Designing for depth

June 10, 2010

If all goes as planned, Seattle, Washington-based Principle Power and Energias de Portugal will deploy a prototype deepwater wind turbine in the Atlantic next year. Russell McCulley talks to Principle Power CEO Alla Weinstein about the company's WindFloat design.

The most striking thing about the WindFloat concept is its apparent asymmetry, with a massive wind turbine planted on one corner of a three-column semisubmersible platform conceived by Marine Innovation & Technology and owned by Principle Power. Given the weight of the power-generating structure it supports, the platform seems light, with plenty of open space between its columns.

That's exactly the point, says Weinstein.

‘The answer is very simple – to minimize cost,' she says when asked about the design. ‘If you put a very heavy structure in the middle, you now have to put a lot of metal in the platform to support it, so you're just going to be reinforcing the structure. By putting a turbine on one leg, we have a monolithic structure. We don't waste any metal, and we don't have to add any more support that might otherwise be required.'

The WindFloat is designed to accommodate most wind turbines; the prototype for the Portugal project will be equipped with a 2MW or greater turbine with a hub positioned close to 90m above water level. The semisubmersible platform consists of three vertical columns spaced 38m apart and interconnected by horizontal tubing and braces, with minimal deck space between the columns to support electrical equipment. The platform has an active ballast water system that transfers water between columns for stability in changing wind speeds or directions. The ballast system can transfer up to 130t of active ballast water in less than 20 minutes using two independent flow paths with redundant pumping capability.

The ballasting mechanism compensates the change in ‘overturning moment' – the point at which outside forces exert enough pressure to topple an object – and ‘gives us the ability to adjust the weight of the structure to the operational draft that we desire,' Weinstein says.

The structure would be moored with four chain and rope lines, two of them connected to the column holding the turbine. In the proposed, follow-on, 150MW wind farm offshore Portugal consisting of 30, 5MW WindFloat systems, medium voltage dynamic electrical cables would connect an array of floating turbines in a lazywave configuration.

Whereas Statoil chose the spar platform design for its HyWind floating turbine prototype, Weinstein says the semisub model is more appropriate for Portugal and the US west coast off Washington state, where the company has signed an MOU for the possible installation of an offshore wind farm. (Weinstein's previous company, AquaEnergy Group, got Federal Energy Regulatory Commission approval for a wave energy demonstration project in 2007. A year after, AquaEnergy was sold to Finavera Renewables and Principle Power founded.)

HyWind ‘took the spar approach,' she says, because Norway's fjords ‘offer very deep ocean very quickly, but they're protected, so the wave conditions are not what they would be in the middle of the North Sea'. Tension leg platforms can also be adapted for wind power, she says, but are not appropriate for areas with extreme tidal variations. And TLPs have a natural resonance frequency of 3-4 seconds, very similar to wind turbines, which can render the two elements incompatible.

‘Semisubmersibles offer you an ability to design such that you will be out of the resonance frequency,' Weinstein says. ‘And semisubmersibles do not require tensioning, though they have catenary mooring, which will probably be longer and have a larger footprint on the ocean floor. But it's not going to be as challenging as, I think, TLPs in terms of securing them to the ocean floor. And the cost will be minimal because you use conventional anchors.'

The biggest challenge in designing platforms for floating offshore wind, as opposed to platforms that support stationary structures, is ‘How do I deal with these forces? How do I not make my platform not flip over?' Weinstein says. ‘How do I make sure that it's stable? Because when that big spinning disk spins, it doesn't like to go sideways, it doesn't like to have a pitch. So your design challenge is very different from floating platforms that support stationary equipment.'

The WindFloat design, its proponents say, reduces wave and turbine induced motion, making it well suited for deepwater applications where jet streams are stronger and more constant. The design also improves economic efficiency because the bulk of construction is done onshore and in port, largely eliminating the need of ‘weather windows' – the long periods required to conduct large-scale construction operations at sea – and the specialized vessels needed to carry out those operations.

‘The WindFloat was designed to be built in port or at the shipyard and towed out, which means you will no longer need big installation vessels,' Weinstein says. ‘You will only use an anchor laying vessel to tow it out. That expands your weather window because you are now looking for a two-day weather window rather than weeks and weeks. And if you have major failures on the turbine, you can disconnect it from the mooring and take it back to harbor and do all your maintenance in a controlled environment, rather than offshore. We took all those things in consideration.

‘Also, the WindFloat was designed to be turbineagnostic,' she says. ‘It doesn't matter what turbine it is. We have validated our models and our ability to integrate and couple all systems. We look at the WindFloat as a unit, a fully integrated device.'

The unit can be scaled, she says, according to metocean conditions and turbine characteristics. ‘We work very closely with turbine manufacturers to make sure that everything is integrated, but the client chooses which turbine to use, and we provide the WindFloat with that turbine.'

Principle Power and Energias de Portugal are seeking additional partners in the joint venture and, if the necessary funding and permits are secured this year, hope to have the prototype in the water late 2011. The JV has begun FEED on the project, Weinstein says, and Portugal's commitment to reduce carbon emissions bodes well for the type of large-scale offshore wind project WindFloat's backers hope to bring to fruition.

Even in good economic times, offshore wind can be a tough sell. ‘Yes, cost is an issue,' Weinstein says. ‘Offshore wind is more expensive than onshore wind, at least from a capital point of view. But if you look at the resource, you have stronger and much more consistent wind resource offshore. So I think as we reach economies of scale, costs will more closely resemble what onshore wind costs today.

‘It isn't going to be today,' she says. ‘It may take a few years. But we will get there.' OE

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