Definition of primary and unloading / reloading stiffness curves to model lateral monopile behavior for design of offshore structures

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4. Definition of primary and unloading / reloading stiffness curves to model lateral monopile behavior for design of offshore structures

The dimensions of offshore monopile foundations are mainly governed by lateral performance, namely the lateral strength and irrecoverable rotation design criteria. Numerical modelling of environmental loading applied to the foundation relies on the definition of soil interaction stiffness curves to derive foundation strength and deformability. The determination of the primary/virgin loading stiffness curve of monopiles under lateral loading has been subject of numerous studies, notably the PISA design approach, supported by finite element (FE) modelling. However, in new geographies and during earlier stages of design, numerical analyses are cumbersome hence an alternative approach to derive rule-based components of the PISA methodology is needed. Moreover, the unloading stiffness is still conservatively assumed in design practice as the initial stiffness, resulting in an overestimation of the permanent rotation caused by extreme wave loading. The determination of the unloading and reloading lateral stiffness can have significant influence on the design of large diameter monopile structures. This article presents a methodology to define both the primary and unloading/reloading lateral monopile curve. The primary loading curve is defined based on a rule-based approach, formulated in total stress space, which adopts the fundamental principles of soil mechanics to match the soil spring data from finite element models. The unloading/reloading lateral behavior of the monopile is modeled using a hysteretic soil damping curve derived from analytical methods, based on the intrinsic relationship between the two physical quantities. The stiffness curves developed through this methodology are benchmarked against results from 3D FE analyses based on typical North Sea conditions.

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

8 - Monopile design to lateral monotonic loads