Lightning strikes wind turbine blades every day. Most of the strikes are dissipated to ground as the lightning protection system (LPS) on the turbine is designed to do; however, some strikes will have enough energy to cause damage to the blades. With the advancement of longer blades, carbon fiber is being used more widely in the blade structure to add strength and stiffness and reduce weight. Since carbon fiber is an electrical conductor, lightning current can conduct through the carbon fiber structure causing major damage.
On this project, the lightning contacted the carbon fiber spar blasting holes through both sides of the spar and burning the structural laminates. Also, the shells on both sides of the blade and the leading-edge joint were damaged badly. Due to the severity of the damages, this blade was deemed unrepairable and a total loss by the OEM and the insurer.
Our repair team and composite engineers reviewed the damages and determined the blade was repairable, and we developed a cost-effective repair procedure. This repair was over 2.5m (8.2ft) long, and included removal of a large area of shell on both sides of the blade to gain access to the structural spar. This image was taken after all the damage was removed, the spar carbon layers were scarfed, and the spar inner biaxial fiberglass laminates were replaced. For removal of the damaged unidirectional carbon fiber layers, we used a 100:1 scarf ratio, which provided gradual load transition to the repaired layers. We replaced several layers of carbon fiber, added reinforcement doubler plies and elevated heat cured the laminates to ensure the resin matrix achieved full structural properties.
Once the spar damage was repaired, the LE joint was rebuilt and additional biax laminate wraps were applied to reinforce the LE joint. We then rebuilt the missing portion of the shells on both sides of the blade, starting with an inner skin lamination, followed by foam core installation and top skin lamination. Once all layers have cured, the repaired surfaces were filled and faired to duplicate the original blade contours. The final steps were application of two coats of polyurethane paint and new LEP tape.
This blade has been in continual service with no issues since it was repaired in 2012. We have performed dozens of these structural carbon spar repairs. To reduce turbine downtime and eliminate crane costs, most of these repairs have been done up tower utilizing our large, stable, wrap-around suspended platform.
For more information on damage repair, please call or email us.