Malta?s 54 MW offshore wind energy hexagon takes shape

Government has initiated a public consultation process on a plan to install a gigantic 54MW hexagonal floating wind-power station, which is being fine-tuned by Stockholm technology developer Hexicon.

Moored in deep waters off Malta’s northeast coast, the structure would bristle with 36 wind turbines – a 120-metre-tall horizontal-axis 6.5MW machine on each of its six corners, and a further 30 500 kW vertical-axis units. It would be the largest single offshore wind farm complex in the world.

Deployment of the first Hexicon is a three-horse race, with sites off Cyprus and Sweden also in the running. The company has its A480 model concept up for funding under the NER300 programme – a new-technology financing scheme managed jointly by the European Commission, the European Investment Bank and Member States – with a decision due this year.

"The Malta project would not necessarily be the first project to go ahead [assuming the funding was secured]," Hexicon’s business development director Percy Sundquis said.

"It depends very much on what each country or indeed a given government department will provide in terms of support [on top of the NER300].

"It is hard to tell which country will be willing to move most quickly to get a first development under way.

"There are certain questions that still need answering… the Swedish Energy Agency has been very supportive of our application," he added.

Hexicon’s offshore wind-power concept is designed in the Olympian-scale tradition of the offshore oil and shipbuilding industries, with a hull 480 metres across and 26 metres tall in the water.

Fully loaded, the 23,000-tonne complex would float with a draught of 18 metres, and be able to produce power while riding out waves of up to 25 metres.

The main turbines, which could be up to 130 metres tall, would be chosen according to local wind conditions, Sundquist said.

"The main average wind speeds off Scandinavia versus the Mediterranean are very different.

"We’d like to use a Vestas [horizontal-axis] unit if we were looking at a development site with lower average wind speeds, but Bard, Siemens and REpower would be the manufacturers we would favour for higher-wind-speed sites."

Hexicon’s "preferred choice" to supply the vertical-axis turbines is Swedish company Vertical Wind.

"The key idea is that you have four turbines that get clean wind 365 days a year, whereas many traditional wind turbines get this 60-70 days a year and spend the rest in the ‘shadow’ of other towers," Sundquist said.

"Then we have two aft [rear] turbines getting 45% efficiency and then the vertical-axis units [which are omnidirectional] boosting capacity up to 54MW." A souped-up version of the design bolts on 27 wave-energy converters for an extra 15MW.

Topside, there is a mobile crane for hoisting and lowering turbine components, a docking area and a heliport.

At the hub of the platform are the bridge and a lodging area to accommodate two eight-member crews.

The Hexicon is built around a Fagerdala Marine Systems hull – a closed-cell steel-and-foam sandwich construction proven in tanker and yacht designs, featuring a patented composite outer shell that protects against the effects of corrosion, vibration and fatigue on the platform "while optimal indoor climate is achieved".

Choosing the FMS, which cuts the need for maintenance and lengthens the unit’s lifespan, has led to Hexicon being given design approval by certification body Germanischer Lloyd. "This is a very important feature of our design," Sundquist noted.

"Any of our competitors, unless they are planning to use something similar, will have to go through a long process before they have equivalent certification."

At the six angles of the hull, beneath each main turbine, is a column – shaped like the forward part of a ship’s hull – outfitted with a ballast system, which is activated to lower the platform in the water to improve stability under varying weather conditions and to simplify hull maintenance and repair.

Mooring for the Hexicon marries a six-point pre-tensioned system using lightweight synthetic cables and six granite gravity anchors.

Because the mooring lines are mated with the hull at a central swivel-turret in the hub, it is possible for the platform to turn around its central axis to position its turbines into the prevailing wind, helped by a set of six azimuth thrusters.

This ‘weathervaning’ boosts energy capture and lessens "the detrimental effects of turbulence, wake and fatigue factors" on the floater.

"A Hexicon would be tailored very much to the water depth of a given site," Sundquist said.

"Depths in the Baltic Sea might be 40-50 metres, off Cyprus 60-70 metres, and off Malta you are talking anything down to around 300 metres, so the configuration of the platform, its draught, and of course the moorings would be adjusted to suit the environment."

Power take-off is via a feeder cable and connections, including substation, that are fitted within the swivel hub, with production flowing down export cables to the seabed, and then along a 132kV line to shore.

ABB will supply all electrical equipment.

Construction of each Hexicon is expected to take just over two years. Work on the first unit is slated for "late 2013 or early 2014," Sundquist said, with Hexicon targeting float-out by 2015.

Karl Stagno,