On the Influence of the Soil Stratigraphy on the Monopile Deflection Investigated With FEM in a PISA Framework

Abstract

Large diameter monopiles are the most commonly used foundation type for offshore wind farms. The accurate representation of their response to lateral loading to optimise the required pile penetration plays a crucial role in the design of offshore wind farms. Recently, the PISA method proposed by Burd et al.[6] and Byrne et al. [5] has gained importance in the design of monopiles as it more accurately represents the interaction between the monopile and the soil than standard methods. In the PISA project, sand as well as soft and stiff clay were investigated through monotonic pile tests and the PISA parameters to represent the monopile-soil interaction curves were calibrated for each soil type investigated. Normally, the PISA parameters are derived separately for each soil type and then combined in a 1D model to include the soil stratigraphy. However, soil conditions on site are typically heterogeneous and the interaction between different soil layers with different strengths and stiffnesses should be captured. In order to gain a deeper understanding of the influence of soil stratification on pile deflection curves, this paper presents numerical simulations of a large diameter laterally loaded monopile embedded in layered soil using the finite element method (FEM) including a combination of soft clays and dense sands. Based on the FEM results, PISA parameters for the monopile-soil interaction curves are derived and implemented in a 1D beam model to calculate the pile deflection. The results are compared with a 1D model where the PISA parameters are first derived for the individual soil types and then combined to include the same soil stratigraphy. It is shown that the influence of soil stratification on pile deflection is significant. This paper highlights the need for calibration of PISA parameters considering the actual soil stratigraphy present on site.