ABSTRACT

The lateral load response of a monopile is often modelled by p-y curves. However, the traditional formulation of p-y curves is unable to describe the behavior of liquefied soil. This paper presents a numerical investigation of how to develop p-y curves for monopiles installed in liquefiable soil. A monopile is modelled in the 3D finite element software package PLAXIS 3D, where a soil model able to capture liquefied soil behavior is applied. The numerical results are used to extract relevant data to assess liquefaction in the soil. Based on this information, p-y curves are generated for the liquefied soil.

INTRODUCTION

Most offshore wind farms are located relatively close to the coastlines in Northern Europe, where the seismic activity is generally low. However, the interest in offshore energy has emerged in areas with high seismic activity, such as in East Asia, which requires a foundation design that is able to withstand the effects of seismic loading.

The seismic load from e.g. an earthquake is cyclic by nature. This type of loading can lead to generation of excess pore pressure in the soil. In loose (contractive) soils, the generated excess pore pressure may reduce the effective stress level to zero. Hence, the soil strength and stiffness are lost. This is the liquefaction phenomena (Andersen 2009, 2015, Nielsen et al. 2013, Nielsen 2016), which can be critical for the foundation (Japan National Committee on eathquake Engineering 1964, Hsein Juang et al. 2005, Cubrinovski 2013).

The most used foundation concept for offshore wind turbines is the monopile foundation concept, as illustrated in Figure 1. One of the governing design criteria for a monopile is the rotation of the foundation. To assess the rotation, the lateral displacement at seabed must be evaluated. The lateral displacement of a monopile is typically calculated using p-y curves, where the soil-structure stiffness is modelled using springs. These p-y curves have been developed to describe the monotonic load-displacement behavior. A typical p-y curve for cohesionless soil has an upward convex shape (Det Norske Veritas 2011), where the tangential stiffness of the soil decreases as the pile displaces towards the soil. Different design practices (Ashford et al. 2011) suggest to apply a multiplier (mp) to the p-y expression for the non-liquefied soil to assess the effect of liquefaction by simply decreasing the stiffness and ultimate resistance of the soil. Nevertheless, laboratory triaxial tests show that the stress-strain behavior of post-liquefaction monotonic loaded soil follows an upward concave shape (Rouholamin et al. 2017), where the stiffness increases as the strains increase. Standard p-y formulations will therefore not give a good representation of the post-liquefaction soil behavior. In a design where there is a risk of liquefaction, the engineer therefore requires a representation of the soil-structure interaction, which better captures the liquefaction in comparison to what is given by standard non-liquefied p-y curves.

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