Numerical investigation on bearing capacity of OWT foundation with large diameter monopile under Seismic load

Abstract

Dynamic characteristics of seabed soil under seismic load arevery important for the overall stability of the offshore wind turbine foundation. In this study, the horizontal bearing capacity and its corresponding soil deformation around the large-diameter monopile, where withstandsa transient high frequency seismic load, are simulated using a numerical method. During the numerical analysis in this paper, first a dynamic boundary surface model of saturated soil is derived instead of the empirical strength degradation method, and then a pile-soil contact model is considered instead of no-slip tie assumption. On the basis of the numerical results, an empirical degradation coefficient for the horizontal ultimate bearing capacity of monopile is proposed in this paper, which can well evaluate the effect of seismic load on the bearing capacity. Along the axis direction of the monopile, an intensive study about deformation law and failure mechanism of the seabed soil is analyzed by comparing somekey points at different crossing sections.Finally, some parameters which may affect the bearing capacity and the excesspore pressure are discussed. It can be found that the horizontal bearing capacity of the monopile can be obviously improved by increasing the buried depth than the diameter of monopile, and the seismic load has great influence on the soil deformation law around the monopile.

Introduction

In the context of the pressing need for clean and sustainablesources of energy, some countries are adoptingambitious policies for the branch of renewable energies, and the harvest of wind energy through theusage of offshore wind turbines is likely to play a key role, sothe offshore foundations are back into the spotlight of engineeringresearch again.Monopiles are the most commonly used foundation system forsupporting offshore wind turbines (OWT), which are single largediameter open-ended tubular steel piles. With the increase of impeller diameter and tower of offshore wind turbine, the buried depth and the diameter of monopile foundation also increase accordingly.But, there are several special phenomena inherentto the offshore environment compared with the terrestrial environment. On the one hand, the effects on the foundationof a permanent exposure to cyclic loads from wind, wave and seismic load,which are still not well understood and do require considerableefforts in research (e.g. Randolph et al, 2005; Raffaele,2018); On the other hand, it must also be pointed out that apart fromthe excess pore pressure generated by the low frequency wave load and wind load acted on the turbine, the effect of high frequency seismic load may result in a net porepressure accumulation and soil shear strength attenuation within a very short period of time (e.g. Luan et al, 2009; Kaynia,2019; Hu et al,2020), which could causeits foundation to sink, tilt or destruct. So the frontier engineering issues faced by the geotechnicalspecialists for the marine foundation of such offshore wind turbines arefar from closed, and the remarkable feature are aggravated by their large dimensions and especiallyby their high ratios of lateral to vertical load, which makethem unprecedented in the offshore engineering field.

Now, the large monopile foundations of about8m in diameter arecurrently state of the art and plans for future wind farms (Achmus et al, 2009). The behaviour of such foundations underthe effects of a short-term seismic load is currently a subject ofintense research; a typical monopile schematic diagram of offshore wind turbine in two-dimensional planeis shown in the Fig. 1 and is adopted in this paper.However, most design considerations for dynamic seismic effects arebased on pseudo-static approaches where the foundation'sresponse is calculated from a static analysis using degradatedsoil strength properties (e.g. Taiebat 2000, Pablo et al 2014). It can be noted that such‘dynamic degradation'is often just an empirical techniqueby experts to merelyestimate the magnitude of accumulated displacements and doesnot necessarily reflect the actual physical characteristics of the soil due to dynamic cyclic load.

A number of investigations using scaled model test and numerical analysis have been conducted to study the response of pile-soil interaction under environmental cyclic loads (e.g. Brown&Reese,1987; Sadek & Isam,2004; Mayoral et al,2005; Kallehave et al,2012; Lu & He, 2014; Zdravkovic et al, 2015; Byrne et al, 2017; Versteijlen et al, 2017; Kementzetzidis et al, 2019). Moreover, numerical simulation can provide further insights into the failure mechanisms and explain some observations, so using numerical methods to study the soil dynamic deformation and excess pore pressure of foundation have been presented by a number of researchers (e.g. Al-Homoud & Al-Maaitah, 1996; Zhou& Randolph, 2009; Omarov, 2010; Mayoral et al, 2016). In most cases, when seabed soil is withstood cyclic load, some dynamic mechanical characteristics such as the cumulative plastic deformation, the stiffness softening and the strength reduction, usuallyappear clearly (e.g. Tileylioglu et al,2011; Pecker et al, 2012; Li et al,2020). And due to of the complex behaviour of pile-soil interaction, some researchers employed a mathematical models in form of a $p-y$ curve or modified dynamic $p-y-\dot{y}$ curve to simulate the soil stress-strain constitutive relationship shown in Fig. 2, where $p$ is the cyclic horizontal resistance per unit length of the buried pile and $y$ is the corresponding horizontal motion, $\dot{\mathrm{y}}$ is the rate of horizontal motion (e.g. Matlock et al, 1978; Randolph, 1981; Naggar et al, 1995; Mohammad et al,2012; Chatterjee et al,2015; Murphy et al,2018). Meanwhile, in order to consider the cyclic weakening of soil shear strength and its corresponding accumulated plastic shear strain, Einav & Randolph (2005) proposed a shear strength degradation curve and an empirical equation. Bayat et al. (2016) suggested a set of theoretical equations to reflect the influence of load rate on soil-pile interaction. And Zhang et al (2019) proposed a simple formula to calculate the dynamicdegradation relation of soil resistance under cyclic numbers as shown in formula (1) below.

$$R_n\left(N\right)=0.25+\frac{1.25}{1+e^{\frac{N-\pi }{2\pi }}}$$

Where, the parameter $N$ expresses the numbers of dynamic cycles, and $R_n\left(N\right)$ means the degradation coefficient of soil resistance.

Actually, when subjected to high frequency dynamic seismic load, the horizontal bearing capacity of large-diameter monopile is different from the quasi-static condition; and the soil deformation law and the failure mechanism surrounding the monopile have obvious influence on the horizontal bearing capacity. Moreover, under the dynamic seismic load, the seabed soil tends to exhibit some dynamic characteristics such as the stiffness softening andthe strength weakening, which also affects the interaction between pile and soil.Based on the assumption that the seabed soil presents elastic-plasticity deformation under dynamic load, a dynamic constitutive model of the seabed soil was derived by using the boundary surface theory in this paper; andan interface contact algorithm is also proposed by assuming that there are both tangential and normal constraints on the interface between pile and soil. Then the horizontal ultimate bearing capacity and its corresponding soil deformation law due to cyclic seismic load are analyzed numerically.