**6. Conclusions**

In this paper, we propose a probabilistic framework for updating structural reliability of offshore wind substructures based on new information from digital twins. The digital twin information is consistently included in the framework by updating the uncertainty related to structural dynamics and load modeling and propagating this uncertainty to the fatigue damage accumulation. The resulting uncertainty is then converted into uncertainty of the stress ranges, which is included in a probabilistic model on structural reliability. The proposed framework is applicable to offshore wind substructures whose lifetimes are governed by fatigue damage accumulation.

The framework is applied to two case studies, where the potential for improved decision models for existing and new structures is demonstrated. In the former case, updating soil stiffness and wave loading is considered to investigate the potential for lifetime extension of fatigue critical joints. In the latter case, the framework is applied to optimize new structures by using Bayesian pre-posterior theory for future wave load calibration.

**Author Contributions:** Conceptualization, D.A., M.D.U. and J.D.S.; methodology, D.A., M.D.U. and J.D.S.; formal analysis, D.A., M.D.U. and J.D.S.; writing—original draft preparation, D.A., M.D.U. and J.D.S.; writing—review and editing, D.A., M.D.U. and J.D.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** Innovationsfonden: 8053-00146B; Rambøll Fonden: A0.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The study does not report any data.

**Acknowledgments:** The work presented herein is financially supported by Ramboll Foundation, Ramboll Energy and Innovation Fund Denmark. The financial support is highly appreciated. The first author would like to thank colleagues from Ramboll for their continuous support.

**Conflicts of Interest:** The authors declare no conflict of interest.
