What a field repair can do
Up-tower repair, by rope access or from a suspended platform, is the normal way to deal with most inspection findings. Done by a competent team in the right conditions, it handles:
- Leading edge protection and coating. Filling, fairing and recoating eroded leading edges, replacing protection tape or shells, repairing gelcoat cracks and crazing. This is the largest volume of blade repair work and it is well within field capability.
- Lightning receptors and down conductor terminations. Replacing damaged receptors and checking continuity.
- Small laminate repairs. Removing damaged plies over a limited area, scarfing into sound laminate and laminating replacements, for surface damage in the sandwich panels and small areas of the shell.
- Small trailing edge repairs. Rebonding short splits outboard of maximum chord and re-establishing the edge.
- Small delaminations. Where the delamination is shallow, limited and away from the spar cap.
- Drainage, vortex generators and add-ons. Replacing lost or damaged aerodynamic add-ons and clearing drain holes.
What limits a field repair
The limits are physical, and a repair provider who is honest about them is worth keeping.
Cure conditions. Repair resins and adhesives cure within a temperature and humidity window. Up-tower, the environment is whatever the day provides, and a laminate cured cold or damp is not the laminate the design assumed. Heated blankets extend the window; they do not remove it.
Access and geometry. A rope access technician works one-handed on a curved surface at height. Large scarf repairs, thick laminates and anything requiring vacuum consolidation over a large area are difficult to do well in that position.
Load. The blade on the turbine is under its own weight and whatever the wind does. A repair that needs the structure unloaded, or needs the two shells held in a fixed relationship while adhesive cures, cannot be done with the blade in the air.
Verification. A structural repair should be verified by non-destructive testing before the blade returns to service. Ultrasonic testing of a spar cap repair up-tower is possible in principle and rare in practice.
Documentation. A field repair is often recorded as a line in a log. A structural repair needs a method statement, material certificates, cure records and inspection results, because a certification body, an insurer or a buyer will later ask for them.
What needs the blade down
Blade-down repair, on the ground beside the turbine or in a repair facility, is required for:
- Spar cap damage. Any transverse crack or delamination in the cap. The cap carries the bending load and its repair is engineering, not maintenance.
- Root damage. Cracking in the root laminate, insert problems, water in the root. The root joint carries the whole blade and cannot be repaired in the air.
- Long bond line failures. An open trailing edge or web bond over a significant length, particularly inboard of maximum chord.
- Structural lightning damage. Where the current has gone through the laminate rather than the conductor, and the delamination extends beyond what is visible.
- Anything that needs controlled cure, vacuum consolidation over a large area, or post-repair ultrasonic verification.
Some of these are not repairs at all. A spar cap that has failed from a manufacturing wrinkle may be repairable in a factory, but if the wrinkle is in every blade from that production the decision is about the fleet, not the blade, and repair of one is not a plan.
Who should decide
The repair decision has three parties with three interests. The repair provider is paid for repairs and has a natural view of what can be repaired. The owner wants the turbine running and the crane avoided. The insurer, the lender and any future buyer want the structure restored to what the design assumed, with evidence.
In our judgement the decision on any finding graded serious or critical should be taken by an engineer with access to the blade's design basis, who is independent of the repair provider. The questions that engineer answers are:
- What is the finding, structurally? A category on an inspection report is an opinion, not a diagnosis.
- What caused it, and is the cause still present? A repair that does not address the cause will be repeated.
- Does the proposed repair restore the load path the design assumed, and can that be shown?
- Can the repair be executed and verified in the proposed location and conditions?
- What documentation will exist afterwards, and will it satisfy the warranty, the insurer and a buyer?
The general decision framework for when damage requires repair sits above this; this article is about the how and the where once repair has been decided.
After the repair
A repair is not finished when the technician descends. It is finished when it has been inspected by someone other than the person who did it, when the documentation is filed against the blade serial number, and when the next inspection has confirmed it is holding. Repairs that fail usually fail at the scarf boundary or the bond, within the first year or two, and a repair that is not re-inspected is a defect waiting to be rediscovered.
Frequently asked questions
Can leading edge erosion be repaired up-tower?
Yes, and it is the most common field repair. The limit is when erosion has cut into the laminate and started a delamination or reached a bond line, at which point the repair is structural and needs assessment.
Can a spar cap crack be repaired on the turbine?
Not reliably. A spar cap repair needs the structure unloaded, controlled cure, and ultrasonic verification, none of which are available up-tower. It is a blade-down repair, and often a replacement.
Should the inspection company also do the repairs?
It is common and it creates a conflict on the scope. Where it cannot be avoided, the scope for any serious or critical finding should be set by an engineer independent of the provider.
What documentation should a structural repair produce?
A method statement, material certificates, cure records, the identity of the technicians, inspection results before and after, and photographs, all filed against the blade serial number. A buyer or an insurer will ask for these later.
How Apex Wind can help
We are an independent blade engineering consultancy in Denmark. We do not carry out repairs, we sell no materials or hardware, and we take no share of the work our findings lead to, so our view of what a blade needs is not connected to what anyone will be paid for. Our blade design and structural assessment assesses whether a proposed repair restores the structure the design assumed and whether its documentation will satisfy a certification body, an insurer or a buyer. Where the question is what caused the damage and whether other blades share it, our blade failure investigation answers it. If you have a repair scope to decide, contact us.