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Research Papers: Ocean Renewable Energy

Model Tests on the Long-Term Dynamic Performance of Offshore Wind Turbines Founded on Monopiles in Sand

[+] Author and Article Information
Zhen Guo

Research Center of Coastal and
Urban Geotechnical Engineering,
College of Civil Engineering and Architecture,
Zhejiang University,
Hangzhou 310058, Zhejiang, China
e-mail: nehzoug@163.com

Luqing Yu

Research Center of Coastal and
Urban Geotechnical Engineering,
College of Civil Engineering and Architecture,
Zhejiang University,
Hangzhou 310058, Zhejiang, China;
Power Transmission Division,
Zhejiang Electric Power Design Institute,
Hangzhou 310012, Zhejiang, China
e-mail: yuluqingcugb@126.com

Lizhong Wang

Professor
Research Center of Coastal and
Urban Geotechnical Engineering,
College of Civil Engineering and Architecture,
Zhejiang University,
Hangzhou 310058, Zhejiang, China
e-mail: wlzzju@163.com

S. Bhattacharya

Professor
Department of Civil and
Environmental Engineering,
University of Surrey,
Guildford, Surrey GU2 7XH, UK
e-mail: s.bhattacharya@surrey.ac.uk

G. Nikitas

Department of Civil and
Environmental Engineering,
University of Surrey,
Guildford, Surrey GU2 7XH, UK
e-mail: g.nikitas@surrey.ac.uk

Yuelong Xing

Power Transmission Division,
Zhejiang Electric Power Design Institute,
Hangzhou 310012, Zhejiang, China
e-mail: xylzepdi@126.com

1Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received November 17, 2014; final manuscript received May 10, 2015; published online June 9, 2015. Assoc. Editor: Colin Leung.

J. Offshore Mech. Arct. Eng 137(4), 041902 (Aug 01, 2015) (11 pages) Paper No: OMAE-14-1142; doi: 10.1115/1.4030682 History: Received November 17, 2014; Revised May 10, 2015; Online June 09, 2015

The dynamic response of the supporting structure is critical for the in-service stability and safety of offshore wind turbines (OWTs). The aim of this paper is to first illustrate the complexity of environmental loads acting on an OWT and reveal the significance of its structural dynamic response for the OWT safety. Second, it is aimed to investigate the long-term performance of the OWT founded on a monopile in dense sand. Therefore, a series of well-scaled model tests have been carried out, in which an innovative balance gear system was proposed and used to apply different types of dynamic loadings on a model OWT. Test results indicated that the natural frequency of the OWT in sand would increase as the number of applied cyclic loading went up, but the increasing rate of the frequency gradually decreases with the strain accumulation of soil around the monopile. This kind of the frequency change of OWT is thought to be dependent on the way how the OWT is cyclically loaded and the shear strain level of soil in the area adjacent to the pile foundation. In this paper, all test results were plotted in a nondimensional manner in order to be scaled up to predict the consequences for prototype OWT in sandy seabed.

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References

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Figures

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Fig. 2

Shanghai Donghai Bridge OWTs

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Fig. 3

Power spectral densities versus forcing frequencies for a three bladed 3 MW Sinovel OWT

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Fig. 4

Methods for the scaling of small scale test

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Fig. 6

An actuator used to supply dynamic loads in previous tests

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Fig. 7

(a) Novel balance gear system and (b) circular motion of a mass point

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Fig. 8

Sketch of the principles of the balance gears system: (a) gears system for cyclic loadings and (b) relation between resultant force and the rotation angle

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Fig. 9

(a) Shear wave velocity method and (b) acceleration signals recorded by Acc 1 and Acc 2

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Fig. 10

Setup of the model test: (a) general arrangement, (b) dimensions of model OWT, and (c) dimensions of model tank

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Fig. 11

Flow diagram of the model tests

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Fig. 12

Cyclic loadings acting on the model OWT in model tests: (a) tests MTR-1 to MTR-4 and (b) tests MTR-5 to MTR-7

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Fig. 13

(a) Records of the OWT acceleration and (b) frequency response using Welch's method

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Fig. 14

Change of the model OWT's first-order natural frequency: (a) tests MST-1 to MST-4 and (b) tests MST-5 to MST-7

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Fig. 15

Subsidence of the sand surface around the embedded monopile: (a) N = 0, (b) N = 52,404, (c) N = 77,177, (d) N = 85,752, (e) N = 114,336, and (f) N = 157,212

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Fig. 16

The relationship between shear modulus and cycle numbers for different strain level [20]

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