Abstract

HybridLaser is a process that combines a laser with an arc welding process, a combination that provides superior properties when compared to each process in the stand alone position. Here we study a new way of welding the longitudinal seam of an offshore pipe in X70 material with a wall thickness of 16 mm plus a cladding layer on the inside of 3 mm in thickness. The standard joint preparation of this joint is a U-prep with 3 mm nose, a bottom radius of 5 degrees and a joint prep of 20 degrees included. The different steps of producing the pipe the conventional way are: a) machining of the joint preparation, b) forming of the pipe, c) tack welding with GTAW, d) root welding with PAW, e) hot pass with GMAW, f) fill & cap with SAW. To produce 1 pipe of the standard length of 12 meter takes 2 h 10 min.Use of the HLAW process was initialized by a series of test welds to find the optimal joint preparation in order to be able to do the welding in one run. The chosen preparation was a nose of 8 mm and a V-preparation of 8 degrees included. The new way of producing the pipe is as follows: a) machining of the joint, b) forming of the pipe, c) tack welding GTAW, d) final welding HLAW. To produce 1 pipe of the standard length of 12 m now takes 18 min.

The welding parameters for the HLAW process are 12 kW laser power, and a CO2-laser was used. For the GMAW a 1,0 mm ER 70S – 6 wire was used, the welding current was 225 Amp, the voltage was 28,5 Volt and the stick-out was 15 mm. As a shielding gas 70 % He, 25 % Ar and 5 % CO2 was used, the flow rate was 20 l/min and all gas was distributed through the welding torch. The interdistance between the focal point of the laser and the point of arc on object was set at 3 mm. The angle of the welding torch was 20 degrees forehand welding and the set up was for this application done with leading laser. For other applications it is advantageous with a leading GMAW.

The quality and integrity of the HLAW welded joint is very high and the micro hardness values are well within specification.

The complete cost calculation that will be presented in the full paper shows that with this new way of welding using HLAW, the welding time will be reduced with a factor 6,6 and that the total welding costs will be reduced with a factor of 2,7. This calculation is based on all investment costs for both the conventional welding method and for welding using the HLAW process and it is based on the production of 1.000 pcs of pipes.

Key words: Hybridlaser. X70. Longitudinal weld. Cost comparison.

Introduction

The Hybridlaser welding process is relatively new to the welding industry, looking at the perspective of welding history, but nevertheless the process has already proven its benefits in solving many challenges such as reducing deformation, achieving required mechanical properties, bridging of gaps, increasing productivity and much more. The concept of the hybrid process is based on the combination of two established welding processes, the processes in this case being laser welding with GMAW. For reference it shall be mentioned that the HybridLaser welding process can also be a combination of laser and GTAW or laser and PAW. The laser itself can be a CO2 laser, Nd:Yag, diode, Yb:YAG disc or Yb fibre. In the welding head, that is carrying both the optics and the welding torch (GMAW, GTAW, PAW), the two processes meet each other and are joined into the same weld pool meaning that we now have a new process that is built up of two sub-processes. There are properties from each process that will be combined in to one. The disadvantages normally found in each single process now being compensated by the advantages from the combined new process. The end result being the excellent new HybridLaser welding process, which has the ability to weld over gaps, requiring minimal joint preparation while providing very high levels of productivity.

Figure 1. Gap bridging capabilities.

As seen in figure 1, it is possible to achieve a good gap bridging capability with the process and at the same time maintain a high welding speed. This is one of the advantages with the process that is the result from the joining of laser and GMAW welding. Process stability, speed and penetration come from the laser and the gap bridging properties comes from the GMAW.

Other significant properties associated with the HLAW process is the reduction in heat input to the base material. This in combination with the greatly reduced weld joint volumes is responsible for minimizing the distortion of the welded objects. This contributes to a major reduction in the need for post weld work, i.e. straightening of plate and objects and this reduces the overall costs significantly.

Welding head

Depending on the choice of laser source, the integration of the laser and the arc welding process in the welding head will be different; the design of heads may vary a lot. There is also different optics available on the market and there are different control systems for the welding process, this will also influence the design of the welding head. The most important feature is that the size of the head is not hindering access to the welded object and the attachment of the object in the welding fixture. If the welded object does not give any restrictions in limiting the height of the object a straight optics can be chosen.

Photo 1

Photo 1 shows the welding head with straight optics.

Photo 2

Photo 2 shows an angled optic that will give a lower total height of the welding head, which greatly enhances the accessibility in confined areas.

Longitudinal joining of pipe

There are different production philosophies for how to perform the longitudinal seam welding of a pipe when it comes to joint preparation and the choice of processes. In this case we have looked at pipes for the offshore industry and in particular, so called raiser pipes. Raiser pipes are used for the transport of oil/gas from the well to any production site. The pipes are made from cladded material, stainless or Ni-base on the inside of the pipe, the composition of the cladded material is dependant on the corrosiveness of the media that is transported in the pipe. The pipe material is selected to meet the requirements and specification laid down by the client. The pipe tested in this investigation is an API 5L X-70 pipe.

C = 0,25 – 0,37; Mn = 1,31 – 1,53; Nb = 0,082 – 0,095 and Ti = 0,008 – 0,015. This particular steel is a TMCP type. The cladding layer in this case is a 309 material and the thickness of the layer is 3 mm. In this investigation we only look at the welding of the longitudinal joint, not the welding of the cladding layer after the joining of the long seam, as this will be exactly the same procedure irrespective of the way the longitudinal joint is performed.

The wall thickness of the pipe (API 5L X-70) is 16 mm and the thickness of the cladding layer (309L) is 3 mm. The length of a pipe is 12 meter.

Present joining procedure

The current method of producing the pipes is according the following schedule:

• Joint preparation (milling)
• Forming of the pipe
• Tack welding of the joint (GTAW)
• Root pass welding (PAW)
• Hot pass welding (GMAW)
• Fill & cap (SAW)
• Quality control

From the point where the tack welding starts, up to the final weld station, the cycle time is 135 minutes, this includes all handling time and changing of the welding equipments between the different beads.

The joint preparation is done in an automatic milling station with a following quality station where the tolerances of the preparation is checked. After welding there is also a quality check with MP and ultrasonic tests, before the pipe is approved and stamped with OK sign.

There are different ways of designing the preparation for this joint and what is unavoidable is that irrespective of which preparation method and processes are used, a back bead must be done from the inside, with the same consumable composition  as that of the cladding layer, in order to assure a sound inside.

Figure 2

Figure 2 shows the joint design that is used today.

The cycle time for the welding operation is as follows:

• Tack welding (GTAW) 6 min
• Handling 5 min
• Root pass welding (PAW) 48 min
• Handling 5 min
• Hot pass (GMAW) 26 min
• Handling 5 min
• Fill & cap welding (SAW) 40 min

Hybridlaser joining procedure

During the trials for the HLAW process a number of different joint preparations were evaluated before the final was chosen. The new way of welding is done according to the following schedule:

• Joint preparation (milling)
• Forming of the pipe
• Tack welding of the joint (GTAW)
• Final welding (HLAW)
• Quality control.

Instead of 4 welding operations, in the conventional welding method, we now have only tack welding and the final welding operation. This reduces the welding cycle considerably and also takes away a number of welding equipment changes that are necessary with the old method. The total heat input to the joint is now also much lower which reduces the amount of time spent on straightening of the pipe after welding.

Figure 3

Figure 3 shows the preparation that was chosen for the HLAW process.

The welding that was previously carried out with different processes in multi bead welding is now done in only one operation and in one run.

Photo 3

Photo 3 shows the cross section of the HLAW joint with full penetration and welded in one run with a welding speed of 1,0 m/min. Mechanical properties are all meeting the requirements including the micro hardness that is also well within the allowed tolerance range.

Cost calculations

Conventional way of welding:

This is the cost for 1 pipe. The cost for producing 1.000 pipes will add up to 763.850:- SEK

HLAW way of welding:

The cost for 1 pipe. The cost for producing 1.000 pipes will add up to 279.700:- SEK

This is a time saving with a factor of 6,6.

This is a cost saving with a factor of 2,7.

Conclusion

The investment in a new and expensive process like HLAW is often dismissed due to the cost. But when using the process for an application where it is possible to utilize all benefits from the process, so much time saved, the production cost of the product being welded will be greatly reduced and this will make the investment very sound. The payback time for this investment will be approximately 1,5 years and the depreciation time calculated for the investment is 5 years, resulting in a very good margin after 1,5 years (the payback time).