An abundant renewable energy source is represented by ocean waves. However, wave-energy converters must be able to withstand strong ocean waves without tipping over in order to make the best use of this natural resource.
Researchers at Texas A&M University have created analytical methods that can be used to describe how floating but moored wave-energy devices move. They said their method is quick while yet being precise enough to predict whether wave-energy devices will turn over in a constantly-changing ocean environment, as opposed to complex models that are expensive and time-consuming.
“Wave-energy converters need to take advantage of large wave motions to make electricity. But when a big storm comes, you don’t want big wave, wind and current motions to destroy these devices,” said Dr. Jeffrey Falzarano, professor in the Department of Ocean Engineering. “We have developed much simpler analytical tools to judge the performance of these devices in a dynamic ocean environment without necessitating massive amounts of simulations or physical model tests that take a lot of time to run and are cost-prohibitive.”
The mathematical tools are described online in the journal Ships and Offshore Structures in July.
Wave-energy devices function in two modes. In “normal mode,” they convert the energy from tidal waves into electricity. Thus, this mode largely determines whether the design of the wave-energy device is economically efficient.
Wave-energy converters need to take advantage of large wave motions to make electricity. But when a big storm comes, you don’t want big wave, wind and current motions to destroy these devices. We have developed much simpler analytical tools to judge the performance of these devices in a dynamic ocean environment without necessitating massive amounts of simulations or physical model tests that take a lot of time to run and are cost-prohibitive.
Dr. Jeffrey Falzarano
However, in “survival” mode, or when incident waves create significant motions in the wave-energy devices, a system of moorings that anchors the equipment to a place at the bottom of the body of water greatly influences the operation of the wave-energy devices.
Moorings can be placed in a variety of ways and come in a variety of types, including wharfs and anchor buoys. In addition, wave energy devices come in a wide variety of shapes, making it difficult to anticipate whether a certain device may capsize.
“Ships come in a variety of shapes and sizes; tankers, for example, are very different from fishing vessels or other military ships. These different geometries affect the ship’s motion in the water,” said Falzarano. “Similarly, the shape of wave-energy devices can be quite diverse.”
For the analysis, Hao Wang, Falzarano’ s graduate student, used a cylindrical wave-energy device. The researchers were able to expand their analysis to additional wave-energy converters with a similar shape because to the generic form’s ability to simplify the prediction challenge. He also gave three mooring arrangements some thought.
Hao used two analytical methods, the Markov and Melnikov approaches, to predict the risks of turning over under random excitation. More specifically, the techniques produce a graph with an envelope-like region utilizing data from the geometry of the wave-energy device, the setup of the mooring system, and tidal wave characteristics. It makes sense that if the floating vessel leaves this envelope during really large waves, such as those that occur during a storm, it will most certainly flip over.
The fact that the analytical models almost produced the same answers despite being very different from one another was recognized by the researchers as evidence of their validity and correctness. They added that the performance of other floating devices, such as floating wind turbines, can be evaluated using their mathematical approach.
“The platform for a floating wind turbine is the same as the one for wave-energy devices, and so floating turbines can also pitchpole or turnover if the waves are very high,” said Falzarano. “My group has been leaders in developing methods for predicting ship stability. We’re now looking at applying those approaches to renewable, floating energy devices.”
