Analysis Of The Accident With A Wind Turbine

Introduction

The accident happened in a wind farm which is 20kms from Vetlanda in Jönköping which contained 32 identical wind turbines in it. The accident took place on the 24th of December 2015 and the wind turbine 15 collapsed to the ground. The accident was first noticed by farmer who owns the farm.

Causes of the accident

1. Screw connections and washers: (M64 standard EN 14399-4) All the flanges are different from one another as the structure narrows as we go to the top. The bottom section/flange is different from the others as it is fixed to the ground and the screws, nuts and washers were placed all around the flange horizontally. The screws failed because of excessive fatigue experienced by them because of their arrangement.
2. The lubricant: the lubricant used in between the screw connections and the flanges was just a normal lubricant but not the corrosion protective lubricant. They have found that there was an accumulation of the lubricant at one part of the flange end and was unevenly distributed causing one side of the screws and washers to corrode faster than the others.
3. The bottom flange was still holding its position on the ground firmly even when the whole structure was coming down with such enormous force pulling out the screws between the flange 1 and 2. So the failure was completely in the screws of flange 1 (M64).
4. Pre-tension: the major reason for the screws to fail was the pre tension was very low which resulted in failure due to fatigue. The bolts were often subjected to environmental factors like heat, snow, rain etc. which resulted in low pre-tension in them. The water (rain) washed away the lubricant applied and caused more friction between the surfaces and hence more corrosion.
5. Installation tools: The tools used for the installation were not the ones recommended by the manufacturer, the installation tools with a different torque were used by the workmen who wasn’t experienced in such kind of installations in the past. So in this case a manual error gave rise to a new problem of technical error.
6. Software management: the software used to run the wind turbines was frequently changed which put additional vibrational loads on the structure more often which might be one of the reasons for a drastic change in pre tension.
7. There were many human errors which played a major role in the accident but these human errors might just be the ignition for the technical material failure which followed. The human errors like maintenance and installation play a major role in the functioning of these structures and that too which were running continuously without break.
8. Environmental causes: environmental causes like the weather conditions affected the structural performance to a severe level. Caused corrosion and wear and tear of parts because of winds. The friction was more because of the lubrication defects.

Prevention steps taken after the accident

1. The inside of the turbine structure should be maintained dry.
2. The fasteners should be stored and used in dry conditions.
3. The time between initial and final pre loading of the screws should be less than seven days.
4. Regular inspections (physical and visual).
5. Many other human mistakes to be rectified and workers with some expertise are to be hired or proper training has to be given to those who are going to work on the project.

Conclusion

I hereby conclude this report saying that this accident would have been avoided if the screws haven’t failed due to the environmental factors, the manual lubrication methods and the lubricants used for the purpose. The steel screws were not corrosion resistant and under severe environmental conditions, they are under a constant of failure after certain time. Proper life time of the individual parts should be calculated precisely or at least close to approximate values. The material failure occurred due to fatigue caused by reduction in pre tension.