There is a tremendous shortage of skilled welders, leading to delays in projects and deterioration in the quality of welding. All over the world, for decades, shipyards have been using the variable speed, all position carriages for mechanisation of welding and cutting for improving productivity and quality at their yards.

Till 2004, there was almost no automation in welding and cutting in shipyards in India. The yards were labour intensive, and the man hours per ton were high.

The only major improvement in shipyards was the increasing usage of CO2/MIG welding machines which slowly started replacing the tedious MMAW (manual metal arc process), hence, marginally improving productivity with the same manpower. SAW (submerged arc) process also saw a growth in areas such as panel butt welding, and butt welding on ship decks. Cochin Shipyard Ltd was the only shipyard in India to have an automatic welding line for one side butt welding of plates. This process was not only very fast, but also saved cumbersome mechanical handling i.e. turning the plates over for welding on the other side.

Garden Reach Shipbuilders & Engineers Ltd (GRSE) Kolkata was the first shipyard in India which seriously started looking at higher productivity through welding and cutting automation. They identified two types of variable speed, all position, welding and cutting carriages for their hull shop, and for joining the blocks. Goa Shipyard Ltd (GSL), Goa was soon to follow with identical selection of equipment.

Seeing the improvement in welding productivity and quality after the induction of welding and cutting automation in these two shipyards, other shipyards followed. They were: HSL, GML, Tebma Shipyards Ltd. L&T, CSL, ABG, Pipavav Shipyard Ltd, and many more.

The basic objective of the paper is to highlight the use of low cost welding and cutting systems for increasing "arc time", reducing man-hours, and, at the same time, improving quality in welding and cutting operations. Arc time is one of the management tools for measuring the shipyard's output in terms of efficiency.

With the deployment of welding and cutting equipment, there is also a significant reduction in "reworking" time, saving in consumables, reduction in skilled labour, lesser crane handling operations etc.

Welding and cutting automation in shipyards, all over the world, normally, use the following equipment.

Automatic Cutting

CNC Machines for High Speed Cutting

For fast and high quality accurate and high speed cutting of steel with complex shapes, the CNC cutting processes used are oxy-acetylene, plasma, laser, water jet, etc.

High Speed Portable Mechanical Edge Bevelling Machines

High speed portable mechanical edge bevellers are common sights in most shipyards. These high speed and accurate bevellers, due to their portability, are extremely handy as they can be moved to the job and save valuable handling time, which is very expensive, and also, very often, a bottleneck. These high speed mechanical bevellers can accurately bevel carbon steel, aluminium, stainless steel and titanium (limited use) up to 50 mm thickness, and at speeds of 2.4 metres/minute to 3 metres/minute. Bevelling with the mechanical beveller, being a non-thermal process, reduces distortion.

Automatic Welding

Automatic Panel Welding Lines

These lines consist of multi-robots on a gantry which weld the fillet welds on the long seams of the stiffeners to the panel, cross members, and the and the vertical welds on the cross members. The welding process used is the MIG process.

These are highly productive panel lines, which can automatically weld stiffeners or bulbs onto the panels at speeds of over 2 metres per minute. The welds are of high quality, dimensionally perfect, with controlled heat input, and the panels are completed with minimum distortion. These highly automatic panel lines save time for fitting up, tacking, crane handling etc.,

considering these operations take up 80% of the time as compared to welding, the equipment is very effective and saves gangs of fitters and welders.

Automatic "One Side" Butt Welding Panel Lines

These lines consist of gantries with multi-submerged arc welding heads. The plates to be butt welded are placed on a bed consisting of magnets on which grooved copper backing strips are placed. The magnets keep the plates to be butt welded (on the bed) clamped onto the grooved copper backing strips on the bed during the welding operations. The submerged arc welding heads weld the butt welds on the plates at high speeds from one side of the plate. The seams to be butt welded are placed along the length of the grooved copper backing strips. The butt welds are welded from one side by the submerged arc welding heads suspended from the variable speed gantry (which provides the variable welding speed of the welding heads) by using high currents. The copper backing strips on the underside of the weld seam, prevent the weld from cutting through, and also give the weld beads a convex shape due to the groove on the copper backing strip.

Excellent x-ray quality welds are produced at very high speeds, with minimum heat input, as the heat is dissipated very fast due to the continuous high speed of welding.

These highly automated Panel Lines save fit- up time, crane handling etc. for butt welding operations.

Laser welding is getting popular, and replacing the SAW welding process due to the high speeds, narrow and deep penetration with narrow heat affected zones. This process is a boon for heat sensitive steel used in building submarines.

For using Automatic Panel Lines, smooth flow lines for moving the welded panels have to be meticulously planned for ensuring quick loading and unloading of the panel beds. One has to ensure high usage of "arc time' or else the equipment lose its effective productivity.

Robots

Only the advanced shipyards have inducted robots for welding inside the blocks, and during the block assembly stage. At present, robots in shipyards are only in the automatic panel lines. They can be seen at Pipavav Shipyard Ltd.

Electrogas & Electroslag Welding

These are "one side" vertical 3G welding processes, which eliminate back chipping/grinding/gouging. Welding is carried out from one side of the weld seam, forming excellent, x-ray quality welds. These welds can be carried out at high speeds with heavy weld deposits in one pass.

Electrogas uses the MIG process, and the Electroslag the submerged are process. Multiple wires are used on heavy plates.

In India, Cochin Shipyard had used the Electrogas process some years back. For some reason, it is not being used now a day

The Electrogas and Electroslag processes, though efficient, have limitations. They cannot be used on curved profiles, and also cannot be used on heat sensitive steels due to the high currents required for these processes.

Automatic Carbon Arc Gouging Process

Carbon Arc Gouging has been used in India for over four decades. Unfortunately, the quality of the gouging depends on the welder's skill. Secondly, the arc being very intense makes it difficult for the welder to operate it in long cycles, especially in the hot and sultry conditions in India.

Variable Speed All Position Carriages, coupled with electronic, through the arc controls have added a new dimensions to the Carbon Arc Gouging

Microprocessor-based, Variable Speed, all-Position Carriages.

These variable speed, all-position carriages are widely used by most of the shipyards, all over the world, for mechanisation of welding and cutting to increase productivity and quality. The paper highlights the effectiveness of these carriages in shipbuilding.

Seam Tracking & Laser Vision Systems

For butt welding, and full penetration fillet welding, seam tracking plays a very important role, specially, in high speed applications and robotics. The robots are programmed and "taught" to follow a weld seam. In case of distortion the robot cannot correct itself unless it is guided by a laser seam tracker, which goes ahead of the arc, follows the weld seam, and sends signals to the X-Y slides to automatically correct the path of the welding head. In high speed operations the laser vision systems are a mandatory requirement.

Ceramic Backing Strips

Radiographic butt welding requires back chipping/grinding/gouging on the reverse side to the root of the weld, and again re-welding. This is a very slow process and creates bottlenecks, not to mention extra mechanical handling of weldments, and very often large fabrications (as they have to be turned over).

These ceramic backing strips are sintered alumina ceramic tiles pasted onto aluminium foils, which adhere to the reverse side of the weld seam.

These ceramic tiles are grooved, and allow the molten welding pool to form a bead on the reverse side of the weld seam, hence creating a homogenous weld.

Objective of the Article

In this article, focus is on the programmable, variable speed, all position welding and cutting carriages, high speed mechanical bevellers, automatic carbon arc gouging systems and ceramic backing strips.

a) Trackless Friction Drive, Programmable, Variable Speed Fillet Welding Carriage - Single Or Dual Torch

The variable speed trackless Fillet Welding Carriages are very simple to operate electronic or microprocessor based carriages with tacho feedback controls. These carriages are guided by adjustable idler guide rollers, which track the vertical member of the fillet, or lap, to be welded. One or two torches can simultaneously be used for welding. Only one operator is required for controlling two torches.

X-Y slides are provided along with a torch tilt mechanism for accurate positioning of the torch before welding, and for minor adjustments during the welding process.

The Fillet Welding Carriage can be retrofitted with an oscillator accessory. The controls are designed for continuous welds, and also, for "stitch welding".

Multiple carriages, each, fitted with twin torches can be automated with a PLC unit, and controlled by one operator. Up to 16 arcs are known to be controlled by a single control. The 16 arcs are capable of depositing a cumulative weld metal of over 80 kg of weld metal per hour.

The portable fillet welder carriages give accurate leg lengths, and the quality of the welds is excellent. This is important, as "over welding" is expensive, i.e., instead of a 5 mm leg length, if 6 mm is welded; the extra weld metal used is in excess of 78%. "Over welding" not only increases the HAZ and causes distortion, etc., but also slows down the welding speed, hence defeating the purpose of automation.

The above is an example of basic simple automation, which can increase arc-time and weld deposition tremendously. A single operator can control a dual torch carriage easily, and deposit over 11 kg/hour of weld metal. The skill of the welder has been transferred to the equipment. The equipment reduces operator fatigue, and, at the same time, the equipment can be used continuously at high arc-times.

Typical applications: Welding of stiffeners and bulbs to panels, fillet and lap welds on the deck, etc.

An example of the productivity of the Dual Torch trackless, friction drive variable speed carriage is can be seen below:

Taking productivity for a 5 mm fillet weld in an 8-hour shift as an example

Manual welder welds approximately = 40 metres

MIG welder welds approximately = 60-65 metres

The dual torch carriage (one operator) approx: = 220 metres @ 50% arc time. = 308 metres @ 70% arc time.

1b) Magnetic Base, Trackless Friction Drive, Programmable, Variable Speed Fillet Welding Carriage

(Refer to photo 1f) The principle of working of the magnetic base carriage is identical to 1a except that it has a magnetic base facilitating the track to be used for all position welding and cutting operations. It is a very useful carriage for certain "out of position" applications, but has the disadvantage of picking up electrode stubs, steel filings etc, onto its magnetic base, which requires periodic cleaning.

Benefits

• Light, portable and user friendly. No skilled operators required
• Minimum set-up time required
• Faster weld deposition, and high arc times up to 80%
• Comfort to the welder, as he is away from the fumes and the heat of the arc
• One operator can operate dual torches
• High quality, consistent and accurate fillet welds, leading to savings in consumables
• The parameters can be preset before welding, hence, leaving no room for "over welding"
• Minimum heat input and controlled HAZ
• Lesser distortion, faster heat dissipation, and lesser stops and starts, hence reduction in stresses
• No "stays" required hence elimination of tacking, grinding the tacks etc, which is a very expensive operation due to time consumption in the "dressing up" after the removal of the "stays"
• Higher output with lower manpower, hence a direct contribution to lower man hours per tonne
• One operator can control two Moggys, or more, (as long as they are near each other)
• Several Moggys can be connected with one another and be controlled through a single PLC, hence
• increasing the number of arcs per operator
• The Moggy can be used for fillet, butt and lap welding in the 1G and 2G positions
• The Magnetic Moggys have the above features, with the exception, that they cannot be used with two torches, and
• The added feature of the Magnetic Moggy is that it can be used for "out of position" welding.

2). Programmable Microprocessor-based all-Position Variable Speed Carriages

These positive drives, programmable microprocessor based, all position variable speed, carriages run on rigid aluminium or flexible (spring steel) tracks with magnets, which are mounted directly onto the weldment. The flexible tracks lend the added advantage of flexibility. They can be bent to follow the contour of the weldment, making it easy for the variable speed carriage to follow the profile of the weld seam.

The carriages have sophisticated microprocessor controls; with tacho feedback loops, for accurate speed control, in all welding positions, even while carrying the load of a wire feeder up to 45 kg. The preset speed remains constant at any position of the carriage.

Several variations in programmes are available in order to add flexibility to the automation process. Some carriages have programmes for "stitch welding", sequence of operation, i.e., "full speed start", "park", "arc start" (after a small time lag), "weld", "carriage stop" etc. A typical programme can automate a repetitive production process.

Similarly, a carriage can be fitted with an oscillator by means of simple bracketry, and micro-fine rack arms and rack boxes. Controls are provided for presetting the width of the weave, frequency of oscillation, side and centre dwells, carriage speed, and wire feed delay start. All the above variables can be preset prior to the start of the weld cycle. Once the parameters are set, the operator has to simply press the start button, and the arc will start (after stabilizing for a few seconds) with the oscillation, and carriage movement. Two types of oscillators are available, viz., Linear and Radial to provide.

All the parameters can be preset as per the WPS, before starting the welding process. No skilled welder is required for operating the carriage. The operator would be required to ensure accurate seam following, correct arc length, angle of the torch etc., by means of simple X-Y slides, i.e., rack arms and rack boxes. The equipment automatically controls the rest of the preset parameters. 

The oscillator is used in automatic mechanized operations for minimizing defects, such as, poor penetration, incomplete fusion, and undercut. These carriages can be upgraded by retrofitting electronic seam trackers and laser seam trackers for following the welding seams automatically. The carriages can also be upgraded for cladding operations with the indexer unit, which can also be retrofitted. All the carriages have output ports for receiving commands from PLCs for automation applications. Typical applications: All position butt welding of the blocks, horizontal welds on the hull, out of position welds on the stern, butt welding on the decks, etc., and all types of fillet and lap welds, i.e. fillet welding of the long welds of the Bilge Fender etc. to the hull. The applications are numerous. 

They can also be used for gas cutting. One very important application is "in situ" cutting and bevelling the "Green" on the blocks. The flexible tracks can be fixed parallel to the edge of the block so that a perfect and accurate cut can be made, followed by a bevel. This reduces the amount of grinding required when the gas cutting is carried out manually. The joints on the blocks are consistent, which helps in welding automation, and also reduces extra weld deposition and distortion.

Typical Comparison Between MMAW and MIG Welding, with and without Oscillation

Vertical Welding (3G) - Without Automation Weld Deposition Rates:

MMAW - Average 60 electrodes per 8 hours shift = 2 kg/8 hr shift

MIG - 1.2 dia FCAW wire deposition rate at 180-200 amps = 2.8 kg/hr

1.2 dia FCAW deposition (at 23% - arc-time) = 5.15 kg/shift

Average arc time for MMAW in the country is 18-19%

Average arc time for manual MIG/FCAW in the country is 23 % (without automation)

Vertical Welding (3G) - With Automation Weld Deposition Rates:

MMAW welder weld deposition per shift @ 18% arc-time = 2 kg/8 hr shift

MIG welder weld deposition per shift @ 23% arc-time = 5.15 kg/8 hr shift

MIG automatic welder's weld deposition/shift @ 50%-60% arc-time with oscillation = 11.2-13.5 kg/8 hr shift

One can clearly see the increase in weld deposition rates by simple automation. Arc times over 70% can be easily achieved in long welding joints. (Though the example about cites arc times of 50-60%).

Now, let us examine typical deposition rates, using MMAW, MIG & MIG automatic process, without oscillation.

Butt Welding (1G) Position - Without Automation Weld Deposition Rates

MMAW 5 mm electrodes, average weld deposition, 8 hr/shift = 3.2 kg /8 hr shift MIG - 1.6 mm dia FCAW weld deposition @ 300 amps = 5.2 kg/hr MIG - 1.6 mm dia FCAW weld deposition, 8 hr/shift (23% arc-time) = 9.6 kg/8 hr shift Butt Welding (1G) Position - With Automation 1G (Oscillation) Weld Deposition Rates MIG-1.6 mm dia, FCAW, 8 hr/shift (50-60% arc-time) = 21-25 kg/shift From the above, it is evident that the weld deposition rates increase considerably by the deployment of a simple automation process. One can imagine the savings in manpower, equipment, power etc, and the increase in arc-time by using several of these carriages in a fabrication shop or at a construction site.

From shipbuilding to heavy wall oil storage tanks, hortonspheres, penstocks, girders, etc., all are being welded at construction sites all over the world using the "all position", programmable, variable speed carriages and oscillator combination units running on flexible tracks. These carriages, with their high weld deposition rates, and high arc times are very productive in "out of position" welding applications in fabrication shops and at construction sites. To cite an example of weld deposition with oscillation in "out of position" welds; A 30 mm, single 'V' butt joint in the 3G position, with ceramic backing, requires only 11 runs to complete the weld!! As mentioned earlier these can comfortably be used at 70%-80% arc time on long weld seams.

Benefits

• Programmable, hence can be preset as per the WPS
• Highly productive
• Very simple to use, and operator friendly
• Removes the need of a skilled welder
• Consistent x-ray quality welds with controlled heat input and reduced HAZ
• Due to the higher speeds in welding and quicker heat dissipation, distortion is lower
• Can be used for butt, fillet and welding in "all positions"
• Can be used with multiple torches (with only one operator)
• Same equipment can be used for "all position" cutting with oxy acetylene, plasma waterjet, etc., and cutting processes
• The equipment can tow a wire feeder up to 45 kg in any position, with no change in the preset carriage speed, and
• For large weldments, 3-4 carriages can be used in tandem, to speed up production.

3). High Speed, Portable, Mechanical Edge Bevelling Machines

One cannot omit the importance of the high-speed mechanical, portable bevelling machines. These portable bevellers can bevel plates at speeds of 3 metres/minute and produce excellent and accurate bevels. The plates can be bevelled, over a range of 6 mm to 58 mm thickness and the maximum bevel that can be cut is 28mm. A range of bevellers is available for bevelling different thicknesses of carbon steel, aluminium, stainless steel, and titanium.

Important Features

• Can be used for removal of the heat-affected zone from plasma cut edges
• Mechanical bevelling, being a non-thermal process, has the important metallurgical advantage of not inducing a heat affected zone, nor causing thermal distortion
• Noiseless and vibration-free process
• The equipment, being portable, can be easily wheeled to the job
• No real skill required for operation
• The self propelled machine follows the edge of the plate while bevelling
• The spring loaded base allows the machine to run smoothly on an undulating floor
• It is provided with a graduated scale for adjustments for setting the root face, and
• No further grinding required for welding.

Bevellers are also available for upside down bevels. For large-scale production, one beveller can bevel the topside of the plate, while a second one follows, bevelling the underside of the plate. This way a double "V" can be achieved without turning the plate over.

These bevellers are used for bevelling plates for the hull and other areas, in order to save the CNC machines from being overloaded with these simple jobs. Secondly, the equipment being portable can be easily wheeled to the job, hence saving expensive handling costs. These bevellers are also used during the erection stage of the blocks.

4). Automatic Carbon Gouging Carriage

In the Automatic Gouging System, high resolution voltage capability regulates accurate gouging depth to "machine like" tolerance of +/- 0.015 (0.4mm) even on a warped plate.

The controls of the automatic gouging equipment are electronic, and are interfaced with the variable speed all position, carriage to ensure total automation. Once the start button on the carriage is initiated, the arc starts, and, after a few seconds, the carriage travels at the preset speed. Controls are provided for inching the rod up and down.

For continuous gouging of long seams the lightweight gouging system carriage (22kgs) can run on a track in the flat, horizontal and vertical position.

The jointed copper coated carbon rods have male and female type connections for facilitating easy in joining of the carbons during the gouging process. Excellent gouged profiles can be achieved at very high speeds. No need of grinding after welding, as it is not an oxidation process.

The gouging systems work on constant current power sources from 600 amps to 1200 amps capacity, with carbon gouging electrodes ranging from 10mm to 16mm diameter.

5). Ceramic Backing Strips

With the variable speed carriages and oscillators capable of depositing at high arc-times and high deposition rates, the process of back chipping the root of a weld, and re-welding goes when used with ceramic backing strips, have a distinct advantage in productivity.

For excellent welds and good radiography results, the inherent characteristics of the ceramic backing strip, such as; halogen-free, non-hygroscopic (resistance to moisture pick-up), neutral fibrous materials, granular density, the refractory nature of the material to withstand high welding heat without distorting as a heat absorbing metal bar, the quality of the adhesive tape to resist preheat temperatures, and a host of other positive characteristics to increase productively, has made this product indispensable in shipbuilding.

There are manufacturers who offer ceramic backing strips at throw away prices. One has to be cautioned regarding the dangers of hydrogen pick-up and halogen transfer. There are only a handful of reputable quality conscious manufacturers (of ceramic backing strips) having ISO certification, and approvals such as Lloyd, DNV and Bureau Veritas, etc.

These non-metallic weld backings are available in several segmented shapes and sizes, to suit almost any application requirements. The ceramic tiles are normally assembled in 2 ft strips, which are held in place by a heat resistant adhesive coated aluminium tape.

Single 'V' preparations can be carried out from one side only (with ceramic backing on the back-side of the weld) without the need of back gouging/chipping from the reverse side. This saves the expensive requirement of turning heavy and awkward weldments for back-gouging/chipping for welding.

It also saves the need of vertical, horizontal or overhead welding from the reverse side after back chipping, as in the conventional methods. Distortion and the HAZ are controlled with the use of ceramic strips. By the elimination of back-gouging/chipping, expensive, consumables are saved. There is no need to remove conventional backing bars, or chip/grind rough seams. Excellent, 100% x-ray quality root runs with clean and consistent beads are achieved. In today's age of power cuts, the focus is on increasing arc-time; and this is one solution.

Conclusions

One can weigh the advantages of modular welding systems for automation. Can one be competitive in today's world, using manual welding, relying on the skill of the welder, for welding long welds in ship construction, oil storage tanks, hortonshperes, heavy girders, etc? Extracts from a paper "Improvements in productivity and weld quality using the oscillator welding process", authored by Mr D W Millar, Welding Engineering Manager, Kvaerner Govan Ltd, Glasgow, Scotland, is reproduced below:

"When the technique was evaluated against the previous practice, the planned man hours allocated to welding were decreased by 72%. In addition, the visual quality of the weld on both root and cap was of a very high standard. Also, radiography results are consistently better than the previous practice. These benefits could not have been achieved without the following key elements: a reliable oscillation system and a high integrity flux cored wire. To achieve a decrease of 72% in man-hours is no mean task. Dave Millar, with the deployment of automatic welding techniques, made it possible."

The total focus is on increasing arc-time and maintaining high weld deposition rates. All this cannot be achieved w