Rolling conical tower sections
Most modern wind turbine towers are conical tubular steel towers, as already said. This image from a tower manufacturer's workshop shows how a steel plate is rolled into a conical subsection for a wind turbine tower. It is a bit tricky to achieve the conical shape, since the tension (pressure) of the steel rollers has to be different at the two sides in order to make the plate bend properly. Towers are assembled from these smaller, conical subsections which are cut and rolled into the right shape and then welded together.
Towers are usually manufactured in 20 to 30 m sections, the limiting factor being transportation on roads or rail. Typical modern tower weights are 40 metric tonnes for a 50 m tower for a turbine with a 44 m rotor diameter (600 kW) and 80 metric tonnes for a 60 m tower for a 72 m rotor diameter (2000 kW).
Designed by the turbine manufacturer
Towers for wind turbines are generally designed by each turbine manufacturer, since the entire wind turbine has to be type approved as a unit . So even if some towers are manufactured by independent producers, they are always specific for each manufacturer. Independent tower manufacturers are often also manufacturers of oil tanks or pressure vessels, since the machinery and safety inspection procedures are very similar.
Tower weights (per installed power in kW) have declined by about 50% during the past five years due to more advanced design methods. Still, towers are a fairly heavy part of the wind turbine, so transportation costs are important. For larger markets it generally does not pay to transport towers more than 1000 km (600 miles) by road. In case the distance is larger (and the project is a large one), towers are usually manufactured locally.
Banana peel shaped plates
In order to end up with a cone-shaped section, the plate used for rolling has to be curved along the longest edges and the short edges are not parallel. Most
tower manufacturers use programmable laser cutting tools in order to obtain the appropriate shape for the steel plate.
Steel sections are powder yielded
Each tower section is welded with a seam lengthwise, plus a circular welding seam to connect to the next section of the tower. This is done by placing the tower sections on a rolling bed which slowly rotates the tower, while an operator with a powder welding machine welds the sections from the outside and another operator welds a corresponding set of seams on the inside.
Welding seams in towers are checked using ultrasonic or x-ray devices. Important seams are checked 100%, while other seams are checked on a sample basis.
Attaching towers to their foundations
Towers are usually bolted onto the concrete foundations on which they are placed. There are other methods, however, as in this case where part of the bottom section of the tower is cast into the concrete foundation, and where the lowest section of the tower is subsequently welded together directly on the site. This method requires that the tower be fitted with special guides and clamps to hold the two tower sections in place while the welding is being done. It also requires a small mobile tower factory including a generator welding gear, and x-ray inspection equipment for checking the welding seams.
Flanges and bolt assembly
Wind turbine tower sections are bolted together using hot rolled steel flanges, which are welded to the end of each tower section. The flanges are made from killed steel. The first image shows a pair of flanges.
The second image shows how the tower sections are bolted together inside the tower. The quality of the flanges and the bolt tensions are important parameters for the safety of wind turbine towers.