۱- Introduction

۲- Experimental setup

۳- Measurement of soil stiffness and pile stiffness

۴- Dynamic response of monopiles

۵- Effects of dimensions of pile: diameter, embedded depth, and free length

۶- Influence of soil density on pile response

۷- Comparison to 1 g tests

۸- Theoretical prediction and experimental data

۹- Conclusion

References

Abstract

Monopiles are one of the most commonly used offshore foundation for wind turbines. Their static capacity, p-y curve and cyclic loading behaviour have been studied using 1 g tests and centrifuge tests, but there is little experimental data regarding their natural frequency, especially using centrifuge testing. The design of offshore wind turbine foundations is largely governed by natural frequency as resonance due to cyclic loading can cause damage and even failure. Understanding the dynamic response of the monopile under free vibration is thus critical to design. This paper presents the results of novel monopile (large diameter) and single pile (small diameter) tests in a centrifuge to for the first time directly determine the natural frequency (fn) of the pile-soil system. An experimental methodology was used to define the natural frequency via measured acceleration and force time histories and their fast Fourier transforms (FFT) under a force applied at a controlled frequency. The effects of pile diameter, embedded length, free length of the tower and soil density on fn were investigated in the centrifuge tests. The same models used in the centrifuge test at 50 g were also tested at 1 g in order to assess the relevance of earlier 1 g investigations into system behaviour. The measured natural frequency of wind turbine monopiles in centrifuge models during harmonic loading from a piezo-actuator, confirmed that soil structure interaction at an appropriate stress level must be taken into account to obtain the correct natural frequency. The experimental data was compared to theoretical solutions, giving important insights into the behaviour of these systems.

Introduction

Wind energy is becoming increasingly more attractive as a source of renewable energy and has widespread potential for application in different regions of the world. Wind turbine technology has been continuously improving, particularly with respect to mechanical and electrical innovations, leading to progressively larger and more powerful turbines And consequently tower heights have increased. This trend looks set to continue, with Wiser et al. [3] suggesting that the hub height will reach 160 m by 2030. In this scenario, the dynamic response of the tower and foundation will be of the utmost importance. The European Offshore Wind Energy Association [4] reported that 3018 MW of offshore wind energy was installed in European waters in 2015. By 2016, Europe had 81 wind farms with 3589 turbines and the cumulative installed capacity reached 12,631 MW. Although the majority of wind turbine capacity is being built in Europe, America and China have also established targets to develop 3305 MW and 10,000 MW of offshore wind energy by 2020 respectively [5]. Offshore wind turbines will hence play a significant role in the global electricity market in the future. Currently monopiles account for 80% of offshore wind turbine foundations with gravity bases accounting for 9% [4]. The remaining foundations include jackets, tripods, tri-piles and floating foundations. The monopile is a short and rigid circular steel pipe pile, which has a slenderness ratio of approximately 5 [6]. It is a common foundation design for offshore wind turbines because it is economical at shallow water depths (10–۲۵ m). The typical dimensions of a monopile are a 3–۶ m outer diameter and 22–۴۰ m length [7]. Although a monopile is a simple foundation design concept, understanding the dynamic soilstructure interaction (SSI) of a wind turbine on a monopile foundation is a complex task.