Technologies for the Shipbuilding/Offshore Industries - Power & System Performance Are What's Called For

By MarEx 2012-04-23 11:32:05

System solutions weld more economically

In shipbuilding, as in other metalworking industries, weld processes have to combine top-quality results with high cost efficiency. What users find most helpful is system solutions that are specifically geared to the needs of the shipbuilding and offshore plant construction sector. This calls for system performance, meaning the interplay of functionality, efficiency and reliability. Although it would go beyond the scope of this article to look in detail at all possible processes, versions and aspects, it does describe a number of significant solutions by way of example.

Digital welding processes have brought about great improvements in economical processing of metals, and in the technical quality of the joins made between them. The applicational range of gas metal arc (GMA) welding systems covers everything from steel and its alloys to aluminium and other metals.

Being in contact with seawater, ships' hulls are mainly made from Grade A steel. For less aggressive environments, Grade B, D, and E steels are also commonly used. High-strength steels such as A 32, E36 or E 40 are typical in welded constructions. In fact, the grades of steel used are as diverse as the many different requirements they address: high-temperature steels are suitable for steam and pressure boilers; heat-treated fine-grained structural steels and nickel-alloy steels meet the requirements for high toughness at low temperatures; and austenitic steels are the material of choice for cargo tanks. The plate thicknesses range between 4 mm and 40 mm.

A ship's steel pipework may have wall thicknesses of as much as 25 mm. These pipes, too, are usually made of high-strength steel; unalloyed grades are sufficient for normal thermal loading, while enhanced thermal loads call for alloyed grades. In the same way as for the hull, the ambient conditions influence the choice of materials in this case as well: high-strength alloyed steels with manganese, molybdenum and/or chromium for high toughness, and nickel-alloy or austenitic steels for low temperatures. Piping made of high-alloy steels can be found on cargo tanks and pressure boilers. Slide-bearing components on the rudder are also made of high-alloy steel, as are the wetted parts of chemical tankers and all ship components needing to withstand corrosive attack. The most frequently welded non-ferrous metals are aluminium and its alloys, in both cast, rolled and drawn forms. These are used for e.g. the hulls of yachts and speedboats, and in air-inlet and exhaust systems ('funnels'), ship superstructures or pipework. Other non-ferrous materials include Invar (36% Ni), copper and its alloys, and nickel alloys. These materials are particularly in demand in the construction of liquefied-gas (LNG/LPG) carriers, and pressure lines for capacitors and heat exchangers.

A typical requirements profile in shipbuilding is for long seams, a high welding speed of e.g. 150 cm/min with an 'a'-dimension of 3.5 mm, and high cost-efficiency. The technology of the TimeTwin Digital tandem process from Fronius is putting this all into practice in the panel line at P+S Werften's yard in Wolgast, Germany. The basic idea here is “two wires, twice the welding speed”.

Various considerations

Steel, then, is the predominant material. As such, it calls for a variety of special solutions, as necessitated by different technical and economic aspects and situation- or user-specific requirements. Although the fabrication of ship panels has some of the features of series production, the assembly operations - especially those inside the hull structure - are still entirely manual in nature. The ambient conditions are also very different in each case: Unlike in enclosed shipbuilding hangars, when working outdoors there are great variations in wind, moisture and temperature. The interaction of these influencing factors with aggressive seawater is what causes the particular set of conditions that typify the maritime environment and that severely test the endurance of both man and machine.

Steel Transfer Technology and TIG welding systems

Most of the steel joins made in shipbuilding are welded by manual welders. They need high-performing 'all-rounder' systems whose components are designed for complete interoperability. The VarioStar, VarioSynergic, TransSynergic and TransPulsSynergic MIG/MAG systems from Fronius incorporate several decades' worth of empirical knowledge. A noteworthy innovation here is the digital, microprocessor-controlled inverter power source TransSteel. Its developers have fitted it out with a package of characteristics, known as 'Steel Transfer Technology', that is specifically tailored to the needs of steel welding. The main benefit for users in their everyday work performing metal-active gas (MAG) welding is that it lets them concentrate on the essential aspects of their workflows. These machines' intuitive operator functions, innovative wirefeed, ergonomically shaped torch and ruggedly designed housing are all deliberately focused on steel welding. The TransSteel systems stand apart for the stable arc that they deliver regardless of which characteristic has been selected. Depending on the task to be tackled, the user either chooses one of the three characteristics for flux-cored wire - rutile, basic or metal powder - or for solid wire. Specifically for the conditions commonly found in the shipbuilding industry, Fronius has developed the following additional characteristics: 'Steel Prime' is designed to facilitate welding over primer coatings, 'Steel Root' is for good gap-bridging ability and roots, while 'Steel Dynamic' is ideal for deep penetration and small included angles. A powerful filter protects sensitive vital components, enhancing system availability in dust-laden environments.

Specially designed for shipyard use, the Yard Edition of the TransSteel comes with welding programs which are all tailored to highly productive joining with standard to high-alloy steels, with the usual filler metals and types of gas, flux-cored wires and electrodes.

A very helpful innovation here is the integral media guidance, from the power source via the VR 5000 Yard wirefeeder all the way through to the easy-change torch. The feeder unit has a 'sleigh' dragging base on one side and a built-in gas-flow regulator; it is detachable, and designed for mobile deployment. These features provide maximum manoeuvrability even in changing, hard-to-access locations. The welder can switch over to MMA operation at the push of a button.

Solutions for aluminium with TransPuls Synergic

A task that is specific to the welding of aluminium is to prevent an oxide skin from forming during the fusion process. This layer would hinder and even stop the entire arc process. Among the other challenges posed by aluminium are that it has nearly twice the thermal elongation of steel, and thermal conductivity that is three to four times as high.

From funnel installations for cruise liners all the way to entire yachts made of aluminium, high-grade weld-seams are needed throughout. When it comes to careful joining of aluminium materials, the MIG arc-welding systems of the TransPuls Synergic series, with their special programs, are ideally suited. Together with these machines, Fronius provides their users in the fields of shipbuilding and offshore platform construction with characteristics for the most commonly used alloys and filler metals.

The MagicWave series

For many applications in the shipbuilding, yachtbuilding or boatmaking fields, tungsten inert-gas (TIG) welding is often the ideal method for joining aluminium. Equipped with ActiveWave technology, the all-digitised systems of the MagicWave series provide a stable arc with low noise emissions, all while being 'lightweights' in their class.

High-performance welding systems: TimeTwin

Tandem welding is a high-performance process in which two wire electrodes melt simultaneously into a single weld-pool under a shielding-gas atmosphere. This basic idea, of significantly boosting productivity and efficiency by using two wires instead of one, is put into practice very successfully by the TimeTwin Digital welding system. Welding with two wires, one following the other, has the further qualitative benefit that the second arc improves dilution in the fluid pool, greatly reducing fusion defects and porosity. On small fillet welds of dimension a = 3 mm to 4 mm, it permits a doubling of welding speeds in the horizontal-vertical welding position. Another advantage will be found useful during multi-pass welds: At the end of the seam, when the torch path reverses direction, the TimeTwin Digital control system automatically switches over the leading and trailing wire electrodes. The perfect start-up phase and optimum crater-filling both help to shorten the cycle times.

In the TimeTwin process, each power source has its own control and adjustment unit, and a separate wirefeeder. The welding system is made up of two separate TransPuls Synergic GMA systems that are coupled together. The high welding speeds keep the thermal input relatively low, minimising distortion and reducing the amount of post-weld machining needed. Deposition rates of up to 30 kg/h are possible.

Repair-welds on propellers are a regular part of shipbuilders' day-to-day business. Manual welding predominates here, particularly when smaller areas need repair.

Laser beam combined with GMA arc

The laser-hybrid process is suitable for joining both steel and aluminium. It is ideal for long seams where great welding depth and extremely solid joins are required. In these cases, this combination of a digital GMA process and a laser beam puts up a convincing performance. The welding speed of the Fronius-developed LaserHybrid process is two to three times higher than in GMA welding alone. The laser beam delivers concentrated - i.e. locationally tightly restricted - thermal input, great weld penetration depth and high speed. The GMA process which follows the laser provides good gap-bridging ability and simple weld-seam preparation. The relatively high power requirements that are typical of lasers are limited to the deep-weld effect, which supports the joining of thick steel plates. This means that the investment volume needed for the expensive laser system is smaller than that for an all-laser welding installation. Both processes concentrate their energy on the same process zone and thus greatly increase the welding depth and speed as compared to either of the processes used on its own. Thanks to its lower energy input, the hybrid process minimises weldment distortion and causes far less spatter. This provides marked benefits in panel production, for example. The optimised characteristics, and modified hybrid welding heads with up to 10 kW of laser power, make work simpler and easier for the welding practitioner.

Thermal overlaying: conditioning and cladding

A typical process in the industry, overlay welding is used either for upgrading heavily stressed surfaces or for repairing damaged areas. In both cases, the overlaid material enters into a metallurgically intimate intermixture with the base metal. In the first application, also known as 'cladding', a higher-grade layer is welded onto a lower-grade base metal. The second application, known as 'conditioning' and used mainly in repair work, involves like-on-like overlaying.

Cladding high-alloy (and thus more expensive) steel onto less expensive low-alloy steel saves on both materials and expense. As well as assuring a protective function in aggressive environments such as salt water, cladding is also used on sealing faces and slide-bearing surfaces. A typical application is the overlaying of weld filler metal (S-CU 6100 and S-CU 6327 to DIN EN 14640) onto the copper alloys of ship propellers.

With conventional thermal overlay processes, the problem has always been one of controlling and compensating for the distortion resulting from one-sided warming. Compared to conventional GMA processes, an innovative, 'cooler' process has been proving beneficial to users: CMT (Cold Metal Transfer) from Fronius. The greatly reduced heat input in this process leads, firstly, to less distortion yet sufficient penetration with the same deposition rate, and secondly, to significant resource savings.

CMT has also proven highly advantageous in cladding-applications, either for upgrading surfaces or for enhancing the quality of sealing/sliding faces. In order to achieve the stipulated purity in the applied high-grade material, welders using the conventional GMA process have to clad the relevant places repeatedly (as many as 5 times). This is because of the mingling of the base and filler metals in the molten zone. With the 'cooler' CMT process, by contrast, there is less melting of the base metal, and - right from the surface zone of the very first layer - it leaves behind a base-metal component approaching zero.

Mechanisation of the welding operations in the longitudinal and radial directions

In fields such as vehicle manufacturing and general mechanical engineering, higher productivity and greater efficiency can be achieved through robot-based automation. In shipbuilding, where the 'lot-size' is just one, this approach is not feasible. Instead, intelligent mechanisation solutions for reproducible travel paths (i.e. welding-tracks) offer some attractive possibilities here. Two typical fields of use are in the panel-production and pipework-mounting operations.

The battery-powered traversing units are ideal for mechanised utilisation of the GMA process on longitudinal fillet welds in the horizontal-vertical and vertical positions, and also with integrated oscillation. Their compact, lightweight design makes them particularly suitable for use in panel production or block construction. These appliances can be combined with the TransSteel Yard and a conventional manual welding torch. Program buttons for the travel path, for segment welding and for crater filling provide a high degree of flexibility and operator convenience.

In the pipework fabrication and mounting operations, single- to multi-pass circumferential seams need to be welded, as necessitated by the wall thicknesses of the pipes to be joined. Orbital welding systems with suitability for steel, CrNi and Cu materials are ideal for these tasks. The use of intelligent control systems and power sources, coupled with weld-data monitoring, makes for maximum process reliability and outstanding welding results.

Conclusion and outlook

Shipbuilding is undergoing great change, throughout the world. This process is being driven by the need for efficiency-gains in the building of large carrier vessels, and by the boom in cruise liners and luxury yachts. The market for waterborne craft is also being transformed by the large numbers of ever more diverse special vessels needed in the offshore field. The sophisticated and differentiated types of demand generated in this way create a need for excellent, well-thought-out weld processes and applications. The challenge for the manufacturers of such welding equipment is to foster this growth trend from both the technical and business angles, in an ecologically sustainable manner. In welding, it is system solutions that dictate the overall direction and application, and this, in turn, is the reason for the increasing prevalence of comprehensive, integrated offerings.