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Heave motion response of a floating offshore wind turbine with damping plate

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Author(s)
고혁준
Issued Date
2014
URI
http://dcoll.jejunu.ac.kr/jsp/common/DcLoOrgPer.jsp?sItemId=000000006851
Abstract
A rigid damping plate attached to the bottom of a cylinder has a distinct advantage in reducing the motion response of a floating circular cylinder by increasing the added mass and the damping coefficient. The added mass plays an important role in determining the location of the resonant frequency, and also the radiation damping reduces the motion amplitude at resonance. Furthermore, the porous holes of the permeable damping plate induce flow separation and vortices agitation, causing the increase of the viscous damping and energy loss. To obtain better heave characteristics, the present research proposes dual damping plates: a porous upper-damping plate attached to the side wall of the cylinder and a rigid lower-damping plate to the bottom.
Analytical and experimental studies are carried out to investigate the heave motion response of the cylinder according to the characteristics of the damping plate such as depth ratio, diameter ratio, and porosity. The analytical method by using Matched Eigenfunction Expansion Method (MEEM) has been developed for the heave motion analysis of the floating circular cylinder with the single rigid or dual (a rigid and a porous) damping plates in the context of linear potential theory and Darcy's law (the normal velocity of fluid passing through a thin porous disk is linearly proportional to the pressure difference across it). To apply the MEEM, the fluid domain is divided into three regions, and both the diffraction and the radiation potential in each region are expressed by the Fourier Bessel series. The unknown coefficients in each region are determined by applying the continuity of the pressure and the normal velocity at the matching boundaries. With the assumption of an inviscid fluid in the potential theory, a heave viscous damping is calculated by the non-dimensional damping coefficient obtained from a heave free decay test. In order to confirm the analytical solutions, experiments have been conducted in 2-D wave flume for the test in regular waves, and in large scaled wave flume for the test in irregular waves, with varying wave conditions. The analytical results are in good agreement with the experimental results in both regular and random waves, and the heave motion response of the cylinder is decreased drastically in the heave resonance region by the proposed dual damping plates.
In the application study of the damping plate to the Floating Offshore Wind Turbine (FOWT), the NREL offshore 5MW baseline wind turbine and spar type floating support structure are adopted to perform computational simulations using FAST code. The hydrodynamic input data of the floating support platform with a single rigid and dual (a rigid and a porous) damping plates for HydroDyn, hydrodynamics module of FAST code, is calculated by the MEEM. The heave motion and the variation of the axial forces of the FOWT are considerably decreased at around the heave resonant frequency by the damping plate. It would be a great advantage to extend the fatigue life of the FOWT. In addition, it shifts the heave natural frequency to the lower frequency region due to the increase of the added mass. However, the decrease in the heave motion of the FOWT hardly affects the power generation.
Department
대학원 풍력특성화협동과정
Advisor
조일형
Awarded Date
2014. 8
Table Of Contents
Chapter 1 INTRODUCTION 1
1.1 Background and literature review 1
1.2 Objectives 8
1.3 Layout of thesis 10
Chapter 2 ANALYTIC SOLUTION 11
2.1 A circular cylinder with a damping plate 11
2.1.1 Diffraction problem 13
2.1.2 Radiation problem 19
2.2 A circular cylinder with a rigid and a porous damping plates 22
2.2.1 Diffraction problem 24
2.2.2 Radiation problem 30
2.3 Equation of heave motion 32
2.3.1 Frequency domain analysis 32
2.3.2 Time domain analysis 33
Chapter 3 DISCRIPTION OF EXPERIMENT 38
3.1 Free decay test 38
3.1.1 Introduction 38
3.1.2 Experimental set-up 40
3.2 Model test in regular waves 47
3.2.1 Introduction 47
3.2.2 Experimental set-up 47
3.3 Model test in irregular waves 50
3.3.1 Introduction 50
3.3.2 Experimental set-up 50
Chapter 4 RESULTS AND DISCUSSION 57
4.1 Introduction 57
4.2 Comparisons 58
4.3 Free decay test 64
4.4 Heave motion response in regular waves 67
4.5 Heave motion response in irregular waves 73
Chapter 5 APPLICATION TO A FLOATING OFFSHORE WIND TURBINE USING FAST CODE 78
5.1 Introduction 78
5.2 Model 80
5.2.1 Wind turbine 80
5.2.2 Platform 81
5.2.3 Environmental conditions 85
5.3 Results 88
Chapter 6 CONCLUSIONS AND FUTURE WORK 99
REFERENCES 101
ACKNOWLEDGEMENT (IN KOREAN) 108
Degree
Doctor
Publisher
제주대학교 대학원
Citation
고혁준. (2014). Heave motion response of a floating offshore wind turbine with damping plate
Type
Dissertation
Appears in Collections:
General Graduate School > Multidisciplinary Graduate School Program for Wind Energy
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