Bolt failures on wind turbines can be catastrophic, ranging from the separation of components such as rotors and blades from the structure to the total collapse of the tower. Although such accidents rarely result in human injury because of their relative isolation, the costs of replacement or repair can be extremely high - and every such failure requires investigation which may mean shutdowns and other interruptions to supply.
In most circumstances, turbine towers are constructed with a circular flange at the base which are secured by means of a series of threaded bolts that are embedded in heavy concrete foundations. Once the tower is lowered into place and levelled, nuts are screwed down on to the flange to prevent any movement.
In some cases, the entire flange, all the bolts and the nuts are then encased in a protective grout, in other cases the bolts and nuts are left exposed to allow for testing and visual inspection. Where grout is used, any problems with security of the bolts is completely hidden from view and failures in the bolts may not be identified until too late. In recent years in the UK there have been a number of tower collapses from incorrect installations and poor quality grout which have caused the bolts to fail.
Exposed nuts and bolts, as in the case study illustrated below, are vulnerable to the effects of weather and the environment but, as long as they are regularly inspected and some form of protection is applied, they can be safely maintained for many years without any risk of failure. Nevertheless, constant vigilance is required and, although a number of solutions exist for bolt protection on wind turbine foundations, none has met all the necessary criteria for long-term protection without constant remedial action. Recently both in the UK and the USA, we have been looking at ways that Enviropeel might provide an effective system for such bolt protection.
A close look at the base bolts shows how much corrosion can affect them
The photographs show an application in the US. Prior to application the bolts can be seen to have significant corrosion on the threads and there is a variation in bolt length which makes it difficult to apply a 'one size fits all' solution such as bolt caps. Because Enviropeel is spray applied, any variation in length or shape of substrate can easily be accommodated, so application to the bolts is very simply accomplished.
As most readers will be aware, application of Enviropeel not only provides a barrier to the effects of the environment - preventing moisture getting on to the substrate - but, through its continuous release of inhibitors, it also provides active protection against galvanic and other forms of corrosion which are endemic in bolted systems.
For this application the Enviropeel USA crew used the US-designed and built MA25, a larger capacity variant of the original MA10 workshop unit. Although heavier than the MA10, the MA25 is still easy to manoeuvre and can easily be moved around a work site. The image shows the operator recirculating the material from the application gun into the tank while adjusting for the perfect flow pattern.
The next image shows the Enviropeel being applied to the bolts. Using a standard two-coat system, the material is flowed on to the bolts forming a perfectly-fitting barrier against deterioration. Following application, excess material around the bases is trimmed and all waste material is returned to the application unit for re-use.
Following the application, all the bolts are fully protected from the effects of corrosion while, at the same time, they remain easily accessible for inspection or adjustment.
A short video of the application can be viewed by clicking this link