With offshore wind going global, developers and blade and turbine manufacturers need data about how blades will behave in the environmental conditions in new markets, in particular how blades could be affected by leading edge erosion
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 predicting and preventing leading edge erosion, 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 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. 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 to the offshore wind industry 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 told OWJ it 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.
Experts at the Offshore Renewable Energy Catapult (ORE Catapult) in the UK told OWJ that the lack of data – and the fact most of it comes from the North Sea – is a symptom of the UK being a leader in developing offshore wind. It currently has the most installed capacity – not just in the North Sea, but around the UK’s coastline.
“It is logical that learnings not just about leading edge erosion, but all aspects of windfarm operations and maintenance would come from existing, operational windfarms, particularly those in the UK, ” they told OWJ.
As blades grow in size, so tip speed increases, potentially influencing erosion
Turbine manufacturers are modifying their turbines for potential typhoon conditions in markets such as Asia with a focus on Taiwan, but how much is known about environmental conditions in emerging markets such as Taiwan that might affect leading edge erosion? OWJ asked them.
“The central weather bureau in Taiwan will collect onshore environmental and precipitation data, which is the same position for all European countries, which can then be used to predict the effects on the erosion rate using modelling, such as that used by ORE Catapult,” they said.
“We believe that during extreme weather events, such as typhoons, it is likely that turbines will be shut down. Therefore, turbine modifications are more likely to focus on engineering and overall strength improvements to withstand extreme weather events, rather than resistance to degradation which occurs when in operation (such as erosion).
“Clearly, there is a lot of work ongoing in the industry looking at not only gaining a better understanding of the impact of rain erosion on wind turbine blades, but also investigating leading edge protection materials and other potential blade manufacture materials that can be used. We would expect these learnings would be applied to the Asian markets, as well as similar investigations carried out by OEMs wishing to export to Asian markets.
“As for characterisation of the offshore environment, including measuring precipitation data, clearly this work will be ongoing for Asia as it is in European markets,” the ORE Catapult’s experts said.
“As well as environmental conditions, there are many other aspects that influence the rate of erosion including tip speeds, blade materials, surface coatings and the quality of blade manufacture.
“All of these must be understood and taken together to get a fuller picture of the impact of rain erosion. Collecting data on precipitation rates and other environmental conditions in Asian offshore markets will enable this data to be used in existing modelling programmes to allow more accurate estimations of erosion potential.”
Asked whether turbine manufacturers and blade makers are sufficiently aware of the issue, the ORE Catapult said European OEMs are performing considerable research and development trying to understand the causes of leading edge erosion. They are also working on R&D looking at developing potential new solutions and mitigations, including new materials, new coatings and new manufacturing techniques. “Chinese manufacturers are also aware of the issues for their markets,” the ORE Catapult team said.
In March 2019, DNV GL announced it had launched a joint industry project to analyse damage caused to wind turbine blades by leading edge erosion.
Apart from DNV GL, the COmprehensive methodology for Blade Rain erosion Analysis (COBRA) project also includes 10 commercial partners, Vestas, Siemens Gamesa Renewable Energy, LM Wind Power, Ørsted, Mankeweicz, Akzonobel, Aerox-CEU, Polytech, Hempel and PPG.
The outcome of the project will be recommended practice for designing a protection system against erosion. It is due to be published by July 2020.
The cross-industry working group aims to identify and define relevant material properties for a protection system; develop a methodology to handle and derive design loads from rain data; develop a model to conduct raindrop impact analysis; and develop a design methodology for leading edge protection systems.
Siemens Gamesa head of blade materials Steffen Laustsen said, “With the trend to building larger turbines, more research is required to provide more protection against rain erosion. The high blade tip velocities associated with large blades make the impact of rain especially challenging.”
Interested parties can still join the project, subject to approval by the steering committee.