For the engineers on the Maine Tidal Power Initiative, the site-specific questions focus on turbine form and function of energy conversion. For the past several years, Peterson and Richard Kimball, a Maine Maritime Academy associate professor and a UMaine external graduate faculty member in the Department of Mechanical Engineering, have been collaborating on research focused on in-stream cross-flow and axial turbine designs at test facilities on campus and at a federally approved site tidal energy demonstration and evaluation site near Castine.
Also as part of the Maine Tidal Power Initiative, the engineering research includes the little-consider interaction between the seabed and foundation structures that support the energy conversion technology. Sites for offshore energy technologies are in highly complex geological areas with variable bedrock depths and soil properties. And no matter how the technology is secured to the seabed for fixed or floating devices, the foundation must withstand any number of loading scenarios — from large lateral forces from tidal currents over multiple tidal cycles to extreme storm scenarios.
“A disconnect exists between technology designers and developers, and the operational environment,” according to civil and environmental engineers Melissa Maynard at UMaine and James Schneider at the University of Wisconsin, in a paper on geotechnics for developing offshore renewable energy in the U.S., published in Frontiers in Offshore Geotechnics II. “Little attention has been given to seabed-foundation interaction, with the exception of offshore wind, and site investigation, soil properties and foundation are largely ignored until final development stages.”
However, designs that incorporate mooring and foundation system responses with metocean conditions — meteorological and oceanographic — likely result in increased efficiency of power production and cost, say Maynard and Schneider. Ocean energy conversion devices such as tidal turbines require more engineering in both structure and foundation to maintain stability when stressed by large lateral loads.
“Developers have a misconception that you can put technology on the seabed and not a lot of engineering is required,” says Maynard, whose research includes land-based and offshore site characterization, physical modeling of soil-structure interaction and foundation engineering. “But soil is an engineering material. We have to understand how the material will interact with new loading scenarios.”
It’s important for developers, contractors, and regulators who work on the entire process to understand that offshore geotechnics and soil structure interaction play significant roles in stability of the turbine in the high-energy environment, she says.
“Tidal environments are challenging,” says Maynard, who was at the University of Western Australia’s Centre for Offshore Foundation Systems this summer conducting in situ testing of soil samples under loads for site characterization of the Gulf of Maine seabed. “When currents are so high, the question is how to economically and efficiently characterize the site to investigate the engineering properties of the seabed. The equipment needed for a site characterization, including collecting samples from the bottom for laboratory testing, can be very expensive, and water depths, high currents, and poor weather increase costs. My ultimate goal is to incorporate deepwater site characterization methods for seabed drilling for oil and gas and learn how to decrease the costs for deep-sea offshore energy technologies.”
With the largest tidal energy resource in the continental United States and its land-grant and sea-grant university assertively pursuing research to help ensure responsible, cost-effective and sustainable development, Maine is poised to be a leader in tidal energy, Peterson says. For Maine, the biggest tidal energy-related benefits are in the form of tax revenue, jobs and alternative renewable energy — in that order.
“The electricity is going to be important — I can see it helping, especially out on the ends of the grid — but the most important thing we’re going to see out of tidal energy is the development of a new industry with jobs that will produce companies that pay taxes. What I would like to see is tidal energy providing reliable, local power that could be used for heating. And if we switch the state of Maine from oil to electricity using heat pumps, we’ve got the potential to very effectively reduce the cost of doing business and living in Maine.”
Think about it, Peterson says. Why did the U.S. auto industry all end up in Michigan? One company got started, then another moved to the same area to take advantage of existing support sectors — the axle manufacturer and foundry. Similarly in Maine, small companies supporting paper and other industries, for example, can diversify to meet the needs of emergent tidal energy and offshore wind industries, he says, both in state and beyond.
“I just see this as the start of a very exciting industrial cluster,” says Peterson. “We already see where there’s a group of people on the mechanical side — independent engineers, machining and assembly companies. What that really means, then, is the next company that comes in is already going to have an engineer who knows how to do marine design for this particular application. And we’re not trying to do it all in-house. Our goal here is to build the independent companies that already exist in Maine. We’re now looking on the electronics side, where we’ve got this same model working, where we’ve got design and assembly people.”
In 10 years, Peterson says, the tidal energy industry will be more mature and exploring the next frontier, such as wave or freshwater in-stream turbine technology. With the multidisciplinary approach of the Maine Tidal Power Initiative, Maine’s tidal energy infrastructure will be in place — and keeping pace.