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LORC tests and demonstrates technology for harvesting renewable energy offshore

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Meeting industry’s needs for environmental testing of large components for offshore use is now one step closer with the ordering of a climatic chamber for a new LORC test centre.

Read more about the environmental testing of structures here

 

 

 

Puff your foundation into place

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onsdag 20. april 2011

FACT BOX

MBD Offshore is owned by

• DONG Energy (majority)
• Novasion
• Aalborg University

By Karsten Prinds
At first, the idea actually appeared useless.

Researchers at Aalborg University, Denmark got a request: Why not look into suction anchors – a known technology from the offshore oil industry – and use them for wind turbines? Suction anchors are used to fasten oil rigs in deep waters. No digging or hammering – Just a pump to install. ‘Put a tower on the anchor, and wind turbines would be installed a lot easier,’ was the idea.

- But it was no good. The anchors tilt too much. It does not matter if an oil rig anchor is a touch offlevel, but this makes a huge difference for wind turbines, says Lars Bo Ibsen, professor at the Geotechnical Engineering Research Group, Aalborg University.

He is one of the researchers who was ready to reject the idea. But instead, the researchers improved it. It took some effort:

- We had to think unconventionally and outside the box. I think a lot people would have given up because the solution could not be found in their own professional area, says Lars Bo Ibsen.

Promising
Now, more than ten years after the idea was first floated, the researchers and the development company behind the suction bucket monotower has reached the peak so far.

Britain’s Carbon Trust fund selected the concept among 103 other competitors as suitable for economic support for building further prototypes. MBD Offshore, the development company behind the suction bucket monotower, has calculated that the installed price for 100 5MW turbines at 30 meters depth will be 0.35 million euros per MW. That made the difference for Carbon Trust:

- The bucket suction foundation is the most challenging concept with largest cost saving potential, says Helge Gravesen, technical project leader of the Carbon Trust foundation concepts competition, stage 1. The confidence of the British fund pleases Lars Bo Ibsen:

- Of course we’re very proud. But this has also been achieved because we have been thinking about economics all along. It’s important for us that the structure can be made out of standard goods using standard methods. The simpler, the better, he says.

Turbine and Professor – both with buckets. The suction bucket monotower prototype was installed in 2002 in Frederikshavn, Denmark. Photo: Michael Bo Rasmussen

The solution
Going back in time and back to the drawing board, the concept of suction buckets struck the researchers as simple and intriguing. ‘Bucket’ is meant quite literally: Placed bottom-up, a cylindrical steel structure will sink into the soil because of its own weight. It will stop sinking when it reaches the point at which the soil can carry the bucket. Applying a vacuum will make the bucket continue to sink until it is completely covered by soil.

The major diffculty was keeping it completely vertical. As a bucket sinks into the soil, the only means of controlling its movement is by controlling the vacuum. Nozzles were the solution. They resemble the nozzles on a pressure washer that you might use at home, but there are hundreds of them mounted on the bottom of the skirt, at the very bottom of the whole support structure. They act as hundreds of small drills, and when controlled in sections like a compass, they can ensure that the suction bucket monotower is steered into the soil completely vertically. If it begins to tilt to one side, the nozzles on the opposite side ‘drill faster’ by flushing more quickly. An operator behind a computer screen steers the nozzles basically by looking at a two-dimensional bubble level and using a joystick. It is almost like landing a helicopter in slow motion. Installation progresses at the rate of two meters per hour. And there you have it: New technology made by combining an oil industry concept, your everyday pressure washer, and a steering interface resembling a bubble level.

Inside the bucket on the seabed, Lars Bo Ibsen can read out the tilt of the support structure over the last eight years. It hasn’t moved more than permitted by the turbine producer. The prototype carries a Vestas V90 turbine. Photo: Michael Bo Rasmussen

Three installations a day
However simple it sounds, installation is still the aspect that needs proving. One installation failed when a barge in Wilhelmshaven bumped into the bucket during suction. It collapsed like an empty beer can.

- We have plenty of documentation for the durability of the support structure once it is installed. It lasts, and it stays in place. But we need to show that the installation can be carried out smoothly”, says Kelly Langkilde, CEO at MBD Offshore. The latest prototype was installed in 24 hours. The goal is to install three support structures in that time.

- It can be done. And without expensive vessels. A barge with a crane can set the monotowers roughly in place on site, and another vessel carrying the suction engine and controls can move from monotower to monotower, sucking them into place, says Lars Bo Ibsen.

Ambitious, yes, but if new prototypes are installed successful, it may be that the offshore sites of the future will be installed to the humming of a vacuum engine rather than the hammering of a bulrush.

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