New system reforms prefabrication in shipbuilding
November 20, 2003
The Jos. L. Meyer GmbH shipyard in Papenburg, Germany, built its first wooden sailing ship in 1795. Today the six-generation, family-owned shipbuilding company designs and builds large metal cruise liners, gas tankers, and ferryboats. These floating behemoths can be as large as 162.5 by 975 feet (50 by 300 meters), and 162.5 ft. (50 m) high.
|Many miles of weld must be performed on ships fabricated at the Jos. L. Meyer GmbH shipyard. Ships can be as large 162.5 by 975 ft. and 162.5 ft. high.|
Many miles of weld must be performed on ships of this magnitude. Like other companies in the shipbuilding industry, Meyer had used manual gas metal arc welding (GMAW) or sub-arc welding to join the sidewall plates and deck panels. These thermal processes created a wide heat-affected zone (HAZ), and as a result, the plates were buckled, warped, and out of square. Therefore their fit-up was unpredictable in the subsequent assembly and joining processes. Usually the plates had to be flattened, and large gaps caused by warping and shrinkage had to be filled.
After the plates were joined, reinforcement elements, called stiffeners, were welded to the plates. Positioning and size of the stiffeners varied to accommodate openings cut out of the panels for stairs, escalators, doorways, and utility piping.
In an effort to shorten the time required to build its ships, the company sought a method that would allow it to prefabricate 65- by 65-ft. (20- by 20-m) deck panels from individual plates 13 by 32.5 ft. (4 by 10 m) wide and up to 5/8 inch (15 millimeters) thick, and with stiffeners of various sizes welded on the panels at variable locations.
Using only GMAW, the shipyard could not penetrate the plate thicknesses required without multiple passes and distortion. It had to flip the panel and weld halfway on both sides to achieve full penetration without distortion.
To reduce production time and eliminate the additional cost of equipment needed to rotate the heavy (up to 65-ton) plates for two-sided welding, the process would have to be capable of achieving full-penetration fillet-joint (T-joint) welds and square butt seams in 15-mm-thick plate from one side.
"This is the only way we could process panel sizes up to 4,300 sq. ft. (400 sq. m) and stiffening elements up to 65 ft. (20 m) long," said Meyer's Hermann Lembeck, who is responsible for developing and applying new technologies.
Since 1994 Meyer has been using lasers in its shipbuilding operation to produce I-core panels, which are "sandwiched," or layered, welded-steel components. The benefits the company received from laser welding included small weld shrinkage and good technological properties of the weld seam.
In 1999, it began investigating the development of a totally automatic laser hybrid welding line to prefabricate deck panels and sidewalls. Although energy costs of laser welding were higher than those of GMAW, Lembeck anticipated that the increased welding speed, elimination of secondary operations, and reduced use of filler metal would counterbalance the costs, resulting in lower overall costs.
"We wanted to develop a process that would improve weld seam properties, lengthen service life, increase the capacity for welding variable plate thicknesses, and increase production speed," Lembeck said.
The laser-GMAW hybrid welding process the shipyard developed uses GMAW to fusion weld, or melt the seam edge and filler metal near the surface, and laser welding to perform the deep-penetration welding. In other words, GMAW fills the gap, while the laser reaches the bottom of the part, or the root of the weld.
To evaluate the possibility of expanding its laser usage, the shipyard, working with the Institute for Laser Technology in Aachen, Germany, test-welded 1-m-long panels. In addition, DNV (Det Norske Veritas) monitored destructive testing for tensile strength, stress, and bending results. Once Lembeck and his team thought they had developed a safe and efficient welding process using the company's own testing unit, a 10-m-capacity Schuler Held Lasertechnik PEDILAS system, they began to search for suitable welding line manufacturers.
They selected Schuler Held Lasertechnik for the welding systems, Graebener Maschinentechnik for the milling processes, and Dornieden Anlagentechnik for the material handling systems.
Lembeck did not want to rely on laboratory results obtained from welding 1-m-long panels to predict fabrication of a 65-ft.-long (20-m) panel. So once the welding line manufacturers were selected, the next step was additional testing on a larger scale. Meyer ran intermediate tests in its production bay using its PEDILAS machine for both butt and fillet-joint laser welding on plate 32.5 ft. (10 m) long to study the effects of stress, clamping, and the environment.
Lembeck and his team tested the effects on variable plate heights and different tilting angles using GMAW and laser welding to determine the necessary adjustments to get a precise weld.
With lengthy welds, gaps occur that require special clamping fixtures. Lembeck considered it necessary to study the operating parameters of long welds with increasing gaps. For example, even with heavy clamping equipment, the gap can increase from 0 to 1 mm. To compensate, sensor technology was introduced to control wire feeding. If a gap widens, it requires more filler to be closed. Sensors permit computing the volume of filler needed as a gap varies, and then adjusts the filler feed rate. This helps maintain the weld head velocity and provides additional material in the weld joint.
Based on the results of its research, the company decided to proceed with plans to implement laser-GMAW hybrid welding in its butt and fillet welding operations.
Each of the four multiaxis laser welders of the new production line—two butt welders and two fillet-joint welders—was fitted with a Fronius 450-amp GMAW source and a TRUMPF 12 kilowatt, CO2 laser. Seven computer numerically controlled (CNC) axes were integrated for GMAW and laser welding in the butt joint working head.
By the beginning of 2002, the Meyer shipyard became the first to use an automated production line with laser hybrid welding to produce and fabricate large ship sections, according to the company.
A large butt welder with integrated longitudinal milling machine fabricates and joins the strips into larger panels.
The key elements of this system are the four laser hybrid welding machines, two plasma cutters, and two milling machines.
The process begins as the first plasma cutter trims raw plate to 32.5 by 13 ft. (10 by 4 m). Then the strip moves into the first butt welding machine by a conveyor system. A small laser butt welder with an integrated transversal milling machine fabricates and joins two 32.5- by 13-ft. (10- by 4-m) plates into 65- by 13-ft. (20- by 4-m) strips.
Numerous hydraulic cylinders ensure that the clamping devices hold the plates tightly in place during butt welding.
This larger strip moves by conveyor system to the larger butt-welding machine with integrated longitudinal milling machine that fabricates and joins five of the strips into panels as large as 65 by 65 ft. (20 by 20 m) (see Figures 1 and 2).
These assembled, welded panels continue on the conveyor system to the second, 20-m-wide plasma cutter, which cuts holes in the panel for utility pipework, stairways, doorways, and escalators. A small fillet-joint welder adds stiffeners to sidewalls, and a large fillet-joint welder attaches stiffeners to the large deck section (see Figure 3). At the end of the line, a completed panel emerges, with openings cut out and with stiffeners attached (see Figure 4).
Edge preparation plays an essential role in the production process of tailor-welded plates. Combining plates with different thicknesses and different materials requires appropriately tapered edges in the transition area. A zero gap is the ideal required to ensure a good weld. The milling systems that are integrated in the automated welding line clamp the plate edges to be joined and machine both opposing edges simultaneously.
Fillet-joint welding machines weld the stiffening elements onto panels and walls.
Rather than use a laser to bevel the edges, Lembeck decided to mill the edges, because he determined that it would be less expensive and quicker. Unlike the typical I-bevel used to laser weld a butt weld, the laser hybrid welding bevel is Y-shaped. The opening at the top of the Y is 12 degrees, with the panels very close together at the bottom.
It is largely the precise edge preparation that enables the use of automated laser hybrid welding in shipbuilding, Lembeck says.
Once the large panels are welded and fitted with the stiffeners, a gantry crane transports these large panels to where they are fitted together. Meyer pre-assembles 600-ton-sections before assembling them on the ship.
Every cruise ship is uniquely designed. Even when an order is placed for more than one ship or gas tanker, multiple identical parts are not produced and put on the shelf. No two panels are alike, with differences in material thickness and type, and with variations in the shape and size of the stiffeners. Therefore, ships are produced in a lot size of one.
Panels as large as 65 by 65 ft. can be prefabricated by achieving full penetration with one-sided, fully automated welding.
"That means that all the machines involved in the production process have to calculate a new program from the CAD data for each individual part," Lembeck said.
Even before the first strip is cut, it is already predetermined in which position, and in which panel, that strip will be located at the end of the welding line. Every plate and every stiffener must be prepared on a just-in-time basis to be available and in place right at the time they are needed to be welded in the panel production line.
"Intermediate storage of the panels, not to mention sections, is just not feasible," Lembeck said.
The welding line is linked directly to the corporate CAD system by the engineering personnel who produce the CAM programming. Then individual CNC programs are generated and transferred to the appropriate CNC machine controller.
The fillet-joint welding machine has 27 CNC axes and is controlled by two CNCs—one for the welding, and the other for the material handling equipment. The material handling equipment consists of five units, each with three CNC axes that are used to position the stiffeners.
According to Lembeck, the new technology has made significant productivity improvements at Meyer. As expected, the one-side welding accomplished the company's goal of eliminating the need for the heavy-plate panels, up to 65 sq. ft., to be flipped over. Also, the minimal heat-induced distortion has resulted in flat fabricated panels. No longer wavy and buckled, they do not require flattening or removal of excess filler material, according to Lembeck.
In addition, welding speeds are up to three times faster, compared to the GMAW or sub-arc welding the company was using, he added. Thinner plates up to 14 in. (5 to 6 mm) can be welded at roughly 99 to 118.8 inches per minute (IPM) (2.5 to 3 meters per minute) (MPM). The 15-mm plate can be welded at 47 IPM (1.2 MPM).
Because the laser welding edge angle is small, (approximately 6 degrees compared to 30 to 45 degrees created by GMAW), filler wire usage has been reduced by an estimated 80 percent.
In addition, the accurate edge preparation has improved the quality of the weld seams. The increased fitting accuracy of the fabricated ship sections has resulted in less alignment and adjustment work, which Lembeck said he considers as a significant advantage over the conventional production concepts. He added that the precision of the work performed gives him better control over estimating and planning procedures.
"Welding procedures previously done largely by hand, or at best with certain mechanical aids such as tractors [robots], now are performed automatically. This saves us time and has improved quality," Lembeck said. "In general I would say that we have reformed the entire process of prefabrication."
Ralf Moeller is sales manager with Schuler Held Lasertechnik, Industriestrasse 26, D-63150, Heusenstamm, Germany, 49 6104 9633 0, fax 49 6104 9633 46, Ralf.Moeller@shl.schulergroup.com, www.schulergroup.com. Stan Koczera is sales manager with Schuler Inc., 7145 Commerce Blvd, Canton, MI 48187, 734-207-7222, fax 734-207-7222, firstname.lastname@example.org, www.schulerinc.com.
Photos courtesy of Schuler Held Lasertechnik, Heusenstamm, Germany.