Considerations for a Reliability-based Foundations Design in Varying Rock Mass conditions – A Review from the Saint Brieuc Offshore Windfarm

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The St. Brieuc Offshore Windfarm site is located within the Massif Armoricain comprising shallow Precambrian rock subject to varying degrees of weathering. The Wind Turbine Generator (WTG) foundations consist of jacket substructures supported by drilled and grouted piles. The main foundation design risks were related to 1) defining a design method to reliably account for the rock mass conditions, 2) accounting for the ground variability considering the presence of faulting zones and a dyke swarm and, 3) accounting for the implications of the adopted installation method into the pile load capacity. To assess the design reliability, the equivalent working stress design factors of safety were assessed to envelope representative scenarios that could arise from the uncertainties associated to these design risks. This study was focused on confirming that in all the foreseeable scenarios, the resulting safety margins would be within acceptable limits. The outcome of this study was crucial during the pile installation phase to substantiate the decisions offshore as the drilling performance was subject to the varying ground conditions with occasional requirement for additional design substantiation work to confirm that the as-built conditions were compliant with the design requirements. Finally, the as-built records on the seabed conditions and drilling activities were assessed and compared against the site ground model to conclude whether the aimed foundation design reliability levels met the target threshold. The findings described in this paper are considered of interest to similar industry projects working towards reducing the Levelised Cost of Energy of offshore Windfarms.

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

17 - Design Standards, Certification and Risk Mitigation