Coupled Mooring Analyses for the WEC-Sim Wave Energy Converter Design Tool: Preprint PDF Download
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Languages : en
Pages : 0
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A wave-energy-converter-specific time-domain modeling method (WEC-Sim) was coupled with a lumped-mass-based mooring model (MoorDyn) to improve its mooring dynamics modeling capability. This paper presents a verification and validation study on the coupled numerical method. First, a coupled model was built to simulate a 1/25 model scale floating power system connected to a traditional three-point catenary mooring with an angle of 120 between the lines. The body response and the tension force on the mooring lines at the fairlead in decay tests and under regular and irregular waves were examined. To validate and verify the coupled numerical method, the simulation results were compared to the measurements from a wave tank test and a commercial code (OrcaFlex). Second, a coupled model was built to simulate a two-body point absorber system with a chain-connected catenary system. The influence of the mooring connection on the point absorber was investigated. Overall, the study showed that the coupling of WEC-Sim and the MoorDyn model works reasonably well for simulating a floating system with practical mooring designs and predicting the corresponding dynamic loads on the mooring lines. Further analyses on improving coupling efficiency and the feasibility of applying the numerical method to simulate WEC systems with more complex mooring configuration are still needed.
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Languages : en
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Book Description
A wave-energy-converter-specific time-domain modeling method (WEC-Sim) was coupled with a lumped-mass-based mooring model (MoorDyn) to improve its mooring dynamics modeling capability. This paper presents a verification and validation study on the coupled numerical method. First, a coupled model was built to simulate a 1/25 model scale floating power system connected to a traditional three-point catenary mooring with an angle of 120 between the lines. The body response and the tension force on the mooring lines at the fairlead in decay tests and under regular and irregular waves were examined. To validate and verify the coupled numerical method, the simulation results were compared to the measurements from a wave tank test and a commercial code (OrcaFlex). Second, a coupled model was built to simulate a two-body point absorber system with a chain-connected catenary system. The influence of the mooring connection on the point absorber was investigated. Overall, the study showed that the coupling of WEC-Sim and the MoorDyn model works reasonably well for simulating a floating system with practical mooring designs and predicting the corresponding dynamic loads on the mooring lines. Further analyses on improving coupling efficiency and the feasibility of applying the numerical method to simulate WEC systems with more complex mooring configuration are still needed.
Author: Junhui Lou
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Category : Computational fluid dynamics
Languages : en
Pages : 174
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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.
Author: Kelley Ruehl
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ISBN:
Category : Ocean energy resources
Languages : en
Pages : 9
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To promote and support the wave energy industry, a wave energy converter (WEC) design tool, WEC-Sim, is being developed by Sandia National Laboratories and the National Renewable Energy Laboratory. In this paper, the WEC-Sim code is used to model a point absorber WEC designed by the U.S. Department of Energy's reference model project. Preliminary verification was performed by comparing results of the WEC-Sim simulation through a code-to-code comparison, utilizing the commercial codes ANSYS-AQWA, WaveDyn, and OrcaFlex. A preliminary validation of the code was also performed by comparing WEC-Sim simulation results to experimental wave tank tests.
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Languages : en
Pages : 0
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WEC-Sim (Wave Energy Converter SIMulator) is an open-source code for simulating wave energy converters, which has been actively developed and applied to simulate a wide variety of device archetypes and has become a popular tool since its initial release in 2014. WEC-Sim is developed jointly by the National Renewable Energy Laboratory (NREL) and Sandia National Laboratories (SNL) within the MATLAB/SIMULINK environment. Figure 1 illustrates a general wave-to-wire model which begins with a deployment site resource characterization, which is used to complete the hydrodynamic simulation of a single WEC (or array), with the power generation profile imported to a grid simulator to understand the influence on the local electrical network. While modelling the entire wave-to-wire is difficult and encompass multiple time scales and physics, WEC-Sim is focused on the hydrodynamics simulation to predict, analyze and optimize WEC dynamics and power performance. WEC-Sim simulations are performed in the time domain based on the radiation and diffraction method using hydrodynamics coefficients derived from boundary element method (BEM) based-frequency-domain potential flow solvers (e.g., WAMIT, NEMOH, Capytaine, or ANSYS-AQWA). Within this level of modeling fidelity, WEC-Sim can handle floating body hydrodynamics, mechanical and electrical power generation methods, advanced control implementation, mooring systems, and other unique applications such as desalination. Table 1 lists additional WEC-Sim functionalities, which are created using prebuilt Simulink blocks and MATLAB scripts that can simulate a wide range of floating systems and the corresponding auxiliary subsystems.
Author: Gianmaria Giannini
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Category :
Languages : en
Pages : 0
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The main advantage of the developed tool, compared to existing codes, is that this is tailored to the specific case of analysis and design for Earth-reacting wave energy converters. The new generic methodology proposed showed to be useful and suitable for analysing and design wave energy converters by including the mooring system as an integral component.
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Languages : en
Pages : 0
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A wave energy converter (WEC) must be designed to survive the extreme sea states that it will be subject to throughout its lifetime. Although there are many analysis methods and codes available to accomplish this, there are currently several engineering challenges to WEC survival design. Foremost, the computational design approach will typically involve a trade-off between accuracy and computational efficiency. Additionally, most computational fluid dynamics (CFD) codes are not ideally suited to modeling extreme events for WECs with multibody dynamics, power-take-off systems, and mooring systems. Finally, although WEC design standards and CFD guidelines are emerging, with the current immaturity of the WEC industry, they are not yet well established. In this study, loads on a 1:35-scale, moored, multibody WEC are evaluated with CFD. The CFD results are compared with results obtained from a computationally efficient, midfidelity model based on linearized potential flow hydrodynamics. For these model verification comparisons, both operational and survival configurations are considered. The extreme load results obtained, using both codes, indicate that the survival configuration successfully sheds loads during extreme sea states. It is also found that WEC-Sim, when appropriately applied, can provide reasonable load results, at a fraction of the computational expense of CFD. However, for the more extreme sea states, and for higher-order effects not included in the WEC-Sim model, the linear-based results have significant errors in comparison to the CFD-based results, and should be used judiciously.
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Languages : en
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WEC-Sim is a mid-fidelity numerical tool for modeling wave energy conversion (WEC) devices. The code uses the MATLAB SimMechanics package to solve the multi-body dynamics and models the wave interactions using hydrodynamic coefficients derived from frequency domain boundary element methods. In this paper, the new modeling features introduced in the latest release of WEC-Sim will be presented. The first feature discussed is the conversion of the fluid memory kernel to a state-space approximation that provides significant gains in computational speed. The benefit of the state-space calculation becomes even greater after the hydrodynamic body-to-body coefficients are introduced as the number of interactions increases exponentially with the number of floating bodies. The final feature discussed is the capability toadd Morison elements to provide additional hydrodynamic damping and inertia. This is generally used as a tuning feature, because performance is highly dependent on the chosen coefficients. In this paper, a review of the hydrodynamic theory for each of the features is provided and successful implementation is verified using test cases.
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Languages : en
Pages : 0
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This study explores and verifies the generalized body-modes method for evaluating the structural loads on a wave energy converter (WEC). Historically, WEC design methodologies have focused primarily on accurately evaluating hydrodynamic loads, while methodologies for evaluating structural loads have yet to be fully considered and incorporated into the WEC design process. As wave energy technologies continue to advance, however, it has become increasingly evident that an accurate evaluation of the structural loads will enable an optimized structural design, as well as the potential utilization of composites and flexible materials, and hence reduce WEC costs. Although there are many computational fluid dynamics, structural analyses and fluid-structure-interaction (FSI) codes available, the application of these codes is typically too computationally intensive to be practical in the early stages of the WEC design process. The generalized body-modes method, however, is a reduced order, linearized, frequency-domain FSI approach, performed in conjunction with the linear hydrodynamic analysis, with computation times that could realistically be incorporated into the WEC design process. The objective of this study is to verify the generalized body-modes approach in comparison to high-fidelity FSI simulations to accurately predict structural deflections and stress loads in a WEC. Two verification cases are considered, a free-floating barge and a fixed-bottom column. Details for both the generalized body-modes models and FSI models are first provided. Results for each of the models are then compared and discussed. Finally, based on the verification results obtained, future plans for incorporating the generalized body-modes method into the WEC simulation tool, WEC-Sim, and the overall WEC design process are discussed.
Author: Michael J. Lawson
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Category : Hydrodynamics
Languages : en
Pages : 8
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Wave energy converters (WECs) are commonly designed and analyzed using numerical models that combine multi-body dynamics with hydrodynamic models based on the Cummins Equation and linearized hydrodynamic coefficients. These modeling methods are attractive design tools because they are computationally inexpensive and do not require the use of high performance computing resources necessitated by high-fidelity methods, such as Navier Stokes computational fluid dynamics. Modeling hydrodynamics using linear coefficients assumes that the device undergoes small motions and that the wetted surface area of the devices is approximately constant. WEC devices, however, are typically designed to undergo large motions in order to maximize power extraction, calling into question the validity of assuming that linear hydrodynamic models accurately capture the relevant fluid-structure interactions. In this paper, we study how calculating buoyancy and Froude-Krylov forces from the instantaneous position of a WEC device (referred to as instantaneous buoyancy and Froude-Krylov forces from herein) changes WEC simulation results compared to simulations that use linear hydrodynamic coefficients. First, we describe the WEC-Sim tool used to perform simulations and how the ability to model instantaneous forces was incorporated into WEC-Sim. We then use a simplified one-body WEC device to validate the model and to demonstrate how accounting for these instantaneously calculated forces affects the accuracy of simulation results, such as device motions, hydrodynamic forces, and power generation.
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Category :
Languages : en
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WEC-Sim is a midfidelity numerical tool for modeling wave energy conversion devices. The code uses the MATLAB SimMechanics package to solve multibody dynamics and models wave interactions using hydrodynamic coefficients derived from frequency-domain boundary-element methods. This paper presents the new modeling features introduced in the latest release of WEC-Sim. The first feature discussed conversion of the fluid memory kernel to a state-space form. This enhancement offers a substantial computational benefit after the hydrodynamic body-to-body coefficients are introduced and the number of interactions increases exponentially with each additional body. Additional features include the ability to calculate the wave-excitation forces based on the instantaneous incident wave angle, allowing the device to weathervane, as well as import a user-defined wave elevation time series. A review of the hydrodynamic theory for each feature is provided and the successful implementation is verified using test cases.
