PO093

THE MINI JACKET FOUNDATION; Get bigger, by thinking smaller

Tijmen Gombert, Simon Lembrechts
Temporary Works Design, Rotterdam, The Netherlands

Abstract

In a continuous attempt to reduce the costs related to offshore wind energy, the output of individual turbines keeps increasing. Larger turbines, often installed further offshore and in deeper water, push conventional foundation types to their boundaries. Currently, a vast majority of offshore wind turbines is founded on monopiles, typically a more economical solution than expensive jacket foundations. However, recent and near future monopile designs have reached unprecedented weights and diameters. With only a few factories capable to manufacture and a few vessels big enough to install these XL-monopiles, it seems that the supply chain is struggling to follow the trend of ever increasing foundation sizes.

By analyzing the current supply chain, available installation methods and upcoming wind projects, requirements were set for a new and competitive foundation design. A design that uses existing technologies, is scalable and ready for the future: the Mini Jacket. By eliminating the weaknesses of monopile and jacket foundations and by combining their strengths, the Mini Jacket proves to be a competitive solution. The Mini Jacket, designed to be manufactured and installed globally with the existing supply chain, could help offshore wind reach its goal as an economical and reliable power source.

 

Method

To identify the requirements for this alternative foundation, a market analysis was performed, investigating Europe's supply chain and vessel availability. To maximize cost reduction, not only a structurally optimal solution had to be designed, it also had to be installable by available vessels and suitable to be manufactured by a wide range of suppliers.
With these boundary conditions a detailed design was made, using a North Sea wind farm as benchmark for 6 and 8MW turbines. During the design phase the structural performance and ease of installation were addressed. Besides, scalability and the possibilities to standardize fabrication have been investigated. To verify the economic impact of the design, a cost analysis was performed together with the Belgian contractor GeoSea, comparing the Mini jacket to conventional solutions.

Results

The Mini jacket is designed for 30-50m water depths. Three inclined piles are driven through a piling template, which is mounted to a hang-off frame over the side of a jack-up barge. After driving the piles through the sleeves of the template, a swaged connection permanently connects the piles to the template. This way, the template becomes the permanent interface with the turbine tower.
The three inclined piles are structurally more efficient than a single monopile, resulting in a significant weight reduction. The Mini jacket foundation is constructed with 4 components that individually don't exceed the weight of 450 tons when supporting an 8 MW turbine.

Conclusions

Compared to jacket foundations, the amount of complex and expensive steel is reduced with a factor 3. The modular components suit much more fabricators and simplify the onshore logistics, opening possibilities for fabrication in the far East. Going offshore, the foundations can be efficiently stacked on the vessel and are installed up to the tower interface in a single campaign, as pre-piling is not required.
The modular foundation is installable by most of the currently available wind turbine installation vessels, from which the majority is outdated regarding the installation of XL-monopiles. Altogether, these foundation optimizations result in a cost reduction of 30 to 40% compared to the fabrication and installation of conventional alternatives.

Objectives

In our fast growing industry the supply chain follows the developers. This means continuous investments, investments that depreciate rapidly. New jack-up barges are just able to lift the piles of their first project and require make-overs shortly after being launched. The Mini Jacket brings supply and demand together and allows further growth of turbines and cost reductions within the boundaries of the existing supply chain.

By looking at what you already have and an increased involvement of the supply chain in the development of the wind farm, a bigger and faster growth can be achieved, while still reducing the costs. Looking globally, these lessons could enable upcoming offshore wind markets to efficiently develop 8MW+ offshore wind within the capabilities of their own local supply chain.