A B53 blade broke off a turbine at MidAmerican Energy's 200MW Eclipse wind farm in Iowa on 4 April, followed by a similar incident at Pattern Energy's 265MW Ocotillo project in California on 16 May.
Siemens launched an investigation and curtailed all turbines using the same B53 blade, most of them in the US.
"The analysis revealed that the root cause of the two incidents is not related to the design of the blade," commented a Siemens spokesperson.
The problem was that precast segments used to construct the blade root, which the turbine maker gets from a number of suppliers, separated from the fibreglass laminate of the main blade. The adhesive bonding failure was caused by "insufficient surface preparation" of the root segments, the spokesperson said.
As of Wednesday, all wind turbines with B53 blades have been inspected and the majority have been returned to operation. Some blades will be replaced because indications of delamination were discovered, the spokesperson added.
As a precautionary measure, Siemens plans a minor field modification to enhance the bond between the pre-cast root and the fibreglass laminate on all existing B53 blades that are not being replaced.
Pattern Energy shut down the Ocotillo project after the blade failure and will now bring the turbines back online one at a time, said spokesperson Matt Dallas. The work is expected to continue at least through the end of July.