November 12, 2008

Wind Power

Turbines tricky for crane operators

IAN HARVEY

correspondent

Growing demand for sustainable energy from wind turbines is presenting potentially dangerous, wind-powered challenges for crane operators involved in turbine construction and repair.

“The problem is that you want to put these turbines up in places where the wind blows,” said Ron Kohner, president of Landmark Engineering Services in Roseville Minnesota, who has been involved in lifting projects internationally, including in Canada.

Adding to that challenge, he noted at a recent workshop, is that the wind velocity at the top of the turbine tower is likely to be much greater than the velocity at ground level. Special caution and planning must be taken when planning a lift in such conditions, since wind effect can have a disastrous result.

A veteran of the crane industry, Kohner is also a commissioner on the U.S. National Commission for the Certification of Crane Operators.

Because wind speed at ground level is often much less than the velocity at the boom tip, he said measuring and understanding wind effects before undertaking a lift is crucial. It will likely mean large loads with large surfaces will have to wait for lower winds before being lifted.

“You can’t just go by the (standard) charts, you have to go further than that,” he said.

“If the wind speed doubles from five mph to 10 mph, the effect of the wind is four times greater,” he said, noting cranes should be equipped with an anemometer at the tip as well as the ground level.

Hand-held wind speed indicators are cheap and useful for checking wind speed when no other instrument is available, he said.

Terrain also makes a difference in elevated wind speed compared to the ground level wind speed – especially in downtown areas, because of the canyon effect of tall buildings. Shapes, be they curved or flat slabs, also have an impact.

“A 15 by 15 by 70 foot box will have 1,500 pounds pushing on it in a 20 mile per hour wind,” he said, which will cause the load to swing out much more and it could push the crane beyond its capacity. “Comparatively, a 15-foot by 70-foot cylinder has an effect of 650 pounds, so the swing out is much more manageable.”

This is important, he said, because booms are designed to handle compressive loads but not extreme side loads due to swing out. The bottom line, he said, is that the bigger the load surface, the lower the wind velocity must be to allow a lift.

Because each crane manufacturer has its own rules around capacity and wind, he said, it’s important to check the manual.

Finally, he said, lifting in wind should require the use of tag lines – anchors to keep the load in check – but he warned that they also come with another set of issues.

“Tag lines to control loads in the wind and their anchors must be sized adequately to withstand the planned wind forces,” he said. “Because, pulling hard on a steep guyline to control a load in high winds may add enough vertical force to overload the crane.”

The best practice, he said, is to schedule lifts of large objects with large surface areas when the wind is lowest – which may require a seasonal approach – and use taglines with caution.

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