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Uncharted waters

Written by  Elaine Maslin Monday, 01 June 2015 00:00

A milestone is about to be reached in the tidal energy industry off the cold harsh coast of Scotland. Elaine Maslin talked to the project director behind the MeyGen tidal array development.

Atlantis Resources’ AR1500, artists’ illustration. Images from Atlantis Resources.

First, assemble a generator, gearbox, electronics and instrumentation with an 18m-diameter rotor to power it. Then attach it to a rocky, 30m+ deep seabed, scoured clean by a 5m/sec mass of turbulent energy rushing from the Atlantic to the North Sea and back. Then expect it to operate for 25 years.

It’s a challenge, but for an engineer, it’s also pretty interesting work. It’s a job David Collier, project manager on the MeyGen project, the world’s largest planned tidal development project, is relishing. When fully completed, MeyGen will see 269 tidal turbines, totaling 398MW, installed in the Inner Sound of the fast flowing Pentland Firth, between mainland Scotland and the Orkney islands.

Earlier this year, construction started on the onshore part of the first phase (1A) of the project. Offshore, Phase 1A will see four turbines, totaling 6MW capacity, installed on the rocky seabed and tied into the grid 2km away onshore. Phase 1 will see a further 61 turbines installed, with the final project working up to 269 turbines.

The MeyGen project, which is owned by Atlantis Resources, will deploy two different turbines in Phase 1A. Norway-based Andritz Hydro Hammerfest is supplying three of its HS1000 turbines and Atlantis Resources will supply one of its AR1500 devices, to be built by Lockheed Martin.

All four will sit in 31.5-36m deep water. First power to grid is expected to be delivered in 2016.

Humble beginnings, early learnings

Atlantis Resources began life as a turbine technology developer (see “Spotlight,” page 62). In 2010, the firm was part of a consortium awarded the development rights for the Inner Sound of the Pentland Firth by the Crown Estate. In 2013, the MeyGen project secured its final regulatory consents, making it the largest, fully consented tidal project in Europe. Not long after, Atlantis Resources took 100% control of the project. Last year, the firm listed on the London Stock Exchange and then secured a £50 million funding package for Phase 1A.

Turbinefarm – How the array could look on the seabed. 

Front-end engineering has been completed and the firm is still working through some finer details, including cable stability. For Phase 1A, the power export cables (one each per turbine) will travel from the onshore power conversion building under the rocky cliffs to the seabed via four, 550m-long, 404mm-diameter bore holes, with 315mm plastic liners, directionally drilled from shore. The bores are lined with 315mm-HDPE ducts, pushed through the bore hole from shore. The remaining 2km lengths of the 100mm-diameter, 4.4kV export cables, and future array cables, will be laid directly on the rocky seabed, which over the years has been scoured bare of all lose sediment due to the combination of fast currents and waves running through the Pentland Firth, and be fixed so that they remain stable for up to 25 years.

“We are trying to lay a stable cable in a very unstable environment,” Collier says. “From our research, this has not been attempted before. From an engineering point of view, it’s a probably one of the most challenging aspects of the project. I’ve worked in oil and gas for 15-20 years, and there were plenty of challenges, but most of the routes to the solutions had been trodden. That’s been the tricky bit on this project. It’s new territory.”

Testing turbines

MeyGen Phase 1A will use three HS1000 turbines and one AR1500 turbine. The design for both devices is similar in terms of requirement – an 18m rotor diameter with 3-bladed propellers – but they also have significant differences. For one, Andtriz Hydro Hammerfest has decided to use steel propeller blades on its 200-tonne HS1000, whereas the 160-tonne AR1500 is sticking with lighter composite blades.

“The steel blades have some advantages and some disadvantages,” Collier says. “They are very robust, but then you have to deal with fatigue and manufacturing them can be quite difficult. But, Andritz Hydro has a history of large hydro plants, so beating metal is not scary to them. It also makes it a bit heavier so you have higher inertia, which can be a benefit and a disadvantage. The AR1500 has stuck with composite, which is proven and quite successful. It is a bit less robust, but lighter and easier to handle.”

The two turbines also have different generators. The AR1500, described by Atlantis as one of the largest capacity single-rotor turbines built, has a permanent magnet generator with a two-stage gear box and the HS1000 an induction generator with a three-stage gear box. Both turbines have a common foundation, which is a three-legged, gravity-based structure that the turbine sits on with a stab connection and is maintained with gravity and the same size propellers.

The propeller blade diameter would ideally be bigger, just as in wind turbines where greater size means greater capacity. But, while some of the site could accommodate up to 20m-diameter blades, MeyGen’s research found that 18m was a good optimum, Collier says. To work at their rated power, 3m/sec flow is needed. Up to 3m/sec, the power output gradually increases. Once 3m/sec is exceeded, power availability has to be dumped.“When we started, we had 1MW devices,” Collier says. “We looked at the best optimization based on the flow profile and rated power and it turned out to be fairly flat between 1.4MW and 1.6MW, so we went for 1.5MW, which is achievable with an 18m-diameter rotor.”

At 18m-diameter, the turbine’s propeller tips will reach about 5.5m from the seabed and minimum 8m from the surface to ensure adequate clearance for any shipping (the tidal range is 2-3m). The turbines will include an active pitching system and full nacelle yawing capability that will be developed by Lockheed Martin, to enable them to adjust to the current strength and direction.

Atlantis Resources’ AR1500, artists’ illustration. 

Testing, testing, testing

Both turbines to be used in Phase 1A have already been tested offshore, at the European Marine Energy Centre, Orkney, for one- to two-year trials. This identified some improvements that were needed around electronics and reliability and redundancy. The real test will be the longer-term operation in the harsh environment of the Pentland Firth. A key lesson learned from the trials was, “you can never have too much instrumentation,” Collier says. “We are still in a phase where we need to know more about performance. What happens to the generators and gear boxes after 5-10 years of operation?

“They [the devices] have been tested at EMEC, which is a less harsh environment. Our site has more turbulence, more waves and slightly higher flow that they will be subjected to. We need to learn more about how these different elements will work in combination. And we are designing for 25 years, so we have to manage what that means in terms of design of the turbines.”

The firm will monitor the turbines and see the effects the external environment has on them so that the next time the same situation occurs, the loading can be adjusted. The sub-structure, nacelle, bearings, etc., will be monitored for fatigue. Another form of monitoring will be around marine wildlife, birds and seals for example, to monitor the interaction with the turbines and to ensure environmental consents are met.

“We don’t want to over design, but we don’t want to under estimate it either,” Collier says. “Getting it right is very important. On wind turbines, to some extent, you can always get access to the moving parts, electronics, etc., by boat. We do not have that option. If anything goes wrong with these turbines, we can’t do anything about it from shore. We have to pick it up out of the water and bring it back to shore. That, in itself, is a relatively expensive operation, losing potential revenue.”

As the project scales up, the MeyGen team will also have to learn how the turbines interact with each other, just as wind turbines interact according to the wind patterns created by other turbines around them, to design an optimum layout. Work is already under way. Patrick Farrell, now a research fellow at the University of Oxford, was commissioned by MeyGen to engineer the best shaped array layout. MeyGen found that, although an idealized solution can be found, when specific site conditions, such as the seabed bathymetry, are applied, the dynamics change. It’s a 3D problem, Collier says.

The research fellow, however, has written an algorithm for turbine array optimization. “It is quite amazing. It takes into account local flows and flow boundaries, etc.,” Collier says. “The most interesting outcome for us is that the final optimized arrangement of turbines is not intuitive, so it really makes us think.” The project also has to balance reliability, cost and risk. “And they are not complementary,” Collier says. “Part of the project is to demonstrate we can make money doing this. But, we also have to do it at a low risk, so nothing breaks from day one, to also prove it is reliable.”

Phase 1A will prove the concept and reliability. Future steps for the project will be about reducing costs. “We are always going to be looking for cost reduction,” Collier says. “Part of that will be doing everything cheaper next time and finding ways to get the cost of the turbine down.”

As good as it could be

So far, the project is going as well as it could do, Collier says. “The nice thing about what we have done, having been there from the start, is that we have the project design about right,” he says. “No one has said we cannot do this or they want to change that. From a personal point of view, that’s pleasing.

“We had a plan, and we said we were prepared to listen to other ways of doing it, which we did, and we changed a few things, and it has gone about as well as it could do. The process is to learn as much as we can along the way and feed that into the next array.”

For a nascent industry with nascent technology, MeyGen is showing the way.

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