Carlos Michelen (in Sandia’s Water Power Technologies Dept.) and a team of National Renewable Energy Laboratory (NREL) researchers (Michael Lawson, Braulio Garzon, Fabian Wendt, and Yi-Hsiang Yu) won the Offshore Mechanics and Arctic Engineering (OMAE) 2015 Hydraulic Modeling Competition during the 34th ASME International Conference on Ocean, Offshore, and Arctic Engineering in St. Johns, Newfoundland, Canada, in June. The NREL and Sandia team entered the competition and modeled the experimental device (described below) using both
- WEC-Sim, an open-source wave energy converter (WEC) simulation tool and
- FAST, an aerodynamic open-source code often used by the wind industry.
“FAST is old and well-developed, but WEC-Sim is a fairly new program,” Carlos said. “This competition validated our models—and proved the value of a mid-fidelity, fast-running model.”
Studying the motions of a rigid body in fluid has a significant impact on technical applications and scientific research. Regardless of the main purpose of the rigid body, dynamic modeling is an essential part of control design and preliminary testing. The hydrodynamic modeling competition didn’t focus on any particular type of the rigid body (e.g., WEC, marine surface vessel, or underwater vehicle). The competition considered a device with a generic shape of the wetted surface; the device is neither an underwater vehicle nor any particular WEC. The figure (below, left) illustrates the geometry of the chosen device—a submerged horizontal cylinder with domed ends, adapted to have surface-piercing geometry, and connected to a mooring system. The resonance period of the device is around 2.2 s for the heave motion. The moorings of the submerged cylinder are vertical lines with a clump mass arranged so that a resonance also occurs in surge.
The competition’s sponsor, the Centre for Ocean Energy Research (COER) at Maynooth University in Ireland, asked participants to model the WEC’s motion when subjected to an incident irregular wave. The research teams were challenged to predict the dynamic response of a floating rigid-body device that was experimentally tested in a series of wave-tank tests. Specifically, COER set up a blind competition, where the device specifications and test conditions were released, but the experimental results were kept private until all competition participants submitted their numerical simulation results.
Our Sandia/NREL team modeled the device using both the WEC-Sim and FAST numerical modeling tools. WEC-Sim was developed by NREL and Sandia specifically to simulate the performance of WEC devices. FAST is a code developed by NREL over the past several decades to model land-based and offshore wind turbines, although it is capable of modeling a wide variety of offshore structures that comprise aerodynamic, hydrodynamic, elastic, and servo components.
The numerical tools were used to predict the surge- and heave-decay frequencies of the device as well as the device response in regular and irregular wave fields. The numerical results for the irregular-wave simulations, which were obtained without a priori knowledge of the experimental results, were compared to the experimental results. Overall, the numerical and experimental data were shown to be in good qualitative and quantitate agreement in surge, whereas the agreement was not as good in heave.
“Creating accurate, open-source modeling tools is a priority for wave energy researchers,” Michelen said. Developers can use open-source codes, such as WEC-Sim, to minimize the cost of testing and deploying new wave energy conversion technologies. “The wave industry is small and most companies are only in the research stage. Because companies are very protective of their models, it is difficult for new developers to enter the market. Open-source codes such as WEC-Sim enable new developers to model new ideas.”
The Ocean, Offshore, and Arctic Engineering division of ASME’s award provides independent validation of the Sandia/NREL modeling tools.