Perhaps no American is better positioned to discuss the state of offshore wind technology than Habib Dagher. A structural and civil engineer with a PhD from the University of Wisconsin, Dagher heads the University of Maine’s Advanced Structures and Composite Center, the site of landmark research and testing on wind turbine blades and floating wind systems. Dagher co-founded the center in 1996. He holds more than 80 patents, most on technologies related to various aspects of floating wind power, including methods of construction, hull designs, and buoys.
One signature achievement: The VolturnUS, which the Center designed and built. It was the first grid-connected offshore wind turbine in the US and a pioneer in the offshore floating wind, employing revolutionary designs that utilized floating concrete hull technology. That one-eighth scale model prototype was launched in 2014. The Center is now developing a full-scale, floating offshore wind turbine, scheduled to be constructed next year and launched in 2024.
While the center’s first wind-related work was on composite materials wind blades, the institution 14 years ago became the first research center in the US to begin investigating floating wind technology. That was, in part, a necessary outgrowth of Maine’s geology. Unlike most states elsewhere along the Atlantic Coast, Maine’s continental shelf is quite deep and ill-suited for fixed-bottom wind turbines. To harness the ample wind off the Maine coast, floating wind turbines will be a necessity.
The center now boasts of multiple laboratories that total 100,000 square feet and include a unique facility that can simultaneously test both wind and wave stresses on turbines.
Dr. Dagher spoke to the American Journal of Transportation twice over Zoom from his office in Orono, Maine. The conversations were edited for space.
AJOT: Taking offshore wind power as a whole, wherein the technology curve are we?
Dagher: There are two types of offshore wind technologies, fixed-bottom turbines, and floating turbines. And they’re quite different.
In the fixed bottom world, Europe built the first offshore wind farm in 1991. They built a very significant industry. They have over 5,000 turbines installed in Europe already.
When you talk about fixed-bottom turbines, you are also talking about what goes along with them. So, for example, the turbine designs and manufacturing are all in Europe. There is no offshore wind turbine manufacturer in the United States today.
AJOT: What about floating turbines?
Dagher: When the water exceeds roughly 150 feet, floating becomes the technology you look at because building a 150-foot foundation down to the seabed will cost you a lot of money. [In 2009] Equinor built a single two-megawatt unit off the coast of Norway, and that became the very first floating turbine in the world. Since then, they built a project off Scotland, using five, six-megawatt units. And now they’re doing one in an oil and gas field [in the Norwegian North Sea] that is even bigger than that, but not really commercial scale yet.
With floating technology, it’s a wide-open field. There are over 40 technologies in the floating world right now. Proposed solutions for floating wind turbines fit into three different major categories: Spar buoy, semi-submersible, and tension-leg platform. And there are also hybrids of these designs.
This is where we have focused our research at the University of Maine for the last 14 years and have built the leading research team in floating technology in the United States.
The design that U Maine has developed is semi-submersible, and that’s the most common technology being proposed across the world today. The one that Equinor designed is called the Spar-buoy, but this requires a very, very deep draft, 250-to-300-foot drafts for these units. So, it’s very difficult to fabricate them dockside because you don’t have port facilities with 300 feet of water unless you’re in Norway.
What we’ve developed is a semi-submersible that has less than a 25-foot draught dockside, so you can actually make the hull in a lot of places around the US.
Also, the hulls are stable without tension legs, without the mooring lines. The mooring lines just keep them on the station, so they don’t fall away.
So, the advantage of the semi is it’s stable on its own and has a relatively small draft. You can actually fabricate it onshore and throw it out to sea.
AJOT: Are there cost advantages of the semi-submersible as well?
Dagher: Every technology has its pros and cons — where you are in the world, what manufacturing infrastructure you have, what port facilities, what geophysical conditions you have offshore. how deep the water is, what kind of soil conditions you have. All determine which technology may or may not be better than the other. There’s really no single solution that fits all. But you can’t make the Spar-buoy in the US, there is no port facility where you can make it.
AJOT: Do the blades and the nacelle, the turbine itself, present technological challenges in floating that are different from fixed?
Dagher: When you go to floating technologies, you have different vibration characteristics, motion characteristics of the hull. The hulls are moving around a lot more when they’re floating than when they’re fixed. They’re moving at different frequencies. You want to make sure that the motions of the hull, as the hull moves around in the waves and the winds, don’t eventually damage the turbine from a fatigue perspective or strength perspective. You don’t want to over-stress the blades or over-stress the gears inside the turbine nacelle.
AJOT: Will it be necessary for a totally new blade design or nacelle design? Or is it likely to be an adaptation of what is now being produced?
Dagher: We’ve worked with all the turbine OEMs. Right now, we’ve been able to adapt the current turbines to the hull that we have. We can accept turbines from almost any supplier.
What we would have to do from an adaptation perspective is really optimize the turbine for the hull. How you control the pitch and power generation of the turbine is very important because if the turbine base is moving, you have to account for that in the control center. So, the controls of the turbine are being adapted to the hulls and the hulls are being adapted to the turbine controls.
Does that mean that you couldn’t design a better turbine that’s specifically optimized for the home? That’s a whole other question. You could. But is it worth it? Is it worth it to go out and reinvent the industry? You can argue one way or the other.
AJOT: What are some other ways to design a turbine?
Dagher: Right now, the most common design is the horizontal axis turbine. That’s called an upwind turbine. Vertical axis turbines have been looked at for floating technologies as well. These are turbines that we call downwind turbines.
AJOT: Please explain the difference.
Dagher: Upwind, the wind always hits the turbine before it hits the tower. There’s a yaw mechanism. If you think of the blades, they always see the wind first before it gets to the tower. That’s called an upwind turbine. A downwind turbine actually has the blades on the other side of the wind, so the wind hits the tower first and then goes to the blades. That design can have a yaw system, but you can have the whole hull yaw. The whole hull actually yaws back and forth, so that the turbine is always optimized to the wind. These designs have a single mooring system so that instead of yawing just the turbine itself, you’re yawing the whole hull.
AJOT: Are engineers investigating other innovations for offshore wind turbines?
Dagher: There are some designs that are looking at putting the generator down at the base.
AJOT: You’ve said that with floating wind turbines, you can adapt current fixed turbine technology. Does that include the new generation of 12 and 15MW turbines?
Dagher: Yes. When we started in this business, we were doing two-megawatt turbines. The one that we’re designing today, that we’re putting off the coast of Monhegan Island in Maine, is 11 megawatts. And we’ve been able to adapt our designs, our current turbines, to that. We are also looking at 15 and 20-megawatt turbines as we speak and looking at adaptation. At least going from two to 11 [megawatts], it has been possible. Can we go from 11 to 15 and 15 to 20 [megawatts] cost-effectively? Going from 11 to 15, I’m very optimistic. That’s very doable. Going from 15 to 20, I think I’ll reserve judgment.
AJOT: In terms of commercial adaptation of floating wind, we’re still talking 2028, 2029, at the earliest, and that’s probably being generous. Does it frustrate you that adaptation is taking so long? Is there a way to speed it up or is this just the way things are?
Dagher: There are technology developments that are needed, physical infrastructure, development of port facilities that are needed for the industry. Finally, there are permitting requirements. You’ve got the technology, you’ve got port facilities and you’ve got permitting. These are three different, separate issues, that really result in the schedule you’re talking about. But the good news is that back in 2013, we actually put the first floating turbine off the US coast. We’ve learned a lot since then and now we’re in the middle of doing one that’s full-sized, bigger than the Washington Monument.
AJOT: You mentioned the need for advances in port facilities. Why can’t we utilize existing facilities or the ones now under development for fixed bottom offshore wind? Is it too difficult to adapt fixed-bottom facilities to floating?
Dagher: It’s not just difficult. In some cases, it’s impossible. You’ve got to fabricate floating turbines dockside and these units are bigger than the Washington Monument. How do you get those in the water? You need deepwater ports. You need facilities that have water depths of 30 feet or more because some of these will draft 25 feet. Maybe you could dredge some existing facilities, but that’s a whole other project.
The other thing is the fabrication methods that you need for floating wind turbines are different from fixed bottom. You need the port facilities to fabricate and assemble these hulls and they’re different. With fixed bottom, you typically use mono-piles. These are essentially 20-25 foot diameter steel towers that you can bring [to a port] and assemble. It doesn’t take the same amount of space and real estate that it takes to assemble a floating hull, which are much bigger and much more complex. You need dedicated facilities that can fabricate big hulls like that. The existing fixed-bottom facilities haven’t been designed specifically to do that. You will need quite a bit of acreage if you wish to build a commercial floating facility, 50 to 100 acres, and more, depending on what you are trying to do.
AJOT: Because the hull is made of concrete, is one advantage that it can be fabricated in Maine or wherever?
Dagher: It certainly could be fabricated locally. We want a technology that creates local jobs, and that’s really why we went to the concrete hull, we have no ability to mass-produce these hulls at this time. And that’s why we went in that direction, right? The first of the concrete hulls we have are less expensive.
AJOT: Are you concerned that transmission issues, the technology, and the infrastructure necessary, may lag behind the development of floating offshore wind?
Dagher: The difference between a floating turbine and a fixed-bottom turbine is that the floating turbine has what we call a dynamic cable. It floats in a water column, has buoyancy modules, and is fixed to the seabed. Each turbine is going to have a dynamic cable and it’s typically a 66KV cable. So, it’s not low voltage, but it’s not extremely high. You’re going to daisy chain these, one to the other, connecting five up to 10 turbines on one line. So, each group of 10 turbines then has one of these cables coming out of it, bringing it together [with others] into an offshore substation. That offshore substation then increases the voltage from 66KV to something over 200KV. And the reason we do that is you have a lot fewer losses in the line by having a higher voltage.
Particularly if you’re 30 miles offshore, you need a substation there that will pick up your voltage and send the voltage out in one or two cables for the whole farm to shore. Then onshore, you connect that to a control substation.
So, the technology for these dynamic 66KV cables already exists. The technology for the offshore export cable already exists. The technology called the inner array cable that daisy chains the cables to get the turbines together already exists. What doesn’t exist today is the high voltage dynamic cable from a floating substation to the ground, and that’s being worked on as we speak. That’s what is left.
