The Netherlands-based offshore engineering company Maridea BV has unveiled the design of an innovative foundation for floating offshore wind turbines that was developed with mass production and cost-effectiveness in mind
Model tests suggest the Moray Base has excellent motion characteristics and is capable of supporting a 15-MW turbine in a harsh environments. Maridea said the patent-pending design has low capex and opex costs compared with other solutions and would require minimal maintenance.
The large diameter, thick-walled tubes that form the basis of the Moray Base are designed to deal easily with turbine loads during bad weather and with thrust from a turbine. The long radius segmented elbows have superior strength and fatigue behaviour compared with the joints of semi-submersible or jacket-type units. Maridea said this makes the structure less prone to fatigue and will reduce the need for inspection and repair work. The location of the turbine on the corner of the structure allows for access with crane vessels if a turbine needs maintenance or repair.
Named after the moray eel because of its shape, the Moray Base is fabricated from steel tubes and would not require additional structures such as braces. Maridea said the absence of structures such as braces allows for the adoption of proven, highly efficient monopile production techniques.
A 15-MW turbine could be supported using a 12-m diameter steel tube, a diameter that is suitable for production using existing manufacturing technology. The company believes that a 15-MW turbine on Moray Base foundation would be much less expensive than the same turbine on a spar, semi-submersible or tension-leg platform.
Other benefits of the Moray Base include an absence of nodes, reducing fatigue hotspots. The thick-walled steel structure does not need close-spaced framing and uses single curved plating in its segmented ‘elbows.’ Construction would not require the use of a drydock, allowing turbine erection and commissioning in inshore waters.
Maridea has designed two versions of the Moray Base: an ‘active’ configuration that is compact and has a ballast system; and a larger ‘passive’ configuration, without a ballast system, which is more maintenance friendly. The mooring system used with the innovative floater would be site specific, but for water depths of 100-200 m, Maridea plans to use a semi-taught system consisting of chain and rope.
Maridea managing director Jeroen Lusthof told OWJ, “Most floating wind concepts rely on conventional offshore foundations as used in the oil and gas industry. But in the offshore oil and gas industry, foundations were produced on a case-by-case basis, and they were designed to support a large deck structure.
“This has led to complex structures that were never intended to be produced in series, as is needed for offshore windfarms. In the wind energy industry, you only need to support a tower and be able to mass produce identical units. It is essential to reduce the cost of a foundation for series production and adopt a simplified design.
“Jacket foundations were never really designed for series production,” said Mr Lusthof. “They are complex, and the fabrication process is difficult. Details are prone to fatigue. Jackets were designed for what are essentially deck loads, and it is challenging to integrate the load from a turbine into a structure with three or four legs.
“In contrast,” said Mr Lusthof, “monopile foundations have other characteristics that are ideal for wind turbines. They are a simple concept, designed for series production and production efficiency is high. They are also easy to install, and it is easy to integrate a turbine into the foundation. Monopiles are also simple and robust and because they allow for mass production, they are generally preferred over jackets.”
Mr Lusthof told OWJ that in model testing, the performance of the Moray Base exceeded the company’s simulations. “Testing is complete, and our numerical models have been fine-tuned with the results of the tests,” he explained.
“We have presented the design to several developers and the feedback has been very positive. Above all, they recognise our focus on developing a design that can be industrialised. We haven’t spoken to turbine OEMs yet, but our design is suitable for any standard wind turbine and is based on the industry standard ‘IEA Wind TCP Task 37: Definition of the IEA Wind 15-Megawatt Offshore Reference Wind Turbine.’
“The technology readiness level of the Moray Base is between 4 and 5. At this point we are working towards approval in principle with Lloyd’s Register, a process that will be concluded shortly, and have engaged in discussions with fabricators to develop a fabrication and assembly process to ensure delivery of one or two units per week.”
Mr Lusthof said the next step in the process is the production and testing of a prototype. Maridea is building a consortium to complete this phase of the project and discussions are ongoing with investors to develop a pilot project.
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