Abstract

Offshore wind turbine is highly affected by wind load unlike general offshore structures. When fatigue analysis is performed for predicting fatigue life of that structure, coupled effect of wind and wave load should be considered at the same time. If it is needed to take into account two loads which have different frequency ranges in fatigue analysis, time domain analysis gives the most accurate result. But it is not preferable in real engineering due to high time cost. That makes many researchers suggest more efficient ways to estimate the fatigue life on that structures. In this paper, numerical simulations are performed to assess the fatigue damage of a floating offshore wind turbine by approximate methods. The results of these methods are compared by those of time domain method in few design load cases to check whether suggested methods are proper in estimation on fatigue life of offshore wind turbine.

Introduction

Due to the increasing energy consumption and pollution caused by fossil fuels, various alternative energy resources have been developed in many countries. Among these resources, offshore wind power are highly favored and actually operated in a few nations. In general, offshore wind turbine is installed in harsh environmental conditions where wind speed is high for maximizing the amount of power production. However, in point of structural safety, these environmental condition could have an adverse effect on fatigue life of this.

Offshore wind turbine could be divided into hull and turbine parts. Each part is made by different venders and fatigue analyses of each part are also performed separately due to lack of communication between vendors. The fatigue damage of turbine part is generally calculated by time domain approach. At first, time series of turbulent wind field are generated by using turbulence model in site and applied force at rotor could be calculated in every time step. Fatigue damages in hot spots are determined with rain-flow counting method. In case of hull part, like ships and offshore platform, spectral fatigue analysis which is based on frequency domain are widely used to estimate the fatigue damage of hull part. This separated process could make it different to estimate accurate fatigue life of hull part and requires additional way to combine the effect of wind and wave loads.

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