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Frequency Domain Modeling and

Multidisciplinary Design Optimization of Floating

Offshore Wind Turbines

by

Meysam Karimi

B.Sc., Persian Gulf University, 2010

M.Sc., Amirkabir University of Technology, 2012

A Dissertation Submitted in Partial Fulfillment of the

Requirements for the Degree of

DOCTOR OF PHILOSOPHY

in the Department of Mechanical Engineering

c© Meysam Karimi, 2018

University of Victoria

All rights reserved. This dissertation may not be reproduced in whole or in part, by

photocopying or other means, without the permission of the author.

ii

Frequency Domain Modeling and Multidisciplinary Design Optimization

of Floating Offshore Wind Turbines

by

Meysam Karimi

B.Sc., Persian Gulf University, 2010

M.Sc., Amirkabir University of Technology, 2012

Supervisory Committee

Dr. C. Crawford, Supervisor

(Department of Mechanical Engineering)

Dr. B. Buckham, Supervisor

(Department of Mechanical Engineering)

Dr. R. Dewey, External Member

(Ocean Networks Canada)

iii

ABSTRACT

Offshore floating wind turbine technology is growing rapidly and has the poten-

tial to become one of the main sources of affordable renewable energy. However, this

technology is still immature owing in part to complications from the integrated de-

sign of wind turbines and floating platforms, aero-hydro-servo-elastic responses, grid

integrations, and offshore wind resource assessments. This research focuses on devel-

oping methodologies to investigate the technical and economic feasibility of a wide

range of floating offshore wind turbine support structures. To achieve this goal, inter-

disciplinary interactions among hydrodynamics, aerodynamics, structure and control

subject to constraints on stresses/loads, displacements/rotations, and costs need to

be considered. Therefore, a multidisciplinary design optimization approach for min-

imum levelized cost of energy executed using parameterization schemes for floating

support structures as well as a frequency domain dynamic model for the entire cou-

pled system. This approach was based on a tractable framework and models (i.e. not

too computationally expensive) to explore the design space, but retaining required

fidelity/accuracy.

In this dissertation, a new frequency domain approach for a coupled wind turbine,

floating platform, and mooring system was developed using a unique combination of

the validated numerical tools FAST and WAMIT. Irregular wave and turbulent wind

loads were incorporated using wave and wind power spectral densities, JONSWAP

and Kaimal. The system submodels are coupled to yield a simple frequency domain

model of the system with a flexible moored support structure. Although the model

framework has the capability of incorporating tower and blade structural DOF, these

components were considered as rigid bodies for further simplicity here. A collective

blade pitch controller was also defined for the frequency domain dynamic model to

increase the platform restoring moments. To validate the proposed framework, pre-

dicted wind turbine, floating platform and mooring system responses to the turbulent

wind and irregular wave loads were compared with the FAST time domain model.

iv

By incorporating the design parameterization scheme and the frequency domain

modeling the overall system responses of tension leg platforms, spar buoy platforms,

and semisubmersibles to combined turbulent wind and irregular wave loads were de-

termined. To calculate the system costs, a set of cost scaling tools for an offshore

wind turbine was used to estimate the levelized cost of energy. Evaluation and com-

parison of different classes of floating platforms was performed using a Kriging-Bat

optimization method to find the minimum levelized cost of energy of a 5 MW NREL

offshore wind turbine across standard operational environmental conditions. To show

the potential of the method, three baseline platforms including the OC3-Hywind spar

buoy, the MIT/NREL TLP, and the OC4-DeepCwind semisubmersible were compared

with the results of design optimization. Results for the tension leg and spar buoy case

studies showed 5.2% and 3.1% decrease in the levelized cost of energy of the opti-

mal design candidates in comparison to the MIT/NREL TLP and the OC3-Hywind

respectively. Optimization results for the semisubmersible case study indicated that

the levelized cost of energy decreased by 1.5% for the optimal design in comparison

to the OC4-DeepCwind.

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Contents

Supervisory Committee ii

Abstract iii

Contents v

List of Tables ix

List of Figures xiii

Acknowledgements xix

Dedication xxi

1 Introduction 1

1.1 Background and Motivation . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 FOWT System Components . . . . . . . . . . . . . . . . . . . . . . . 3

1.3 Dissertation Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.4 Research Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2 A Multi-Objective Design Optimization Approach For Floating

Offshore Wind Turbine Support Structures 11

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.2 Design analysis methodology . . . . . . . . . . . . . . . . . . . . . . . 17

2.2.1 Inviscid platform hydrodynamics . . . . . . . . . . . . . . . . 18

vi

2.2.2 Viscid platform hydrodynamics . . . . . . . . . . . . . . . . . 18

2.2.3 Wind turbine properties . . . . . . . . . . . . . . . . . . . . . 21

2.2.4 Mooring line loads . . . . . . . . . . . . . . . . . . . . . . . . 21

2.2.5 Frequency-domain dynamic model . . . . . . . . . . . . . . . . 22

2.3 Support structure parameterization . . . . . . . . . . . . . . . . . . . 24

2.3.1 Platform topology . . . . . . . . . . . . . . . . . . . . . . . . 24

2.3.2 Mooring system . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.3.3 Platform mass and ballast . . . . . . . . . . . . . . . . . . . . 30

2.4 Optimization problem methodology . . . . . . . . . . . . . . . . . . . 31

2.4.1 Objective functions . . . . . . . . . . . . . . . . . . . . . . . . 33

2.4.2 Design constraints . . . . . . . . . . . . . . . . . . . . . . . . 35

2.5 Time-domain verification of dynamic model . . . . . . . . . . . . . . 38

2.6 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

2.6.1 Environmental conditions . . . . . . . . . . . . . . . . . . . . 40

2.6.2 Single-body platforms . . . . . . . . . . . . . . . . . . . . . . 40

2.6.3 Multi-body platforms . . . . . . . . . . . . . . . . . . . . . . . 43

2.6.4 Full design space exploration . . . . . . . . . . . . . . . . . . . 46

2.6.5 Sensitivity analysis . . . . . . . . . . . . . . . . . . . . . . . . 47

2.7 Conclusions and future work . . . . . . . . . . . . . . . . . . . . . . . 52

3 A Fully Coupled Frequency Domain Model for Floating Offshore

Wind Turbines 54

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

3.1.1 Time domain models for FOWTs . . . . . . . . . . . . . . . . 57

3.1.2 Simplified FOWT modeling techniques . . . . . . . . . . . . . 58

3.1.3 Proposed model . . . . . . . . . . . . . . . . . . . . . . . . . . 60

3.1.4 Chapter outline . . . . . . . . . . . . . . . . . . . . . . . . . . 61

3.2 Frequency domain model framework . . . . . . . . . . . . . . . . . . . 61

3.2.1 Wind turbine and platform description . . . . . . . . . . . . . 62

vii

3.2.2 Wave and wind inputs . . . . . . . . . . . . . . . . . . . . . . 64

3.2.3 Linearizing FOWT dynamics using FAST . . . . . . . . . . . 65

3.2.4 Assembling the frequency domain model . . . . . . . . . . . . 68

3.3 Fatigue load analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

3.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

3.4.1 Environmental and simulation conditions . . . . . . . . . . . . 77

3.4.2 System DOF reduction . . . . . . . . . . . . . . . . . . . . . . 79

3.4.3 OC3-Hywind spar buoy case study . . . . . . . . . . . . . . . 81

3.4.4 MIT/NREL TLP case study . . . . . . . . . . . . . . . . . . . 86

3.4.5 OC4-DeepCwind semisubmersible case study . . . . . . . . . . 88

3.4.6 Comparison of 22 DOF FAST and 6 DOF frequency domain

model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

3.5 Conclusions and Future Work . . . . . . . . . . . . . . . . . . . . . . 95

4 Multidisciplinary Design Optimization of Floating Offshore Wind

Turbine Support Structures For Levelized Cost of Energy 98

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

4.1.1 FOWT time domain dynamics modeling . . . . . . . . . . . . 101

4.1.2 FOWT frequency domain dynamics modeling . . . . . . . . . 102

4.1.3 Design optimization studies . . . . . . . . . . . . . . . . . . . 102

4.1.4 Cost models . . . . . . . . . . . . . . . . . . . . .