Effects of Monopile Installation on Subsequent Lateral Response in Sand. I: Pile Installation

Abstract

Monopiles are widely used as the foundation to support offshore wind turbines (OWTs). The response of monopile-supported OWTs is strongly affected by the natural frequency of the system relative to the load spectrum, and in turn, the natural frequency is governed by the initial stiffness of the foundation. Both fatigue limit state (FLS) and serviceability limit state (SLS) design are also influenced by the foundation stiffness. The stiffness of the foundation during operational loading is a function of the initial soil state postinstallation, which is influenced by the installation effect. This is the first part of a pair of companion papers that investigates the effect of different installation methods on the subsequent response of monopiles under lateral loading through extensive numerical analyse. The present paper focuses on the quantification of the effect of pile installation on the soil state in sand for three different initial relative densities. The numerical model is first validated against purpose-designed centrifuge tests. Subsequent analyses demonstrate how the soil state, including the void ratio and stresses, is altered from in-situ conditions during pile installation and is markedly different depending on whether the pile is jacked or impact driven. The results presented here and in the companion paper highlight the need to account for the effects of the installation process on the lateral pile response.