An Assessment on Permanent Rotation of Monopile Foundations for Large WTGs under Operational Loading Conditions in North Sea OWPs Using the High-cycle Accumulation Model

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4. An Assessment on Permanent Rotation of Monopile Foundations for Large WTGs under Operational Loading Conditions in North Sea OWPs Using the High-cycle Accumulation Model

The design of monopiles is mainly driven by pile head movement, either in the form of maximum allowable deflection for bearing capacity check or permanent rotation during the lifetime of the foundations. Among the different assessments required for these checks, the permanent rotation due to long-term operational loading is highly complex. This loading scenario and its consequent soil ratcheting around the foundation is, in sandy soil, mainly happening in drained conditions for which no clear methodology has been presented in the industry standards. In this study, a case study of a monopile in a sandy soil profile in the North Sea was considered to evaluate the permanent rotation of the foundation under operational loading, containing packages of cycles with different average loads and amplitudes. The High Cycle Accumulation (HCA) model, which accounts for strain accumulation under drained cyclic loading, was adopted with 3D Finite Element (FE) analysis to carry out this assessment. It was observed that the major contribution to the permanent deformation due to the cyclic loading effect comes from load packages with high average loads and a high number of load cycles. Load packages with high cyclic amplitude have fewer load cycles, which lead to insignificant cyclically accumulated strain. Therefore, they act more similarly to a constant loading.  

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

9 - Monopile design to cyclic loads: quasi-static, dynamic and seismic loads