Are Lockdowns Leading to a Critical Lack of Essential Maintenance?
Whilst globally it is agreed that just over 40% of wind turbine losses are as a result of blade damage we are seeing an increase in gearbox failure claims being reported, and the frequently found cause of the loss is increasingly as a result of poor maintenance. The typical material damage claim costing in excess of €350,000, plus business interruption and all other related fees and expenses. We are also seeing the down time significantly increased as suppliers are in lockdown and boarders are closed, meaning delivery times for items not held in local stock are increasing further augmenting the insurance claim period.
The gearbox is 13% of the overall cost of a typical wind turbine, making it a costly and heavy part of the wind turbine. If a gearbox has to be replaced, the plant outage can last between a few days to as much as two months, depending on parts availability. As stated this matter that is now being exasperated at the moment lock downs and closed national boarders across the EU as a response to the COVID outbreak.
The gearbox itself, connects a low speed shaft to a high speed shaft and increases the rotational speeds from say 30 to 60 rpm, to around 1,000-1,800 rpm; the speed required to generate electricity, There is no one size fits all approach to bearings as such we see many different designs.
The configuration of a turbine’s mechanical power transmission depends on many factors. These include the available space within the nacelle, the configuration of the gearbox and the specific operating regime of each unit. That said all bearings have the same goal to support and stabilize the rotational shaft. So that when these bearings fail, the whole gearbox is compromised and indeed can in extreme cases lead to the total loss of the turbine. The turbine bearing losses is often called axial cracking.
Whilst there are numerous causes for gearbox failures, a common feature of premature bearing failures is a network of small, white decorated cracks below the surface of the bearing called "white etching cracks" (WECs), or small axial cracks visible at the raceway surface.
Up until now it was assumed that these WEC′s were the root cause of the failures. However, research has shown that these cracks actually occur at the end of the failure chain and are a natural consequence of crack networks in prematurely failed bearings.
In other words shaft bearing axial cracks or WECs are only a visible symptom of the failure, rather than part of the underlying cause. At the heart of measures to prevent these events is profound periodic maintenance.
The perceived wisdom is that wind turbine drive-trains naturally undergo severe transient loading during start-up operations as well as shut-downs, emergency stops or during grid connections. It is this load casings which results in torque reversals that can shorten the bearings effectiveness utility.
We have seen numerous cases of rollers skidding during the sudden relocation of the loaded zone. Seals and lubrication systems must be suitable to work constantly over a varied temperature range to prevent the ingress of dirt and moisture, and once again, this is a question of maintenance.
Although there is no single reason why gearboxes fail, it is true that the engineering challenges of assessing the non-torsional loads that pass through the gearbox, lie at the heart of the matter. That said all efforts are needed to keep up with servicing the assets, from the basics of eliminating soiled or water-contaminated lubrication, to managing the units as best as possible to counter transient loads that lead to sudden accelerations and load-zone reversals, as well high edge stresses from improper bearing settings.