Threat to Wind Turbines – bad vibrations

Medium-sized waves such as those occurring in even small storms, can snap offshore wind turbines like matchsticks.

For wind turbines at sea with a cylinder diameter of eight metres, the most endangering waves are those that are more than 13 metres high and have an 11-second interval between them. Such storms and waves are frequent in North Sea storm conditions.

Wind Turbines

When waves above 13 metres hit wind turbines, an unfortunate force arises at the rear of the turbine. This is called ringing. John Grue is now looking for a general mathematical formula that can explain the special phenomenon.

Professor John Grue of the Department of Mathematics at the University of Oslo has discovered this wave phenomenon called ringing, which is a unique vibration that occurs when choppy waves hit marine installations creating a resonance vibration that in turn creates metal fatigue, such as can some about when choppy waves hit marine installations. Ringing is not confined to wind turbines alone – it is already a big problem facing the oil industry as designers of the YME platform found at a cost of £1.3bn when the phenomena was not taken into account.

An analogy is when soldiers in step are ordered to break step when marching across a bridge. Should the vibration match the structures resonant frequency the bridge could collapse. All structures have their own vibration frequencies, whether they be wind turbines, bridges, oil rigs or vessels.

“If we do not take ringing into consideration, offshore wind turbine parks can lead to financial ruin”, warns Professor Grue. “Thus far it has not been possible to measure the force exerted by ringing. 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” said Professor Grue. “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.”

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