What the standard analysis does
Lifetime extension frameworks, of which DNV's lifetime extension standard is the most used, ask two things of a component: an analytical part, comparing the fatigue loads the design assumed with the loads the site has actually delivered, and a practical part, an inspection of the component's condition. The analytical part for blades uses the design load spectrum, the site's wind data over the operating years, and the blade's fatigue utilisation from the design documentation, to estimate how much of the design fatigue life has been consumed. A site with a milder climate than the design class shows margin; a site with more turbulence, wake or higher mean wind shows a deficit.
That calculation is worth doing and it is built on an assumption that is rarely stated: the blade being assessed is the blade the design described. Undamaged, built to the drawings, with the laminate the calculation assumed.
Why blades are different
The fatigue life of a composite blade is not a property of the design. It is a property of the design as built, in a specific blade, with whatever the manufacturing process put into it. A wrinkle in the spar cap can halve the compressive fatigue life of that section. A starved trailing edge bond line has a fatigue life measured in a fraction of the design's. Delamination from a lightning strike ten years ago, a field repair whose cure was never verified, erosion that has opened the leading edge laminate to water: none of these appear in the design load spectrum, and all of them change the answer.
The full-scale test that validated the design was run on one new blade. A twenty-year-old blade on a turbine being assessed for extension is a different object, and the assessment has to treat it as one.
What the blade assessment must cover
The design basis, and where the margin actually is
Which sections of the blade were fatigue-critical in the design, and by how much. This is in the design documentation and it is what turns a site load comparison into a blade-specific statement. A blade with a large margin at the root and a small one at maximum chord is assessed at maximum chord.
The as-inspected condition, internally
An external drone survey is the minimum and is not sufficient. A lifetime extension assessment needs an internal survey of a representative sample: bond lines, shear webs, the inner skin over the spar caps, the root laminate. Where the blade type has a known weakness, the sample is chosen to test for it, and where a defect is suspected in the laminate, ultrasonic testing confirms it. The inspection checklist sets out what is looked at; the point here is that the extension decision rests on it.
The inspection and repair history
Every inspection report over the operating years, read for trend rather than for the latest snapshot: which findings have grown, which repairs have held, which blades have been repaired repeatedly. And the repair records themselves, by serial number, with method statements. A structural repair without documentation is an unknown in the assessment, and it should be treated as one.
Known serial issues for the blade type
Technical bulletins, field notices, retrofit campaigns and public failure history for the blade type and the plant that built it. A blade type with a documented history of root insert movement or trailing edge disbond is assessed for those specifically, and the extension is conditioned on them being addressed.
Leading edge and lightning protection
Leading edge erosion that has reached the laminate is structural, and its rate over the remaining years is part of the assessment. Lightning protection continuity is checked on every blade, because a lightning defect on a blade being asked to run another decade is a defect with a decade of strikes ahead of it.
Root condition
Bolt tension on a sample, the root face for moisture, the root laminate around the inserts for cracking or whitening. The root is where a blade separates from the turbine in one event, and the root joint has carried more cycles than any other part of the structure.
Material ageing
Composite laminates take up moisture over decades, adhesives lose strength with temperature cycling and ultraviolet degrades exposed resin. The design values were for new material. The assessment should state what allowance has been made for ageing and on what basis, rather than silently reusing the design values.
What the assessment should produce
Not a single remaining-life number. A remaining-life number for the blade as designed, and then a list of the conditions on which that number depends for this fleet: which defects must be repaired first, which blades are excluded until inspected internally, which findings must be re-inspected at what interval, and which serial issues change the picture if they appear. An extension granted on those conditions is a decision. One granted on the loads analysis alone is a hope.
Frequently asked questions
Can blades be life-extended like the rest of the turbine?
Yes, and the method has to be different. The turbine's loads analysis assumes the blade is the one the design described. A blade assessment has to establish what the blade actually is after twenty years: its as-built defects, its damage, its repairs and its ageing.
Is a drone inspection enough for a blade lifetime extension?
No. The defects that limit blade life are in the spar caps, the bond lines and the root, none of which a drone sees. A representative internal survey, with ultrasonic testing where a laminate defect is suspected, is the minimum.
What if the blade type has a known serial defect?
The assessment tests for it specifically and conditions the extension on the finding. A blade type with a documented weakness is not excluded from extension, but it is not extended on the fleet average either.
Who should carry out the blade assessment?
Someone with the design basis, the inspection record and no interest in the outcome. The manufacturer has the first, the inspection provider has the second, and neither has the third.
How Apex Wind can help
We are an independent blade engineering consultancy in Denmark, independent of every turbine and blade manufacturer, and we sell no inspections, repairs or retrofits. Our blade technical due diligence is the review a lifetime extension decision needs: the design basis against the as-inspected condition, the inspection and repair history read for trend, the known issues for the blade type, and the conditions an extension should carry. Where a design question or a proposed retrofit is involved, our blade design and structural assessment covers it. If a fleet is approaching the end of its design life, contact us before the loads analysis is commissioned, not after.