At issue are marine hydrokinetic devices, which convert the mechanical energy of the ocean’s tides, current and waves into electricity.

Researchers have created a new modeling tool that can be used to help develop ocean-based hydrokinetic energy projects. The tool can be used both to help design more robust marine hydrokinetic technologies and to inform risk assessments that are essential for securing financing and permitting of commercial projects.

At issue are marine hydrokinetic devices, which convert the mechanical energy of the ocean's tides, current and waves into electricity.
"Marine hydrokinetic technologies hold promise for being a significant contributor to sustainable energy portfolios in the future," says Anderson de Queiroz, co-author of a paper on the work and an associate professor of civil, construction and environmental engineering at North Carolina State University. "However, the same currents and waves that allow hydrokinetic devices to generate electricity can also damage the devices during extreme weather events, such as hurricanes or tropical storms. For example, waves generated by high winds could potentially tear devices loose from their moorings and anchoring system."
"Marine hydrokinetic energy projects are expensive to get off the ground," says Mo Gabr, co-author of the paper and Distinguished Professor of Civil Engineering and Construction at NC State. "If a developer is proposing a marine hydrokinetic project, it will need to apply for permits and get insurance - and both of those things will require the developer to do a robust risk assessment. That's where fragility curve estimates come in."
For risk assessments, it is important to know how much force the components of marine hydrokinetic device can withstand before it breaks loose from the ocean floor. And that's exactly what a fragility curve estimate can tell you. Specifically, in this context, a fragility curve estimates how much force a marine hydrokinetic device can withstand from hurricanes or storms before damage occurs due to mooring system failure.
"So, a fragility curve estimate helps you make more informed risk assessments regarding marine hydrokinetic energy projects," de Queiroz says. "However, it also gives you information you can use to design marine hydrokinetic device mooring and anchoring systems that are more robust and better able to withstand extreme weather conditions.
"Our goal with this work, which focused on hydrokinetic devices called ocean current turbines, was to create a model that can do two things," de Queiroz says. "First, allow users to determine the fragility curve estimate for their projects based on the specific characteristics of the devices and mooring and anchoring systems they are using. Second, incorporate a hydrodynamic simulation component that allows users to see precisely how their systems respond to various currents and wind speeds. This second element is particularly valuable for helping system designers develop more robust devices and mooring systems."
With that in mind, the researchers have created a piece of software that allows users to plug in the specific data associated with their marine hydrokinetic projects, such as ocean currents, wind speeds, the physical characteristics of the hydrokinetic devices and mooring systems, and so on. The software can then be used to determine not only how likely a system is to break free from its moorings at the relevant location, but how much current or wind speed the technology could withstand before such failure occurs.
"For example, you could use this tool to determine whether your device would likely break loose from its anchoring and mooring system during a 100-year storm," de Queiroz says.
"But you could also use this tool to modify your device or mooring system in the simulation until its support capacity characteristics are capable of withstanding a 100-year storm - and then use that information to design a system that has the attributes that will provide such capacity."
The researchers plan to make the software program freely available on the GitHub platform.
"We hope our model will be useful for advancing successful marine hydrokinetic energy projects and technologies that will make meaningful contributions to renewable energy," de Queiroz says.
The paper, "Bayesian Modeling and Mechanical Simulations for Fragility Curve Estimation of The Mooring System of Marine Hydrokinetic Devices," is published in the journal Applied Ocean Research. Corresponding author of the paper is Victor de Faria, who received his Ph.D. from NC State earlier this year. The paper was co-authored by Neda Jamaleddin, a Ph.D. student at NC State.
This work was done with support from the North Carolina Renewable Ocean Energy Program under grant number 16065 11-0127.