Dynamic response of monopiles in sand using centrifuge modelling

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.