DESIGN OF HYDRODYNAMIC TEST FACILITY AND SCALING PROCEDURE FOR OCEAN CURRENT RENEWABLE ENERGY DEVICES. William Valentine


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1 DESIGN OF HYDRODYNAMIC TEST FACILITY AND SCALING PROCEDURE FOR OCEAN CURRENT RENEWABLE ENERGY DEVICES by William Valentine A Thesis Submitted to the Faculty of The College of Engineering and Computer Science in Partial Fulfillment of the Requirements for the Degree of Master of Science Florida Atlantic University Boca Raton, Florida August 2012
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3 ABSTRACT Author: William Matthew Valentine Title: Institution: Design of Hydrodynamic Test Facility and Scaling Procedure for Ocean Current Renewable Energy Devices Florida Atlantic University Thesis Advisor: Dr. Karl D. von Ellenrieder Degree: Master of Science Year: 2012 Simulations have been carried out to validate a hydrokinetic energy system nondimensional scaling procedure. The requirements for a testing facility intended to test such devices will be determined from the results of the simulations. There are 6 simulations containing 3 prototype systems and 2 possible model facility depths to give a range of results. The first 4 tests are conducted using a varying current profile, while the last 2 tests use a constant current profile of 1.6 m/s. The 3 prototype systems include a: 6 m spherical buoy, a 12 m spherical buoy and a turbine component system. The mooring line used for the simulations is a 6x19 Wire Rope Wire Core of diameter 100 mm and length 1000 m. The simulations are implemented using Orcaflex to obtain the dynamic behavior of the prototype and scaled system. iii
4 DEDICATION I d like to dedicate this thesis to my loving family, friends and God for supporting me through my master s degree. Their faith and guidance helped me through all the obstacles in my path. I d like to thank my family and girlfriend for being patient and believing in me at all times.
5 DESIGN OF HYDRODYNAMIC TEST FACILITY AND SCALING PROCEDURE FOR OCEAN CURRENT RENEWABLE ENERGY DEVICES List of Tables...viii List of Figures...xi Nomenclature...xvi 1. Introduction Problem Statement Background Hydrokinetic Systems and the Gulf Stream Relevant testing facilities Towing Tanks Linear Rotating Arm Planar Motion Mechanisms Water Tunnels Cavitation Tunnels Wave Making Basins Flow Generation Methods Wave Generation Methods 15 iv
6 Mooring Systems Dynamics of Moored Systems Scaling Mooring Lines Proposed Contribution Approach Evaluation of Company Testing Needs Testing Parameters of Existing Facilities Information about Existing Hydrokinetic Energy Systems Results of Evaluation of Testing Needs Proposed Scaling Procedure Determining Facility Size Requirements and Constraints Mission Requirements Performance Requirements Constraints Evaluation of the Scaling Procedure Environmental Parameters Moored Prototype Systems Spherical Buoy System Spherical Buoy System Turbine System Prototype Mooring System Testing Facility of Depth D m = 10 m v
7 Scaled Quantities for Spherical Buoy System Scaled Quantities for Spherical Buoy System Scaled Quantities for Turbine System Dynamic Behavior Results Testing Facility of Depth D m = 40 m Scaled Quantities for Spherical Buoy System Scaled Quantities for Spherical Buoy System Scaled Quantities for Turbine System Dynamic Behavior Results Conclusions Recommendations for Future Work...65 A.0 Appendices.66 A.1 Prototype Environment 66 A.2 Prototype Mooring Line...66 A.3 Prototype System A.4 Prototype System A.5 Prototype System A.6 Prototype System 1 Simulation 71 A.7 Prototype System 2 Simulation 72 A.8 Prototype System 3 Simulation 73 A.9 Test Description...75 A.10 Test 1, 3 and 5 Mooring Line...75 A.11 Test 2, 4 and 6 Mooring Line...76 vi
8 A.12 Model Environment.76 A.13 Model System A.14 Model System A.15 Model System A.16 Model System 1 Simulation.83 A.17 Model System 2 Simulation.97 A.18 Model System 3 Simulation A.19 Model System 1 NonDimensional Scaling Parameters 126 A.20 Model System 2 NonDimensional Scaling Parameters 127 A.21 Model System 3 NonDimensional Scaling Parameters 128 Bibliography vii
9 LIST OF TABLES Table 1: Blade Geometry...37 Table 2: Prototype Environmental Parameters..66 Table 3: Prototype Mooring Line Parameters 66 Table 4: Prototype System 1 Parameters...67 Table 5: Prototype System 2 Parameters...67 Table 6: Prototype Lumped Turbine Parameters...68 Table 7: Prototype Turbine Components...68 Table 8: Prototype Turbine Blades 70 Table 9: Prototype System 1 Positions..71 Table 10: Prototype System 1 Static Results.71 Table 11: Prototype System 2 Positions 72 Table 12: Prototype System 2 Static Results.73 Table 13: Prototype System 3 Positions 73 Table 14: Prototype System 3 Static Results.74 Table 15: Test Specifications.75 Table 16: Mooring Line Parameters for sc = Table 17: Mooring Line Parameters for sc = Table 18: Test 1, 3 and 5 Model Environmental Parameters.76 Table 19: Test 2, 4 and 6 Model Environmental Parameters.77 viii
10 Table 20: Test 1 System 1 Parameters...77 Table 21: Test 2 System 1 Parameters...78 Table 22: Test 3 System 2 Parameters...78 Table 23: Test 4 System 2 Parameters...79 Table 24: Test 5 Model Lumped Turbine Parameters...79 Table 25: Test 5 Model Turbine Components...80 Table 26: Test 6 Model Lumped Turbine Parameters...81 Table 27: Test 6 Model Turbine Components...82 Table 28: Test 1 Model System 1 Positions...83 Table 29: Test 1 Model System 1 Static Results...84 Table 30: Test 2 Model System 1 Positions...85 Table 31: Test 2 Model System 1 Static Results...86 Table 32: Test 3 Model System 2 Positions...97 Table 33: Test 3 Model System 2 Static Results...98 Table 34: Test 4 Model System 2 Positions...99 Table 35: Test 4 Model System 2 Static Results.100 Table 36: Test 5 Model System 3 Positions.111 Table 37: Test 5 Model System 3 Static Results.112 Table 38: Test 6 Model System 3 Positions 113 Table 39: Test 6 Model System 3 Static Results.114 Table 40: Test 1 NonDimensional Parameters Comparison..126 Table 41: Test 2 NonDimensional Parameters Comparison..126 Table 42: Test 3 NonDimensional Parameters Comparison..127 ix
11 Table 43: Test 4 NonDimensional Parameters Comparison..127 Table 44: Test 5 NonDimensional Parameters Comparison..128 Table 45: Test 6 NonDimensional Parameters Comparison..128 x
12 LIST OF FIGURES Figure 1: Linear Towing Tank Test Facility....7 Figure 2: Rotating Arm Towing Tank Test Facility....8 Figure 3: PMM Towing Tank Test Facility Figure 4: Water Tunnel Test Facility Figure 5: Cavitation Tunnel Test Facility..11 Figure 6: Wave Making Basin Test Facility..12 Figure 7: Varying Current Profile with Depth...33 Figure 8: Turbine Performance Curves...37 Figure 9: Turbine Component Top View...37 Figure 10: Dynamic End A Tension (6m, sc = 40) 44 Figure 11: Dynamic End A Tension (12m, sc = 40)..44 Figure 12: Dynamic End A Tension (Turbine, sc = 40) 45 Figure 13: Dynamic End B Tension (6m, sc = 40) 45 Figure 14: Dynamic End B Tension (12m, sc = 40)..46 Figure 15: Dynamic End B Tension (Turbine, sc = 40) 46 Figure 16: Fluid Incidence Angle at End A (6m, sc = 40).47 Figure 17: Fluid Incidence Angle at End A (12m, sc = 40)...47 Figure 18: Fluid Incidence Angle at End A (Turbine, sc = 40).48 Figure 19: Dynamic X Position of End A (6m, sc = 40) 48 xi
13 Figure 20: Dynamic X Position of End A (12m, sc = 40)..49 Figure 21: Dynamic X Position of End A (Turbine, sc = 40).49 Figure 22: Dynamic Z Position of End A (6m, sc = 40).50 Figure 23: Dynamic Z Position of End A (12m, sc = 40)...50 Figure 24: Dynamic Z Position of End A (Turbine, sc = 40).51 Figure 25: Dynamic End A Tension (6m, sc = 10) 56 Figure 26: Dynamic End A Tension (12m, sc = 10)..57 Figure 27: Dynamic End A Tension (Turbine, sc = 10) 57 Figure 28: Dynamic End B Tension (6m, sc = 10) 58 Figure 29: Dynamic End B Tension (12m, sc = 10)..58 Figure 30: Dynamic End B Tension (Turbine, sc = 10) 59 Figure 31: Fluid Incidence Angle at End A (6m, sc = 10).59 Figure 32: Fluid Incidence Angle at End A (12m, sc = 10)...60 Figure 33: Fluid Incidence Angle at End A (Turbine, sc = 10).60 Figure 34: Dynamic X Position of End A (6m, sc = 10) 61 Figure 35: Dynamic X Position of End A (12m, sc = 10)..61 Figure 36: Dynamic X Position of End A (Turbine, sc = 10).62 Figure 37: Dynamic Z Position of End A (6m, sc = 10).62 Figure 38: Dynamic Z Position of End A (12m, sc = 10)...63 Figure 39: Dynamic Z Position of End A (Turbine, sc = 10).63 Figure 40: Test1 Current Profile 76 Figure 41: Test 3 Current Profile...76 Figure 42: Test 2 Current Profile...77 xii
14 Figure 43: Test 4 Current Profile...77 Figure 44: Test 1 End A Force...86 Figure 45: Test 1 End A Tension Separate 87 Figure 46: Test 1 End B Force...87 Figure 47: Test 1 End A Force Proof.88 Figure 48: Test 1 End A Tension Proof.88 Figure 49: Test 1 End A Tension Separate Proof..89 Figure 50: Test 1 End B Force Proof.89 Figure 51: Test 1 End B Tension Proof.90 Figure 52: Test 1 End A Fluid Incidence Angle Proof..90 Figure 53: Test 1 End A X Direction Motion Proof...91 Figure 54: Test 1 End A Z Direction Motion Proof 91 Figure 55: Test 2 End A Force...92 Figure 56: Test 2 End A Tension Separate 92 Figure 57: Test 2 End B Force...93 Figure 58: Test 2 End A Force Proof.93 Figure 59: Test 2 End A Tension Proof.94 Figure 60: Test 2 End A Tension Separate Proof..94 Figure 61: Test 2 End B Force Proof.95 Figure 62: Test 2 End B Tension Proof.95 Figure 63: Test 2 End A Fluid Incidence Angle Proof..96 Figure 64: Test 2 End A X Direction Motion Proof...96 Figure 65: Test 2 End A Z Direction Motion Proof 97 xiii
15 Figure 66: Test 3 End A Force.100 Figure 67: Test 3 End A Tension Separate..101 Figure 68: Test 3 End B Force.101 Figure 69: Test 3 End A Force Proof Figure 70: Test 3 End A Tension Proof Figure 71: Test 3 End A Tension Separate Proof 103 Figure 72: Test 3 End B Force Proof Figure 73: Test 3 End B Tension Proof Figure 74: Test 3 End A Fluid Incidence Angle Proof 104 Figure 75: Test 3 End A X Direction Motion Proof.105 Figure 76: Test 3 End A Z Direction Motion Proof.105 Figure 77: Test 4 End A Force.106 Figure 78: Test 4 End A Tension Separate..106 Figure 79: Test 4 End B Force.107 Figure 80: Test 4 End A Force Proof Figure 81: Test 4 End A Tension Proof Figure 82: Test 4 End A Tension Separate Proof 108 Figure 83: Test 4 End B Force Proof Figure 84: Test 4 End B Tension Proof Figure 85: Test 4 End A Fluid Incidence Angle Proof 110 Figure 86: Test 4 End A X Direction Motion Proof.110 Figure 87: Test 4 End A Z Direction Motion Proof.111 Figure 88: Test 5 End A Force.114 xiv
16 Figure 89: Test 5 End A Tension Separate..115 Figure 90: Test 5 End B Force.115 Figure 91: Test 5 End A Force Proof Figure 92: Test 5 End A Tension Proof Figure 93: Test 5 End A Tension Separate Proof 117 Figure 94: Test 5 End B Force Proof Figure 95: Test 5 End B Tension Proof Figure 96: Test 5 End A Fluid Incidence Angle Proof 118 Figure 97: Test 5 End A X Direction Motion Proof.119 Figure 98: Test 5 End A Z Direction Motion Proof..119 Figure 99: Test 6 End A Force.120 Figure 100: Test 6 End A Tension Separate 120 Figure 101: Test 6 End B Force Figure 102: Test 6 End A Force Proof.121 Figure 103: Test 6 End A Tension Proof.122 Figure 104: Test 6 End A Tension Separate Proof..122 Figure 105: Test 6 End B Force Proof.123 Figure 106: Test 6 End B Tension Proof.123 Figure 107: Test 6 End A Fluid Incidence Angle Proof..124 Figure 108: Test 6 End A X Direction Motion Proof Figure 109: Test 6 End A Z Direction Motion Proof 125 xv
17 NOMENCLATURE A Crosssectional cable area a Imposed motion amplitude at the top of frequency w A b Buoy projected surface A c Crosssectional area of bottom lying part of the cable B o Net buoyancy of buoy in water BCM Buoyancy Compensation Module C d Normal drag coefficient C db Buoy drag coefficient C f Frictional drag coefficient D Water depth d Stretched cable diameter D B Buoy diameter DDHT Dual Ducted Hydrokinetic Turbine D x Buoy drag force in xdirection D Static buoy drag force in xdirection ( x 0 ) xs b D z Buoy drag force in zdirection E Cable Young s Modulus e Local Strain xvi
18 E c Young s Modulus of bottom lying part of the cable FFF (, ) Distributed external force vector per unit stretched length t n F n Normal drag force F n0 Normal tangential drag force (v = 0) F n1 Dynamic normal drag force F t Tangential drag force F t 0 Static tangential drag force (u = 0) F t1 Dynamic tangential drag force GHT Gorlov Helical Turbine H Horizontal static force applied at the ends k eq Equivalent line stiffness k s Inserted spring constant L Suspended cable length l c Length of bottom lying part of the cable M Mass of buoy m Mass per unit unstretched length Ma Added mass n local unit normal vector p Tangential displacement p 1 Lower end displacement p 2 Upper end displacement xvii
19 PMM Planar Motion Mechanism q Normal displacement RTT Rotech Tidal Turbine S Unstretched Lagrangian coordinate measured from the lower end of the cable up to the material point of the cable SMD Soil Machine Dynamics T Tension vector along local tangential vector t Local unit tangential vector T 0 Static tension in cable T 1 Dynamic tension in cable t 0 Static tangential direction of the cable T a Static cable tension T i Total tension in cable segment i T Total tension in cable segment i+1 i 1 U Water/current velocity Vuv (, ) Material point velocity vector w ( ) ga  Immersed cable weight per unit length c w c ( xb, yb, z b) x, y, z coordinates of the buoy ( x, y, z ) Buoy velocity b b b ( x, y, z ) Buoy acceleration b b b Kinematic viscosity of water xviii
20 c Cable density w Water density B or b Buoy density Imposed motion frequency n Natural frequency twodimensional cable added mass 0 1 Angle between horizontal and local tangential direction at cable material point 0 Static components of, means angle between the horizontal and the static tangential direction of the cable 1 Dynamic component of, means the dynamic deviation from the condition of static equilibrium i Angle in segment i Angle in segment i+1 i 1 0, i 0, i 1 angle relative to horizontal xix
21 1. INTRODUCTION Due to the energy crisis, there is an increasing demand on natural energy production. This demand includes natural resources such as wind, waves, thermal, biological, tidal and current. A large amount of interest is in ocean currents. The largest ocean currents that provide the best potential for energy production in the world are the Gulf Stream, Kuroshio current, and Agullas current. An advantage that South Florida has is that the Gulf Stream is close to its eastern shoreline. The Gulf Stream starts at the southern tip of Florida and flows northward along the coast of Florida. It then follows a northeastern course along the rest of the United States and Newfoundland. This is a warm Atlantic Ocean current that is caused by westernintensification. After the Gulf Stream crosses the Atlantic Ocean, it splits into two parts. One part flows over Northern Europe, while the other part flows southward off the western coast of Africa. The Gulf Stream is an ocean current highly driven by the trade winds. The flow rate in the Gulf Stream varies depending upon location between 30 Sv 150 Sv (30 million 150 million cubic meters per second) [1]. An area of interest for energy production in this current is the portion of the Gulf Stream that passes between the coast of Florida and the Bahamas. This area is estimated to have an average kinetic energy flux of somewhere between 25 GW [2] and 20.6 GW [3]. This current starts at the surface and decreases in speed to a depth of 300 m as can be seen in figure 16 from [3]. The seafloor depth in this area can vary from 300 m to 800 m. 1
22 Systems that harvest this natural energy resource are called hydrokinetic energy systems. Due to the increase in demand for natural energy production, more companies are beginning to research and develop systems to harness this energy resource. Once these systems have a planned design, companies will want to test their systems at scale to make sure that their design works and is efficient before spending the money to build a fullsized system. These scaled systems will need to be tested in a facility capable of simulating the environmental conditions, the system may encounter during deployment, and to evaluate the performance of the system under those conditions. The testing parameters and conditions with which the tests are conducted for the model are crucial for an accurate system performance test. If the testing conditions do not accurately simulate the environmental conditions within which the full scale system will encounter, then the results obtained from the test can t be used as validation for the performance of the system under different conditions. Testing is of great importance when designing hydrokinetic energy systems. The most important part of a test is knowing what to test and simulate, otherwise time and money may be wasted performing meaningless tests. Thus, most testing facilities are designed to simulate environmental conditions that are unique/dominant in a specific deployment location Problem Statement Design a testing facility to test hydrokinetic energy devices. The facility should be designed with the intent of testing the hydrokinetic energy devices that will someday be implemented in the Gulf Stream. The design of this testing facility will have four parts: 2
23 1. Evaluate the testing needs of companies that produce hydrokinetic energy devices. 2. Provide a scaling procedure for hydrokinetic energy systems. 3. Evaluate this scaling procedure. 4. Provide a concept design of a testing facility that best meets these needs Background Hydrokinetic Systems and the Gulf Stream There are many hydrokinetic energy systems in existence today, although mainly in the early deployment/evaluation stages. Some widelyrecognized companies and their hydrokinetic energy systems are: Hydro Green Energy and their DDHT (array) [4]; Hydrovolts and their class II and class III turbines [5]; Sea Generation Ltd and their SeaGen system [6]; OpenHydro Ltd and their Open Center Turbine [7]; IHC Engineering Business and their Stingray [8]; GCK Technology Inc. and their GHT [9]; Lunar Energy Ltd and their RTT [10]; SMD Ltd and their TidEl [11]; Verdant Power and their Rite [12]; and Bourne Energy and their OCEANSTAR [13]. Not all of these systems are specifically designed for deep water applications; however the above systems provide only a minimum depth of operation and state their system can work in any depth greater than the minimum depth. None of these systems have been deployed and remained operational in deep water applications for more than a day. This is due to the challenges in which companies face when designing these systems. 3
24 Systems that will be placed in the Gulf Stream encounter problems such as deployment, mooring, maintenance and retrieval. These issues may be encountered due to the location of the Gulf Stream in the vertical water column above the seafloor and the velocity profile of the Gulf Stream. The location of the Gulf Stream offshore causes problems also for the deployment and retrieval process. Deploying a large system can require a complex system of cables and winches. Retrieving a large system that is moored to depths near 400m 800m requires detailed thought and planning. The systems that harvest the Gulf Streams energy will be designed to remain at sea for long periods of time before maintenance is required. In order to remain at sea, the system must be placed in the Gulf Stream and remain there on its own. This causes anchoring/mooring problems due to the velocity profile of the Gulf Stream with increasing depth. Systems will want to reside in the portion of the stream within which the optimum amount of energy can be produced [3]. In order to accurately decide on the depth of operation for the system, investigation of the Gulf Streams velocity profile is required. Velocity Measurements of the Gulf Stream (location Lat: N, Long: W) were taken over a 13 month time span, which started at February 2009 and ended at March 2010 [3]. These measurements show that the velocity of this current isn t always constant and can vary between 0.4 m/s to 2.5 m/s over time for a depth below the surface of 20 m. These data provide an average flow speed profile which can be seen to linearly decrease with increasing depth. From this datum, the optimum operational depth range for a system in this current would be m [14]. 4
25 Mooring problems arise when designing a system to reside in this depth range. The system can t be directly anchored to the sea floor since the seafloor depths range from 300 m to 800 m in the Gulf Stream. Thought about the twisting of the mooring lines needs to be taken into account. A worst case environmental analysis needs to be done. The systems designed to harvest the Gulf Stream need to be extensively tested before placed into operation. These systems can be very costly to build at fullscale, therefore it is recommended that companies perform extensive testing so that they are confident that their design can operate efficiently in the Gulf Stream environment. To perform these tests, a testing facility that is capable of simulating the Gulf Stream environmental conditions is required Relevant Testing Facilities The most useful testing facilities are those which are wellsuited to the particular requirements of the systems to be modeled/tested. There are several facilities that have been well established for testing in specific areas of interest. Since the testing area of interest is broad for current/tidal extraction devices, this paper will consider several testing facilities that are most beneficial for fluid flow generation. The facility and tank types of interest in this paper are towing tanks, water tunnels/channels/flumes, cavitation tunnels and wave making basins. Each facility s average size, relative flow speeds, and flow generation ability/capability is described in detail in the subsequent sections. The testing ability/capability of each type of facility will be provided in the approach section of this report. 5
26 Towing Tanks These tanks are usually very long in comparison to their width and depth. A carriage is mounted on either a monorail or a dualrail that extends the length of the tank. When testing in a towing tank, the test model is mounted and suspended from the carriage. The model can either be submerged or on the surface of the water during the test. The carriage will accelerate to the desired testing velocity in which the test and data collection will begin. The carriage will maintain this velocity for the entire length of the testing section of the tank. The carriage initiates the braking sequence once past the end of the test section, thus ending the test run. The towing carriages and their driving system vary for each tank. A towing carriage typically can reach speeds ranging from 3 15 m/s depending on the system driving the carriage. The driving system for these carriages can either be mechanically or electrically driven. Mechanically driven carriages are the slower of the two types, ranging from speeds of m/s. Electric driven carriages have an electric drive system with drive wheels coupled to electric motors and horizontal guide wheels. The key issues that are encountered when designing any towing tank are: the connection of the model to the towing carriage, the instrumentation must be mounted on the towing carriage in order to continuously gather data, limited space on the towing carriage due to the previous issue, short test run times, need a very long test section in order to get a decent amount of data, difficulties in obtaining PIV or flow visualizations, the time taken and distance needed for the ramp up and ramp down speeds of the towing carriage. There are 3 main types of towing tanks that are used for more specific purposes, they are: Linear Towing Tanks, Rotating Arm Tanks, and Planar Motion Mechanism Tanks. 6
27 Linear Linear towing tanks are the most basic type of towing tank. These tanks roughly range from lengths of 20 m to 320 m, widths of 2 m to 15 m and depths of 2 m to 9 m. Most of these tanks have wave making devices on one side of the tank and a beach on the other side of the tank. The benefit of these wave making devices in a towing tank is to measure your systems overall performance in a wave environment. The draw back of this is that in the actual ocean environment, your system will encounter more than just waves in one direction. The information for this section was obtained from references [15] [22]. Figure 1: Linear Towing Tank Test Facility [53] Rotating Arm Rotating Arm towing tanks are usually circular in shape as opposed to other towing tanks. These tanks roughly range from lengths of 30 to 50 m, widths of 20 to 50 m and depths of 5 to 12 m. These tanks have towing carriages that have an extended arm attachment system. The model is mounted to the end of this arm and the carriage begins to rotate the model about the center of the carriage. Wave making devices are used in these tanks to measure a system s performance as it encounters waves at many different 7
28 angles. These tanks allow full control of the yaw angle, surge speed and sway speed during testing. The information for this section was obtained from references [23], [24] and [56]. Figure 2: Rotating Arm Towing Tank Test Facility [56] Planar Motion Mechanisms Planar Motion Mechanism towing tanks are similar to linear towing tanks in size. These tanks perform the same tests as the rotating arm tanks except in a narrower and longer tank such as the linear towing tank. The PMM can measure a models velocity derivatives, rotary derivatives and acceleration derivatives by producing a transverse oscillation at the bow and another transverse oscillation at the stern of a model as the model is towed with a constant velocity. The information for this section was obtained from references [25] [29]. 8
29 Figure 3: PMM Towing Tank Test Facility [59] Water Tunnels A water tunnel is a recirculating fluid flow testing facility. These facilities are closed so the Reynolds Number is most favorably used for model scaling. These facilities are typically around 50 m long, 20 m wide and 10 m tall. The flow can be generated by a pump system or an impeller/propeller system. These facilities are typically capable of generating flow speeds of around 1 5 m/s. These tanks need several honeycombs, screens and corner vanes in order to have an accurate uniform flow velocity profile in the testing section of the tunnel. The downside to having such accurate flow profiles by using these facilities is that the tested model is not easily accessible to make changes or modifications to once placed in the test section. An example of water tunnel testing facility with a unique conveyer belt addition will be discussed. The flow is initially set into motion by turning on the motors which turn the impeller/propeller. The conveyer belt will also begin to move at the same time as the impeller/propeller. The conveyer belt forms the floor of the test section and moves at the flow speed to minimize the boundary 9
30 layer. The water flows through a diffuser then into a honeycomb system to straighten the flow and reduce lateral turbulence and velocity. The then enters the first turning vane system, which turns the flow 90 degrees while maintaining flow uniformity and avoiding boundary layer separation. Next, the flow enters the second turning vane system to turn the flow another 90 degrees. The flow then enters another honeycomb system to again straighten the flow before the test section. After flowing through the test section, the flow passes through another two sets of turning vanes and then into the impeller/propeller where the process is repeated. The key issues encountered when designing water tunnels are: accessibility to the test section, slow flow speeds, obtaining a proper flow characterization (by means of honeycombs, screens, turning veins, etc.), how and where the model will be mounted, difficulties in cleaning and maintaining, how will the data be collected and where will the instrumentation be mounted. The information for this section was obtained from references [30] [33]. Figure 4: Water Tunnel Test Facility [57] 10
31 Cavitation Tunnels Cavitation tunnels are pressurized water tunnels. Reynolds Number is most favorably used in these facilities. Cavitation tunnels are usually 15 m long, 10 m wide and 10 m tall. The flow in cavitation tunnels is generated in the same way as the flow generated in water tunnels. The flow generation requires pumps or impeller/propeller systems, honeycombs, screens and corner vanes for accurate fluid velocity profiles in the test section of the tunnel. These tunnels consist of a contracting and expanding piping system with some noise insulation systems and pressure control systems. The pressure control systems allows for pressure changes to be made in the test section of the tunnel. The key issues encountered when designing cavitation tunnels are: accessibility to test section, precision of machined components since this a pressure chamber, how and where will the model be mounted, difficulties in cleaning and maintaining, how will the data be collected and where will the instrumentation be mounted. The information for this section was obtained from references [34] [39]. Figure 5: Cavitation Tunnel Test Facility [58] 11
32 Wave Making Basins Wave Making Basins are large facilities having a wide range of testing capabilities. These basins are around 80 m long, 50 m wide and m deep. Wave Making Basins have model towing capabilities, current generation capabilities and multidirectional wave generation capabilities. Models can be towed in the basin by means of an electric winch system at around speeds of 3 4 m/s. Current generation can be achieved by a nozzle manifold system at variable depths and speeds of around.5 m/s (possibly up to 1 m/s). Waves can be generated by different wave generating devices located at any or all sides of the basin. The wave height and period depends on the type of wave maker used, but typical wave heights of 280 mm and periods of seconds. The key issues encountered when designing wave making basins are: slow flow speed generation capabilities, how will the model be pulled or move about within the basin, how will the data be collected and where will the instrumentation be mounted, difficult to obtain flow visualization and PIV measurements. The information for this section was obtained from references [40] [43]. Figure 6: Wave Making Basin Test Facility [42] 12
33 Flow Generation Methods Since the testing facility will be designed to simulate the Gulf Stream s environmental conditions, the facility should have the ability to produce a current stream that is a scaled representation of the Gulf Stream. A flow generation method is needed in order to produce a current stream. Several methods that have been used to generate flow in testing facilities will be examined: 1. Mechanically towed by a towing carriage, 2. Pumps, 3. Motor powered propeller, and 4. Multiport jet nozzle/manifold. The characteristics of each method will be discussed below in more detail. The first method isn t technically flow generation, but rather apparent flow generation. In this method the fluid inside the tank remains still and the model being tested is dragged through the water by the towing carriage. For systems that need high speed tests, this is the most feasible method to use. The downside to this method is length of time in which this flow can be maintained. The apparent flow can only be maintained for very short intervals of time. A method that uses pumps is also used for flow generation. There are four main types of pumps: positive displacement pumps, hydraulicram impulse pumps, centrifugal pumps and axial flow pumps. Positive displacement pumps are commonly categorized as either rotary type or reciprocating type. Rotary positive displacement pumps create a vacuum from the rotation of the pump which captures and draws in the fluid. These pumps are very efficient but can only produce slow flow rates. Reciprocating positive displacement pumps use oscillating pistons to drive the fluid movement. These are used for systems that need to pump high viscous fluids. Hydraulicram impulse pumps take the 13
34 fluid in at a certain flow rate and pressure, then expels the fluid at a higher flow rate and pressure. These are used for pumping a fluid to a higher elevation. Centrifugal pumps use a rotating impeller to increase the flow rate and pressure of the fluid [44]. It is very easy to precisely control the flow rate at low speeds which is a big reason why these are commonly used in flow generation applications. Axialflow pumps cause a pressure change between the upstream and downstream sections of the pump [44]. These pumps operate at low pressures and high flow rates. The performance curves for these pumps are less than the performance curves for the centrifugal pumps [44]. The thrust, head, and discharge coefficients for the above pumps are defined as:,, [44]. In the above formulas, the discharge rate is Q, the pressure head is H, the diameter of the impeller is D, and the rotational speed in revolutions per second is n. Another method for flow generation is the use of a motor powered propeller residing in the flow. The motor rotates the shaft thereby rotating the propeller attached to the end of the shaft. This rotation causes the fluid to be sucked through and accelerated by the propeller. This method is commonly used in recirculating flow applications. The speeds in which could be produced vary widely based on the motor size, propeller size, propeller pitch and propeller RPM s. The nondimensional advance ratio, thrust and torque coefficients for a propeller are:,, [45]. The last flow generation method is the multiport jet nozzle/manifold. An array of jet nozzles can be oriented to generate the flow shape and conditions for the desired flow 14
35 stream. This stream would be easy to predict and control. A manifold system can be used whereby multiple arrays of these nozzles can be placed at different depths. This would allow for the production of any desired flow stream at any depth Wave Generation Methods Current flow isn t the only environmental flow condition that is needed to simulate the Gulf Stream environment. Since the velocity profile of the Gulf Stream starts at the surface of the ocean, then waves will be a factor on systems that are surfaced moored or in the deployment process. Waves in testing facilities are generated using wave makers. There are several types of wave makers: pistontype, flaptype and plungertype [46]. The pistontype and flaptype are 2D wave makers whereas the plungertype is a 1D wave maker. There also exists a 3D wave maker known as a snake [46]. These wave makers have equations that use their stroke length S, the tank depth h and the wave number k. The equations for the 1D and 2D wave makers are found by equating the volume of water displaced over a whole stroke and the volume of water in a wave crest. The pistontype wave maker uses a piston driven motion. The piston oscillates horizontally keeping its vertical position constant. The shallow water condition formula is [46]. The flaptype wave maker uses a flap driven motion. The flap is hinged at the bottom of the tank and oscillates about this hinged point. There is a reduction in the volume displacement for the stroke motion due to the limits in the 15
36 motion caused by the hinge. The shallow water condition formula is [46]. The plungertype wave maker uses a cylinder driven motion. A cylinder oscillates vertically about the mean water line. If the cylinder has a radius R and the stroke length is R, then at full stroke the cylinder will range from fully emerged to half submerged. The shallow water condition formula is [46] Mooring Systems Mooring systems are needed for any system that desires to remain in one location in the ocean for a period of time. For systems that will be used to harvest energy from the Gulf Stream over long spans of time, the use of either a surface buoy system or a subsurface buoy system will be employed. Each of the buoy systems require an anchoring point on the sea floor that is connected to the mooring buoy through a cable system, single point or multileg. Surface buoys can be designed such that the environmental conditions on the oceans surface have either a great impact or little impact on the behavior of the buoy. These buoys can have a great range of shapes. A single point surface moored buoy system can have slack in the line allowing the buoy to move with the waves and current until the slack is diminished, or it can have a taut line that stretches some due to the motion of the buoy on the surface. A buoy is considered a surface buoy until the center of that buoy is 100m below the sea surface. This is due to the diminished effect that surface excitation has at 100m or more below the sea surface [47]. 16
37 A system in which the buoy is 100m or more below the sea surface are called subsurface buoys. The largest forces, on average, that these buoys experience are due to ocean currents. These buoys can be moored with slack in the line or with a taut line also. For a more detailed understanding of buoys and their purpose refer to [48] Dynamics of Moored Systems For a system moored in the ocean that is harvesting energy, it is crucial to know the dynamic behavior of that system. In order to have an accurate system behavior, the nonlinear effects on the cable should be considered. The cable is assumed to have a negligible bending stiffness and is modeled as a slender rod. The Lagrangian coordinates are measured from the bottom most end of the cable up to a material point on the cable. In the equations below: m is the mass per unit length, s is the unstretched Langrangian coordinate, v(u,v) is the velocity vector, T is the tension vector, t is the tangential unit vector, n is the normal unit vector, F(F t,f n ) is the external force per unit stretched length, e is the local strain, is the added mass, w is the submerged weight per unit length and is the angle between the horizontal and the local tangential direction of the material point. The equations of motion of a cable only expressed along the local tangential and normal directions of the moving dynamic reference are [49]: u T (1) m[ v ] wsin( ) Ft (1 e) t t s v (2) ( m ma) ( m) u T wcos( ) Fn(1 e) t t s 17
38 (3) e u v t s s ; (4) (1 ) v e u t s s ; (5) w ( ) c w gac; (6) T EAe A linear tensionstrain relation (eq. (6)) is used. Using the separation principle, the external forces can be obtained [49]: (7) 1 Ft d wcf ( U cos( ) u) U cos( ) u 2 (8) 1 Fn d wcd( Usin( ) v) Usin( ) v 2 where ρ w is the water density, d is the stretched cable diameter, U is the water velocity, c f and c d are the frictional and drag coefficients for the cable. In eq. (7), F t is obtained using the separation principle by which a relation of the tangential force is made to the relative fluidcable tangential velocity. In eq. (8), F n is obtained in a similar way using the normal force and the relative fluidcable normal velocity. The outofplane motion is, to first order, decoupled from the inplane motion of this cable configuration [49]. Projecting the nonlinear eq. (1) (4) onto the static reference system, requires a relationship between the moving and static reference system using the velocity components and displacements [49]: (9) p q u cos( 1) sin( 1) t t ; (10) p q v sin( 1) cos( 1) t t where p and q represent the tangential and normal displacements and 1 is the dynamic angle. 18
39 Using equations (9) and (10), the unit vector relation for the 2 coordinate systems, subtracting the equilibrium equations, and assuming small displacements, the simplification equations, and assuming small displacements, the simplification procedure can be established [49]. The simplification procedure yields: (11) (12) p T d m T F t s ds t1 2 q 0 1 T0 T1 1 ( m ma) T 2 1 T 0 1 Fn 1 1 T 1 t s s s s s (13) p d q 0 T 1 s ds EA (14) d ds q 0 p 1 e0 s (1 ) The external forces are obtained by the following: F ( F F )(1 e) F (1 e ) ; (16) Fn 1 ( Ft 1 Fn1)(1 e) Fn0(1 e0) (15) t1 t n1 1 t0 0 In these equations, Ft 0 and Fn 0 are found by setting u v 0. Neglecting the last 2 terms in eq. (12), assuming excitation frequencies are well below the first elastic frequency of the cable and using the hypothesis of large static tension relative to the total weight of the cable, then eq. (11) becomes trivial thus [49]: (17) q q q d ( ) t s s ds m ma T 2 0 T 2 1 T 2 1 Fd (18) 1 q q F d c U U U 2 t t 2 d w d[( N ) N N] 19
40 L (19) UN Usin( ) d 0 ; (20) T1 keq[ p2 p1 q ds] ds 0 where (21) k k ( EA/ L) eq s The above equations are for a cableonly analysis. Considering a buoy within 2 segments (i and i+1) of a line, the following equations of motion are obtained [49]: (22) ( M M ) x T 1cos( 1) T cos( ) D a b i i i i x (23) ( M M ) a zb Ti 1sin( i 1) Tisin( i) Dz B0 1 Dx Ap wcbdvr( U x b) (24) 2 ; (25) 1 Dz Ap wcbdvrz b 2 V ( U x ) z ; (26) 2 2 r b b Now applying a similar simplifications procedure, the following is obtained [49]: (27) ( M M ) p cos( ) ( M M ) q sin( ) T cos( ) T sin( ) a i a i 1, i 0, i 1, i T D cos( ) D sin( ) 1, i 1 xb 0, i 1 z 0, i 1 (28) ( M M ) p sin( ) ( M M ) q cos( ) T sin( ) T cos( ) a i a i 1, i 0, i 1, i T D sin( ) D cos( ) 0, i 1 1, i 1 xb 0, i 1 z 0, i 1 D D D (29) xb x xs 0, i 0, i 1 ; (30) x p cos( ) q sin( ) p cos( ) q sin( ) b i 0, i i 0, i i 1 0, i 1 i 1 0, i 1 (31) z p sin( ) q cos( ) p sin( ) q cos( ) b i 0, i i 0, i i 1 0, i 1 i 1 0, i 1 (32) 20
41 Scaling Mooring Lines The scaling of mooring lines for model testing has given many people trouble over the years. When scaling these lines, many factors need to be taken into account. These factors are discussed in NonLinear Cable Response and Model Testing in Water [49] and Deep Water Mooring Dynamics [50]. The first paper describes the dynamic response of cables in water. The model testing of cables in water ratherthen in air is highly important due to the vastly different response properties of cables in water as opposed to in air. The scaling of cables can be accomplished once the nondimensional quantities are known. There are nine nondimensional parameters for the analysis of the cables on [49]: L D ; d L ; wl T a ; Ud on which c f, c d and depend; c ; EA T ; a 2 2 a d ; L ;. ( T /( A)) w a c There are three more parameters in addition to the nine parameters above, when considering the analysis of a cable with a buoy attached [50]: a D ; B ; w B UDB on which c bd and M a depend. Papazolou and Mavrakos have shown that proper sealing can be achieved for the 2 cable only analysis [50]. They provide a parameter,, from which the response of cables primarily depends. 2 EA wl 2 ( )( ) where represents the ratio for the elastic H H stiffness to catenary stiffness of a mooring line. For small the response is dominated 21
42 by elastic stiffness where stretching occurs in the line [49]. For large the response is dominated by catenary stiffness where no stretching occurs. When modeling mooring lines, the scaling methods to be taken are not clear. To ensure the slenderness of the cable, d L should be matched for the model to the prototype. For proper static modeling, L D and wl T a must be kept the same [49]. For dynamic modeling, EA similitude is essential for the dynamic response, but is not needed for the T a static response. The problem occurs in finding the materials at these very small scales that will match the EA T a [49]. Papazoglou and Mavrakos show that by inserting buoys and springs in series with the cable at its lower end, they could artificially produce the required elastic stiffness (EA/L) needed for proper sealing [50]. Additional natural frequencies are introduced by these springs. With quasistatic stretching for small Ks compared to elastic stiffness the following is obtained for the natural frequency 3k approximation [50]: s n. If n comes out to be within the testing range of ml frequencies, then the test and results are invalid because the system excites a natural frequency. This occurs when the forcing frequency is near the natural frequency. 22
43 1.3. Proposed Contribution The goal of this project is to provide a system level design of a testing facility that meets the testing needs of companies. Companies will be allowed to pay in order to test their hydrokinetic energy systems in this facility. The contribution that will be made will consist of five parts: 1. Detailed Evaluation of the testing needs of companies 2. Trade Study to downselect from the possible tanks that are able to suit these needs 3. Scaling procedure for hydrokinetic energy systems 4. Evaluation and proof of the proposed scaling procedure 5. Design of a testing facility that best suits those need 23
44 2. APPROACH A test facility must meet the testing needs of companies, thus research about the needs and testing process of companies is required for a proper analysis. The size of the facility will be evaluated by the comparison of the dynamic tensions from several OrcaFlex simulations. Once the testing needs of industry and reasonable facility size are known, the facility requirements and constraints for the testing facility can be established Evaluation of Company Testing Needs Testing Parameters of Existing Facilities Research on the testing parameters that existing facilities offer was carried out. Since there are many different types of facilities that can be used to test hydrokinetic energy systems, the testing parameters of each type of testing facility were collected and combined with facilities of the same type. This will provide information on what each type of facility most commonly tests. 1) Towing tanks a) Linear used for force and resistance measurements on systems or ship hulls, propulsion systems, propeller or turbine performance in open water, sea keeping 24
45 and maneuvering in head and following waves, mooring simulations and wake surveys. Information collectively obtained from references [15] [22]. b) Rotating Arm used for force and resistance measurements on submersible systems (submarines, torpedo s, etc.), accurate measurements of rudder performance, sea keeping and maneuvering simulations, all of these tests can be performed at high speeds. Information collectively obtained from references [23], [24] and [56]. c) Planar Motion Mechanisms used for force and resistance measurements on systems or ship hulls, measuring velocity derivatives, measuring rotary derivatives, measuring acceleration derivatives, propulsion systems, propeller or turbine performance in open water, sea keeping and maneuvering, experiments that require vertical and horizontal motions, rudder performance, mooring simulations and wake surveys. Information collectively obtained from references [25] [29]. 2) Water Tunnels used for cavitation tests of propellers or turbines, force and resistance measurements, measure noise produced by a system, measuring vibration produced by a system, flow visualization tests, deep water mooring and anchoring systems. Information collectively obtained from references [30] [33]. 3) Cavitation Tunnel used for cavitation tests of propellers or turbines, measuring the response and behavior of the system or the flow over a system to fluctuations in pressure, measurements of the noise produced by a cavitation propeller or turbine, performance measurements of hydrofoils and tidal energy turbine systems. Information collectively obtained from references [34] [39]. 25
46 4) Wave Making Basins used for extensive mooring system tests, sea keeping and maneuvering, deployment and retrieval process simulations, characterization of the dynamic behavior of the system, force and resistance measurements on systems or ship hulls and measurements of waves generated by ships. Information collectively obtained from references [40] [43] Information about Existing Hydrokinetic Energy Systems Research was carried out to gather information on as many hydrokinetic energy systems that were made available through the internet. The systems which provided the most available information were chosen and placed into a spread sheet. A total of 13 systems were used (references [4] [13] in addition to 3 other systems), since the most information was publically available for these systems. Out of the 13 total systems: 8 were horizontal axis turbine systems, 4 were vertical axis turbine systems and 1 was an oscillating system. The system rotor diameter for the 12 turbine systems ranged from 1 m 25 m with the average being 10 m. The designed flow speeds for all 13 systems ranged from 1.5 m/s 3.5 m/s with the average being m/s. The information provided about the mooring systems is minimal and generalized for 9 of the 13 systems. The deployment wasn t given for 3 out of the 13 systems. The operational depth for most of these systems is greater than 30 m, which means they are deep water hydrokinetic energy systems. 26
47 Results of Evaluation of Testing Needs Due to the limited information available, some assumptions are made about the testing needs of companies. Researching the testing parameters offered at existing facilities and what information could be obtained about existing hydrokinetic energy system designs, allows some assumptions to be made about the testing needs of companies. Due to the limited facilities that offer detailed mooring tests along with the limited information companies provide online about their systems mooring plan, the performance of the system and the systems mooring were chosen as the most important testing needs of companies. 2.2 Proposed Scaling Procedure The scaling procedure proposed for hydrokinetic energy systems in this design facility will be described in detail below. Scaling systems as a whole is complex due to having to simultaneously scale the energy device and the mooring system. The proposed method uses a force scaling procedure. The most significant factors that need to be taken into account are depth of operation (D p ) for the prototype system and depth of testing facility (D m ). These two factors can be used to determine the required scale factor, which is (33) sc = D p /D m. The next scaling parameter of importance is the drag force (thrust force) on the moored system. The prototypes drag or thrust force will be used to estimate the End B tension, the lower end tension in the mooring line, later on in the steps thus the 27
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