Model Tests on the Frequency Responses of Offshore Monopiles

He He,Zhu Zhu

doi:10.3390/jmse7120430

Abstract

Monopiles are widely used to support offshore wind turbines as a result of the extensive development of offshore wind energy in coastal areas of China. An offshore wind turbine is a typical high-rise structure sensitive to dynamic loads in ocean environment such as winds, water waves, currents and seismic waves. Most of the existing researches focus on elastic vibration analysis, bearing capacity or cyclic degradation problems. There’re very few studies on vibration of monopiles, especially considering the influence of static loads with different amplitudes, directions, and loading-unloading-reloading processes. In this paper, laboratory-scale 1 g model tests for a monopile in dry sands were carried out to investigate the frequency responses of the monopile under different loading conditions. The bearing capacities of the model monopile were obtained as references, and dynamic loads and static loads with different amplitudes were then applied to the monopile. It was found that (1) the first resonant frequency of the monopile decreases with the increase of dynamic load amplitudes; (2) the first resonant frequency of the monopile steadily increases under the lateral static load and loading-unloading-reloading processes; (3) the frequency responses of the monopile with static loads in different directions are also quite different; (4) damping of the monopile is influenced by the load amplitudes, load frequencies, load directions and soil conditions. Besides, all the tests were conducted in both loose sand and dense sand, and the results are almost consistent in general but more obvious in the dense sand case.

Highlights

Monopiles have been widely used in offshore wind engineering for their low costs, short construction periods and small environmental constraints
Offshore wind turbines are typical high-rise and flexible structures with low natural frequencies, which can be very close to the frequencies of offshore dynamic loads such as winds, waves, currents and seismic waves (Figure 1a)
Vibration characteristics or dynamic impedances of pile foundations are of great concern to designers

Summary

Introduction

Monopiles have been widely used in offshore wind engineering for their low costs, short construction periods and small environmental constraints. They are recommended by DNV (DET NORSKE VERITAS) code to be the well suited foundation type in the offshore wind power industry for water depths below 25 m [1]. Offshore wind turbines are typical high-rise and flexible structures with low natural frequencies, which can be very close to the frequencies of offshore dynamic loads such as winds, waves, currents and seismic waves (Figure 1a). Numerous methods are available in the literature to obtain these vibration characteristics, including: Analytic methods: Winkler type model—The soils around the pile are simplified into a series of independent 1D (one-dimensional) springs [8]; Plain strain model—The soils around the pile are treated as infinitely thin and independent 2D (two-dimensional) layers [9]; Virtual pile-soil model—The soil-pile system is decomposed into a fictitious pile and an extended 3D (three-dimensional) half-space [10]; Integral equation method, which is the most accurate analytical method to study 3D shell type foundations in offshore engineering—The theories of Cauchy singular and Fredholm integral equations are applied to solve the Green’s function of pile and soil [11,12]; He et al [13,14] obtained a rigorous analytical solution for coupled horizontal and rocking vibration of a monopile embedded in a porous seabed, and found that the effect of vertical shear stress on the monopile caused by horizontal loads and moments cannot be ignored

Methods

Results

Conclusion