Stability and Loads Validation of an Ocean Current Turbine
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The design of a moored ocean current turbine presents many engineering challenges; among them are accurately predicting the stability and loads of the device. To validate computational loads and stability prediction tools, Aquantis Inc. designed, built, and tested a 1/25th scale model of their ‘C‐Plane' dual‐rotor moored ocean current turbine. This effort was conducted in cooperation with the US Naval Surface Warfare Center at the David Taylor Model Basin and was funded in part under a grant awarded to Dehlsen Associates by the U.S. Department of Energy. This multi‐stage testing effort included both a captured singlerotor test and a dynamic, moored test of the complete dual‐rotor C‐Plane. The test data is subsequently used to validate a variety of stability and loads simulations including the Navy's DCAB Code and Tidal Bladed v4.4. Specialized testing methodologies were developed for this purpose and the results are compared with computational model predictions. This testing effort investigates many aspects of moored ocean current turbine design. The captured test was essential to characterize rotor loads and stability coefficients at various blade pitch and cone angles, as well as measure rotational stall delay and unsteady rotor loads due to upstream structure wakes. The dynamic test validated stability and loads predictions of all anticipated modes of deployment and operation, depth keeping and loads avoidance, yawed flow behavior, and various failure modes. An extensive suite of sensors is employed on the C‐Plane test model including: 6 degree‐of‐freedom (DOF) load cells, 6‐DOF inertial measurement and heading sensors, rotor torque, rotor rpm, rotor position, static pressure/depth, tow speed, and mooring tension. These sensors provide a comprehensive understanding of the C‐Plane motion and essential loads during testing. A 400Hz sample rate is utilized to accurately capture transient events. The model rotors have a high degree of controllability including rampup/ ramp‐down, counter‐rotating synchronization and phase‐shift, and constant tip‐speed‐ratio regulation. Many challenging aspects of testing a moored ocean current turbine have been addressed in this effort, such as: very low Reynolds number scaled rotor design and fabrication, development of a mooring test rig capable of yawed flow, and simulating the motions of a dual rotor moored device. This test program has proven that the CPlane design has a high degree of stability in a wide range of flow conditions and computational models are capable of accurately predicting CPlane behavior.