The need to be competitive in the shipbuilding market
has led to new fabrication techniques, process and working
practices. David Millar looks at the positive roll that welding automation
has had in improving production efficiency.
Over recent years shipbuilding has seen an enormous increase in gross tonnage requirement, with Shipyards in Canada, USA, UK, Australia, and India having a major increase in Naval shipbuilding orders.
As a result of this increase in demand the shipyards have had to review their approach to welding to meet the new production targets and costs.
As a means of reviewing the importance of welding automation in shipbuilding, Sumitomo Heavy Industries Oppana Shipyard provided information on the breakdown of the various elements of ship construction and concluded that welding represented around 25 – 28% of all shipbuilding operations.
As such the shipyard’s competitive edge can be greatly improved if they can produce welds faster and with a higher quality than their competitors, this is incredibly significant when 28% of ship manufacturing is involved in the welding of these vessels.
As the global shipbuilding market has increased, the availability of labour to meet these new demands has become problematic in many countries around the World, this situation has generated a need to greatly improve the welding and fabrication processes to accommodate the lack of skilled workers, and the increased economic market demands.
The main theme of these improvements was simple, the welding processes had to be modified or redeveloped and the use of automation in welding must be raised to an all-time high.
In Japan in the 1980’s working groups were created between the shipyards welding technology departments and the welding machine and wire manufactures. The agenda for these meetings was to form partnership agreements that would develop the next generation of welding processes and use welding automation to compensate for the reduced skilled labour availability.
Welding Processes and Welder Training
Before we look at the introduction of welding automation into fabrication areas it is vitally important to start with the introduction of welding processes that will complement, and be suitable for adaption into automation, the first natural step in this journey is the use of semi-automatic welding.
Processes such as Gas Shielded Flux Cored Arc Welding (GSFCAW) and Gas Shielded Metal Cored Arc Welding (GSMCAW) are the first major upgrades to shipyards that have traditionally only used the Shielded Metal Arc process (SMAW), and as in any welding process the key to the success of this change over is in the quality of training that is provided to the operators involved, and GSFCAW and GSMCAW are no different from any other welding operation.
However, for companies that have traditionally only ever used the SMAW process the transition to semi-automatic welding must be handled with great care.
In SMAW the set-up is very robust and simple, a power source (generally with a single step up/down control), a welding cable with welding electrode holder and a return cable.
When using the GSFCAW and GSMCAW process the set-up is slightly more complicated, with levels of complexity varying in the type of machines selected. In these processes the power source will generally require settings in amps, volts and shielding gas pressure, and for the inexperienced and untrained welder any errors in any of these settings will have significant consequences in the finished weld quality.
The key to success for the introduction of GSFCAW and GSMCAW is to have a controlled phase in of the processes into the workplace, and this must include not only the workforce but also within the management and maintenance departments.
Elements for success:
- Keep it simple: Carry out a review of what equipment will be best suited for the work that it will be used for, there are many machines to choose from – if in doubt seek advice before you make your selection.
- Set up some of the machines in a training school, or sectioned off area in the workplace, appoint trainers who have been fully trained in the equipment and use consumables that have been selected, this training will generally be available free of charge from the supplier of the welding wire and equipment.
- Machine familiarisation: It is essential that any welder being trained in these processes fully understands the functions of the machines being used, not just what amps, volts and gas pressure they have to use for a given weld, but also the consequences and results of getting any of these parameters wrong. It is useful to incorporate simulated errors during training i.e. change the shielding gas pressure, change round the wire induction etc and get the welder to fault find and correct. It is also recommended that every welder involved in the process is issued with a gas flow gauge, Fig 1 This simple device allows the welder to set the gas flow accurately at the end of the torch, and for the small price they cost to purchase they can not only ensure that the gas flow conforms to that stated in the welding procedure but will also save the company a huge amount of money in wasted gas.
- Controlled evolution: To ensure a smooth introduction into production it is advisable to introduce any new equipment gradually. Initially select a small area of the shop and set up one or two machines, this will assist in the yard’s acceptance of the new process and will ensure that a control of the welds being produced can be closely monitored.
- Training follow up: After the offline training of the welders is complete (this time scale will be different for every welder dependent on their ability) and they are put into the production environment, it is highly recommended that close follow up is carried out on every welder as soon as possible after leaving the training area. If this is not done it can result in costly repairs if the welder gets it wrong, and this follow up will provide a reassurance for the welder and will strengthen the confidence of the process within production management.
- Unlike the SMAW process the equipment used in GSFCAW and GSMCAW will require more maintenance and calibration. If this is not carried out in a controlled and regular basis it will result in not only a danger to the quality of the welds being produced but will also shorten the life expectancy of the equipment.
- Once the introduction of the GSFCAW and GSMCAW has been established, and as the confidence in these processes is developed, then the fabricator can take the next step in the use of these processes, that is in using these with automation.
Semi-automatic welding provides production with a great tool to improving welding efficiency, however the real rewards are found when the welding torch can be taken out of the welder’s hand and taken to the next stage of welding automation.
To realise the full benefit of automated GSFCAW and GSMCAW management must prepare for the introduction of these new processes, build method scripts, welding preparations etc. must be changed to accommodate these new welding methods.
Welding Automation
The first stage of the welding automation evolution is to remove the welding torch from the welder’s hands wherever possible, an action that will improve efficiency improvements at a stroke.
To maximise welding automation its critical to ensure that the welding preparation being produced is of a high-quality standard, if this is not achieved then the final weld quality will be compromised.
Particularly on submerged arc and Robotic welding the aim should be to provide quality weld preparation consistency, including:
- Preparation angle.
- Root face.
- Root gap.
This can be achieved on the cutting table, where oxy-fuel or plasma cutting can provide consistent preparation finishes, or the use of portable bevelling machines can also be used Fig 2 these provide attractive health and safety benefits as they provide accurate welding preparation, they are portable, silent running (no vibration) and with no heat or fume generated, and one operator can run a number of these machines simultaneously.
Fig 3 shows the use of a very simple low-cost welding solution with the welding torch being placed in a small light weight fillet tractor. The wire for these machines being provided by either small portable wire feeders, or from 200kg wire pay off packs and wire feeders suspended from the roof Fig 4.
It is possible for a welder to operate two or more of these machines simultaneously and with double headed torches welding fillets on both sides can be carried out, with the use of the 200kg wire drums reducing welding downtime to change wire.
These tractors can also be set for stitch welding Fig 5, providing consistent intermittent weld lengths, with burn back control to prevent crater cracks, and running at full speed between welds to maximise process efficiency, and with magnetic bases on the tractors it is also possible to weld in all positions, without the need of track Fig 6.
Single sided welding using ceramics
In the 1980’s the Health and Safety directives in the UK started to be more stringent on the level of noise that was created in the workplace. In those days in shipbuilding the main means of preparing the second side for welding was by using a pneumatic caulking hammer, Fig 7, and the noise levels were extremely high for those in the immediate vicinity. Yards were forced to look at new means of back gouging that were less noisy.
Arcair gouging and grinding were available, however their noise levels were also at a level that was not acceptable coupled with the high level of dust and fume produced – health and safety issues – vibration.
The best solution was to find a way of producing welds that did not require back gouging at all. Work was developed in the use of ceramic backing tiles for producing welds that did not require any back treatment, and from this work the single sided welding process was born.
In essence the weld preparation is produced by leaving a gap (typically 6 mm) and a ceramic tile is placed on the back side, Fig 8. The tile is then secured in place by an adhesive strip and in situations where the steel is dirty or damp this can be supplemented with magnetic backing straps Fig 9.
The ceramic tile then acts as a support for the root weld metal, and the tile also provides mould like qualities which leaves an optimum shape for the finished root of the weld Fig 10 a,b,c. The tiles are only used once and are discarded after use, however even with having to pay the additional cost for the tile, this is far outweighed by the savings in not requiring to back gouge the second side, and then reweld.
Generally, very little root rework is required when welding against a ceramic backing tile, and over the years various shapes of tiles have been designed to cover many different welding applications and preparations.
Improvements in Productivity and Weld quality using the Oscillator Welding Process
It has long been considered that the most productive and cost-effective welding position is the flat position (PA). To satisfy this, great emphasis is put in at the design stage to maximise the use of this position. However, even when adopting this philosophy, welding must be carried out in a variety of other positions.
At a Shipyard in Glasgow, major benefits were found when welding a specific section of the ship hull in the overhead position (PE).
The yard had a block build strategy where large block sections on the ship were welded together on the building berth, Fig 11, On each section there are many webs, girders and longitudinal connections to be made, in addition to the main hull connection. The bottom shell of the hull was traditionally welded from the inside using a single V preparation, 600 inclusive angle, 6mm gap feather edge with flat ceramic tiles on the outside to produce a single sided weld.
This weld requires no back gouging treatment, and for a time was the most effective way of completing the connection. However, there were disadvantages such as:
- Welder access.
- Obstructions from webs, girders, etc.
- Potentially higher defect levels on multi pass runs passing through access gaps (‘Mouse / Rat holes’)
The alternative was to weld the connection from the outside, where a free run with minimum restrictions was available. This required reversing the preparation discussed earlier and welding the bottom shell of the ship in the overhead using a Kat welding oscillator Fig 12.
To achieve this the following was carried out:
- Joint rooted from the inside against a round ceramic tile, Fig 13
Welding carried out from the outside using a Kat weld oscillator and flux cored welding wire in the overhead (PE) position, Fig 14.
As the welding moves from the bottom shell to the side shell it undergoes the transition from the overhead (PE) to the vertical position (PG) position. The continuity of the weld is achieved using a flexible track. Welding is then continued up the side of the ship in the vertical position to the main deck, Fig 15.
When the technique was evaluated against the previous practice, the planned man-hours allocated to welding were deceased by 72%. In addition, the visual quality of the weld on both root and cap was of an extremely high standard Fig 16 (PE weld) Also, radiography results were consistently better than the previous practice.
These benefits could not have been achieved without the following key elements:
- A reliable welding oscillation system.
- High quality all positional FCAW wire
The use of the welding oscillator machines within the yard has been increased Fig 17 This enables the torch to be taken out of the welder’s hands, and therefore is a significant improvement in arc on time (natural fatigue being a major drawback for the welder).
Procedures are now available for almost all general welding positions Fig 18 This includes the horizontal position (PC) Fig 19 which in the past has been particularly problematic, especially when rooting onto ceramic backing tile. A further use of the oscillator carriage and track can be to attach either an oxy fuel or plasma cutting torch for plate preparation, and after welding this set up can be used as an aid to radiographic inspection.
The X-ray head can be mounted onto the carriage and moved into position for shooting as required.
The use of welding oscillators can also be used on pipe welding, Fig 20 a fixed track can be attached to most diameters of pipe, and from there one or more Pipe Kats can be operated to complete the joint.
Again, this system not only speeds up the welding process, but also provides high quality welds, with good surface weld profile appearance.
Panel Line Systems / Robotic Welding
When installing a new panel line submerged arc seam welder (Seamer) if space permits the largest width available should be selected, in one shipyard in Japan a single side high speed sub arc machine was installed Fig 21. A significant feature of this machine was its size (24m wide, including run on / off tabs) It was built to accommodate as large a steel plate as was being produced by the steel mills, this feature helped to reduce the number of welds required to produce unit panels.
The next requirement was to increase the welding speeds, which was achieved by using the submerged arc welding process onto flux backing, using the four-wire method.
This technique allowed the capability to weld plate thicknesses from 10mm to 40mm single sided, with a typical 25mm thick plate being welded at one metre / minute.
The design of the panel line should be created to provide a natural fabrication flow, with each stage having automatic panel transport and handling built into each operation, thereby reducing the labour requirement, and ensuring maximum efficiency.
The next stage on the panel line is the fitting and welding of stiffeners, to increase the welding speed of the stiffening bars a machine was designed that could weld up to five longitudinal’ s simultaneously Fig 22 This incorporated the Large-Scale Welder from Nippon Steel Welding with five welding heads – twenty metal cored wires welding at the same time.
This process utilises metal cored wire, two 1.0mm electrodes in one pool, using Co2 shielding gas, on primed plate. A 5mm leg length can be on deposited on both sides of the bar at 1.4m/min.
The machine was designed to enable welding to be performed in both the girder running direction and the perpendicular lateral carriage direction.
On completion of installation and commissioning trials, the process proved to be extremely reliable, and the single person operation required little training or expertise.
However, it is always vital that all other areas of fabrication are also either automated or upgraded, this includes the fitting of stiffeners in preparation for welding, this is required to prevent bottlenecks occurring due to the increased welding speed used on the bars being welded – Fabrication advantages can only be obtained at the rate of the slowest part of the operation.
There are several simulation packages available which can be used to identify and then rectify the potential bottlenecks. Some of these simulations have shown that there is a short term need to slow down the welding speed until a solution to the bar fitting bottleneck, for example, has been resolved.
Robotics are now taking a much larger place in shipbuilding, and although the use of Robots in this sector is not new, the simplicity of operation and operator ease of use are making these systems extremely attractive.
Historically Robots were to a large extent programmed using CAD drawings, however this demanded the need of capable backend engineering for translating the given design into welding instructions, thereby increasing time and cost for each of the welding operations, this in turn demanded extremely strict tolerances at the assembly stage for the parts to be welded.
In the development stages of Robotics, the Robot was physically taught the welding operation, this process was slow and laborious, and an operator was required to manipulate the Robot head around the area to be welded, and then store the data for use with future welds of this type, in addition as joint configurations changed, the library of programs increased, and even small changes required additional time consuming programming work.
The days of manually programming Robots have thankfully long since passed, and the need of offline programming, transfer of CAD drawings, manual selection of objects, and back-end engineering are also a thing of the past, in the example shown Fig 23 the Inrotech-MicroTwin is delivered as a “plug-and-play” unit and is fully operational once the rails, safety curtain or fencing has been installed and the system has been connected to power, shielding gas and compressed air.
The operation of this type of equipment cannot be any simpler – Place the items to be welded randomly within the workspace footprint and press the start button on the intuitive touch panel Fig 24.
Scanning of the panels can now take place and typically this will be less than 5 minutes to complete, with a scanning speed of around 8m/ min to allow a ready to weld situation.
Once the scan has been completed, the exact position of each stiffener / profile is verified by the laser sensor, which is integrated in a housing that also has the welding torch with fitted on gun fume extraction system.
The welding can now take place without any further input by the operator, this means simply placing the panels in the work area and pushing the start button.
With camera systems also build in it is possible that this welding can be operated remotely, and with the additional inbuilt modem system, (and if it is required) this provides the facility for the Robot manufacturer to carry out diagnostics and fault finding remotely, thus adding to the peace of mind for the operators.
Welding parameters can be input by pre-programmed instructions, with each wire used having its own unique information available for selection, training for the operators of this type of equipment is minimal, and a typical return of investment for this kit would be around 1 to 2 years.
New Robotic systems of course speed up the welding operation and save many hours of welding time, However, it also requires that designers and planners must now prepare fabrication drawings and build method scripts with Robots in mind to ensure the full potential of this production tool is maximised.
Weld Repairs, Weld cap removal
Even with the best, and most modern pieces of welding automation errors can still occur, and in such circumstances, automation can still be an important factor of getting repairs carried out efficiently.
In Fig 25 the Kat Arcair-Matic Gouging system can provide an accurate gouge depth and width on welds that have to be repaired, jointed gouging carbons from 7.9mm to 19.1mm can be used to provide a continuous gouging operation, This process provides gouging speeds of up to five times faster than manual gouging, and ten times less grinding is required when complete, It also produces a far higher quality standard than if this operation had been carried out manually Fig 26.
And as a bonus of this controlled uniform depth and width of gouge, re-welding time will also be less than that of manual gouging and will require far less welding time and welding consumables to complete the finished repair weld.
Another area that is growing in popularity in shipbuilding is the removal of weld caps after welding, as an example of this for a ship to improve its fuel efficiency there is a trend to reduce the hydrodynamic drag of the ship through the water in way of the bow. To assist in this the welds around the hull can have these caps smoothed down to leave no discontinuities between the weld and shell plating, and this should also help in maintaining the paint integrity in these areas.
If this operation were to be carried out in the traditional manner for removing weld caps, where grinding or linishing machines were used, this would be very time consuming and labour intensive, and have serious health and safety issues with noise and a large amount of dust and fume, there is also the other issue of lack of uniformity on the finish when grinding or linishing is carried out.
To speed up this task there is now an automatic weld shaver Fig 27 that can be used, this is a piece of equipment that can be attached to the hull area by track, and this can remove excess welds without any noise, dust, or heat.
Weld caps can be removed at speeds of up to 37 metres per hour Fig 28a & b.
The Future of Shipbuilding Fabrication – Welding Cobots.
Cobot – Definition
‘’Cobot, or collaborative robot, is a robot intended for direct Human Robot interaction within a shared space, or where humans and robots are in proximity. Cobot applications contrast with traditional industrial robot applications in which robots are isolated from human contact. Cobot safety may rely on lightweight construction materials, rounded edges, and inherent limitation of speed and force, or on sensors and software that ensures safe behaviour’’.
With the need of shipyards to look at being able to weld faster, with a smaller less skilled workforce there is now a requirement to have available a flexible automatic welding machine that can work alongside a workforce safely, and can also provide a means of welding remotely, where in some applications it would be unsafe to have welders present (i.e., where there is danger due to a toxic working environment, and or in tight workspaces)
As an example, Robotic Technologies of Tennessee (RTT) have developed a High-Mobility Manufacturing Robot (HMMR) Fig 29 for targeted weld applications for industries including shipbuilding, oil and gas and energy production. The HMMR consists of a six degree of freedom lightweight welding arm on a mobile climbing platform. The system provides a man-portable, fully mobile robot platform that can be quickly transported and deployed to perform a large variety of welding tasks or other manufacturing tasks. The HMMR can operate in the flat, vertical, or horizontal position Fig 30 and can transition from flat to vertical or horizontal to horizontal planes.
The HMMR has onboard lidar for real-time mapping and built-in obstacle avoidance allowing an operator to focus on manufacturing tasks, and can be operated remotely using a touch panel, where all welding tasks can be controlled Fig 31.
In a project supported by the National Shipbuilding Research Program (NSRP) in the USA, RTT have developed and demonstrated fillet welding tasks in the 2F and 3F positions to weld out compartments and a variety of stiffeners. The weld requirements were identified, validated, and tested by the Edison Welding Institute.
It is clear that as the world market for shipbuilding tonnage continues to rise, then the need to provide new faster and more efficient welding processes need to continue to be developed to match these efficiency expectations.
Literature:
Boekholt, R.: Welding mechanisation and automation in shipbuilding worldwide, Abington Publication, Cambridge/UK, 1996.
Millar, D. W.: Improvements in productivity and weld quality using the oscillator welding process. Welding and Metal Fabrication, February 1995.
Millar, D. W.: Welding automation in Japanese shipbuilding. Welding and Metal Fabrication, March 2000.
Bøgner, T.: Inrotech-MicroTwin Presentation.
Canfield, S.: The Rise of Robots – Technology and trends in Cobots (Collaborative Robots)
Eur Ing David W Millar CEng, MPhil, CEWE, CIWE, FWeldI.
Worldwide Business Development Manager
Gullco International.
Email: david@gullco.com
