Yb Fibre Laser/MAG Hybrid Processing for Pipeline Welding

Paper published in ISOPE-2005, Seoul, Korea, 19-24 June 2005

Abstract

TWI has been processing materials with high power (4kW and above) Nd:YAG lasers since 1997 and has a history of welding structures with lasers extending back some thirty years. The work carried out at TWI for the pipeline industry using lasers has moved from early autogenous work with high power (9kW) fibre delivered Nd:YAG laser to hybrid procedures with Nd:YAG and MAG through to recent work carried out with a 7kW Yb fibre/MAG procedure capable of delivering deep penetration girth welding procedures at 1.8m/min travel speed. The paper describes the technical progress made throughout this work in terms of process developments to meet the exacting productivity requirements along with those necessary to meet technical requirements such as impact toughness and other pipeline code acceptance criteria.

Introduction

There is continual emphasis on reducing the costs associated with new pipeline development, particularly for large diameter gas transmission lines. In particular, Markland (2000) noted that BP expects to be involved in building over10,000km of onshore pipelines for transporting oil and gas with capital expenditure estimated as exceeding £11 billion. Over half of the world's undeveloped hydrocarbon reserves are remote from potential users and very large pipelines, up to 1.42m (56') diameter are required to transport the fuel to market. The welding process used to make the site girth welds has a significant bearing on the total cost per kilometre of pipeline and is one of the areas that TWI has been involved with over the last few years. Current practice is to use either mechanised or automated MAG and in the short term there are probably further cost reduction opportunities offered by incremental improvements to this operation. The multipass MAG process, however, requires a high manning level and the costs of providing this and the necessary support in fairly remote regions are a significant component of the overall costs.

Laser welding, and in particular fibre delivered laser welding, has now been developed to the stage where it presents opportunities for cost savings, which arise from reductions in labour content, despite perceived high capital costs. Yb fibre lasers are now commercially available with power levels of up to 10kW. These lasers are particularly suitable for pipe girth welds because the beam is delivered to the workstation through a single optical fibre and their efficiency is high enabling the development of more portable welding systems. It has also been demonstrated (Howse, 2002, Booth 2002) by TWI that the concept of high power laser welding of land pipelines is entirely feasible. Welding procedures have been developed that produce good quality welds with satisfactory tolerance to joint fit-up. Additionally techniques have been developed for welding around 360o and for ensuring a good weld at the start/stop weld overlap position. With the Yb fibre lasers currently available, a procedure involving laser and MAG offers an attractive commercialisation route.

Laser technology

Until quite recently, there have been two main types of industrial laser used at high powers for deep penetration keyhole welding. These were CO ₂ gas lasers and Neodymium doped Yttrium, Aluminium Garnet (Nd:YAG) lasers. Ireland (1999), noted that, historically, CO ₂ lasers were the first to be developed capable of delivering higher powers needed to process relatively thick structural steels (>6mm thickness) while Nd:YAG lasers have been more limited in terms of power. However, over recent years, developments in Nd:YAG laser technology have resulted in higher power systems, up to 6kW, becoming commercially available. In terms of materials processing, the principal difference between Nd:YAG lasers andCO ₂ lasers is the difference in wavelength of the light emitted. Nd:YAG lasers produce light of 1.06mm wavelength that can be transmitted to the workpiece by a fibre optic cable. This is a much more flexible system of beam delivery than for CO ₂ lasers (10.64mm wavelength) that must be transmitted to the workpiece by more cumbersome reflective or transmissive optical systems. This makes Nd:YAG lasers much more attractive as flexible manufacturing tools compared to CO ₂ lasers. Certainly for orbital welding of land pipelines, fibre delivered lasers would be the preferred choice.

Larson (1999) has shown that lamp pumped Nd:YAG lasers have established themselves as reliable processing tools capable of delivering a precise heat source in high volume, flexible manufacturing environments such as the automotive industry in Europe. Kincade and Anderson (2005) have also shown that the trend is for the world market to grow for solid state lasers generally. Forecast worldwide sales for 2005 are $520M, this compares with $602M for CO ₂ lasers in an expanding materials processing market worth an estimated $1.5 billion.

Although Nd:YAG lasers compare favourably to CO ₂ lasers in terms of reliability and ease of processing, they have a significant drawback for some manufacturing applications in that although they are relatively compact they are inefficient, only converting around 3% of the input energy to produce the laser beam power. This is not a major issue for most manufacturing applications, but the use of lasers for welding cross-country pipelines relies on the portability of the process. Although the Nd:YAG laser process can be containerised, the low efficiency and high capital cost of the process make it very difficult to justify economically. One of the major advances in laser technology in recent times is the introduction of ytterbium (Yb) fibre lasers. The lasing medium for these lasers is contained within the fibre itself and individual units generating 2-300W can be combined to produce single lasers with up to 10kW power and beyond. Alternatively, higher power single mode fibre lasers are also commercially available at powers up to 1kW. These lasers have a similar wavelength to Nd:YAG lasers and the laser light can be transmitted to the workpiece via a flexible optical fibre. Yb fibre lasers are approximately 20% wall plug efficient and are much more compact than Nd:YAG lasers. This makes them very attractive for applications such as pipelay where they need to be portable. Although these lasers are just becoming commercially available, at powers in excess of those available for lamp pumped Nd:YAG lasers, there has been very little practical work carried out to investigate their suitability for keyhole welding and their performance. Also, although having enormous potential, they need to be thoroughly evaluated for reliable and economic industrial use. To this end, TWI has recently added a 7kW fibre bundled Yb fibre system to its laser processing facilities. The laser is capable of delivering any power up to 7kW via a 0.3mm diameter, 20m length, optical fibre cable and has the capability of producing a focused spot with a power density of 5.6 x 106 W/cm ² .

Laser processing for pipeline applications

CO ₂ laser processing

Bonigon and Geertsen (1998) reported development work carried out in this area by Bouygues Offshore, looking at the application of CO ₂ laser welding for S-lay of offshore pipelines. This system used a CO ₂ laser delivering power up to 20kW and was designed to weld up to 20mm wall thickness in a single pass. Not all the results from this work have been made public, but those that have show that although the processing tolerances are acceptable for producing welds in the 5G position (pipe axis horizontal and welding direction vertical up or down). It was concluded that it was not possible to use all commercially available pipeline compositions and product forms to satisfy existing code requirements, particularly in meeting hardness requirements for sour service and toughness requirements at the same time. Similarly, work was also carried out by Gainand Y. et al(2000) at AXAL/ITP to develop a fully automatic laser welding system to improve the speed of offshore pipeline welding. This system also used a CO ₂ laser delivering power up to 20kW and was designed to weld up to 20mm wall thickness in a single pass. Welds have been made in wall thickness up to 15.9mm but although the weld met non destructive tolerances to API 1104, detailed mechanical property data is not available.

Lamp pumped Nd:YAG laser processing

For a number of years TWI has also been involved in initiatives looking at girth welding of pipelines with high power Nd:YAG lasers. Initially a review project was carried out by TWI for BP to identify potential options for reducing welding costs associated with land lay of pipelines. This work identified improvements to existing arc welding technology and also highlighted the potential of high power Nd:YAG lasers in reducing the cost of pipeline fabrication. Very early studies investigated autogenous laser welding in the 2G position ( Fig.1). Although these welds demonstrated that welding was possible in wall thicknesses up to 12.7mm, the welding speeds were still relatively slow. Higher speeds of around 1.0m/min, needed for the process to maintain productivity, were used but the process was not tolerant to variation in joint gap. In addition, the autogenous process produced welds with very low impact toughness. In order to solve the problems with poor tolerance to fit up at high speeds and the low toughness, the development of the process concentrated on procedures using hybrid Nd:YAG laser/MAG welding reported by both Howse (2002) and Booth (2002).

Fig.1. Macrosection of BOP weld made on X52 pipe steel, 324mm outside diameter and 12.7mm wall thickness. Power at the workpiece 8.2kW, travel speed 500mm/min. The laser source was three combined lamp pumped Nd:YAG lasers.

Hybrid Nd:YAG laser/MAG processing

For this work, the initial proposed fabrication sequence was for the MAG pass fill stations in a land based pipe lay spread to be replaced by a hybrid Nd:YAG laser/MAG welding station, whilst still retaining the MAG root. An example of this type of joint, but made autogenously with 9kW of Nd:YAG laser power, is shown in Fig.2.

Fig.2. Section through pipe wall, showing internal MAG root run and 9.0kW autogenous laser fill made at 0.7 m/min. Pipe grade API 1104 X70 14.3mm wall thickness (mm scale shown). The laser source was three combined lamp pumped Nd:YAG lasers.

Two separate laser power sources/hybrid welding heads or a single Nd:YAG laser system with the beam switched between two heads positioned on either side of the pipe, may potentially be used to produce this weld. The intention was to use the laser in conjunction with a MAG wire fed consumable to increase the processing speed, tolerance to fit-up and generate appropriate microstructures with acceptable impact toughness.

The laser apparatus used to make these welds comprised two 3kW Trumpf HL3006D Nd:YAG lasers and one 4kW Trumpf HL4006D Nd:YAG laser combined in an optical beam combining unit to deliver 9kW at the workpiece. The output from the lasers was transmitted into the beam combining unit using a step index fibre optic of core diameter 0.6mm and length 30m. A single fibre optic of core diameter 1mm and length 15m transmitted the combined laser power from the beam combining unit to the laser output housing. A Lincoln Electric Power wave 455 MAG power source was also used. A synergic pulsed welding programme appropriate for 1mm diameter wire and Ar-CO ₂ shielding gas mixtures was selected for the welding trials.

A purpose built fixture was used to hold both the laser focusing head and MAG torch. The fixture permitted accurate and repeatable control of the relative position and orientation of the two heat sources. Figure 3 shows the laser output head and MAG torch attached to the hybrid fixture, with the laser co-axial gas nozzle and MAG shroud removed.

Fig.3. Laboratory demonstration of hybrid Nd:YAG laser/MAG welding for pipelines

If the welding conditions used were carefully chosen to give fully penetrating welds, the weld was clear of solidification defects. It was evident that if full penetration was achieved, defect free welds above 8mm penetration were possible at processing speeds of 1m/min ( Fig.4).

Fig.4. Macrograph of a hybrid Nd:YAG laser/MAG weld in X60 pipeline steel showing an acceptable weld with 11mm full penetration. Laser power at the workpiece 8.0kW, travel speed 1.0 m/min. The laser source was three combined Nd:YAG lasers.

In summary, this work showed that it was possible to use high power Nd:YAG laser welding combined with the MAG process to produce deep penetration welding passes in commercially available pipeline steels that met the requirements of pipeline specifications such as BS 4515 and API 1104 in terms of acceptance criteria for imperfection limits. The welds also showed acceptable hardness values and good, low temperature toughness ( Tables 1 and 2). The acceptance criteria used for was for a minimum average vale of 40J and minimum individual value of 30J impact energy at -10°C using full sized (10mm x 10mm) Charpy specimens and a maximum permitted hardness of 275HV10 in the weld metal. It can be seen that although the samples met the minimum requirements, there was some scatter in the Charpy results with a single lower value in the heat affected zone. One disadvantage of using a single pass welding process is that the heat affected zone does not undergo microstructural refinement in the same way that a multipass weld will. It is possible that the scatter is due to sampling of a region of unrefined grain coarsened microstructure. Subsequently, much more Charpy testing was carried out within this programme of work and values in the weld and heat affected zones consistently met acceptance criteria. It would be recommended however, that a full qualification of the process for pipeline applications should include heat affected zone CTOD testing.

Table 1. Hardness values for a hybrid Nd:YAG laser/MAG weld in API 5L X60 pipeline steel.

Table 2. Charpy impact results for a hybrid Nd:YAG laser/MAG weld in API 5L X60 pipeline steel. Charpy values quoted are values for full size (10x10mm) tested at -10°C.

Although the results of this project work showed that fibre delivered lasers were capable of producing orbital welds in pipe materials that would meet productivity targets, give acceptable mechanical properties and low defect levels, the economic performance could not be justified. One of the main drawbacks in the proposed use of lamp pumped Nd:YAG laser technology for the site welding of pipelines was the process' poor efficiency and lack of portability.To achieve these welds would require two separate laser power sources being combined and operating together. The poor efficiency and high capital cost of the process effectively prohibited their use compared to existing arc technologies.

Hybrid Yb fibre laser/MAG processing

In late 2003, TWI took delivery of a 7kW Yb fibre laser. The 7kW laser at TWI delivers the power to the workpiece through a 0.3mm diameter optical fibre. In addition, the wallplug efficiency of the laser at this power has been measured as 20%, much higher than competing lamp pumped Nd:YAG technology. The wavelength that the laser operates at for these powers is 1070nm, with similar, good material interaction to Nd:YAG lasers. The laser is also relatively compact with length, width and height dimensions of 0.8, 1.2 and 1.6m respectively. Figure 5 is a photograph of the laser with one of the front panels open. The 200W modules, the outputs of which are combined to provide 7kW, are clearly visible. In this laser we have a single power source with high efficiency, which is capable of being containerised and used for pipeline applications.

Fig.5. The IPG YLR7000 Yb fibre laser installed at TWI (Yorkshire)

With this in mind, TWI carried out some initial trials welding API 5L X80 linepipe material to investigate the potential of this technology. Fig.6 is a photograph of the processing cell. The laser processing head is mounted on a Kawasaki ZX130L 6 axis robot. Hybrid laser-arc welding capability is achieved using the Yb fibre laser in combination with an ESABAristoMIG 450 programmable arc power source, AristoFeed 30 and an MA6 controller. A 0.3mm fibre delivers the laser beam to the workpiece. The processing heads used in the trials held a 250mm length focusing lens, producing a 0.6mmdiameter focused spot and a power density of 2.5x10 ⁴ W/mm ² at 7kW.

Fig.6. The Yb fibre laser processing cell at TWI (Yorkshire)

Fig.7. ASME 9 welding positions

The X80 linepipe material was prepared with a compound preparation angle with a 6mm root face and welds were made on pipe girth section in three positions; flat (1G), vertical up (3G) and overhead (4G) to simulate the quadrants in pipe girth welding 5G vertical up ( Fig.7).

The procedure used for all three positions is as follows:

• Travel speed = 1.8m/min.
• Wire = 1mm diameter BS 2901 pt1:1983 A18.
• Wire feed speed = 10m/min.
• Source on synergic setting.
• Average pulsed current = 215A.
• Arc voltage = 25V.
• Arc power = 5375W.
• Laser Power = 7kW.
• Spot size = 0.6mm.
• Position of the focal plane at the surface of the preparation.

Cross sections of the welds are shown in Figs.8-10.

Fig.8. Macrosection of weld W70 transverse girth Yb fibre/MAG hybrid weld in X80 linepipe steel made at 1.8m/min using 1mm A18 wire with a wire feed speed of 10m/min welded in the overhead (PE/4G) position

Fig.9. Macrosection of weld W69, transverse girth Yb fibre/MAG hybrid weld in X80 linepipe steel made at 1.8m/min using 1mm A18 wire with a wire feed speed of 10m/min welded in the vertical up(PF/3G) position

Fig.10. Macrosection of weld W71, transverse girth Yb fibre/MAG hybrid weld in X80 linepipe steel made at 1.8m/min using 1mm A18 wire with a wire feed speed of 10m/min welded in the flat (PA/1G) position

These welds were inspected both visually and by radiography and were found to be of good quality with no internal defects. Samples were also taken from these welds and used for hardness, tensile and Charpy impact testing. The results of this testing are shown in Table 3 . It can be seen that the samples produced acceptable toughness and that tensile failure occurred in the parent material in all cases. The hardness of these welds is high and the use of this technology for sour service applications would need further consideration.

Table 3. Mechanical testing results for Yb fibre laser/MAG hybrid welds made in X80 C-Mn line-pipe steel at a travel speed of 1.8m/min. Charpy values quoted are actual values for sub size (7.5x10mm) specimens tested at-10°C.

Discussion

The developments within high power laser materials processing have been extremely rapid within the last ten years and the potential for welding pipelines with lasers has improved with each new product that has come onto the market. CO ₂ laser technology, although reliable and available at high powers has disadvantages in terms of its delivery. The relatively long wavelength of CO ₂ generated laser light (10.64µm) means that the light cannot be transmitted through silica based optics and hence glass fibre optical delivery systems. Although the beam quality of these lasers is good and high power densities and deep penetration welds can be achieved, the manipulation of the beam around a pipe to make a girth weld is not easy. Lamp pumped Nd:YAG technology however with shorter wavelength light (1.06µm) and power delivered to the workpiece via optical fibres has much more flexibility. Initial trials with this technology have demonstrated the potential of fibre delivered lasers to produce deep penetration orbital girth welds. However, both the CO ₂ and lamp pumped Nd:YAG laser have significant drawbacks. The first being that the wallplug efficiency is relatively low at much less than 10%. This means that the systems not only require large amounts of energy to operate them but that they also require chiller equipment to extract waste heat. These characteristics mean that the lasers are not readily portable.

Another issue highlighted relatively early in the work with both CO ₂ and Nd:YAG lasers for the welding of steel pipeline material is that the autogenous microstructures generated within the welds are brittle and have poor impact performance. Pipeline application standards require that the welded joints have adequate impact toughness at the appropriate ambient temperature, which can be low for cross country pipelines in cold regions. The work carried out has shown that this can be overcome by using laser-archybrid technology which allows filler wire additions with higher oxygen content to be added to the molten pool, which generates more favourable microstructures with better impact resistance. Using hybrid technology, acceptable weldmetal impact values have been consistently been achieved at test temperatures of -10°C. The use of hybrid laser arc processes has further benefits in that the arc acts to bridge joint preparation gaps and gives stability to the process at high welding speeds.

The introduction of high power Yb fibre laser technology to the marketplace has further widened the scope for materials processing and particularly for application of hybrid laser-arc processing in pipeline welding applications. These lasers are capable of delivering high powers from a single source with good beam quality, resulting in high power densities and, therefore, fast processing speeds. The trials reported here demonstrate that these lasers are capable of producing 9mm depth welds at speeds of 1.8m/mmin. These procedures have been demonstrated in fixed positions capable of being translated to circumferential girth welds in pipeline demonstrators. Another relevant characteristic of these lasers is the relatively high efficiency of the process, measured by TWI at 20%. This means that, per kW, there is less input power required and less waste heat generated that has to be removed by some chilling capacity. One of the major unknowns of the process is its reliability in industrial applications. Although the laser at TWI appears to be a relatively robust design and has shown itself to deliver a very high availability for processing in its first year, this is the first commercial system of this size under evaluation and more data will be needed before industry is fully convinced of its capabilities. Similarly, although the technical results of these initial trials are extremely encouraging, further development work needs to be carried out to fully develop an integrated pipe welding system capable of being used in a production environment.

Conclusions

The following conclusions can be made:

• Over the past ten years laser technology has developed to the stage where reliable, high power and efficient Yb fibre laser power sources are now available capable of offering productivity benefits for pipelay.
• The hybrid Yb fibre/MAG process can be used at high processing speed (1.8m/min) to weld a 9mm ligament in a single welding pass with low and acceptable flaw levels.
• Welded deposits have been tested for strength and the impact toughness and have been shown to be acceptable to current pipeline application standards.
• The developed procedure has been demonstrated in the overhead, vertical up and flat welding positions capable of being translated to a 5G vertical up girth welding procedure in linepipe.

Acknowledgements

The authors would like to acknowledge the help and assistance of Anthony Elliot and Tom Woolhouse in making the welds with the 7kW Yb fibre laser facility and also to thank BP Exploration for funding the initial work at TWI with high power Nd:YAG lasers.

References

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