David Foxwell reflects on the challenge posed to offshore wind turbines by leading edge erosion and the potential scale of the challenge in new offshore wind provinces
Blade leading edge erosion is a significant issue for the offshore wind industry. It can cause material to be removed from the leading edge of a blade, leaving a roughened profile that adversely affects aerodynamic performance and, potentially, energy generating potential.
A lot of work is being done on leading edge erosion, on predicting it and on preventing it, but for the time being, there is no thoroughly validated method to relate test results to real-world erosion performance.
A paper presented at the 2019 Wind Europe Conference & Exhibition in Bilbao last week said tests carried out using guidelines from DNV GL showed that those guidelines do not enable accurate comparison of the results from different test rigs and suggested that lifetime prediction models need to be refined.
In the early days of the offshore wind industry, leading edge erosion was recognised as a problem in more extreme environments, such as those on the west coast of Scotland where blades faced massive amounts of rain and wind, but it has become a widespread issue.
The main reason leading edge erosion is becoming more of an issue is because of the high blade tip velocities associated with larger and larger blades, but there are other challenges to address.
To date, almost all of our understanding of leading edge erosion comes from the North Sea. Conditions there can be testing, but they may be even more demanding in parts of the world where offshore wind is an emerging industry. Taiwan is an obvious example, and China is emerging as the world’s largest offshore wind market. The monsoon season there will see a huge amount of rainfall in a concentrated period of time and very large droplets of rain compared to other parts of the world.
Worryingly, environmental conditions are poorly characterised and there are few databases available that provide detailed information about rain droplet size and rainfall intensity and their effect on rotating components. For the time being, we only have data gathered by the offshore oil and gas industry about effects on static installations. This data gap means that, in the real-world, blades may behave quite differently in new offshore wind provinces.
The data gap is being addressed however, and as part of a joint industry project Ørsted is measuring the difference in rain droplet size distribution at Horns Rev 2 in Denmark and an offshore site in Taiwan. These are spot measurements in locations specific to windfarms to be developed by Ørsted.
Blade manufacturer LM Wind Power has adopted a more general approach and has accessed a NASA database collecting rain data on a global scale through satellite observations. Spot measurements like the ones Ørsted is doing can then be used to validate this more general approach. In the end the rain load data gathered through measurements and satellite data will be used in the models manufacturers are developing to predict when erosion will appear and when repairs might be needed.
As the industry goes global, there is a lot more research and development to be done, data to be gathered, new standards to be developed and new products and technology to introduce to ensure wind turbines everywhere perform as expected and that asset owners are not hit by unexpectedly high O&M costs.