Edge erosion experts battle ‘hydrometeors’ and their effects

Blade leading edge erosion has developed into a serious issue in the offshore wind industry and is likely to become much more important as turbines increase in size, blade tip speed increases and offshore windfarms are built outside the North Sea, in monsoon climates.

Higher tip speeds increase the energy with which raindrops and hailstones impact the leading edge of a blade. Higher steady state wind speeds offshore compared with onshore also increase the impact velocity of water droplets. In Asia, which is expected to become a major market for offshore wind, monsoons can see up to 80% of annual rain in a single season. This could mean turbines experience three or four times the amount of rain they may see in northern Europe.

There are several protection systems available to limit the effects of leading edge erosion, but they do not last the lifetime of a turbine and require regular replacement. Another problem is that, currently, there is no model that can predict the lifetime of an edge erosion protection system.

As scientists at the department of aerospace engineering at the University of Bristol and the Offshore Renewable Energy Catapult (ORE Catapult) noted in a presentation at Wind Europe Offshore 2019 in Copenhagen in November, modelling leading edge erosion is important to help understand the characteristics of raindrops or other types of ‘hydrometeors.’

But understanding and predicting leading edge erosion presents challenges, not least because hydrometeor and precipitation intensity is site specific and season dependent. To accurately predict the offshore precipitation environment at a given site using weather radars, an understanding of droplet size distribution is required.

The severity of hydrometeor impact on a blade is governed by kinetic energy. Blade speed provides most of the impact velocity, but the velocity and mass of a hydrometeor are also important.

As rain is the most frequent hydrometeor, its effect when striking a blade is particularly important. Currently, weather radars validated against onshore datasets are used to predict precipitation, but the lack of an offshore dataset introduces uncertainty into radar predictions, and calls into question the accuracy of predictions.

To address this issue, ORE Catapult has been using state-of-the-art measurements from devices known as disdrometers, instruments used to measure the droplet size distribution and velocity of falling hydrometeors. Disdrometers vary in capability – some can distinguish between rain and hail, for instance.

Two have been installed three nautical miles from the coast of Blyth on the east coast of England on an offshore anemometry hub owned by the ORE Catapult and have so far been used to collect 12 months of offshore data about hydrometeors. That data has been used to characterise the offshore precipitation environment and to provide an offshore dataset with which to validate weather radar predictions. The ORE Catapult said it anticipates the results of the project will help improve prediction and modelling techniques and will help to combat leading edge erosion.

The same scientists at the ORE Catapult and University of Bristol have also been researching the way hydrometeors degrade a blade edge and their effect on leading edge protection systems (LEPs) to help identify how LEPs might be improved.

They explained that soft-shell solutions for LEPs show considerable promise, and hard metallic solutions are in development, but both are adhesively bonded and their long-term durability has yet to be demonstrated.

The scientists say producing and applying gelcoats and flexible coatings relies heavily on manual procedures, leaving the coatings vulnerable to defects that can act as initiation points for erosion.

Another solution, leading edge tapes, are manufactured autonomously in a controlled environment. This means they are relatively free of defects and when applied effectively, possess very good erosion resistance. However, poor application can result in forming air pockets and wrinkles that reduce the adhesion of the bond, resulting in the tape disbonding from the blade.

Metallic erosion shields have been shown in accelerated rain erosion tests to possess a lifetime greater than that of an offshore wind turbine blade. However, differences in stiffness between the blade and the metallic shield introduce a risk of the shield detaching under loading and, as a result, the reliance on the adhesive is high.

Integrating a metallic erosion shield into the blade mould would remove an additional manufacturing process and alleviate any aerodynamic concerns caused by a profile step on the leading edge of the blade. A design that can account for the stiffness mismatch between an integrated metallic erosion shield and a blade could form the basis of an effective and long-lasting solution to leading edge erosion.

To develop what they described as ‘step-change solutions’ for leading edge protection, the scientists say the edge erosion process needs to be better understood and connected to the integral properties of potential solutions.

They explained that the factors that affect the degradation process include: the rain impact environment; offshore weathering conditions and degradation processes; the coating stress state; the impact energy ‘wave’ through coating layers; and temperature and humidity effects during coating application, curing and operation. Different levels of elasticity and viscoelasticity also play a role.

In a second presentation at the conference, the scientists also highlighted the importance of connecting the elastic and viscoelastic properties of coatings when tested in a rain erosion test (RET) rig with the rain impact environment offshore. Only if this connection can be made and better and longer-lasting coatings developed will the lifetime of coatings be more accurately predicted in future, they say.

Work the scientists at ORE Catapult and University of Bristol have completed to date demonstrated the elastic and viscoelastic behaviour of LEP coatings can significantly influence RET rig results.

They confirmed that a number of factors cause differences in the effectiveness of LEP coatings. These include droplet spread density; recovery time between droplet impacts and in stop periods; linear speed; and droplet size. These were correlated against material properties including short-term recovery and creep recovery. Using this information, they suggest, industry could design improved LEP coatings.

“For the time being,” they concluded, “the current lifetime prediction methodology does not account for test parameter influence on viscoelastic coatings,” and, “as newer LEP coatings are trending towards viscoelastic behaviour, modifications to the current methodology need to be developed.”

Integrated erosion shield holds promise

In a paper on the subject published in November 2019, scientists at University of Bristol highlighted what they see as the potential of metallic erosion shields as a solution to leading edge erosion.

They explained that the same type of shield has been shown in the helicopter industry and in rain erosion tests to exhibit excellent erosion resistance. They also noted their high impedance ensures any erosion occurring is likely to be seen at the surface, reducing the reliance on the adhesive. However, the long-term adhesive performance remains in question, due to creep and structural loading.

Integrating a metallic erosion shield into a blade mould and co-curing or co-bonding it to the blade substrate would remove a manufacturing step and allow the shield to sit flush with the blade, avoiding any aerodynamic issues, they say.

An integrated metallic erosion shield could provide a solution to leading edge erosion if a design can be found that mitigates the differences in flexibility between the two components and eliminates the risk of the shield detaching and leaving the blade completely exposed, they believe.

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