Final report on investigation of a catastrophic turbine failures, February 22 and 23, 2008

This investigation was conducted following a request from the Energy Agency’s Secretariat for the Danish Wind Turbine Certification Scheme, together with experts from Risø DTU.  [Risø is the national laboratory for energy research, including wind energy.  It is a part of the Technical University of Denmark, DTU.]

1) Catastrophic failure of a Vestas (Nordtank) 600 kW wind turbine at Halling in Hornslet (Denmark) at about 3:20 p.m., February 22, 2008.

The site of the damaged turbine was inspected Tuesday morning, February 26.  It stands as the northern-most turbine in a row of five.  Meetings with Vestas were held in the afternoon.  The chain of events is based on information from Vestas and recordings from TV2 East and the neighbor.

Description of wind turbine

• The turbine is a Vestas (Nordtank NKT 600-180/43).  The turbine entered service on December 23, 1996, as the first in a row of five turbines.

• The turbine is privately owned.

• The turbine is no longer produced.  It was approved by Risø on September 15, 1996 (number B-130-1 rev1).

Chain of events

• Vestas state that service was requested for the turbine, following a malfunction of the brakes (cause: worn brake).  At the last routine major checkup it was noticed that the main gear made some noise.  It was then recommended to have an endoscopic inspection done and a price quote was forwarded.

• A service team from Vestas arrive Friday morning to check the brakes.  The brakes were repaired and checked.

• The braking mechanism is tested eight to ten times from the nacelle [engine house on top of the tower].  Then the turbine is restarted from the bottom of the tower with the intent of putting it back into normal production.  At this point, the wind is very strong.  The airbrakes at the tip of the blades are used to control the speed of the rotor, prior to attaining operational speed, according to Vestas.  That means the tips of the blades are turned out during the start.

• During the start of the turbine, just when it is put back on the grid, noise is heard from the nacelle.  The service personnel press the stop button.  The turbine’s control system starts a controlled shutdown procedure, but a large crash is then heard (probably the gear that fails).  The turbine is shaking strongly and cable assemblies, etc., fall down inside the tower.  The rotor stops abruptly for a moment, but then starts turning again.  The speed of the rotor is rather low, but it is noted that the turbine no longer can be controlled from the control panel and stopped by the brake on the high-speed axle.  The personnel evacuates the tower immediately.  Outside, it is observed that the airbrakes at the tip of the blades have broken off.

• The wind is very strong.  The turbine continues to gain speed, and attains speed beyond its design limit.

• The service personnel contacts the local police and assist in establishing a safety zone of 400 meters (1300 feet) and warn the neighbors.  This continues for 21/2 hours.

• The turbine wrecks.  The events are filmed by a neighbor and later shown on the TV2 station.  The result is that the blades collapse, the tower is hit and is strongly indented.  The nacelle is bent forwards (against wind direction).  Pieces of all three blades are thrown a great distance downwind from the tower, almost 180 degrees.

• The nacelle and the upper part of the tower falls to the ground in front of the tower.  The generator falls out and rests next to it.

• Large pieces of the blades land 200–300 meters (600–900 ft) away, while small pieces appear to have flown 500 meters (1600 ft).  Smaller, lighter pieces are found near a farmhouse, about 700 meters (2200 ft) away, though these may have landed and then been blown further by the wind, as the wind was very strong.

• The lower part of the tower remains standing.  An inspection of the gear shows it to be damaged.

• Nobody was hurt.

Assessing the events

• The likely chain of events is that the gear fails and causes a short, abrupt stop so the engaged airbrakes are broken loose and then thrown off.  The brake on the fast axle is not functioning as no momentum can be transmitted through the gear box.

• The wind is very strong, and without a functional brake, the propeller spins out of control.  The shock of the abrupt stop engaged the vibration sensor which activated the emergency power system in the tower and functions such as the mechanism to turn the nacelle (yaw) are off-line.

• At this time, it was no longer practically possible to stop the turbine and a wreck is inevitable.  The propeller accelerates and the airbrake tips, which now only hang by some lightning ground wires, break off and fall to the ground.

• The movie (filmed from a neighbor’s house) shows the last seconds of the uncontrolled spin of the propeller, where one blade collapses (probably when the tip is thrown off).

• Subsequent investigation of the gearbox reveals extensive damage.  The high-speed part of the gear has been checked, which showed the teeth to have been basically completely ground down by the high-speed cog wheel.  This confirms the listed chain of events.

• A crash report has been made for Vestas Technology R&D Denmark dated June 19, 2008.  The report was written by Morten H. Jensen, consulting engineer and Leif Hansen, Wind Power Technology, Denmark.  The conclusions of this report concur with the above-listed events.

• It is the opinion of the Energy Agency’s Secretariat for the Danish wind Turbine Certification Scheme that there have been a series of unfortunate conditions, which were difficult to foresee.  The events do underline the need for proper maintenance by qualified personnel.

2) Assessment of catastrophic failure of a Vestas V47 660 kW at Sidinge [Denmark] on February 23, 2008

The site at Sidinge was not visited, since the failure was less complicated.

Description of Wind Turbine

• The turbine is a Vestas V47 660 kW.  It entered service in August 2000.

• The turbine is owned by a co-operative.

• The turbine is no longer produced, but was certified by Germanischer Lloyd on June 16, 2000 (GA-02B-2000).

Chain of Events

• Vestas have stated that one blade broke off early in the morning on February 23, 2008.  The failure was noticed by a neighbor.

• The blade broke off where it is attached to the hub and hit the ground about 60 meters (190 ft) from the tower.

• It is observed that the bolts in the hub were broken.

• Nobody was hurt.

Assessing the events

• The inspection performed by Vestas on the broken bolts showed that they broke due to insufficient tension in some of the bolts [they had not been tightened well enough].  Over time, this resulted in metal fatigue and breakage, followed by a domino effect on the remaining bolts.

• Vestas have stated that the insufficient tension could have been there since the turbine was originally erected, and it was regrettably never found at the annual checkups.

• The Secretariat for the Danish Wind Turbine Certification Scheme finds that the described events are likely correct and that failure to properly check the bolts are critical for the turbine safety.

Combined recommendations

• It is recommended that the Consulting Committee for the Secretariat looks at these events soon, and provides guidelines to ensure that the certification of models and projects more precisely shows the required maintenance.

• It is further recommended that requirements for ongoing service and maintenance of wind turbines are very soon considered by the Consulting Committee for the Secretariat.  Together with the industry, they should work to ensure that all wind turbines receive the necessary qualified service and maintenance.

Editor's note: Windaction.org has updated the English translation of this report with special thanks to one of our readers.