The defining feature of the Tetra foundation concept is the modular, “building block” arrangement and the industrialized manufacturing.

The main component of the Tetra concept is the foundation itself, a tetrahedral structure assembled from 4-5 types of modular, tubular steel braces. Some modules are specific to a given configuration, others can be shared over a range of configurations, turbine sizes and site conditions.

All modules are factory-made using automated processes. Even though they differ in specific details, the modular, tubular steel braces and tanks are similar to tubular steel tower sections for wind turbines, and they are easily manufactured by experienced tower suppliers. Consequently, the Tetra concept benefits from an existing, highly efficient, and highly competitive supply chain that has experience from the manufacturing of tens of thousands of tower sections.

Stiesdal has selected Welcon as a supplier of modules for the Tetra foundations. Welcon is one of the world’s leading supplier of offshore wind turbine towers, and its automated production lines are ideally suited for the manufacturing of the tubular braces and tanks for the Tetra concept.

Stiesdal can provide support to the establishment of local manufacturing.

The foundation modules are delivered to the project’s local construction port by land or sea transport.

It is a design target of the Tetra concept that no module should be heavier or have larger diameter than the heaviest and largest part of the turbine tower, and that no module should be longer than the wind turbine blade. The result of this design target is straightforward – if it is possible to get the turbine to the port of embarkation, then it is also possible to get the foundation there without having to resort to on-site manufacturing.

Once delivered to the port, the structures are assembled at quayside using a proprietary, maintenancefree assembly technology. The assembly in general includes no special processes (welding, painting, etc.), and the infrastructure required for assembly is limited to a flat, open area at the quayside. No dry docks or other special facilities are needed. The harbor depth requirement is 8-10 m.

Following assembly, the foundation is launched using a semisubmersible barge, and the turbine is installed using one or more standard, landbased cranes. The foundation may be temporarily ballasted to sit on the seabed at the quayside, establishing truly “onshore” installation conditions.

TetraSub can be assembled in existing ports, using local supply chains and contractors, no need for large welding crews or specialized facilities. Port requirements are flexible, and assembly setup can be adapted to port specific layout. A single assembly stand requires only 18,000 m².

Deployment to the project site is carried out using ordinary tugs or suitable service vessels. Following hookup to preinstalled anchors the turbine will be ready for final cable connection and start of operation.

Our proprietary Stiesdal Shared Mooring system reduces anchor count by 70-80% at GW projects while maintaining full redundancy. Stiesdal Shared Mooring delivers ~30% lower mooring system costs and ~5 % total project
CAPEX saving (insight based on 500 MW APAC project). Designed for fabrication with regional materials and existing industrial capabilities, the system supports local content while minimizing environmental impact. Read more (PDF).