Effect of the installation process on monopile lateral response

The majority of offshore wind turbines (OWTs) are supported by large diameter monopiles that are installed by impact-driving. OWTs are dynamically sensitive structures, with a narrow design range for the eigenfrequency. Fatigue and serviceability limit states can be critical in design. Therefore, the foundation stiffness plays an important role. The pile installation process results in changes to the soil state. This is investigated here through large deformation numerical analyses using the Coupled Eulerian-Lagrangian (CEL) approach with a hypoplastic constitutive relation to model the sand. The soil state is then mapped to a small strain model to evaluate the effect of the installation process on the lateral response. Complementary physical modelling results from monopile installation and lateral load testing in a centrifuge provide confidence in the numerical model. Analyses for two different sands highlight commonalities and differences. Comparison with results featuring wished-in-place or jacked installation illustrates the effect of the soil state changes on the lateral response. The results characterise the changes in soil state and allow quantification of their effects as impact driving tends to produce a stiffer lateral response, with the expected variations due to pile dimensions and sand relative density. Further research will provide insights also into the role of the pore fluid response during impact driving and investigate vibro-driven piles.