Windcrete, a new Spar-type floating substructure that is designed to carry wind turbines of up to 10MW in deep offshore environments, could provide a high level of reliability and lower installation, maintenance and construction costs than alternatives, says a team of Spanish researchers.

As project co-lead Climent Molins, a professor of civil engineering at UPC-BarcelonaTech, explained, the structure uses reinforced concrete in a monolithic structure with what he describes as a “smooth and efficient geometry”. It can also be built in a drydock before being towed to the installation point in a horizontal position, helping to minimise water resistance. The installation procedure itself can be carried out by pumping water into it and partially flooding the structure – a process that effectively causes the structure to self-erect from a horizontal to a vertical position – while the top of the tower is lowered to a level a few metres above mean sea level.

This means that a wind turbine can be assembled on the tower without the need for large floating cranes, which are quite expensive to contract. After this, the water is replaced with ballast aggregates, increasing its hydrostatic stiffness and stability. “The concept uses a mooring system that handles the various marine and weather conditions it can be subjected to in a deep offshore environment,” said Professor Molins.

Work on developing the structure started in the Department of Civil and Environmental Engineering at UPC-BarcelonaTech in 2010 as part of a project designed to analyse the technology of innovative structures and materials. The team quickly realised the benefits that concrete structures have over steel ones and spotted a clear opportunity for their use in the offshore wind energy sector. Since then, they have filed several patents and funded three PhD theses to develop aspects and design tools to be applied to Windcrete or other similar concepts. Throughout the development of the project, the team has also received investment and support from KIC-InnoEnergy for the AFOSP proof of concept (alternative floating offshore substructure for offshore wind) as part of a consortium that also included Gas Natural Fenosa and the University of Stuttgart.

Alexis Campos, a PhD student at UPC-BarcelonaTech, said “significant” numerical and experimental work had already been carried out on the design and confirmed that the team has achieved the proof of concept through numerical simulations and tests of a scale model in laboratory conditions in the framework of the AFOSP project. “These tests showed the viability of the concept thanks to its reliable stability in the array of conditions in which it was tested,” he told OWJ.

The research has not only been technical, however. The KIC-InnoEnergy AFOSP project included a study on the expected levelised cost of energy, which, in favourable scenarios and taking advantage of the capability of using large wind turbines, has been identified to be competitive at 0.12 €/kWh, he explained.

“Windcrete’s aim is that its users can focus on managing wind turbines and can ‘forget’ about the infrastructure that supports it. In addition, its lower material costs and simpler installation procedure also reduce the initial capital investment required. This combination is quite significant because it not only reduces the operational costs but also the capital ones, which are typically the major challenge in renewables,” said Professor Molins.

“On the other hand, the construction methods required have long been used and are further simplified by its smooth and efficient structure. Furthermore, Windcrete could have more positive impacts on a regional economy than with steel, because concrete is typically more locally produced and requires less skilled labour.”

Mr Campos revealed that the team is now actively seeking funding and industrial partners to build a large-scale prototype to be tested under real conditions and anticipates this could be the final step towards commercialisation.

“Our market is areas with great wind resources and deep water, over 90m, for example, such as the Mediterranean and offshore Hawaii, New England, the Great Lakes, West Coast of the US and Japan. We are excited to see many of these areas have already started investing in deep offshore wind energy, acknowledging its potential. We are currently aiming to start testing in the marine environment in 2018 and start commercial use by 2020,” he concluded.

Source: Offshore Wind Journal