Offshore Wind Turbine Foundations on Rocky Formations – The French Experience

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4. Offshore Wind Turbine Foundations on Rocky Formations – The French Experience

French coasts offer a large variety of geological contexts ranging from soft sediments in the Mediterranean sea to basalt offshore Brittany. The first awarded offshore wind projects were all concerned by rocky formations including weak chalk (with various flint contents), limestone, calcarenite and metamorphic formations. Each of these projects had to tailor solutions for foundation design and installation. Gravity based structures were selected as Wind Turbine Generator (WTG)s foundation type on Fécamps chalk whereas jackets with driven piles were considered on the Treports chalk formations. Jackets with drilled and grouted piles were adopted on the ground dominated by the Neoproterozoic and Lower Palaeozoic rocks of the bay of Sant-Brieuc. Monopiles were considered for projects offshore Saint-Nazaire, Noirmoutier and Courseulles, all involving large diameter drilling operations in carbonated rocks. The purpose of this keynote paper is to describe the soil and rock conditions prevailing on the French coasts and highlight the challenges encountered for the WTG developments. Various aspects will be discussed including site investigations, foundation type selection, foundation installation and performance. International recommendations for the design of offshore foundation in rock being very poor, specific R&D programmes have been conducted such as onshore pile tests in representative rock conditions or large-scale laboratory tests. Innovative solutions were also developed by contractors to adapt the pile installation technique to the different configurations.

5th International Symposium on Frontiers in Offshore Geotechnics (ISFOG2025)

Keynote Papers