Effect of the installation process on monopile lateral response

Most offshore wind turbines (OWTs) are supported by large-diameter monopiles, installed by impact driving. OWTsare dynamically sensitive structures, with a narrow design range for the eigenfrequency. Fatigue and serviceabilitylimit states can be critical in design, so the foundation stiffness plays an important role. The pile installation processresults in changes to the soil state, investigated here through large-deformation numerical analyses using the coupledEulerian-Lagrangian approach with a hypoplastic constitutive relation to model the sand. The soil state is thenmapped 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 provideconfidence 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 changes in the soilstate on the lateral response. The results characterise the changes in soil state and allow quantification of their effectsas impact driving tends to produce a stiffer lateral response, with the expected variations due to pile dimensions andsand relative density. Further research will provide insights into the role of pore fluid response during impact drivingand investigate vibro-driven piles.