Graduate Thesis Or Dissertation

Numerical Simulation, Laboratory and Field Experiments, Analysis and Design of Wave Energy Converter and Mooring System

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  • This dissertation studies the coupled fluid-structure interaction (FSI) of a wave energy converter (WEC) and evaluates the design of a WEC mooring system. The research is conducted in support of conceptual development, field test and performance evaluation of WECs as part of the mission of the Northwest National Marine Renewable Energy Center at Oregon State University. The coupled FSI study focuses on the evaluation of predictive capabilities and computational performance of commercial computational fluid dynamics (CFD) and potential flow codes using laboratory model test results. The evaluations of a WEC mooring system focus on analysis of field test data and evaluations of the anchor movability, fatigue design and extreme load of the Ocean Sentinel (OS) test platform mooring system deployed off the Oregon coast. Numerical data using a commercial mooring system simulation code are conducted to supplement time history data for the calculations of anchor pulling force, fatigue damage and extreme load. Specifically, this dissertation can be divided into three parts. In the first part the performances of a finite element explicit Navier-Stokes (NS) solver (LS-DYNA ALE), a finite element implicit NS solver (LS-DYNA ICFD), and a nonlinear potential flow solver (AQWA) in predicting highly nonlinear hydrodynamic responses of a floating point absorber (FPA) under large-amplitude waves are studied. The two NS solvers calculate the coupled FSI including fully nonlinear inviscid and viscous forces. The nonlinear potential flow solver calculates individual inviscid wave force components (a Froud-Krylov force, a radiation force, a diffraction force and a hydrostatic force) and empirical (Morison equation) viscous force. Comparing numerical results to laboratory experimental measurements, the two NS solvers and the nonlinear potential flow solver are found to be capable of providing accurate predictions of the nonlinear motion responses of the FPA. FSI coupling algorithms and computational costs of these three solvers are evaluated. Based on the results of the nonlinear potential flow solver at different wave periods, the individual wave force components and the viscous force are studied quantitatively. The nonlinearity of the restoring force and the Froude-Krylov force are found to be important for the FPA responses in all (heave, surge and pitch) directions; the nonlinearity of the viscous force is found to be important in only the heave and pitch directions. The second part first presents a catenary spread mooring system design of a mobile ocean test berth (MOTB), the Ocean Sentinel (OS) instrumentation buoy, which is developed by the Northwest National Marine Renewable Energy Center (NNMREC) to facilitate ocean test of wave energy converters (WECs). Then the OS mooring design, which is similar to a conventional WEC point absorber mooring system, is evaluated through both field test analysis and quasi-static analysis: the field test analysis is based on the extensive data of the OS positions, mooring tensions on the OS and environmental conditions of waves, wind and current, collected during the 2013 field test of the OS mooring system; the quasi-static analysis is based on the analytical catenary equations of mooring chains. Both global characteristics and survivability characteristics of the mooring system are evaluated: the global characteristics include the influence of the OS excursion to mooring tension, positional distribution of the OS, directional control of the OS and environmental contributions of waves, current and wind to mooring tensions; the survivability characteristics include the anchor movability and strength capacities of mooring. Because anchor movement occurred near the end of the field test, a systematic procedure of designing a mooring system with adequate anchor holding capacity is developed and applied to design a new OS mooring system. In the third part, first, the accuracies of a fully coupled method based numerical model in predicting the mooring tensions of the OS mooring system and the OS positions are validated by comparing the numerical results to the field data collected during the 2013 OS field test. Then, the anchor movability, fatigue damage and extreme mooring tension of the OS mooring system are investigated using the mooring tensions predicted by the numerical model. The results of the above studies are summarized as follows: (1) The numerical model provides accurate predictions of the mooring tensions and OS positions under harsh environmental conditions; (2) When the OS drifted significantly near the end of the field test, the bow, port and starboard anchors were likely not dragged, dragged significantly and dragged slightly, respectively; (3) The fatigue damages of mooring lines are predicted for environmental conditions from low to high sea states; and (4) The strengths of mooring lines in the original mooring design are adequate compared to the predicted extreme mooring tensions.
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