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'Ringing' poses significant problem for marine wind farms

26 February 2013

In 1989, an inexplicable wave phenomenon called ringing was discovered in a laboratory. This is a special type of vibration that occurs when choppy waves hit marine installations.

The discovery was made in a 25-metre long wave laboratory located in the basement of the mathematics building at Blindern Campus, Iniversity of Oslo. So far scientists have studied ringing in small and large waves, but as it turns out, ringing is more common in medium-size waves.

For wind turbines at sea with a cylinder diameter of eight metres, the worst waves are those that are more than 13 metres high and have an 11-second interval between them. It is thought that the ringing problem may increase significantly in the years ahead. 

If ringing is not taken into consideration, the huge investment in offshore wind farms may be at risk, warns Professor John Grue from the Department of Mathematics at the University of Oslo.

Today, the largest wind farms at sea are outside the coasts of Denmark and Great Britain. They are nevertheless dwarfed by Statkraft and Statoil's plans for the Dogger Bank. This wind farm could produce as much electricity as 60 to 90 Alta power plants. A wind farm with the capacity of two Alta power plants will be built outside Møre og Romsdal.

"Thus far it has not been possible to measure the force exerted by ringing," says Professor Grue. "Laboratory measurements show that the biggest vibrations in the wind turbines occur just after the wave has passed and not when the wave hits the turbine. Right after the crest of the wave has passed, a second force hits the structure.

"If the second force resonates with the structural frequency of the wind turbine, the vibration is strong. This means that the wind turbine is first exposed to one force, and is then shaken by another force. When specific types of waves are repeated this causes the wear to be especially pronounced. This increases the danger of fatigue."

It is precisely this secondary force that creates ringing which, hitherto, mathematicians have failed to calculate.

The Norwegian University of Science and Technology and the Massachusetts Institute of Technology (MIT) have made a number of calculations of ringing. Ecole Centrale Marseille and the French Bureau Veritas have also made such calculations. Det Norske Veritas is among those who use versions of these models.

"Current models are the best we have, but the estimates are too rough and erroneous," says Grue. "The theories are applied far outside of their area of validity. This means that we cannot calculate the fatigue adequately. Ringing is very difficult to calculate. There is great uncertainty.

"We want more precise descriptions of the physics of ringing. We are now trying sophisticated surface elevation models and complex calculations to reproduce these measurements accurately. We want to show that the ringing force arises systematically according to a general mathematical formula."

An extensive set of measurements of the ringing force in waves, conducted by Saga Petroleum, fit the measurements very well, says Grue. Another variable to consider is whether the installations are in deep or shallow waters, as the structural frequency also depends on the seabed conditions. Rather like a flagpole in a storm, it will vibrate differently depending on whether it is fixed in concrete or in softer ground.

Oil rig damage
Ringing does not just harm wind turbines; it has already posed great problems for the oil industry. The designers of the YME platform did not take ringing into account, and lost NoK 12 billion.

"It is possible to build your way out of the ringing problem by strengthening the oil rigs. However, it is not financially profitable to do the same with wind turbines", says Grue.

Arne Nestegård, chief specialist in hydrodynamics at Det Norske Veritas, has stated that wind turbines at moderate depths may be exposed to high-frequency resonant oscillations if the waves are extreme, but they safeguard against this.

Nestegård says that in the past twenty years, Veritas has developed ringing models and that they are now working on improving the models for wind turbines at sea.

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