At a recent wind energy conference, owners and operators were asked if they ever thought the industry would be able to fully standardize blade damage categories. Specifically, the question focused on if there could ever be a universal guide that was based on simple characteristics of the damage, such as type, length, and location on the blade. Although hesitant to say the definitive “never,” it was clear there was a lot of skepticism that this would be possible even in the far future.
Most owners and operators have a blade expert, like those at ONYX, who review blade damage inspections before repair to ensure the categorization matches the company’s specific risk tolerance. Blade operations & maintenance (O&M) budgets are usually limited each year and this review ensures the limited funds are not used on minor damages that can potentially wait until the next repair season.
A recent survey conducted by a major non-profit energy organization (EPRI) showed a disparity in how various stakeholders in a wind farm’s operation rated blade damages. When presented with an obvious critical defect, such as one meter of the blade tip is missing, a consensus is easily found; this is the most severe damage, the turbine should be stopped, and the blade replaced or repaired immediately. The same consensus can be found on the other end of the spectrum. A small (<100mm) spot of grease found on the exterior blade surface is easily identified as the least critical. This gets a damage category assignment of 1.
The deviations start when you consider the middle tiers of damage, with the highest variation on the middle rating of 3. Why would this be?
Broad trends, if one understands blade design and turbine operation, can be used to evaluate damages on an individual basis. However, the reasons listed below are just a small sample of why a detailed universal standard may be difficult to achieve.
Site conditions and load levels
Wind turbines are designed to specific load envelopes and sites must fit within this envelope for the turbine to be safely installed at that location. However, it is possible that the site conditions present lower loads in some (or all) areas of the envelope and therefore the structural margins of the blade will be even higher than designed based on the site conditions. This may lead to a large defect being acceptable for a longer period of time at some sites, while that same defect would start to grow at other sites where loads are closer to the maximum load envelope.
Manufacturing conditions and composite materials
In composite materials, the material properties are determined when the geometry and final part are being manufactured. The stiffness and strength vary with manufacturing parameters such as local vacuum pressure, humidity, and temperature. While the manufacturers follow tolerance limits from the engineering team and original design intent, variation is inevitable in the final product. Localized regions may have slightly lower strengths than others in a random distribution.
Variation in design-driving load cases – structural margins
Blades are designed against expected load conditions and compared against multiple strength & stiffness criteria. Damage growth is typically driven by fatigue (cyclic) loading on the blade. However, depending on controller strategy, turbine design, and wind conditions, the design limiting criteria may be either static or fatigue loading. This means, in some designs, the blade will have higher fatigue resistance due to high static load requirements and reinforcements needed for those specific load cases. This may be localized to certain regions of the blade or simply a difference between turbine models and control strategies.
While this presents a difficulty in putting together criteria that would cover all possible blade defects for every turbine model, there are many benefits of working towards a base level of standardization. Agreement on terminology will help the communication across the value chain. Increasing the knowledge and available test data regarding composite material damage will help all stakeholders with decisions regarding whether the damage is critical or can be left on the blade for one to two more repair seasons.
Ultimately, a standard needs the right balance of flexibility and detail in improving the recommendations for damage and lowering blade O&M costs across the globe.
With the lack of a global standard, what is the right approach to categorising blade defects being seen in the field right now?
The task of reviewing inspection reports and thousands of photos has been aided immensely by machine learning and image recognition. After an autonomous drone captures images of the entire blade surface, defects are then automatically identified and categorized by an algorithm. A human may then also do a spot check for quality before they are delivered to the customer.
At this stage, there are two options:
- Another review by the customer and independent third party
- Accept categorisation and move directly to repairs
The benefit of strategy 1 is that the damage categorisation can be tailored for customer risk and historical knowledge of damage progression on their turbines. This helps prioritise in-between categories of 2-4 with additional experience regarding blade types, site conditions, risk tolerance, and maintenance strategy. The repairs are then optimised for the season.
If moving directly to accepting categories, critical damages may be overlooked, and minor issues could be pushed for repair. Having a blade engineer’s eyes on the actual damage, coupled with historical data trending, can make the best use of the blade O&M budget by focusing on the right damages.
Correctly categorising blade damages will help reduce blade O&M budgets by building confidence and the data needed to support an industry standard.
ONYX Insight offers a blade damage categorisation review with our blade expert. We can also provide recommendations for repair quality and help with optimising your blade maintenance budget.