Axial Dynamic Stiffness of Tubular Piles in Viscoelastic Soil

Mehdi Bayat,Lars Ibsen,Lars Andersen

doi:10.3390/en9090734

Abstract

Large offshore wind turbines are founded on jacket structures. In this study, an elastic full-space jacket structure foundation in an elastic and viscoelastic medium is investigated by using boundary integral equations. The jacket structure foundation is modeled as a hollow, long circular cylinder when the dynamic vertical excitation is applied. The smooth surface along the entire interface is considered. The Betti reciprocal theorem along with Somigliana’s identity and Green’s function are employed to drive the dynamic stiffness of jacket structures. Modes of the resonance and anti-resonance are presented in series of Bessel’s function. Important responses, such as dynamic stiffness and phase angle, are compared for different values of the loss factor as the material damping, Young’s modulus and Poisson’s ratio in a viscoelastic soil. Results are verified with known results reported in the literature. It is observed that the dynamic stiffness fluctuates with the loss factor, and the turning point is independent of the loss factor while the turning point increases with load frequency. It is seen that the non-dimensional dynamic stiffness is dependent on Young’s modulus and Poisson’s ratio, whilst the phase angle is independent of the properties of the soil. It is shown that the non-dimensional dynamic stiffness changes linearly with high-frequency load. The conclusion from the results of this study is that the material properties of soil are significant parameters in the dynamic stiffness of jacket structures, and the presented approach can unfold the behavior of soil and give an approachable physical meaning for wave propagation.

Highlights

There are more than 7000 offshore structures around the world
It can be noticed that the turning point, at which the concave curve changes into the convex curve, is the same for all different loss factors
In contrast with the results for different values of the loss factor, other material properties, such as Young’s modulus and Poisson’s ratio, do not have any effect on the phase angle, like the results reported in [27,33]

Summary

Introduction

There are more than 7000 offshore structures around the world. Structures to support wind turbines come in various shapes and sizes; the most common are monopile, jacket, tripod, gravity base and floating structures (Figure 1). There are several kinds of platforms, but a fixed structure in a jacket structure is considered in the present paper. The tendency of large-sized offshore wind turbines has increased during the last 10 years. As wind turbines get larger and are located in deeper water, jacket structures are expected to become more attractive. A fixed platform is described as consisting two main components; the substructure and the superstructure.

Objectives

Results

Conclusion