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«A Thesis by ASHWIN LOHITHAKSHAN PARAMBATH Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the ...»

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IMPACT OF TSUNAMIS ON NEAR SHORE WIND POWER UNITS

A Thesis

by

ASHWIN LOHITHAKSHAN PARAMBATH

Submitted to the Office of Graduate Studies of

Texas A&M University

in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

December 2010

Major Subject: Ocean Engineering

Impact of Tsunamis on Near Shore Wind Power Units

Copyright 2010 Ashwin Lohithakshan Parambath

IMPACT OF TSUNAMIS ON NEAR SHORE WIND POWER UNITS

A Thesis by

ASHWIN LOHITHAKSHAN PARAMBATH

Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

Approved by:

Co-Chairs of Committee, Juan Horrillo Patrick Lynett Committee Members, Vijay Panchang Prabir Daripa Head of Department, John Niedzwecki December 2010 Major Subject: Ocean Engineering iii

ABSTRACT

Impact of Tsunamis on Near Shore Wind Power Units.

(December 2010) Ashwin Lohithakshan Parambath, B. Tech (Civil), National Institute of Technology Calicut Co-Chairs of Advisory Committee: Dr. Juan Horrillo Dr. Patrick Lynett With the number of wind power units (WPUs) on the rise worldwide, it is inevitable that some of these would be exposed to natural disasters like tsunamis and it will become a necessity to consider their effects in the design process of WPUs. This study initially attempts to quantify the forces acting on an existing WPU due to a tsunami bore impact. Surge and bore heights of 2m, 5m and 10m are used to compute the forces using the commercially available full 3D Navier Stokes equation solver FLOW3D. The applicability of FLOW3D to solve these types of problems is examined by comparing results obtained from the numerical simulations to those determined by small scale laboratory experiments. The simulated tsunami forces on the WPU are input into a simplified numerical structural model of the WPU to determine its dynamic response. The tsunami force is also used to obtain base excitation which when applied on the WPU would be equivalent dynamically to the tsunami forces acting on it. This base excitation is useful to obtain the response of the WPU experimentally, the setup for

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Performance Outdoor Shake Table (LHPOST). The facility allows full scale experimental setup capable of subjecting a 65kW Nordtank wind turbine to random base excitations. A stress analysis of turbine tower cross section is performed in order to assess the structural integrity of the WPU. It has been determined that the WPU is unsafe for bore/surge heights above 5 m. It has also been postulated that the structural responses could be considerable in case of the taller multi megawatt wind power units of present

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This thesis is dedicated to my grandpa. Alzheimer’s disease made you forget your last meal and even your own home for the last 30 years, but you never forgot the fact that “Achu is studying in America”. It is dedicated to a sweet small girl, Rammu. You laughed a lot the day I told you, “Thoda pane ke liye, thoda khona bhi padtha hain (To gain a little, you have to lose a little).” Never did I know that I had to lose you to learn so much about life. It is also dedicated to my parents and sissy, for their love has been

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I would like to thank my committee chairs, Dr. Juan Horrillo and Dr. Patrick Lynett, for guiding me through this challenging thesis. patience and industry in helping me tackle my problems is greatly appreciated. I would also like to thank him for taking me to Alaska, the most beautiful place on earth, where the idea for this thesis was conceived.

I would also like to thank my committee members Dr Prabir Daripa and Dr.

Patrick Lynett for their valuable feedback and guidance. I would also like to thank Dr.

Vijay Panchang for granting me an opportunity to work under him. I would also like to thank him for all those wonderful parties that his family has hosted for us and also for telling the world that his students (Abhishek and me) had caught a shark!

I would like to thank my family for supporting my decisions in life and stepping in to help whenever things got out of my control. If I ever were to fall on my way, you would unconditionally help me get back on my feet, is all the confidence that I ever need. I would also like to thank my friends and colleagues for all the support that they have rendered in the successful completion of this thesis. My special thanks go out to

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ABSTRACT

DEDICATION

ACKNOWLEDGEMENTS

TABLE OF CONTENTS

LIST OF FIGURES

LIST OF TABLES

1. INTRODUCTION AND LITERATURE REVIEW

2. THEORETICAL CONSIDERATIONS

2.1 Hydrodynamic Model

2.1.1 Governing Equations and Discretization

2.1.2 Temporal Discretization

2.1.3 Spatial Discretization

2.1.3.1 Continuity Equation

2.1.3.2 2D Momentum Equation Discretization.................. 13 2.1.4 Free Surface Tracking, Volume of Fluid (VOF)





2.1.5 Boundary Conditions

2.1.5.1 Wall Boundary Condition

2.1.5.2 Outflow Boundary Condition

2.1.5.3 Velocity Boundary Condition

2.1.6 Stability Criterion

2.1.7 FLOW-3D

2.2 Structural Stress Analysis Model

2.3 Structural Dynamics Model

3. VALIDATION

3.1 Hydrodynamic Model versus Experimental Setup

3.2 Comparison of Results

3.3 Conclusions

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4.1 Establishing a Computational Domain

4.2 Physics Constrains

4.3 Boundary Conditions

4.4 Numerical Schemes

4.5 Simulation

4.6 Discussion of Results

5. STRUCTURAL RESPONSE AND INTEGRITY

5.1 Description of WPU

5.2 Static Structural Analysis

5.2.1 Numerical Modeling

5.2.2 Discussion of Results

5.3 Dynamic Structural Analysis

5.4 Determination of Base Excitation

5.5 Base Excitation of WPU

5.6 Results and Discussions

6. SUMMARY AND FUTURE RESEARCH

6.1 Summary

6.2 Future Recommendations

6.2.1 Response of MDOF Turbine Model

6.2.2 Effect on Components

6.2.3 Experimental Validation

6.3 Conclusions

REFERENCES

APPENDIX A DESCRIPTION OF THE PARTS OF A WIND POWER UNIT.......... 66 APPENDIX B DETERMINATION OF BASE EXCITATION

APPENDIX C TEMPORAL PROGRESSION OF 5 M SURGE

APPENDIX D RESPONSE OF THE WPU TO TSUNAMI LOADINGS

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Figure 1 Near shore WPU susceptible to tsunami hazard, located at Hull, Massachusetts in Boston Harbor

Figure 2 Two dimensional grid cell for the discretization of the continuity equation.... 12 Figure 3 Computational domain with ghost cells and real cells

Figure 4 Typical modal load function

Figure 5 Experimental setup used to validate hydrodynamic model (Arnason, Petroff and Yeh [2])

Figure 6 Forces acting on cylinder from Arnason et al [3] experiment

Figure 7 Forces acting on cylinder calculated by FLOW3D

Figure 8 Computational domain and orientation of the coordinate system

Figure 9 Mesh resolutions near the cylinder

Figure 10 Forces on cylinder due to 2 m bore/surge

Figure 11 Forces on cylinder due to 5 m surge/bore

Figure 12 Forces on cylinder due to 10 m surge/bore

Figure 13 Determination of forces at the structural node

Figure 14 Stress distribution along WPU due to 2 m bore (unit: Pa)

Figure 15 Stress distribution along WPU due 2 m surge (unit: Pa)

Figure 16 Stress distribution along WPU due to 5m bore (unit: Pa)

Figure 17 Stress distribution along WPU due to 5 m surge (unit: Pa)

Figure 18 Stress distribution along WPU due to 10 m bore (unit: Pa)

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Figure 20 Force transformation

Figure 21 Sketch of 65kW Nordtank Wind Turbine, showing observation points for structural response, Adopted from [20]

Figure 22 Representative sketch A) dynamic system with time history forcing; B) dynamic system with base excitation forcing.

Figure 23 Spectral energy of accelerations at J1

Figure 24 Spectral Energy of accelerations at J2

Figure 25 Spectral energy of accelerations at J3

Figure 26 Parts of a WPU (source: [25])

Figure 27 Simplified 1D dynamic system

Figure 28 Pressure contour 5 m surge (time = 0.3 secs)

Figure 29 Pressure contour 5 m surge (time = 1.5 secs)

Figure 30 Pressure contour surge 5 m (time = 2.5 secs)

Figure 31 Pressure contour surge 5 m (time = 3.1 secs)

Figure 32 Responses at Joint J1 for 2 m bore

Figure 33 Responses at Joint J2 for 2 m bore

Figure 34 Responses at Joint J3 for 2 m bore

Figure 35 Responses at Joint J1 for 2 m surge

Figure 36 Responses at Joint J2 for 2 m surge

Figure 37 Responses at Joint J3 for 2 m surge

Figure 38 Responses at Joint J1 for 5 m bore

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Figure 40 Responses at Joint J3 for 5 m bore

Figure 41 Responses at Joint J1 for 5 m surge

Figure 42 Responses at Joint J2 for 5 m surge

Figure 43 Responses at Joint J3 for 5 m surge

Figure 44 Responses at Joint J1 for 10 m bore

Figure 45 Responses at Joint J2 for 10 m bore

Figure 46 Responses at Joint J3 for 10 m bore

Figure 47 Responses at Joint J1 for 10 m surge

Figure 48 Responses at Joint J2 for 10 m surge

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Table 1 Computational domain specifications for different impoundment depths.......... 26 Table 2 Percentage difference in the estimation of forces (Experimental v/s Hydrodynamic model)

Table 3 Specification of computational domain for determining forces on WPU........... 30 Table 4 Impoundment depth and flooding depth

Table 5 Computation times for various surge/bore cases

Table 6 Properties of 65kW Nordtank Turbine. Source [20]

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Wind power units (WPUs) are primarily of three different types, on shore, near shore (Figure 1) and offshore. WPUs lying within three kilometers to the nearest shore line or lying on the water within ten kilometers from the shore are considered a near shore wind turbine. Near-shore wind turbines benefit from the higher coastal winds caused due to differential heating of land and sea. Since the air is denser it has more energy for the same wind speed compared to on-shore winds. The lower cost associated with the maintenance due to easier accessibility makes near-shore wind turbines preferable over offshore wind turbines. Outside the US, especially in Europe, near-shore wind turbines are common, and it is reasonable to assume that wind energy developments in the US will broadly follow the European growth pattern. With the high number of wind farm installations expected to meet the world renewable energy goals over the next decade, there is a continuous increase in the size of the wind power units. This results in an inevitably increasing risk of these vital facilities to be damage by natural events like tsunamis. Tsunami hazard investigation based on NGDC and USGS has shown that parts of the United States such as Puerto Rico and Virgin Islands and U.S Pacific island territories have very high to moderate tsunami threats [1].

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Figure 1 Near shore WPU susceptible to tsunami hazard, located at Hull, Massachusetts in Boston Harbor Tsunami is a series of water waves, which is caused by the displacement of a large volume of body of water, usually occurring in an ocean or large lake. Earthquakes, volcanic eruptions, landslides and other mass movements which cause disturbances above and below water have the potential to generate tsunamis. With wave lengths of the order of a couple hundred kilometers and wave heights usually less than a meter, tsunamis are usually long waves at the offshore location. As tsunami waves approach the shoreline where water depth is reduced, their wave lengths reduce to ten to twenty kilometers which implies that the celerity of the waves are reduced. This results in an

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breaking of the wave. Once broken these waves propagate on to the shore. Among the various ways in which a tsunami can propagate onshore, the common one is that of a rapidly advancing hydraulic bore which is formed by the breaking of the tsunami wave at the offshore reef or at the shoreline. This type of wave front is one of the most destructive forms of onshore propagation of tsunamis (Arnason, Petroff and Yeh, [2]) and this study proposes to use this type of tsunami propagation onto the WPU structure.

The bore consists of a turbulent front, which being steep exerts a large force on any objects in its path. In general, tsunami bores might propagate over dry land, in which case they are referred to as a surge or over an existing inundation or flowing water in which case they are called wet bores.

In the past, considerable research has gone into predicting the runup of tsunami waves, as these help to estimate the inundation caused in the coastal areas due to a tsunami. Very few studies have however tried to address the problem of estimating the forces exerted by a tsunami bore on coastal structures. With onshore propagation speeds of the order of nearly 10 m/s and surge heights up to 30 m in extreme cases (Arnason [3]), the force imparted to the coastal structures are considerable. Most of these studies however tried to quantify the forces on vertical walls [4] and other structures which are common among coastal regions. As WPUs have been in existence only recently, there are not any known studies which evaluate the temporal variation of the tsunami force

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