Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Fatigue life analysis of monopile foundation offshore wind turbine under coupled multiple loadings
Bin Wanga,b, Zengfeng Sunc, Chenggen Xud, Zhe Tianc,*aKey Laboratory of Far-shore Wind Power Technology of Zhejiang Province, Hangzhou, China
bPowerchina Huadong Engineering Corporation Limited, Hangzhou, China
cCollege of Engineering, Ocean University of China, Qingdao, China
dCGN Rudong Offshore Wind Power Co.,Ltd. , Nantong, China

foundation OWT and pointed out that wind load was the main factor
affecting OWT fatigue. Ju Shenhaw (2019) proposed a fatigue analysis
and design method, which is suitable for non-contact support structuresby using the method of parallel calculation to determine the fatigue life.
Li Haoran (2018) studied the short-term fatigue failure properties ofSPAR floating OWT under random wind and wave loadings, andconsidered the effects of simulated length, wind and wave misalignment,wind effect and wave effect on fatigue damage. Lin Chen (2018)
proposed a method to describe wave flow interaction model and use it toSPAR OWT fatigue analysis. Rui Teixeira (2019) studied the uncertaintyof the definition of loading spectrum in fatigue calculation of OWT, andalso analyzed the damage density under different environmentalconditions. Zhao Jianbin (2019) compared the time-domain method withthe frequency-domain fatigue analysis method, and came to theconclusion that the wind-induced fatigue damage obtained by thefrequency-domain method using the Dirlik model was close to thatobtained by the time-domain method, but the superimposed total fatigue
damage was smaller than the total damage caused by the combined actionof wind and waves. Qin Peijiang (2017) used frequency domain method
to analyze fatigue of OWT support structure. However, the frequencydomain method is more effective for fatigue damage caused by one kindof loading. Considering only one kind of loading will cause larger error,
and the frequency domain method is not applicable to the fatigue analysis
under the combined action of wind and wave. The time domain method
is more suitable for fatigue analysis of OWT. Sheng Zhenguo (2014)
presented a set of time-domain fatigue analysis method and used it toconduct fatigue analysis of offshore wind turbines in wind and wave
environment. Ma yongliang (2012) used the loading superposition
method to analyze the fatigue of the tower structure of OWT under thecombined action of wind and waves and compared it with other methods
such as full coupling method, stress superposition method, and damage
superposition method were used to analyze the fatigue of OWT.
In this paper, aiming at the difficulty of obtaining the strain on the OWT,a theory of using easily accessible displacement to transform strain isproposed. In addition, the whole process of fatigue analysis is described,
and the fatigue life of the monopile OWT is obtained under the action ofwind, wave and their combination, which is of guiding significance tothe life of the OWT.

ABSTRACT
Offshore wind turbine (OWT) gradually attracts people's attention forsolving energy shortage and environmental pollution. The fatigue
analysis of OWT is our research emphasis, since OWT fatigue damage
is one of its main forms of damage during the long term. This paper
focuses on the fatigue problem of the monopile foundation OWT undercoupled loading. In order to solve the above problems, this paper uses amethod to construct effective strain data. In addition, this paper uses
finite element method (FEM) to conduct fatigue analysis for themonopile foundation OWT model by using S-N curve and Miner’s rule.
The fatigue life of the OWT model is considered under the influence ofwind and wave. In this paper, a complete analysis procedure fromdisplacement to strain and from strain to fatigue damage of OWT isobtained, which is of great significance to fatigue analysis of OWT andmakes it possible to obtain the fatigue damage of actual OWT in realtime.

KEY WORDS: offshore wind turbines; fatigue life; S-N curve;
FEM
INTRODUCTION
With the development of society, energy shortage and environmental
pollution has aroused attention, the utilization of clean energy has
become more and more important. China's abundant wind energy
resources and the maturity of offshore wind power technology make thewind power industry gradually transfer from land to sea. Compared withthe land, OWT has to endure more complex environment, such as wind,waves, currents and corrosion and so on, which put forward great
requirements for the development of the OWT industry. During thelifetime of the OWT, the loading cycle times of reciprocating changes
such as environmental loadings can reach more than 109. Excited by thecyclic loadings, the weak or stress concentration part of the OWT always
occur fatigue failure. As a result, the research to avoid fatigue failure ofOWT become a significance work for scientists. Li Wei (2011)
considered the influence of different loading sequence on fatigueproperties, conducted fatigue analysis on OWT foundation structure, andfurther explained that miner’s law did not consider the influence ofloading sequence. Mo Jihua (2011) analyzed the fatigue of monopile

NUMERICAL METHODS233