VGROW: Validation of a foundation strengthening concept for the repowering of offshore wind turbines

At a glance

• The repowering of older wind turbines at sea will be significantly more sustainable if existing foundation structures can be reinforced and continue to be used.
• In the InGROW project, Fraunhofer IWES developed and patented a concept for increasing the load-bearing capacity of existing monopiles. The VGROW research project is now validating the technical and economic feasibility of the InGROW foundation concept.
• As project coordinator, Fraunhofer IWES is responsible for developing the validation concept and a large-scale demonstrator, among other things.

The challenge

Whereas the replacement of older wind turbines with new, more powerful models at the same site accounts for a growing proportion of the newly planned projects in the onshore sector, repowering of offshore sites remains somewhat rare. One reason for this is the technical challenge involved: the foundations anchoring the turbines in the seabed generally do not have sufficient load-bearing capacity for larger wind turbines. As such, the structures need to be reinforced accordingly.

The InGROW concept developed by Fraunhofer IWES addresses exactly this issue: a jacket-like support structure is installed around the existing monopile and founded on suction caissons. The resulting hybrid foundation system increases the load-bearing capacity and rigidity, allowing replacement of a 3.6 MW turbine with a modern turbine up to around 10 MW. However, implementation of this concept at full scale requires robust validation under realistic operating conditions. 

The solution

This is where the VGROW research project comes in. The aim is to validate the technical and economic feasibility of the InGROW foundation concept and increase the corresponding technology readiness level (TRL) to 5. To achieve this, Fraunhofer IWES, supported by the technical guidance of an accompanying advisory board, is developing a large-scale demonstrator, which will be tested for economic viability and sustainability using a suitable validation concept. Furthermore, design recommendations and a draft of a reference-based verification framework for the InGROW concept will be developed, which represents a major step toward market readiness.

The added value

The strengthening of existing monopiles for continued use establishes the basis for resource-saving repowering and higher efficiency with lower numbers of larger turbines. With the validation of the InGROW concept, development of a large-scale demonstrator, and establishment of a reference-based verification framework, technical, economic, and permit-related aspects are being addressed. This is laying an important foundation for the preparation of pilot installations. In light of the high proportion of monopile foundations in Europe, the InGROW concept has significant scaling potential and can accelerate offshore repowering.