Abstract

In the offshore environment, aerodynamic and hydrodynamic loads induced by wind, wave and current are directly or indirectly applied to the foundations for offshore wind turbines (OWTs). To avoid resonance, the first frequency of the OWT system must keep away from the frequencies of these external excitations. The monopile foundation has a significant effect on the dynamic responses of OWT system, which should be carefully evaluated in the design. In this paper, a comprehensive numerical study has been carried out on the dynamic behaviors of the OWT supported by monopile in sand. In the numerical model, the OWT structure is simplified as an Euler-Bernoulli beam with a series of nonlinear Winkler springs, in which the soil resistance is modeled by p-y, t-z and q-z curves. The influences of dynamic loads, the dimensions of the monopile and tower (length, diameter and wall thickness) are investigated in detail, and some useful conclusions are drawn in the end.

Introduction

Monopile is an economical foundation option for the offshore wind turbine located in shallow water (water depth less than 30m). During its service life, the embedded monopile always sustains complicated environmental loads, i.e. the wind, the wave and other external loads. As the offshore wind turbine is a kind of dynamic-sensitive structure, dynamic loads including the wind turbulences, waves, rotational frequency of rotor (1P) and the blade passing frequency (3P for 3 bladed turbine), need to be considered in the design (Arany, Bhattacharya et al. 2015). A typical frequency range for energy-rich waves varies from 0.05 to 0.5 Hz(Bisoi and Haldar 2014). For variable speed generator in modern OWT system, the operational frequency of the rotor varies from 0.1-0.3 Hz, and thus the 3P frequency lies in the range of 0.3-0.9 Hz. Designers must keep the first natural frequency of the OWT away from frequencies of all these external loads.

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