February 7, 2006
With the recent increases in gasoline and natural gas prices, more attention than ever is focused on alternative energy sources. One fabricator, Aerisyn LLC, investigated manufacturing towers for use in the wind power industry. To produce towers efficiently enough to compete against imports from Asia, Aerisyn relied on an equipment vendor that had experience in demanding fields such as aerospace, nuclear, and wind power.
|A programmable rotary bevel head creates any combination of vertical and bevel cuts up to a 45-degree angle to prepare the cut edge for welding. This eliminates many of the secondary operations previously required before welding.|
The recent surges in gasoline and natural gas prices have reawakened many Ameri-cans' interest in alternative energy resources. Aerisyn LLC, a metal fabricator, faced the energy challenge head-on by opening a new wind tower manufacturing facility in Chattanooga, Tenn., in August 2005.
"Wind energy is the current leader at being price-competitive to fossil fuels," said Walter Thompson, COO and director of marketing for Aerisyn. "Currently 0.8 percent of U.S. energy is derived from wind." Thompson expects that percentage to increase to 8 percent to 10 percent over the next 10 years.
Technology improvements are partly responsible for the growth in wind energy. Thompson has been involved in the alternative energy industry since the 1970s, and he remembers a time when wind towers deserved their reputation for being noisy and ugly. Today's wind towers, however, five or six generations removed from the first designs, are "like gentle giants," Thompson commented.
"They are larger, slower-rotating, and they are very quiet." They also are much more efficient. Taller towers reach higher into the atmosphere to take advantage of stronger air currents, so one new turbine today can produce the same amount of energy as 10 to 20 of the first-generation towers. An average wind turbine today produces between 1.5 megawatts (enough for 600 homes) and 2.8 megawatts.
|An automated submerged arc welding (SAW) column and boom system welds the external seams of the individual cans.|
Fabricating these taller towers, however, can be a challenge. The few existing U.S. wind tower manufacturers cannot meet current demand in North America, so
U.S. energy companies have been forced to purchase towers from Asia. "We could triple capacity and still not meet the demand," Thompson said. To keep the company productive enough to meet this growing demand and efficient enough to compete with Asian manufacturers, Aerisyn developed an automated production line.
Aerisyn's involvement in the wind tower business began four years ago when a vendor approached CEO William Stone with an idea at a tradeshow. Stone was familiar with fabricating large cylindrical tanks for the ethanol industry, so why not look into building wind towers, which are produced as a series of large cylindrical cans welded together?
Aerisyn determined that it wanted to be a "sophisticated fabrication shop providing a very highly consistent welding process to weld material together," Thompson said. To do that, Aerisyn's staff needed to work with a partner with significant expertise in welding. It found a supplier, ESAB, that had experience in fields with demanding requirements—aerospace and nuclear—and wind towers.
Stone intended to build a new facility until he learned of a plant for lease in Tennessee. The facility had been used to build nuclear reactors and for other heavy fabrication. The plant not only offered the space Aerisyn needed, it was also located near railways and waterways. This solved one of the key problems wind tower manufacturers face: transporting the final product. An average wind tower section is 14 feet in diameter, 50 to 100 ft. long, and weighs up to 50 tons. A completed tower stands 260 ft. to 320 ft. high.
|An automated tractor system completes the internal seams.|
The other major challenge Aerisyn faced was the need to keep its operating costs down to compete with the Asian manufacturers.
"We have concentrated on making as sophisticated a process line as you can get, so you can get a high level of quality with fewer people," Thompson said. He is quick to add, however, that people are still a vital part of the equation. The fabrication process still focuses on welding, and Aerisyn knew it needed talented welders who understand what happens when metals melt. Although the new automated equipment allowed Aerisyn to rely on fewer people, the company hired and trained the best talent it could find to operate the equipment.
ESAB not only supplied all the cutting, welding, assembly, and material handling equipment for the plant, it also helped Aerisyn develop the welding procedures and was instrumental in designing the plant layout and material flow. "We redid the plan four or five times," Stone noted. "ESAB stuck with us all the way, often requoting to reduce our costs so we could get the money from venture capitalists."
A key to producing superior-quality wind towers is the ability to produce dimensionally accurate parts with precise weld preparation beveling on all sides. The fabricating process at Aerisyn begins on one of two ESAB Avenger gantry cutting machines. One machine has a 32-ft. by 109-ft. cutting table with twin heads that allow Aerisyn to cut two 10-ft. by 10-ft. high-tensile-steel plates at the same time, while two more plates are loaded onto the table. This machine is equipped with oxyfuel cutting and beveling heads and is used to cut heavier 2.5-in. plate. A second machine has a 14-ft.-wide by 109-ft.-long table and is equipped with plasma cutting and beveling heads to cut plate up to 1 in. thick.
|A video monitoring system allows the operator to observe the weld from the safety of the operator station and make adjustments to the equipment as necessary. A single control manages the welding head, positioning equipment, wire feeders, and the flux recovery system.|
Both machines have programmable capabilities to cut parts and bevel edges in a seamless operation. The software simplifies setup time, and the accuracy and repeatability reduce the need for secondary cleanup operations. These machines are equipped with an optical troubleshooting system that allows an ESAB technician to "see" the machine in operation and instruct the Aerisyn operator on repairs if a problem should arise.
Having the cutting machines in-house reduces the company's lead-time by four to eight weeks, Thompson said. Most of its U.S. competitors have outsourced their cutting operations. If they discover a fault in the metal, it may take weeks to get a new piece cut. Aerisyn can cut a new part and get production back up and running in a matter of hours.
After the pieces are cut, they are run through a plate blaster to remove mill scale and primer, rolled into a can, manually tack welded, and loaded onto an automatic welding station. A column-and-boom submerged arc welding (SAW) system welds the external and internal longitudinal seams. Mounting flanges are also welded onto the can ends at this station. A fully integrated controller operates the welding head and the column and boom, the wire feeders, and the flux recovery system.
The can then moves to a head- and tailstock positioner with motorized rotation and hydraulic height adjustment. The first can is clamped into the tailstock. The next can then is clamped into the headstock in a corresponding manner, and the cans are positioned using the ESAB support roller bed and manipulators. The two cans then are tightened together and tack welded before the automatic SAW equipment welds the circumferential seams. A column-and-boom station welds the external seams while a tractor welder does the internal seams. Can by can, the section grows until it reaches its final 50- to 100-ft. length. A built-in rounding jig maintains the proper roundness. Like the longitudinal seam welding station, the circumferential seam welding station uses an integrated control. An optical monitoring system allows the operator to see the weld seam throughout the process.
After fabrication, all electrical equipment, ladders, lighting, controls, and platforms are added to the tower sections so they are ready to be installed in the field.
A key focus of the wind energy business, according to Thompson, has been to make towers taller but not heavier. For that to happen, the welded joints must be as strong as the parent metal. The weld must become part of the structure.
"The automated equipment doesn't make better welds," Thompson said, "but it allows us to make good welds much more consistently." According to the company, the level of confidence Aerisyn's customers now have in the welds has allowed Aerisyn to reduce its testing from ultrasonic testing of 100 percent of the welds to 20 percent. This is an enormous time-saver for the company. "Our customers who see our welds are saying these are consistently the best welds they've seen from any of their suppliers around the world," Thompson said.
"Fatigue in the metal is 10 times higher on wind towers than on almost any other machine," Thompson explained. To make the weld part of the structure, precise welding techniques are required, such as those used in the aerospace and nuclear fields. "They [ESAB] know how to provide the right procedures and equipment to get certified at these levels," Thompson said.
Aerisyn is currently working on improvements in the welds. The company's goal is to reduce the cost of a wind turbine by 2 percent. This may not seem like much, but this savings will be enough to make wind energy more economical than oil, natural gas, or nuclear.
Aerisyn LLC, 1025 W. 19th St., Chattanooga, TN 37408, 423-648-3884