took a final investment decision on a plan to test it off the Norwegian coast. Testing will begin in 2020.

Shell has increased its share in the project from 33% to 66%. innogy retains 33% in the newly founded project company that will test the foundation, and Mr Stiesdal’s company, Stiesdal Offshore Technologies is contributing to the project with its modular TetraSpar concept and holds the remaining 1% of shares. As technology partner, Siemens Gamesa Renewable Energy will provide the wind turbine and required services. The demonstration project will use a 3.6-MW Siemens Gamesa direct drive offshore wind turbine.

The partners will be part of a project team that will gain detailed, practical insights into the construction, installation and operation of the TetraSpar concept as well as detailed performance data.

Mr Stiesdal said, “Reaching the final investment decision on the deployment and test of our first full-scale demonstration project is a very important milestone for us. We have already benefited greatly from the dialogue with Shell, innogy and Siemens Gamesa Renewable Energy during the project planning, and we look forward to further enhancing the dialogue during the project execution. The benefits of our partners’ experience combined with the competences of our manufacturing partner Welcon will put us on the fast-track for rapid commercialisation.”

Dynamic stability tests on a true-to-scale model have been running since December 2018, using the wave/wind channel at the University of Maine and the wave tank at Force in Lyngby, Denmark. The prototype TetraSpar will be installed approximately 10 km from shore in water depths of 200 m at the test site of the Marine Energy Test Centre near Stavanger, Norway.

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