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 of a pair of companion papers that investigates the effect of different installation methods on the subsequent response of monopile under lateral loading through extensive numerical analysis. The present paper focuses on quantification of the effect of pile installation on the soil states 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 are 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.

Effects of Monopile Installation on Subsequent Lateral Response in Sand. II: Lateral Loading

“…The radius and depth of the soil domain are identical to those used in the pile installation model described in the first of the companion papers (Fan et al, 2020), with a width of 10.8Dpile and a depth of 7.6Dpile. The soil surface post installation (Fan et al 2020) was approximated by a spline, which was revolved for the 3D lateral loading model. The side of the soil domain is restrained from any lateral displacement and the base of the soil domain is restrained from any vertical displacement.…”

“…The properties of very fine UWA silica sand are given in Table 1 of the first of the companion papers (Fan et al, 2020). The sand was modelled using a rate-independent hypoplastic constitutive law by (Kolymbas, 1991(Kolymbas, , 1985 in the form proposed by von Wolffersdorff, (1996) with the enhancement of intergranular strains by Niemunis and Herle, (1997).…”

“…The user subroutine of the UMAT implementation for Abaqus/Standard by Gudehus et al (2008), as available on soilmodels.com, was used. The hypoplastic constitutive model parameters are given in Table 2 of Fan et al (2020).…”

“…To capture the effect of pile installation, the post-installation soil state needs to be taken into account in the lateral loading model. The results (stress and state-dependent variables) obtained from the pile installation model (Fan et al 2020) were mapped to the lateral loading as initial soil conditions following the methodology outlined by Heins and Grabe (2017). Only one-quarter of the full model was simulated during the installation phase, while half of the full model was simulated during the lateral loading phase.…”

“…Monopiles used for OWTs are typically installed into the seabed by impact driving. As shown in the first Effects of pile installation on … Fan, Bienen, Randolph _____________________________________________________________________________________ 5 of the companion papers (Fan et al, 2020), the post-installation soil state, with indicators including stresses and void ratio, are significantly affected by the pile installation process.…”