An investigation analyzing a fallen wind turbine nacelle and blades in the Halkirk region says that they fell to earth because they weren’t tightened to the rest of the structure properly.

A reader forwarded a copy of Capital Power’s “incident report of the turbine T33 nacelle detachment” to the Rural Alberta Report this week. Readers should note Halkirk and area residents were informed in Nov., 2024 that one of the turbines in the brand new second phase of Capital Power’s development had fallen to the ground, complete with all three blades. The nacelle and blades are essentially the engine and propellers, for those less experienced with wind turbine terminology.

DNV, an assurance and risk management firm, analyzed the Halkirk failure on behalf of owner Capital Power; the incident report was prepared by DNV Senior Wind Turbine Specialist Hendrik Schroeder. The report was an in-depth analysis of the investigation undertaken about a year ago into why the top of T33 fell to the ground.

“DNV has reached the conclusion that the focus event (the nacelle and blades detaching and falling to the ground) resulted from use of out-of-tolerance tooling to tension the tower to nacelle bolts,” stated the report, which is dated Mar. 6, 2025. “This conclusion is based on currently available information and evidence at the time of this report issuance.

“The support for this conclusion is the metallurgical analysis of several retrieved tower-to-nacelle bolts from turbine T33 sent to an independent laboratory confirmed that the bolts had no pre-existing damage and that the bolts meet their design requirements, the bolts were lacking typical evidence indicating that they had been properly torqued (i.e., a lack of smearing of the zinc coating under the bolt head or on the washer’s surface), the fractured bolts showed evidence of fatigue crack propagation as well as ductile overload and wear was observed on the unthreaded shank of the bolts, indicating radial loading of the bolts.

Furthermore, “The electric torque tooling (impact wrench) used to torque the bolts for the nacelle installation at turbine T33 was inspected and tested against a calibrated load cell by an independent service provider on 8 Jan., 2025. The tooling was found to be out-of-tolerance providing between 20 per cent to 24 per cent of the intended 1,475 Nm torque specified for the tower-to-nacelle connection bolts.

“(About half) of the bolts self-loosened, a phenomenon that has been identified to result from under tensioned bolts in a highly dynamically loaded connection.

“The torque of the tower-to-nacelle bolts was verified on all remaining project turbines. The tooling used on turbine T33 was used on one additional turbine at the project: turbine T31. On turbine T31 one hand-loose tower-to-nacelle bolt was observed and all bolts of this connection experienced movement ranging from 30 to 60 degrees during the retorquing campaign. Hand-loose bolts were not observed on any other project turbine during this inspection campaign, although a notable number of tower-to-nacelle bolts at turbine T16 experienced movement during the retorquing campaign.”

The report noted that the original equipment manufacturer (OEM) conducted its own investigation into the nacelle and blade fall and came to the same conclusion.

A final cause diagram included in the report summarized thusly: the use of out-of-tolerance tooling caused an installation error, which in turn caused insufficient pre-tension of tower-to-nacelle bolts which resulted in fatigue loading which was followed by the failure of the tower-to-nacelle bolted connection which ended in the nacelle and blades detaching and falling to earth.

DNV noted investigators looked into many possible factors, including manufacturing defects and weather, and the impact wrench explanation fit all evidence.

It was noted in the report that the impact wrenches used for securing the nacelle to the rest of the turbine have two parts, a control box and a handle, that investigation suggested T33’s nacelle was secured by impact wrenches applying about 325 Nm and that “DNV considers it possible that the OEM-specified 1,000 Nm was not or not consistently applied to the tower-to-nacelle bolts at turbine T33 and other project turbines.”