Multi-directional and combined force-moment limit state envelopes for the design of offshore monopile founded in sands

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The geotechnical design of laterally loaded offshore wind turbine monopile foundations usually has two main requirements: 1) design load cases must not exceed the total lateral pile resistance calculated with material partial factors and 2) a limit on pile head deformation and rotation under critical load cases. When resolving and translating load combinations into geotechnical design loads on the foundation, the effect of loads on the turbine structure acting in multiple directions at varying moment arms is often neglected. For ULS design, a maximum load case prescribed to act at a single moment arm is typically assumed as the most onerous static load case. This is done without consideration of the fact that the pile capacity is dependent on the assumed lever arm and without full definition of the limit state envelope. Three-dimensional finite element analyses are well placed to evaluate limit state envelopes so that combinations of load magnitudes, directions and moment arms can be jointly considered. This paper presents a design workflow using a parametric numerical study on a typical monopile founded in sands of different densities to jointly consider these factors. The evaluated response envelopes are proposed as a more rigorous evaluation of geotechnical design limit states and can be adopted in conjunction with a probabilistic treatment of magnitude, height and direction of environmental loads.

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

8 - Monopile design to lateral monotonic loads