design, testing and validation of a scale model

DESIGN, TESTING AND VALIDATION OF A SCALE MODEL SEMISUBMERSIBLE OFFSHORE WIND TURBINE UNDER REGULAR/IRREGULAR WAVES AND WIND LOADS LAURA ROLO PÉREZ

A thesis submitted in partial fulfilment for the requirement of the degree Master of Science Sustainable Engineering: Offshore Renewable Energy

Under the supervision of Professor Alexander Day Department of Naval Architecture, Ocean and Marine Engineering

University of Strathclyde Glasgow, 2014

August 2014

DESIGN, TESTING AND VALIDATION OF A SCALE MODEL SEMISUBMERSIBLE OFFSHORE WIND TURBINE UNDER REGULAR/IRREGULAR WAVES AND WIND LOADS by Laura Rolo Pérez MEng in Civil Engineering [email protected] A THESIS Submitted in Partial Fulfilment of the Requirements for the Degree of Master of Science in Sustainable Engineering: Offshore Renewable Energy Under the supervision of Professor Alexander Day Director of the Kelvin Hydrodynamics Laboratory, Glasgow Department of Naval Architecture, Ocean and Marine Engineering University of Strathclyde, Glasgow, UK

Copyright Declaration This thesis is the result of the author’s original research. It has been composed by the author and has not been previously submitted for examination which has led to the award of a degree. The copyright of this thesis belongs to the author under the terms of the United Kingdom Copyright Acts as qualified by University of Strathclyde Regulation 3.50. Due acknowledgement must always be made of the use of any material contained in, or derived from, this thesis.

Signed: Laura Rolo Pérez Date: 27th August 2014

Abstract Nowadays, Europe is facing an energy security challenge to satisfy its demand, as more than the 50% of the energy consumed has to be imported. Moreover, fossil fuels represent the vast majority of these energy resources, contributing to the greenhouse gas emissions and their consequences. For these and many other reasons, it is pursued a progressive shift to the renewable energies as the first solution for a safe, secure, sustainable and affordable energy. The offshore wind energy stands out as the most promising offshore renewable energy for the next years and decades. Its main advantage in relation to its land-based homogeneous technology is that its energy output is higher and steadier. In addition, wind farms placed in deeper waters present the extra advantages of increasing power output and reducing environmental and social impacts. However, larger water depths require floating wind turbines whose optimal technology is still under research. This work is focused on the performance of one of the offshore floating concepts: the OC4-DeepCwind semisubmersible wind turbine, because of its minimal dynamic coupling between wave-induced and turbine-induced motion and its easier and lower-cost offshore installation. To asses this issue, a 1/80th scale model is designed, assembled and tested under different configurations of regular waves, sea states and wind loads at the Kelvin Hydrodynamic Laboratory in Glasgow. The experimental results confirm the high stability of the floating platform under the waves and wind loads and reveals considerable hydrodynamic nonlinearities which most of the numerical analysis does not display but might play a critical role in certain load conditions.

Key words Offshore, windturbine, floating, semisubmersible, platform, OC4-DeepCwind, model, test, basin, nonlinearities

I

Acknowledgements Firstly, I wish to express my profound gratitude to my thesis supervisor, Professor Alexander Day, for offering his encouragement, guidance and sharing his knowledge which allowed me to take giant steps in this topic. In particular, special thanks for having given to me the opportunity to develop and test this floating wind turbine model, with the great responsibility that entails. This is a great added value I appreciate and retain for my next professional experiences. I would like to specially thank the staff at the Kelvin Hydrodynamic Laboratory: Bill McGuffie and Bill Wright for turning plans into reality; Charles Keay for coordinating all procedures in model fabrication and tests; and Edward Nixon and Grant Dunning for sharing their vast knowledge in basin testing and their company during the long but rewarding testing hours. Special thanks also to Adil Akgül and Steven Martin for their help with data acquisition and interpretation of results. I would also like to send my appreciation and countless thanks to Fundación Iberdrola and Scottish Power Foundation for the Fundación Iberdrola Scholarship I have received, which has allowed me to join this MSc program and venture in this new innovative direction. Sincere thanks to the professors and rest of staff at the University of Strathclyde for their approachability and rich share of knowledge and experiences. Special thanks are to all my friends in Glasgow who made my time at University of Strathclyde an unforgettable experience. Last but not least, I extend my most special and sincerest gratitude to my loved ones for always standing by me and supporting me at this endeavour, especially to my parents to whom I owe everything.

II

Table of Contents Copyright Declaration ..................................................................................................II ABSTRACT ..................................................................................................................... I ACKNOWLEDGEMENTS ..................................................................................................II TABLE OF CONTENTS .................................................................................................. III LIST OF TABLES ......................................................................................................... VII LIST OF FIGURES ......................................................................................................... IX LIST OF NOMENCLATURE ......................................................................................... XIII LIST OF ABBREVIATIONS AND ACRONYMS .............................................................. XVII

CHAPTER 1 INTRODUCTION ........................................................................ 1 1.1 Outline of the Thesis .......................................................................................... 1 1.2 Aim of this Thesis .............................................................................................. 2 1.3 Introduction ........................................................................................................ 4 1.4 Present & Future of Offshore Wind Energy....................................................... 5 1.5 Main Advantages of Offshore Wind Power ....................................................... 7 1.6 Classification of Offshore Wind Turbines ......................................................... 8 1.7 Challenges of Offshore Floating Wind Turbines (Critical Review) ................ 10

CHAPTER 2 LITERATURE REVIEW ............................................................ 13 2.1 Theory of Waves .............................................................................................. 13 2.1.1 General Waves Defining Parameters in Time Domain ............................. 15 2.1.2 Regular Waves ........................................................................................... 17 2.1.3 Irregular Waves ......................................................................................... 19 2.2 Aero-Servo-Hydro-Elastic Analysis of the Offshore Floating Wind Turbine System .................................................................................................................... 22

III

2.2.1 Equation of Motion .................................................................................... 23 2.3 Hydrodynamic Loads........................................................................................ 24 2.3.1 Linear Hydrodynamics ............................................................................... 24 2.3.2 Linear Time-Domain Hydrodynamic Model ............................................. 26 2.3.3 Frequency-Domain Approach .................................................................... 28 2.3.4 Non-Linear Effects ..................................................................................... 29 2.4 Hydrostatic Properties & Stability .................................................................... 31 2.5 Damping and Natural Frequency Response ..................................................... 33 2.5.1 Free-vibration of viscous-damped 6 DOF systems .................................... 34

CHAPTER 3 DESIGN OF THE SCALE MODEL TESTS ................................... 37 3.1 Basin Specifications.......................................................................................... 37 3.1 OC4 – DeepCwind 5 MW Semisubmersible floating wind system ................. 38 3.1.1 OC4 DeepCwind OFWT System Description ........................................... 39 3.1.2 Floating Wind System Natural Frequencies .............................................. 41 3.2 Model Scaling Methodology ............................................................................ 42 3.2.1 Scaling Criteria .......................................................................................... 42 3.2.2 Established Scaling Factors ....................................................................... 47 3.2.3 Modelling of Floating Platform ................................................................. 48 3.2.4 Modelling of Mooring Lines ...................................................................... 48 3.2.5 Modelling of Environment ......................................................................... 49 3.3 Model Dimensions ............................................................................................ 49 3.3.1 Model Fidelity ............................................................................................ 51 3.4 Model Environment Loads ............................................................................... 52 3.4.1 Regular Waves ........................................................................................... 52 3.4.2 Irregular Waves .......................................................................................... 53 3.4.3 Wind ........................................................................................................... 54

IV

3.4.4 Drag Disk Modelling (Rotor) .................................................................... 55 3.5 Test Matrix ....................................................................................................... 56 3.6 Tests Procedure ................................................................................................ 57 3.7 Calibration of Environment .............................................................................. 58 3.7.1 Wind assessment and calibration ............................................................... 58 3.7.2 Waves Calibration ..................................................................................... 59

CHAPTER 4 MODEL TEST RESULTS .......................................................... 61 4.1 System Identification Tests .............................................................................. 61 4.1.1 Inclining Test ............................................................................................. 61 4.1.2 Free Decay ................................................................................................. 64 4.1.3 Only Regular Waves .................................................................................. 71 4.1.4 Only Oblique Regular Waves .................................................................... 76 4.1.5 Regular Waves + Wind.............................................................................. 78 4.2 Station Keeping Test Types ............................................................................. 81 4.2.1 Sea States ................................................................................................... 82 4.2.2 Motions Significant Height........................................................................ 85 4.2.3 Frequency Domain Analysis - Spectral Analysis ...................................... 87

CHAPTER 5 NUMERICAL MODEL ........................................................... 101 5.1 Introduction .................................................................................................... 101 5.2 Data Input ....................................................................................................... 102 5.3 Results ............................................................................................................ 104 5.3.1 Response Amplitude Operators – AQWA Diffraction Tool ................... 104 5.3.2 Resultant Motion Results ......................................................................... 106

CHAPTER 6 SUMMARY AND CONCLUSIONS ............................................ 109 BIBLIOGRAPHY ...................................................................................... 113

V

ANNEX I TEST INSTRUMENTATION......................................................... 119 I.1

Instrumentation required for the Inclining Test ......................................... 119

I.2

Instrumentation required for test in only regular/irregular waves ............. 119

I.3

Instrumentation required for test in regular/irregular waves and wind ..... 122

I.4

Others ......................................................................................................... 124

ANNEX II LABORATORY DIARY ............................................................ 127 ANNEX III CALCULATION OF OFWT HYDROSTATIC PROPERTIES.......... 147 III.1

OFWT Centre of gravity ............................................................................ 147

III.2

Platform Hydrostatic Properties ................................................................ 148

VI

List of Tables Table 3.1. Modelled Designs from 2005 to 2013 by the OC3 and OC4 projects ...... 38 Table 3.2. Floating Wind Turbine System Natural Frequencies (s) according to different authors (with no wind) ................................................................................ 41 Table 3.3. Floating Wind Turbine System Natural Frequencies (s) with wind ......... 41 Table 3.4. Established scaling factors for floating wind turbine model testing ......... 47 Table 3.5. OC4-DeepCwind OWT system prototype and 1:80 scale model dimensions ................................................................................................................. 50 Table 3.6. Difference between target and model (1:80) ............................................ 52 Table 3.7. Regular Waves Tested .............................................................................. 52 Table 3.8. Sea States Tested....................................................................................... 54 Table 3.9- NREL 5MW Wind Environment and equivalent Thrust Forces .............. 55 Table 3.10. System Identification Tests ..................................................................... 57 Table 3.11. Station Keeping Tests ............................................................................. 57 Table 3.12. Wind Flow Assessment with standard Skywatch Xplorer 2 anemometer .................................................................................................................................... 58 Table 3.13. Wind Speed (m/s) Test Parameters ......................................................... 59 Table 4.1. Inclining test results for model without drag disk .................................... 63 Table 4.2. Inclining test results for model with installed drag disk .......................... 63 Table 4.3. Natural Periods (NP), Natural Frequencies (NF) and Damping Ratios (DR) tested under wind and no wind loads and comparison with references in the bibliography ............................................................................................................... 70 Table 4.4. Sea States parameters in full and model scale .......................................... 82 Table 4.5. Statistics for measured JONSWAP spectra .............................................. 84 Table 4.6. Spectral coefficients .................................................................................. 89 Table II.1 Materials used in platform scale model................................................... 128 Table II.2. Extra weight to be considered in the platform model ............................ 129 Table II.3. Spike2 Data entry for Inclining Test I-3 ................................................. 129 Table II.4. Feedback from Inclining Test I-3 ........................................................... 131

VII

Table II.5. Components Masses of the Turbine Model ............................................ 132 Table II.6. Floating Wind Turbine System Model Weight before ballasting........... 133 Table II.7. Floating Wind Turbine System Model Weight after ballasting ............. 133 Table II.8. Spike2 Data entry for Inclining Test I-3 ................................................. 134 Table II.9. Irregular wave configuration .................................................................. 137 Table II.10. Significant NREL 5MW wind speed conditions and correspondent thrust forces in prototype scale and model scale ................................................................ 138 Table II.11. Mean Wind Velocities Measurements (m/s) at 5, 5.5 and 6.5 meters from the funs position ....................................................................................................... 139

VIII

List of Figures Figure 1.1. Installed capacity – cumulative share by country (MW) (EWEA, 2014) . 6 Figure 1.2. Global offshore wind generation and projection by IEA and MTRMR 2012 (IEA, 2013) ......................................................................................................... 7 Figure 1.3 (a) Fixed Offshore Wind Turbines (b) Floating Offshore Wind Turbines (Wiser, R. et al., 2011) ................................................................................................. 8 Figure 1.4Semisubmersible OFWT concepts: (a) DeepCwind, (b) Windfloat .......... 10 Figure 2.1. Superposition of Waves (Thurman, 1997) .............................................. 14 Figure 2.2. Ranges of validity for various wave theories (Kraineest, 2009) ............. 18 Figure 2.3. Platform modes of motion (Chen, 2012) ................................................. 25 Figure 2.4. DeepCWind Offset Column Stability Diagram ....................................... 32 Figure 2.5. Underdamped Oscillation (Rao, 2004) .................................................... 35 Figure 3.1. OC4 DeepCwind Semisubmersible Floating System (Author) ............... 40 Figure 3.2. Plan (left) and Side (right) view of the DeepCwind Semisubmersible Platform (Robertson, et al., 2012) .............................................................................. 40 Figure 3.3. Model with drag disk installed ................................................................ 56 Figure 3.4. Wind sentry set test ................................................................................. 59 Figure 3.5. Results of the wave probe calibration...................................................... 60 Figure 3.6. Screen Capture of the wave maker software used for the irregular waves calibration .................................................................................................................. 60 Figure 4.1. Model without drag disk during the Inclining Experiment ..................... 62 Figure 4.2. Platform motions response in Pitch Free Decay Test (without wind) ..... 65 Figure 4.3. Platform motions response in Roll Free Decay Test (without wind) ...... 65 Figure 4.4. Platform motions response in Heave Free Decay Test (without wind) ... 66 Figure 4.5. Platform motions response in Surge Free Decay Test (without wind) .... 66 Figure 4.6. Pitch Free Decay Data and Fit ................................................................. 67 Figure 4.7. Heave Free Decay data and Fit ................................................................ 68 Figure 4.8. Surge Free Decay data (Spike2 view) ..................................................... 68

IX

Figure 4.9. Parameters used in the log-decrement method to obtain the damping ratio .................................................................................................................................... 69 Figure 4.10. System configuration for only regular wave tests .................................. 71 Figure 4.11. Photography of the model during one test in only regular waves .......... 72 Figure 4.12. From Spike2 raw data representation: (a) Reflected waves, (b) Almost broken waves, (c) Waves not yet stabilized ............................................................... 73 Figure 4.13. Pitch RAO for regular waves with wave height equal to 1, 2, 4 and 6 meters ......................................................................................................................... 74 Figure 4.14. Heave RAO for regular waves with wave height equal to 1, 2, 4 and 6 meters ......................................................................................................................... 74 Figure 4.15. Surge RAO for regular waves with wave height equal to 1, 2, 4 and 6 meters ......................................................................................................................... 74 Figure 4.16. Non-linear effects seen during test simulation....................................... 75 Figure 4.17. System configuration for only oblique regular wave tests ..................... 77 Figure 4.18. RAO for oblique regular waves (wave incident angle 60º) with wave height equal to 2 and 6 meters: (a) Pitch, (b) Roll, (c) Heave and (d) Surge ............. 78 Figure 4.19. System configuration for regular waves + wind tests ............................ 79 Figure 4.20. RAO for regular waves + wind with wave height equal to 2 and 6 meters: (a) Pitch, (b) Roll, (c) Heave and (d) Surge................................................... 80 Figure 4.21. Scale model during test under wave and wind loads. It is noticeable the increment in the heel angle due to the wind load ....................................................... 81 Figure 4.22. System configuration for the sea states’ tests ........................................ 82 Figure 4.23. Theoretical JONSWAP spectra.............................................................. 83 Figure 4.24. Significant Height of pitch for load cases with only waves and waves + wind ............................................................................................................................ 86 Figure 4.25. Significant Height of roll for load cases with only waves and waves + wind ............................................................................................................................ 86 Figure 4.26. Significant Height of heave for load cases with only waves and waves + wind ............................................................................................................................ 86 Figure 4.27. Significant Height of surge for load cases with only waves and waves + wind ............................................................................................................................ 87 Figure 4.28 Theoretical and Measured JONSWAP spectra under wind load (W) when data available .................................................................................................... 92

X

Figure 4.29. PSDs from test data for pitch, roll, heave and surge for an irregular wave only case with Hs = 2 m and Tp = 7.5 sec .................................................................. 92 Figure 4.30. PSDs from test data for pitch, roll, heave and surge for an irregular wave only case with Hs = 2.44 m and Tp = 8.1 sec ............................................................. 93 Figure 4.31. PSDs from test data for pitch, roll, heave and surge for an irregular wave only case with Hs = 3.66 m and Tp = 9.7sec .............................................................. 93 Figure 4.32. PSDs from test data for pitch, roll, heave and surge for an irregular wave only case with Hs = 5.49 m and Tp = 11.3 sec ........................................................... 94 Figure 4.33. PSDs from test data for pitch, roll, heave and surge for an irregular wave only case with Hs = 9.14 m and Tp = 13.6 sec ........................................................... 94 Figure 4.34. PSDs from test data for pitch, roll, heave and surge for an irregular wave only case with Hs = 10.5 m and Tp = 14.3 sec ........................................................... 95 Figure 4.35. Pitch RAO values for the six sea states tested ....................................... 97 Figure 4.36. Roll RAO values for the six sea states tested ........................................ 98 Figure 4.37. Heave RAO values for the six sea states tested ..................................... 98 Figure 4.38. Surge RAO values for the six sea states tested ...................................... 99 Figure 5.1. Representation of the OC4-DeepCwind OFWT system in ANSYS AQWA ..................................................................................................................... 101 Figure 5.2. Geometry transformed in ANSYS DesignModeler ............................... 102 Figure 5.3. Mesh ...................................................................................................... 103 Figure 5.4. Pitch RAO comparison between only regular wave tests and AQWA simulation ................................................................................................................. 105 Figure 5.5. Heave RAO comparison between only regular wave tests and AQWA simulation ................................................................................................................. 105 Figure 5.6. Surge RAO comparison between only regular wave tests and AQWA simulation ................................................................................................................. 105 Figure 5.7. Motions for H = 2.44 m and T = 8.10 sec ............................................. 106 Figure 5.8. Model in regular waves Hs = 2 m and Tp =8.10 sec .............................. 106 Figure 5.9. Motions for H = 5.44 m and T = 11.6 sec ............................................. 107 Figure 5.10. Model in regular waves H = 6 m and Tp =11.3 sec ............................. 107 Figure 5.11. Motions for H = 10.5 m and T = 13.16 sec ......................................... 108 Figure 5.12. Model in irregular waves Hs = 10.5 m and Tp =14.3 sec ..................... 108 Figure I.1. Inclinometer and inclining masses ......................................................... 119

XI

Figure I.2. Tank carriage .......................................................................................... 120 Figure I.3. Wave Maker ........................................................................................... 120 Figure I.4. Equipment Controls and Data Loggers mounted on the carriage ........... 121 Figure I.5. Wave Probe............................................................................................. 121 Figure I.6. a) Qualisys Camera, b) Passive marker balls ......................................... 122 Figure I.7. Video recording camera.......................................................................... 122 Figure I.8a) Skywatch Xplorer 2 Anemometer, b) Wind Sentry Set & c) Clarke CAM6000 Fan .......................................................................................................... 124 Figure I.9. a) Vacuum, b) Laser distance meter, c) Reference balls panel ............... 125 Figure II.1 Model being built in the workshop of the Kelvin Hydrodynamic Laboratory ................................................................................................................ 128 Figure II.2. Weight placed on one of the offset columns’ bottom in order to achieve the 1:80 model weight target .................................................................................... 129 Figure II.3. Inclining test for the semisubmersible platform. The different pictures show the test procedure where the inclining masses change their position. ............ 130 Figure II.4. Calibration of the Qualysis Cameras ..................................................... 131 Figure II.5. Qualisys Track Manager Screenshot ..................................................... 132 Figure II.6. (a) wave Probe situated 10 meters away the wave maker, (b) probe slots .................................................................................................................................. 135 Figure II.7. Model positioned and moored ............................................................... 136 Figure II.8. Roll Free Decay Test ............................................................................. 137 Figure II.9. Anemometer attached to a carbon fiber stick to measure the instant wind speed in the turbine testing position ......................................................................... 139 Figure II.10. Floating system being tested in Irregular Waves ................................ 140 Figure II.

.

odel rotated

and tested under H = 6 m regular waves................. 141

Figure II.12. Free decay tests: a) Pitch and b) Surge ............................................... 143 Figure II.13. Image of the model from the Qualisys Cameras Software .................. 144 V. -

Figure IV.2. DeepCWind Offset Column Stability Diagram ........................ 148

Figure IV.3. Platform dimensions in water plane .................................................... 149 Figure IV.4. Semisubmersible platform hydrostatic parameters .............................. 151

XII

List of Nomenclature Symbols Latin Symbols =

Wave crest height

=

Wave crest depth

( ) =

̅̅̅̅̅

Added mass matrix

=

Random wave amplitudes

=

Swept area of the rotor

=

Wave amplitude

=

Spectral normalizing factor

=

Metacentric radius

=

External damping contribution

( ) =

Radiation damping matrix

=

Power coefficient

=

Thrust coefficient

=

Restoring stiffness

( ) =

̅̅̅̅

̃

=

Root mean squared wave height

=

Significant wave height

=

Wave radiation Retardation kernel

=

Keel-center of gravity distance

=

Fluid length of travel

=

Mass matrix

=

Total mass of the OFWT system

=

Rotor radius

=

Wave steepness

=

Apparent of virtual wave period

=

Period of the damped vibration

=

Energy wave period

=

Spectral zero-up-crossing period

=

Spectral mean wave period

=

Peak wave period

=

Statistical peak wave period

=

Total sampling time in the spectral analysis

Wave excitation force

=

Generalized active forces

=

Generalized inertia forces

̅̅̅̅̅

=

Metacentric height

̅̅̅̅

=

Righting lever

⁄

=

Significant wave height

=

Mean wave height

=

Spectral wave significant height

XIII

()

̇ E (

=

Time zero-crossing period

=

Wave height

=

Ursell number

=

Length

=

Submerged platform volume

=

=

Damping constant

Total number of discrete data in the spectral analysis

=

Critical damping

=

Power

=

Group velocity

=

Radius

=

Wave frequency

=

Reynolds number

=

Signal data in time domain

=

Power spectral density

=

Sampling frequency in the spectral analysis

=

Standard deviation

=

General spectral moment

=

Wave period

=

DOF velocity

=

Mean wind speed

=

Amplitude peak

=

Wind speed

=

Average energy density =

Watt

=

Acceleration due to gravity

Fourier transform in frequency domain

=

Wave number

=

DOF displacement

=

Energy flux

=

Time

=

Rotor thrust

=

Control input

=

Surface elevation =

Mean velocity of the object relative to the fluid

) =

( ) P z

( ) ( )

=

Discrete Fourier transform in frequency domain

=

Phase velocity

=

Froude number

XIV

Greek Symbols λ

= Scale model factor

̃

= Angular velocity of rotor

Wave length =

Apparent of virtual wave length

= Density of air

= Rotor angular speed

= Wave phase

=

Angular spectral peak frequency

= Wave propagation direction

=

Spectral peak shape parameter

= Random wave phases

= Spectral width parameter

Wave angular frequency

=

Random wave angular frequencies

= Offset = Damped natural frequency

= Variable time = ϕh

= Shallow water parameter = Dynamic viscosity

Non-dimensional definitions of structure motions

= Water density

= Heel angle = Damping ratio

XV

List of Abbreviations and Acronyms BC

Base column

CB

Center of buoyancy

cfm

cubic feet per minute

CM

centre of mass

COG

centre of gravity

DFT

Discrete Fourier transform

DNV

Det Norske Veritas

EC

European Commission

EU

European Union

EWEA

European Wind Energy Technology Platform

FFT

Fast Fourier Transform

GHG

greenhouse gases

IEA

International Energy Agency

IPCC

Intergovernmental Panel on Climate Change

JONSWAP

Joint North Sea Wave Project

LVDT

Linear variable differential transformer

MC

Main column

MTRMR

Medium-Term Renewable Energy Market Report

NREL

National Renewable Energy Laboratory

O&G

oil and gas

OC4

Offshore Code Comparison Collaboration Continuation

OFWT

offshore floating wind turbine

OWT

offshore wind turbine

PM

Pierson Moskowitz

PSD

Power Spectral Density

RAO

response amplitude operator

RE

renewable energies

SWL

surface water level

XVII

TLP

Tension Leg Platform

TSR

tip speed ratio

UC

Offset column

UK

United Kingdom

US

United States of America

XVIII

CHAPTER

1 1 Introduction

1.1 Outline of the Thesis This work starts with a brief overview on the state of the art of the offshore floating wind turbine systems and presents the main challenges in the deep-offshore wind industry. Reading through those lines would make the reader to understand the need for further research in the topic which concerns the present thesis and the reasons for having chosen a semisubmersible platform for the offshore wind turbine. Particularly, the offshore floating system chosen comprises the DeepCwind semisubmersible platform with the 5 MW NREL wind turbine. This system constitutes the design belonging to the Offshore Code Comparison Collaboration, Continuation (OC4): Phase II Results of a Floating Semisubmersible Wind System, project under International Energy Agency (IEA) Wind Task 30. Chapter 2 presents a review of the existing literature in order to offer the essential concepts to describe the interaction between the environmental conditions (wind and mostly waves) and the offshore floating wind turbine (OFWT) system. Special attention is given to the hydrodynamic analysis in time and frequency domains and the spectral analysis. Following, Chapter 3 brings the model and environment scale methodology to follow for basin tests. The full scale prototype and model dimensions calculated with Froude methodology are also presented. The chapter finishes with a summary of all the tests carried out at the Kelvin Hydrodynamic Laboratory (Glasgow) and the calibration of the tests environment (wind and waves). 1

CHAPTER 1

Introduction

The results of the environment and model tests are shown and discussed in Chapter 4, the longest chapter of the thesis. It is divided in two sections attending the nature of the model tests: system identification tests (inclining test, free decay, only regular waves, only oblique regular waves and regular waves + wind) and sea states’ ones. The data is properly elaborated in order to present meaningful indicators of the system performance under the different loads. Numerous comparisons with the literature and other authors’ works are also included. A brief numerical analysis of the OC4-DeepCwind semisubmersible platform is carried out with the software ANSYS AQWA and explained in Chapter 5 in order to compare with the experimental results and others authors’ ones. A final Chapter 6 includes last comments, conclusions and further proposed work about the topic particularly. After the list of References and Bibliography used in this work, the Annexes include an interesting diary of the testing days with test procedures, problems solving and chronological description of the tests. The instrumentation used and calculation of hydrostatic OFWT properties are also included in this last part of the document.

1.2 Aim of this Thesis The aim of this MSc Thesis is to learn and contribute to the research of the most promising offshore renewable energy for the next years and decades: the offshore wind energy. In particular, the research is done on one type of the floating wind turbine concepts, the only ones which can be installed in depths greater than 50 meters. It is chosen an existing semisubmersible wind turbine system: the DeepCwind, in order to create a comparative to the few existing reports through hydrodynamic tank experimental testing and numerical analysis. Accordingly, the specific objectives of the thesis are:

2

CHAPTER 1

-

Introduction

Design of a 1/80th scale model of the OC4-DeepCwind semisubmersible wind turbine system according to Froude number methodology

-

Calculation of the platform hydrostatic and hydrodynamic properties through system identification tests in the Kelvin Hydrodynamic Tank

-

Study of the performance and determining of RAO of the floating model for six different cases of sea states tests in the Kelvin Hydrodynamic Tank

-

Comparison of the experimental results with literature, other authors’ works and a hydrodynamic numerical analysis simulated with the software ANSYSAQWA

-

Discuss the validation of the experimental model tests and the observed advantages and drawbacks of the OC4-DeepCwind semisubmersible system

3

CHAPTER 1

Introduction

1.3 Introduction The energy sector is one of the columns of growth, competitiveness and development in our modern economy. Nevertheless, just with safe, secure, sustainable and affordable energy a promising future for the sector can be secured. EU is facing an energy security challenge to satisfy its demand as more than the 50% (European Commission, 2013) of the energy consumed has to be imported, some of the cases from countries who pose a high risk of internal instability (De Micco & Andrés Figueroa, 2014). Moreover, fossil fuels represent the vast majority of these energy resources, and the energy-related emissions account for almost 80% of the EU´s total greenhouse gas emissions (European Commission, 2013), which directly contribute to climate change and its consequences (IPCC, 2014). In addition, this represents a Catch-22 situation: climate change is today directly affecting energy security, as it has the potential to act as a multiplier/accelerant for conflicts and extreme weather conditions which can cause energy disruptions (Umbach, 2009). In contrast, energies from renewable resources (solar, thermal, wind, hydro, tidal, wave, biomass, and geothermal energies) have an essential role to contribute to a safe, secure, sustainable and affordable energy future. Renewable energies allow increasing the energy autonomy of a region and therefore decreasing the sensitivity to international fluctuating energy prices (Molho, 2013). In addition, RE do not considerable contribute to greenhouse gases (GHG) emissions and their operating costs are much lower than for the rest of conventional energies (ARUP, 2011). In this context, the European Council and Parliament agreed to an integrated climate and energy policy and adopted the “Energy Action Plan” to maintain a careful balance between all three parameters: (i) security of supply, (ii) competitiveness and (iii) environmental sustainability (European Commission, 2010). The three 20% targets to achieve by 2020 are:

4

CHAPTER 1

Introduction

 Reduction in GHG emissions by 20% compared to 1990 levels or by 30% if the conditions are right  20% share of energy from renewable sources in gross final energy consumption  20% improvement in energy efficiency In the case of United Kingdom, the Government objective by 2020 is to reduce GHG emissions by at least 34% compared with 1990 levels, increase the share of renewable energy to 15% by 2020 and enhance the energy efficiency of homes, business and transport (HM Government, 2014). Scottish Government gives a further step, and its policy aims to reduce 42% in GHG, generates the equivalent of

% of Scotland’s gross annual electricity consumption

and 11% of its heat by renewable resources (The Scottish Government, 2011).

1.4 Present & Future of Offshore Wind Energy Total wind energy has presently risen to 2.6% global share (IEA, 2013) and 8% share of EU consumption (EWEA, 2014), but just an insignificant proportion comes from offshore wind farms. Globally, just a 2% of total global installed wind power capacity comes from offshore developments (Sawyer, 2012). However, in the case of some European countries the situation of the offshore wind energy is very different, where the total installed capacity across Europe has reached 6,562 MW, producing 24 TWh in a normal wind year, enough to cover 0.7% of the EU’s total electricity consumption1. A total of 2080 offshore wind turbines are installed and connected to the grid in 69 wind farms in eleven European countries, mainly in the United Kingdom (3.7 GW) and Denmark (1.3 GW), with large plants also installed in Belgium, Germany, the Netherlands and Sweden (EWEA, 2014).

1

According to Eurostat’s latest figures, the EU’S gross domestic consumption of electricity was 3,3 Twh.

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Figure 1.1. Installed capacity – cumulative share by country (MW) (EWEA, 2014)

The Global Wind Energy Council (CWEC) states that the potential of offshore wind could meet Europe’s energy demand seven times over, and the United States’ energy demand four times over. Nevertheless, the offshore wind power progress is delayed because it remains expensive and technically challenge. It is known that in offshore projects the turbine accounts for less than half of the investment cost, important difference in comparison to the three-quarters for landbased projects. Offshore projects incur additional expenses for foundation, electric infrastructure and installation costs, which vary with distance from shore and water depth. However, the IEA Roadmap (2013) expects a reduction in wind power costs of 45% offshore by 2050. Looking to the future and according to the more ambitious projections, a total of 80 GW of offshore wind power could be installed by 2020 worldwide, with three quarters of this in Europe (Sawyer, 2012). EC (2012) predicts a scenario of 40 GW installed capacity by 2020 (equivalent to 4% EU electricity demand) and 150 GW by 2030 (14% EU electricity demand).

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Figure 1.2. Global offshore wind generation and projection by IEA and MTRMR 2012 (IEA, 2013)

Policies are another important motive in this context. Thanks to the EU renewable energy targets and incentives for investments such as feed-in tariffs or green certificates, offshore wind power generation has started to expand rapidly in Europe (European Commission, 2012) and for example, offshore wind power has become essential in the UK strategy.

1.5 Main Advantages of Offshore Wind Power The main advantage offshore wind power presents in relation to its land-based homogeneous technology it is that its energy output is higher and steadier. The sea emplacement allows greater rotor diameters, at the same time that the wind flow is much stronger and steadily off the coasts. In addition, offshore breezes can be strong in the afternoon, unlike wind over the continent, matching the time when people are using the most electricity. Another plus it is the limitation by space and visual impact of onshore wind farms or even the depletion of appropriate onshore emplacements in some regions. Moreover, offshore wind farms can be located near large coastal demand centres, often avoiding long transmission lines to get power to demand, as can be the case for

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land-based renewable power installations. This can make offshore particularly attractive for countries with coastal demand areas and land-based resources located far inland, such as China, several European countries and the US. While needing to satisfy environmental stakeholders, offshore wind farms generally face less public opposition and, to date, less competition for space compared with developments on land. Furthermore, offshore wind farms placed in deeper waters present the extra advantages of increasing power output because of the better wind conditions, reducing the visual pollution and the environmental impact on the seabed and decreasing the interferences with marine activities.

1.6 Classification of Offshore Wind Turbines (a)

(b)

Figure 1.3 (a) Fixed Offshore Wind Turbines (b) Floating Offshore Wind Turbines (Wiser, R. et al., 2011)

Up to now, most of the current offshore wind turbines have been built in relatively shallow water (