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Semi-submersible wind turbine is floating into the future

Portuguese energy group EDP and US partner Principle Power have applied for EU funding to help build a 25MW-plus pre-commercial offshore array, following a flawless eight-month run for the world’s first semi-submersible wind turbine.

WindFloat 1 (WF1) has flowed more than 1.7GWh of energy since being moored in the Atlantic in 43 metres of water, 5km off Aguçadoura, near Póvoa de Varzim, northern Portugal, last autumn. The 1,200-tonne floating concept, fitted with a single Vestas V80-2.0MW turbine, rode out storms that buffeted it with gale-force winds and waves of up to 15 metres during its first winter offshore.

The €20m ($25m) WF1 was built by the WindPlus joint venture, which counts EDP, Principle Power, Vestas, Portuguese shipbuilder A Silva Matos (ASM), venture capitalist InovCapital and Spain’s Repsol among its shareholders. The floater is seen as a game-changer both in terms of technology and supply-chain management.

“We have demonstrated and proven the WindFloat technology,” states Principle Power chief executive Alla Weinstein. “The entire system was fabricated using domestic resources, suppliers and infrastructure. Since its connection to the grid, the WindFloat has delivered an energy profile far better than that of local onshore wind installations.

“It has been doing better than expected. It was a conservative design. We have met all the performance criteria that we set out to meet. And we did so even in storms that arrived the minute after we arrived on site — a real ‘welcome to the neigbourhood’. But we came through very well.”

WF1, designed to float half-submerged with a draught of 17 metres in water depths of 40-200 metres, is based around a triangular steel platform structure with columns 30 metres tall and eight metres wide. They are footed with hydrodynamic damping heave plates and part-filled with 230 tonnes of water ballast for stability.

Construction of WF1 by ASM, at the Lisnave facility near Setúbal, southern Portugal, involved 60 European suppliers — 40 of them Portuguese, with French marine contractor Bourbon towing the unit 350km to its current location.

Vestas’ V80-2.0 was chosen as the first turbine for the floater because it was a “known quantity”, with 4,000 of the machines operating around the globe. The rotor, hub and transmission systems are unchanged from the tried-and-tested onshore model, with the one difference being that the WindFloat unit bristles with sensors on its blades, nacelle and tower to gather data on the heavier loads borne by the turbine offshore.

Ulrik Skyttergaard, the Danish turbine maker’s WindFloat project manager, says WF1 has performed “brilliantly” during its first stretch offshore, with an uptime of 92% in wind velocities averaging about nine metres per second (m/s).

“There is nothing [in the early operation] that would compromise our expectations or hopes for the behaviour of these turbines in a deep-water location on this floater,” he says.

Pitch and roll has been less than three ­degrees; heave has been only 250cm, meaning the unit operated “well within the design limits for both the turbine and the WindFloat”.

“To this point we haven’t had any stops on the turbine due to [pitch, roll or heave],” Skyttergaard adds. “We did put the turbine into ‘idle’ during the storm [in November] but it had been running for days beforehand in eight- to 12-metre waves and 22m/s winds with no stops at all.

“The power curves we have seen so far are modelling exactly those we are used to seeing [with the V80-2.0] onshore.”

Testing of WF1 will continue over the next 18 months to build a bank of data on its behaviour in open sea and power production levels, with an eye on integrating five 5MW WindFloats offshore by 2014-15.

“With this data we will have the information that we need to do the next iteration of the design and the pre-commercial array,” says Weinstein. “With Repsol entering the joint venture [this month], WindPlus is now in a position to advance the next phase of the project.

“With possible support from the European Commission NER300 funding scheme [for innovative renewables technology], this second phase would prove the scaleability of the WindFloat design and the intrinsic opportunities to reduce the cost of offshore wind deployments, while creating an entirely new industrial base in Portugal.”

Development of the 25MW array of WindFloats will call for a larger-diameter rotor purpose-engineered for offshore winds, notes Skyttergaard, pointing to the suitability of Vestas’ V112 and V164 designs.

“There is no doubt that the higher the megawatts you are putting on the turbines, the better the business case we will get out of this,” he says. “How this concept will perform over 25 years we don’t yet know, but it will allow [the industry] to explore new offshore regions and water depths.”

For Principle Power, which was formed less than five years ago in Seattle, development of a proven floating turbine has unfolded at a staggering pace. But Weinstein is keen to emphasise that these are still early days.

“We are only the second floating wind turbine operating [after Statoil’s spar-buoy Hywind, off Norway]. It will take many more turbines and many more companies [to build an international floating wind turbine industry].

“We are hoping that our success will stimulate others. We need to move this industry on quickly. This turbine was tiny at 2MW. We are after much bigger turbines.”

Europe remains Principle Power’s primary market “because it exists — other markets still have to be created”, explains Weinstein, but it is looking to the US as well as Portuguese waters for a site for its proposed five-unit array. “Hopefully, both. We are pursuing both.”