Professor John Grue, one of the world’s leading experts on wave research from the Department of Mathematics at the University of Oslo, discovered a wave phenomenon called ringing in 1989. It occurs when waves hit marine installations, causing a special type of vibration.
According to Professor Grue, ringing can break wind turbines and it may produce even more damage in the years ahead, when some of the world’s largest offshore wind farms will be built. If not taken into consideration, offshore wind farms can lead to a ‘financial ruin’, he says.
Offshore wind turbines that have a cylinder diameter of 8 metres are endangered by waves that are more than 13 metres high and have an 11-second interval between them.
Oil and gas industry has already faced issues created by ringing. Professor Grue says that it is possible to strengthen an oil rig to avoid this problem, but this hardly applies to offshore wind, because it is not a financially profitable solution.
“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.” Professor Grue is quoted as saying in an article published in Apollon, a research magazine from the University of Oslo.
The phenomenon is still inexplicable, as very little research has been done on ringing internationally. However, it is discovered that it happens more frequently in medium-size waves.
“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,” Professor Grue explains.
[mappress] Offshore WIND Staff, February 28, 2013; Image: Vattenfall