The making of a DMI wind tower

June 30, 2010

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Figure 1

Gary Williams, DMI Industries general manager, Tulsa, Okla., facility (left), and Stefan Nilsson, the company president (right), stand before one of the company's wind towers.

Take a guided tour of just how it happens at the DMI Industries facility in Tulsa, Okla.

Gary Williams, DMI Industries general manager, Tulsa facility, and Stefan Nilsson, DMI Industries president, plan to strengthen the company's position as a leading wind tower supplier in North America (see Figure 1).

DMI Industries' 285,000-sq.-ft. wind tower production facility in Tulsa, Okla., originally was built in 1999 by a railcar wheel manufacturer. DMI opened it as a tower production facility in 2008. Williams explains how the company builds a wind tower:

1. "We start with materials coming in on one end and the assembled, finished tower sections going out the other end in a U-flow. We're really lean to be cost-effective."

2. "We'll cut the steel plate on a plasma table. The plate becomes either a straight or tapered can, depending on its position in the tower. From the top to the bottom of the towers, diameters can vary from 6 feet to 16 feet."

The plate is then beveled and the surface is shotblasted to prepare it for welding and painting.

3. After cutting, beveling, and blasting are complete, the plate is moved to the rolling machines. "Plate rolling is a critical operation. How well we do here is very important to what we do downstream."

Figure 2

It can take more than 30 cans to comprise a three to four-section tower.

4. After the plate rolling, the seam is carefully aligned to ensure the ends meet properly and welded. The rolled and welded plate, now called a can, continues on to fit-up.

5. In fit-up, seam-welded cans are matched end-to-end. After they're positioned correctly, cans are first tack-welded, then securely final-welded into a section (see Figure 2).

The flanges that must be attached to either end of a section are welded at this time as well. Flanges have holes fabricated into them for holding the bolts that will eventually attach the tower sections together at the project site. The base section flange has holes for the bolts that attach the tower to the ground.

"Welds are crucial to the structural integrity of the tower, so we place great emphasis on having the right equipment and highly skilled welders. In fact, DMI uses its own custom-built equipment to best perform the welding of these very large-diameter sections."

Figure 3

Once the painted section is dry, it is ready for assembly where internal components are installed.

6. Some of the original rails remain in the building and are sometimes used to transport components around the plant. A section is moved by rail into a blast booth to prepare the surfaces for painting. "It takes two or three operators to blast a section—each moves up and down the section, inside and out."

7. Next the section is moved into the paint booth where the entire surface is manually painted. "The painters are no ordinary painters—they have to paint under demanding conditions. There's a tight tolerance on paint. Paint thickness is specified to minute measures by the customer." A forced air furnace heats the air in the booth to 180 degrees to dry the paint (see Figure 3).

8. Because tower designs from different manufacturers vary in terms of the thickness of the plate; numbers of sections; designs on the matching surfaces and degree of taper; and some of the internal components such as ladders, elevators, and platforms, a highly trained team is needed to track and label materials and components. "Some of the tapers between the top and bottom of a section are so slight that they're almost imperceptible. We label everything and have a thorough quality check and testing system in place to ensure accuracy." DMI posts quality metrics as part of its Six Sigma discipline at the Tulsa facility.

"The entire operation takes skill and it takes a while to get up to speed. So, the people who work for us are very valuable resources.

"Our system provides instant feedback. It keeps our work centers very synchronized, very focused on quality."

9. In final assembly, weldments are bolted to the inside of the section. They secure the internal components such as wiring, ladders or elevator guides, and control cabinets, all specified and supplied by the customer
(see Figure 4).

Finally, the tower sections are moved to the receiving area, where customers pick them up and transport them to the project site where they are assembled as a complete tower.

"So that's how we build a DMI wind tower."