Wind Turbine Blades – An Asset with a 20-year Operational Life – Realistic or Optimistic?
Why Is It Important to Determine the Cause of a Defect? The defect effectively is an undesirable feature in or on the part that was not present originally. The defect, if left, may or may not result in deterioration of the blade; however, without finding and understanding the root cause of the defect, it is not possible to accurately state whether the longevity of the blade is or will be compromised by leaving the defect, i.e. not repairing it.
It does not make sense to repair every defect. The repair process may result in the inadvertent removal of good material from the blade, leading to other, additional defects subsequently developing. There is also the financial aspect of the defect’s repair to consider. Minor, non-structural, non-performance-affecting defects (defects that do not require repair), if repaired, will result in costs for the turbine owner/operator.
These costs include, but are not limited to:
• the personnel, equipment and material cost for repairing the defects: the repair team and the turbine operating team;
• client representatives, as required; • •
turbine down-time – a significant cost; independent inspectors, if required; and
• administration (paperwork assessment, etc.)
Therefore, it is important to understand whether a defect should be repaired or not, thus it is essential that the root cause of the defect is determined and acted on appropriately (see Figure 3).
How to Determine the Root Cause of a Defect and the Difficulties and Risks Involved
The lack of specific details from the blade designer and/or manufacturer results in, in the majority of cases, the investigation and assessment of the defect being undertaken from first principles, i.e. without any specific knowledge of the particular design but with an underlying understanding and experience of the general manufacturing processes that are used in the production of the part or parts being assessed.
Once as much information as possible has been obtained via non-destructive methods, it is highly likely that destructive inspection will be required to confirm the non-destructive investigation or to provide additional information not attainable through non-destructive methods alone. This can range from exposure of the surface surrounding the defect to invasive removal of material and/or taking of samples for off-site, specialist investigation (see Figure 4).
For larger defects it is relatively common to use the services of an independent person or body for assessment and recommendations. This helps reduce the risk of an incorrect root-cause assessment.
What Are the Risks of Getting It Wrong? The risks of getting it wrong are as follows:
• greater repair than is actually required: this incurs large costs owing to the time taken, personnel involved and the turbine down-time for the repair;
•
the resolution method developed does not remove the root cause, the defect, or another manifestation of the defect reoccurs after time. The worst case is that this occurs outside of the warranty
52 •
period and the owner/operator of the turbine becomes liable for the rectification cost. The operator/owner may seek to reclaim this cost against the person or body that undertook the original investigation and assessment;
•
the defect is a serial flaw which has not been identified and results in a significant number of defects arising in other components. The defects may not be observed in time, resulting in significant repair costs or even outright failure or uneconomical repair of one or more components; and
the defect is highlighted as a serial defect but is actually a one-off: this could have large cost implications if a campaign of inspection is undertaken to identify other components with ‘the same’ defect.
Are Solutions Out There?
There are several approaches to ‘looking after’ an asset such as a wind turbine blade, from ‘cheap as chips’ through to fully-fledged, experienced inspection and resolution. A team with a ‘bag of ropes’ may be able to gain access cost-effectively, but it is the level of detail and quality of the inspection and reporting process that counts (see Figure 5).
A defect can be observed, but without the knowledge to comprehend it and accurately determine the root cause, the correct method of resolution may not be presented. A missed defect, or a defect that is misinterpreted, can result in, in the worst case, deterioration of the blade and a foreshortening of its life.
Blades and their defects, managed properly by an experienced service provider, may not appear as ‘cheap’ on the outset; however, over the long term the provision of an invaluable source of knowledge ensures maximum potential, performance and longevity is obtained from the blades. Forging a solid relationship with such a provider benefits the asset manager because there is consistency in the processes and a long-term understanding that the asset will be maintained in a way that mitigates defects developing into a potentially serious, costly event, right through until the end of the asset’s life. n
MODERN ENERGY REVIEW – VOLUME 3 ISSUE 2
Figure 5: Blade Failure Owing to Inappropriate Identification of a Defect Root Cause, Resulting in Propagation of the Defect, Leading to Complete Failure of the Blade
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