Drainage Conditions Around Monopiles in Sand Under Cyclic Loading

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Sand, and especially silty sand, is highly degradable under cycles of undrained loading, hence the degree of drainage in a storm fully controls the required pile penetration depth in sand dominated profiles. Undrained cyclic stress-strain curves, representing the equivalent accumulated strain for storm loading at different stress mobilizations, are typically employed for monopile design analysis in sand dominated units based on interpretation of cyclic contour diagrams using site-specific storm Markov matrix data. Such design approaches can also consider the effect of drainage occurring concurrently with the excess pore pressure (epp) generation during each load parcel. Conservative drainage assumptions are typically employed within simplified analytical or empirical models to estimate the degree of epp dissipation. This paper reviews the drainage characteristics (epp build-up and dissipation) around a laterally loaded monopile. In particular, a number of key factors influencing the epp dissipation (load utilisation, pile embedment ratio, sand permeability and presence of a shallow clay layer) were investigated. Fully coupled flow-deformation finite element analyses were performed using a soil constitutive model, calibrated against cyclic contour diagrams from a North Sea dense sand to generate a representative pore pressure field, for a given number of cycles, prior to running the dissipation stage to examine the storm drainage time.
The paper shows that the drainage characteristics of the system can have a substantial impact on the storm drainage time and hence the monopile response. The evolution of pore pressure ratio over time is presented which can be used to inform the degree of epp dissipation that can be considered during a storm for the various sensitivity cases. It is also shown how these results can be used to calibrate an analytical radial dissipation model within the design framework of cyclic envelope stress-strain curve development.

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

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