High wind load, high workload

Back in April 2010, both the European Association of abnormal road transport and mobile cranes (ESTA) and the European Federation of Materials Handling (FEM) issued a warning outlining the dangers of improper consideration of wind loads during lifting operations.

This followed a number of well-publicised wind speed-related crane accidents that saw cranes toppling during lifting operations.

The ESTA and FEM product group for mobile cranes wanted to ensure that crane operators were aware of the exact information needed before tackling lifts in areas where wind speed can play a major factor.

Crane operators across the industry will of course be familiar with the safe maximum working wind speed limit for each crane in their fleet.

This data is often used in conjunction with meteorological data for the area so that the operator knows what to expect in terms of wind speed, as required by the EN13000 European standard for crane design.

However, when planning lifts for heavy components that are designed to catch the wind and maximize the effect across their surface area, additional factors come into play.

Operators must be prepared for sudden gusts of wind and fully aware of the effect that different wind loadings will have on the load lifting performance of the crane being used.

In order to minimise the number of lifts and simplify installation of wind farm turbines, project managers most often prefer to attach the rotor blades to the hub while the components are at ground level.

Although this can save time, lifting a rotor blade/hub assembly in this way can also present potentially dangerous difficulties for the crane operator.

The ESTA/FEM warning pays special attention to the lifting of loads that have a large surface area exposed to the wind in proportion to the load’s mass.

According to guidance in the EN13000 standard, for a typical load the maximum surface area exposed to wind (the ‘sail area’) should be proportionate to the load’s mass in the order of, or less than, 1.2m² of sail area per tonne.

To account for the wind loading in a given situation, the sail area can be multiplied by the particular shape coefficient for that load (known as the C_w factor).

This C_w factor accounts for the drag effect of the wind on that particular load, and when multiplied by the sail area it gives a reference value that can be used in conjunction with the crane’s load charts to determine the lifting capacity weight reduction accounting for the wind load.

One detail that may have contributed to the crane accidents preceding the ESTA and FEM warning is that wind turbine manufacturers did not originally give the correct C_w factor for wind turbine rotor blades in the documentation.

This incorrect C_w factor would have lead operators to believe that the load limits at higher wind speeds were higher than is actually safe.

ESTA general secretary Søren Jansen believes that this vital error was not the result of negligence on the part of the wind turbine manufacturing industry, but due to a difference in perspective.

He explains: “What prompted this safety alert was the realisation that the C_w factor on rotor blades and the complete rotor was indeed a lot higher than anyone expected. You really have to be damn sure that you know the correct C_w factor for that construction, and that was the problem.

“No wind turbine manufacturer actually had the correct figure because their interest is not as much in the C_w 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.”

Jansen says that having being alerted to the error thanks to ESTA and FEM’s warning, wind turbine manufacturers now provide operators with the correct C_w factor for a stationary rotor as opposed to the C_w factor for a moving rotor.

Wind turbine manufacturer Vestas’ director Finn Sandholm is responsible for the execution of construction excellence programmes at the company, as well as group finance and operations.

He agrees that Vestas, along with other manufacturers, needed to take greater measures to make crucial information more easily available to crane operators.

Commenting on the importance of data such as the C_w factor being available to operators, Sandholm says: “It’s not something that we have always been aware of. Right now we’re working on a template for the entire scope of installation, and here we will definitely look into having the right data in it like the C_w factor. We are starting to add this information into our certification system so that the technicians are also aware of it before they start a lift.”

He adds: “It is very important to combine the experience of the technicians with the crane operators. The technicians are very often local, so they know the wind patterns [local to the site].”

Other measures to ensure the availability of data to ease the handling process includes marking nacelles with the individual weight of each nacelle on its housing, rather than the standard weight previously used.

Vestas also now include maximum safe wind speed working limits for turbine components in their manuals. “That is two ways we have also learned how to improve on safety,” adds Sandholm.

This kind of support from the wind turbine manufacturers is much needed, as US crane firm Manitowoc’s director of technical projects in the EMEA, Gerhard Kaupert, explains: “We learned from customer feedback that many companies installing wind turbines are unaware of wind-speed limits.”

However, Sandholm emphasises the point that as ultimate responsibility for each lift lies with the crane operator, they must be proactive in demanding vital data from the manufacturer.

“At the end of the day I think it is the crane operator’s responsibility that it is there,” says Sandholm, “but that said, although there is no doubt about whose job that is when we’re doing a lift, I would still like all the [turbine] manufacturers to provide them with the news and the most correct data.”

This goes for new turbines as well as discontinued models still in service, because as the company’s name is still on the turbine, they have a vested interest inensuring operations run smoothly.

However, in many cases manufacturers will not know the correct C_w factor for older turbines, and Sandholm would expect the operator to perform the calculation, which could be costly affair.

“I know we still have lots of old models out there, turbines we installed back in the 70s or back in the 80s, but we don’t support the model any longer.”

“If it is one of the turbines that we do not support through our service strategy then we could ask the crane operator to calculate it, and in the contract it should be agreed who is paying for what.“

The corrected C_w factors for wind turbine components have meant extra costs for lifting operations, whether it is increased downtime due to the lower wind speed limit or the increased number of lifts that may now be necessary.

While accepting the ultimate responsibility of the crane operator for having all the necessary data, UK crane hirer Bryn Thomas Crane Hire’s sales and technical director, Dean Stanton, thinks that principle contractors on site should consider this aspect more carefully during planning.

He says, “It’s something that the principal contractors on site need to consider more depending on the location and the individual conditions on site. If time does become an element on site where conditions are bad and it actually starts to hinder the health and safety of it, you’ve got to just stand back and respect that.”