￼Trained at the Nelson Mandela Metropolitan University (NMMU) in Port Elizabeth, Jauka joined DCD Wind Towers in November 2013 at the starting point of commercial production. "I did a degree in industrial engineering and, after working at a local Goodyear facility for a few years, I joined DCD to improve the production efficiency of the wind tower fabrication process. To re- main competitive, we need to find ways to cut away non value adding operations and to promote those activities that are directly incorporated into the price of our product," he tells African Fusion.
"The core value adding activity of this facility is welding, and we employ mainly GMAW, FCAW and semi-automatic submerged arc welding," Jauka continues. "But my experience is production, so I am learning about welding all the time," he adds.
Describing the particular requirements for wind tower manufacturing, he says that the key productivity goal is to balance each part of the production line so that the fabrication of each wind tower segment proceeds steadily and efficiently through the plant, without causing delays or inactivity further into the process.
The plate for the first commercial can was cut on Saturday February 15, 2014, for a Vestas V112, 3,075 kW wind turbine destined for InnoWind's Grass Ridge project 25 km South of PE. DCD Wind Towers has now completed and delivered five three-section towers for this project, which was commissioned during January this year. In total, this plant has 20 turbines capable of producing 61.5 MW which, at a capacity factor of 25%, is equivalent to 134.7 GWh of power per year – enough to supply the basic annual needs of up to 40 000 South African households.
"Our current work also involves Vestas turbines for a second wind project dubbed 'Chaba'." The requirement is for seven towers split into 21 separate sections, which will support turbines supplying 21 MW of power to the Great Kei municipality at Komga, up near the Kei River north of East London. The first turbine is due to be operational in July 2015, with the facility reaching full capacity in September. For this project, DCD Wind Towers will locally build all of the wind towers.
DCD's fabrication process, according to Jauka involves fabricating three structural sections for each tower, a top section, a middle section and a base section. Each of these is flanged so that the 84 m towers can be erected onsite by bolting them together. Each individual section is made by welding together a series of cans, which vary in diameter and plate thickness – 5.0 m in diameter and 38 mm for a typical base section and 3,5 to 4,0 m and 16 mm thickness for top sections. "Each section consists of 9, 10 to 11 cans, most of which are tapered, so fabrication has to be done in a strict sequence," he explains.
Four growing lines
The concept being employed to achieve production and cost efficiency is 'grow' each tower section, can by can, on a growing line. "We have four parallel growing lines at this facility, which all need to be busy and balanced for maxi- mum output capacity and efficiency," says Jauka. "For each line, the idea is that we cut, roll and seal a can, then we join that can to the previously completed one. So we join and grow, join and grow until the tower is the required length. Then we add the end flange. But balancing the work on the growing lines is also important. Fabricating bottom sections, for example, takes up 45% of the total welding time, because of the thicker section plate being welded particularly on the three bottom sections with 38 mm plate thicknesses," he tells African Fusion.
"At any one time in the plant, we aim to have one can being joined to the tower section in one of the lines, a can-to can- fit-up being completed on the second line, can rolling on the third line and plate cutting for the fourth," he notes.
At the fabrication starting point a plate is put through a wheel abrator to take off the rough scale. "The abrator blasts grit onto both sides of plate as it enters the facility. From there, a three headed CNC-controlled ESAB oxyfuel cutting system cuts the plate to size, while simultaneously bevelling the edges for welding. Due to the different conical shapes of each can in a segment, each sequential can has a different size, so automatic control of this process is essential and each can has its own unique identifier that is generated be- fore the plate enters the facility," Jauka explains.
The cut plate is then taken to a chamfering table to polish and clean the bevel and take off the dross. "At this stage we have our first quality hold point. The quality controller checks that the plate is dimensionally accurate for the specific can and tower section being fabricated, to within around 1.0 mm of tolerance. If okay, he places a green sticker on it, but the plate will remain at the holding ￼￼station until a green sticker is applied," he adds.
A plate with a green sticker then proceeds to rolling: "Rolling is our critical operation as it relies heavily on the skill of the operator. We currently have two roller operators rolling five to six cans per shift, but we are operating our growing lines on a pull-based production system, so the rolling rate must match the speed of the of the whole line. We are looking for at least two more plate rollers so that we can implement a second shift," Jauka reveals.
To avoid double handling, the can roller operator inserts some initial tacks as soon as the required shape has been established. Then a welder immediately inserts a root run along the seam using the solid wire GMAW process. The can is then moved across to a submerged arc welding station. The outside seam is completed from the top. Then the can is rotated by 180° and the remaining welding is done from the inside at a lower level. Immediately after welding, the can is then re-rolled to correct any distortion that may have been introduced.
"The crane operator then takes this can and places it onto an orange block painted on the floor. "As soon as a quality inspector sees a can on an orange block, he does a visual inspection of the welding and weld-bead profiles. Then an ultrasonic (UT) inspection is done on the seam – and this must pass. If any flaw is detected, the inspector marks it up and the can will be moved aside to a separate repair station. Once repaired, both magnetic particle (MT) and UT are repeated until that can passes all tests.
"Once cleared by the inspector, the can is moved onto the next available green block. These green blocks are used to feed the growing line. Completed cans are moved up towards Green Block 1, which is the placeholder for the next can required for the section being fabricated. Our production target is to have two cans in the correct sequence ready to feed the growing line at all times," Jauka explains, adding that, from a production perspective, "the idea is that a green block is filled as soon as it become empty". In this way, the rate that cans are joined to the tower sections will be matched to the rate that cans are being produced.
On the growing line, ESAB roller beds with built in manipulators are used to support the growing section and the can to be added. A set of hydraulically driven transfer rollers lift the can clear of the ￼￼rotating rollers and shuffle it along and onto a roller bed further up the line. A new can is then loaded and welded, before the whole weldment is shuffled up a further 3,0 m. The welding is done using state-of-the-art ESAB submerged arc welding equipment, with the whole tower section being rotated to complete each circular seam.
"Visual and UT testing is required following each weld, but we have also now introduces magnetic particle testing on some cans to ensure that we don't miss any surface cracks," Jauka says, adding that each section has flanges on its ends, "so the flange is the first and last circumferential joint".
At the top end of the growing line, completed tower sections are lifted and placed on another set of 35 m roller beds for testing. Following final inspection and repair, a team of boilermakers is brought into the section to mark off positions for internal bracketry and fittings. Welders will then collect the relevant bracket kit from stores and, using mostly flux-cored welding, they weld them on.
The final operation on the fabrication side of the line – the black section operation – is to do Easy-Laser® alignment and flame straightening on the flange surfaces. "Flange flatness must be within 2,0 mm across the 4.5 m to 5,0 m diameter to ensure that when bolted together onsite, the flanges seal against each other and the bolts can be tightened without inducing stresses," Jauka explains. "The laser alignment system gives us a printout highlighting twist and distortion, which is then used to determine where flame straightening is needed to bring the flange into tolerance," he says.
From the top end of the line, the completed tower section is brought back down the U-shaped facility on a parallel path – the white section operation. It first enters a wash bay for the removal of any gel, grits and oil. Once dry, grit blasters are used to descale the surfaces inside and out. For cathodic protection across the flange connections of the tower sections, "we are required to metallise an area within about 500 mm of the flange and all around access hatches and doorways with a 40 μm layer of zinc. We therefore look for surface roughness of between SA2 and SA3 from our grit blasting operations, before using the twin-wire arc spray process to deposit the metallic layer," he says. "Grit blasting also has to be done under controlled humidity conditions, so we are currently installing a humidity controller so that we will no longer be dependent on appropriate ambient conditions to complete this task."
Following metalising, the tower section enters one of three paint booths, PB1, PB2 or PB3. The section is placed on a paint rotator supported and driven via the flange rings on either side. This allows access to the full surface for painting.
Once painted, the last stage of manufacture is to fit the platforms, rails, ladders and internal fixtures onto the welded bracketry. These components are also sup- plied in
kit form from Vestas. "We then move the tower section into the yard for a final inspection. When we are happy, we cover the ends attach the lower feet, enclose the whole structure in tarpaulin and arrange for transportation to site," Jauka notes.
"It is currently taking us six to 10 days to complete a top section, seven to 12 days for a middle section and up to 14 days for the bottom sections because of their heavier material thicknesses. But we are not yet at full production. We hope to be running all four lines in the near future, which will allow us to complete six sections per week.
The facility was designed to produce 110 towers per year, or 330 sections. If we can persuade a few skilled people to join us here in Port Elizabeth, we will soon be meeting our production expectations," he concludes.