When the wind blows…

27 February 2012

In March, ESTA will bring together a group of crane and specialised transport industry experts in the field of wind turbine erection and transport to discuss safety. Kevin Walsh looks at the some of the issues to be covered in just one day.

In March, ESTA will bring together a group of crane and specialized transport industry experts in the field of wind turbine erection and transport to discuss the safety problems that have dogged the industry of late.

While crane-related accidents do occur from time to time, and always too often as many will attest, the number of accidents that have happened involving sizable and well-engineered cranes during the installation of wind turbines has caused some alarm.

Cranes overturning or experiencing a boom collapse due to a failure of one crane type would arguably be less of a cause for industry-wide concern, as the problem may be localised to an individual design concept, but this summit—along with warnings from both the FEM and ESTA— will indicate this is far from being the case. This is everyone’s problem, and everyone will have to pitch in to solve it.

Warnings highlighting the importance of crucial data during wind turbine lifting operations were issued in a joint statement released by the FEM and ESTA back in April 2010. The statement focused on two pieces of information, the Cw factor used to determine the wind drag on a static rotor blade during lifting, and the sail area, the maximum surface area exposed to the wind during lifting.

While these values have been provided in the load charts of crane manuals for over 40 years, these were assumed values based on typical loads being lifted which didn’t take into account the nature of turbine rotor design—i.e to maximise the effect of wind on the ‘load’.

The charts have always stipulated that for any loads which did not adhere to the principles behind the assumed values—for example, that for every tonne of weight the sail area was roughly 1m2—if the crane operator obtained the correct Cw factor and sail area for the load they could recalculate for themselves what the correct crane working limits would be for a particular lift, taking into account wind speeds.

Unfortunately, due to a general lack of awareness surrounding the difference in these values during wind turbine erection, along with Cw factors not fit for purpose initially being provided by the turbine manufacturers, a number of accidents have followed.

In April 2010 ESTA’s general secretary, Søren Jansen, explained the reason for the incorrect Cw factors provided by manufacturers, saying, “What prompted this safety alert was the realisation that the Cw factor on rotor blades and the complete rotor was indeed a lot higher than anyone expected. No wind turbine manufacturer actually had the correct figure because their interest is not as much in the Cw factor of the rotor blade itself, it’s more in the rotating rotor. That is what their concern is, that it has as little [wind] resistance as possible, and that was really what contributed to why they didn’t know it.”

But despite turbine manufacturers now being aware of this problem and starting to provide better information, crane operators on site still need to be educated regarding its proper use, says Liebherr’s general manager of its design department, Hans- Dieter Willim.

“For forty years now we have assumed that the area of the wind on a typical load is 1m2 per tonne of load, and we’ve also assumed, as we can’t know the shape of every load being lifted, that the drag factor is 1.2. Now we know in most cases for wind turbines the drag factor is 1.6, so a lot more than in our assumptions, and [using the standard chart values] we assumed that a 20t rotor has a sail area of 20m2, when in reality it has more than 180 or 200m2.

“With these wind charts we say you have to reduce the [safe working] wind speed in such a way that you get the same wind forces on the load as we have in our load assumptions, that’s the point. To reduce the wind speed, there are two charts in the operator manual, or you can give a formula with a square root. Now if customers would go to the operators manual and go in the charts which have been in there for 40 years, they would know they have to reduce the safe working wind speed.

“But when I go over the square root with a crane driver or lift planner, they cannot do the calculation with the charts. Also when they go into the diagrams, for the average crane operator it’s very difficult to read out the allowable wind speed for this wind area. So what many crane drivers do is think that because they don’t have the maximum load on the crane there is a lot left for higher wind speeds. That is their impression but this is completely wrong.”

A turbulent business
By the very nature of the industry, crane operators erecting turbines will always be working in areas where wind speeds are higher than normal, and lifting to heights where wind speeds are more keenly felt by each load.

As the number of wind farms across the world grows, more rapidly each year, prime locations with the highest wind yields become sparser. However, the higher the rotor blades are above ground level the greater the yield, so we can only expect to see future wind turbine designs including hub heights well in excess of 100m.

Willim explains: “In the past we had 80m towers, and there you don’t have the highest wind speeds. The towers are being manufactured taller because for every 1m of additional tower height you get about 1% more power output.

“So 10m extra height brings you 10% more output for the same machine, but also you have 10% more wind on the crane boom during erection, therefore the danger will also increase in the future. If you see the sizes of cranes we are selling now we are coming to heights we have never thought about.”

These higher wind speeds at greater heights also mean that the direction of the wind takes on greater importance, as it can not only make the accuracy of such calculations even more critical, but also harder to perform.

Crane booms designed in accordance with EN13000 or the ASME B30.5 standard are constructed with the assumption that the side loading from any load lifted by the crane will not exceed around 2% of its weight, so when the wind blows from either side of a crane the resultant additional moment of the load and wind loading on the crane boom can be quite significant. To further complicate the situation, Willim says that most load moment indicators, while perfectly capable of registering the wind loading on a crane boom when it comes from in front of or behind a crane, will more often than not fail to register a side wind.

“Another relevant point that I will cover in my presentation is the rated capacity limiter”, says Willim. “It reacts, I would say, when the wind is coming from the back of the crane, then it shows you more load. When the wind is coming from the front it shows you less load, but when the wind is coming from the side it shows you nothing. No increase, no decrease.

“But when the wind comes from the side, the side loading is very critical because then the whole boom bends, and the more side bending of your boom system, the more your load goes out and gives additional moment. Two percent is very little when you compare this to the wind forces that are applied to these big rotors.

“Some tower heights are as high as 150m now, and this means we are getting into areas where the wind is blowing all day long. We will have higher wind speeds when lifting rotors in the future, and as boom systems get longer and longer just to lift to these high hub heights the danger will increase.”

Further contributing to the dangers of side loading, the increased turbine installation heights also reduces the effectiveness of the taglines often used to help secure the rotor/hub assembly’s position as it is being lifted. A more vertical angle between the rotor hub assembly and the ground-level securing points for the taglines means they provide less resistance to wind side loading.

With wind turbine rotors often lifted in their ‘sailing position’, an orientation chosen to minimise wind forces on the rotor that could make the blades rotate during lifting, this simultaneously maximises the surface area on another plane of the rotor/hub assembly.

All of these factors combined can provide such a large load moment, with longer boom system lengths acting as a lever arm, that the outrigger ground bearing pressure underneath the crane is dramatically affected, producing more side load in the process that can eventually lead to overturning.

Failing to prepare
A number of different factors can come into play on any wind turbine erection job, and the differences in conditions between the erection of one turbine on site and another just 100m away can vary wildly.

With the one factor least controllable by lift planners being the wind, even when a weather station is near enough to the job site to provide accurate data on the overall weather conditions during a project, sudden unexpected gusts can still cause delays, with worse consequences still if these changes occur midway through a lift.

This means that in addition to calculating the maximum wind speed at which it is safe to lift, the crane operator needs to calculate the wind speed at the boom tip at the time of the lift, to check its variance from the information provided by the weather station, as well as the three-second gust value.

However, a large part of the ESTA Expert Summit in March will focus on factors more easily controllable by the lifting firms, such as preparation of the site and on-site logistics.

Among the topics will be consideration of the way civil works are undertaken to ensure access to the site—which is often across agricultural land—is level, to support the partially assembled cranes moved around site and other transport onto site. Even a small inclination of the road above 2° can significantly contribute to a crane or transport trailer’s risk of overturning.

Trailer manufacturer Nooteboom is aware of a number of difficulties in this field that it believes can be avoided simply through better communication between stakeholders, as director Han Rekers explains.

Rekers says that on occasion he has witnessed the transport of components for some of the larger turbines on trailers that were not designed for that purpose, and as a result require a number of safety risks to be taken during transport.

He attributes this to a dearth in numbers of trailers designed by manufacturers to cope with such enormous components, which in turn speaks to a lack of communication between the industry and the wind turbine manufacturers.

“Turbine blades are getting longer and longer, and in the beginning no-one wanted to have a discussion about it, they all said we’re just going to wait and we’ll see what happens”, says Rekers.

“In the end there are just a few trailers on the market at this moment to do this kind of transport. The Vestas V112s for instance, but also the 3.XM from Repower and the E101 from Enercom, suddenly they’re all on the market, but there isn’t enough capacity to move all these long blades.

“So what is happening at the moment is that people are changing their existing equipment, with tools and some beams and some counterweights, all kinds of stuff, just to be able to do the transport. But it is not secure, it is unsafe, it is not what [the trailer] was meant for one and a half years ago when it was developed.”

Rekers says the hesitance from trailer manufacturers that led to the low numbers of suitable transport options being available was caused by uncertainty as to which way wind turbines were developing.

He comments that the rumour mill, some years ago, had industry workers split between whether to expect longer blades being produced by turbine manufacturers or blades that would be manufactured in two pieces which could later be assembled at the site.

Rekers explains: “That was something that was said two or three years ago, but the blades are not splitting, they are still long. REpower’s is 61.5m, and they are even starting with 73m, and then when you ask them ‘are you going to split the blades?’ they say ‘no, that is not possible’.

“If we would have invested enormous engineering and innovation capacity to develop a 65m trailer to do it and then they said ‘don’t worry we will split the blades in two parts’ that would be a big problem.

The squeezed middle men
Although a fledgling sector compared to the established power generation industries around coal, gas and nuclear power, a booming industry has sprung up rapidly around the market for providing energy from wind.

As an ever increasing number of countries looks to reduce their domestic reliance on CO2 producing power networks, crane and special transport equipment manufacturers develop new products in anticipation of a wealth of wind farms springing up across the globe.

According to the World Wind Energy Association the pace of this growth is quickening, with 15% more wind turbine installations globally in the first half of 2011 than the previous half year, up from 16,000MW to 18,405MW.

As a result competition is becoming increasingly fierce. Manufacturers in countries such as China and Vietnam, the former already a dominant force in the wind energy market, are allegedly using unfair government subsidies to flood the world market with cheaper turbine components and consequently devaluing it.

While the US Department of Commerce’s International Trade Administration launched an investigation into these purportedly illegal subsidies in January, many already competing in the industry are worried that the gradually declining price of wind turbines could further squeeze profits, which more worryingly could encourage corner cutting during installations to reduce costs.

During his time as director of Danish crane rental company BMS, Jansen says that he came to understand how some of the pressures on tough wind turbine erection jobs could help compromise safety when undertaken by smaller firms.

“Some smaller companies who hire out big cranes needed for the erection of some of the bigger turbines are increasingly coming under pressure from the turbine manufacturers, price pressure,” says Jansen. “They don’t have any negotiation power when they dispatch to a multinational turbine manufacturer or whoever.”

Jansen believes that some smaller firms are forced to walk a dangerous tightrope between conducting lifts safely while keeping to a tight schedule on site.

“At a lift that I did at my former job as president of BMS Denmark, we often saw that if a crane rental company starts asking too many safety-related questions on a job site they are often, unfortunately, considered difficult to work with.

“We had a rule, that still applies, if the crane operator doesn’t feel okay about doing this job, then he doesn’t do it. But the penalties for delays when installing a wind turbine are very big, and for a small company it can kill you. So often they are scared, they don’t dare to stop because they know that the penalties that can be given afterwards can be very big.

“Also they don’t want to ask questions because then they are considered difficult to work with by the site manager, and then they won’t be asked for the next job. I know about companies that have been asked to leave a job site because they were ‘difficult’.”

Many believe that there should be greater importance given to the role of the crane operator on such jobs, so that they not only have received specific training in wind turbine lifting operations, but also so that their word on whether a lift is safe to perform is final, regardless of the cost implications of deviations from the schedule.

The whole supply chain needs to deal with the issues causing this pricing pressure and stakeholders must collaborate to ensure the familiar battle between market forces and operational safety does not tip too far in either direction.

In the end, the use of bigger cranes can solve many of the safety problems currently inherent in wind turbine transport and erection. Now the industry will be hoping that at the ESTA summit everyone will recognise there is no room for shortcuts, and play a part in developing a solution.

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