An updated explicit approach to account for undrained cyclic loading effects in offshore monopile foundations

¹ Seequent – The Bentley Subsurface Company, Delft, The Netherlands
² Department of Geoscience and Engineering, Delft University of Technology, Delft, The Netherlands

^* Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Abstract

Cyclic loading, for example on offshore monopile foundations, affects the bearing capacity compared to static conditions. Rather than simulating the full loading history, the capacity under cyclic loading can be evaluated using an explicit approach, where loads and soil response are expressed as a function of the number of cycles. Along this line, the Norwegian Geotechnical Institute has developed a cyclic accumulation procedure to derive target stress-strain curves, which can be fitted with appropriate constitutive models for the modelling of offshore foundations in the finite element method. A simplified version of this procedure, known as UDCAM-S (which uses the NGI-ADP constitutive model in the background), has been implemented in the finite element code PLAXIS. This procedure has been recently updated to account for different levels of shear strength moblization along a monopile. It employs an iterative procedure where the seabed is divided into several sub-layers, each with its own load history and corresponding parameter set. The NGI-ADP model parameters are fitted to target curves for triaxial compression, extension, and direct simple shear, using a Particle Swarm Optimization algorithm. To speed up the optimization process, a fast surrogate model, pre-trained on tens of thousands of single stress-point laboratory test simulations, was developed. This results in an efficient procedure that provides a more realistic representation of the seabed response for the design of offshore monopile foundations under undrained cyclic loading conditions.