Marcel Stemvers, Global Energy Segment Manager and Gordon Eadie, Global Pipeline Segment Manager, ESAB, Sweden, explain how welding consumable improvements can reduce pipeline construction time.

There are continual developments in the field of pipeline welding products and those involved in the ownership, design, installation and maintenance of pipelines need to be fully aware of such improvements.

Manufacturers of welding consumables and machines, such as ESAB, have continued to invest in research and development. They offer solutions that are technologically advanced, acknowledge the need to keep costs as low as possible and reflect the important criteria of safety, quality and environmental requirements. ESAB is the only welding company to have achieved ISO 14001 and OHSAS 18001 standards in environmental, health and safety management systems across all of its global manufacturing facilities.

Every day vast distances of steel pipelines are installed worldwide for the most varied civil engineering and industrial uses, carrying essential fluids and gases. To comply with all pipeline technical specifications and fulfil the necessary safety requirements, welding

Figure 1. ESAB welders

demonstrate the versatility of the

Pipeweld DH range of vertical

down basic electrodes.

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materials and processes continue to evolve. Whilst welding consumables may represent only a small fraction of the overall cost of pipeline construction, the combination of these consumables and welding processes is critical to a project’s success.

Within the industry, the onus is on the pipeline contractor to propose a welding process that meets the mechanical requirements of the pipeline designer or owner. Pipeline contractors often design the welded joint to suit their preferred welding technique, then approach manufacturers of consumables for tailored welding electrodes or wires that perform as required for the proposed process.

Meeting targetsESAB has recently succeeded in supporting pipeline contractors with a welding technique that significantly reduces welding time and is easy to adapt to specific project demands. The contractors are able to utilise existing staff and equipment as well as meet more demanding requirements. The drive for this change came from increasing demands on weld metal strength as well as the need from the contractor to improve competitiveness by reducing the man-hours on the job. It was clear that adopting the same welding technology would not deliver both targets. However, a fundamental change in welding technology would push the operations into another dimension, with all risks associated.

The technology adopted uses several classical welding solutions, which have recently been upgraded to deliver improved weld metal properties as well as enhanced welding productivity and process control. After depositing the classical cellulose root, filling commences normally with a ‘2 run’ procedure. The suggested alternative is a ‘3 run’ procedure where a modified vertical down basic electrode is used, to overcome the typical shortcomings of the ‘2 run’ procedure, as explained in more detail later.

Thanks to its ease and versatility, the main welding process used to install pipelines is manual welding with coated electrodes. However, to limit costs and increase welding productivity – particularly on long routes – pipeline constructors have adopted the semi-automatic or completely automatic welding process with solid and flux-cored wires. Since protective gases can also be difficult to find in certain countries – and these are necessary in welding with solid or cored wires – manual metal arc (MMA) remains attractive.

CharacteristicsTo describe the benefits of the recently modified ESAB Pipeweld DH series the typical process characteristics should be reviewed. These Pipeweld DH products are offering 120% recovery, against 90% of the cellulose electrodes. This leads to deposition rates over 3 kg/hr in vertical down position, very close to flux-cored wire vertically up. The main added benefit from welding vertical down is the inherent safety against excessive heat input. Impact toughness and tensile properties of weld metal and heat affected zone are generally better than when welding vertical up, which is further supported by the low oxygen content of the basic weld metal. Another benefit is the low hydrogen content of the deposited weld metal, the Pipeweld DH range is classified H4R, providing the highest possible security against cold cracking or porosity. This low hydrogen content is guaranteed from the ESAB VacPac electrode packaging, avoiding costly baking and electrode handling.

The practical limitations from using vertical down basic electrodes has been the requirement to apply a starting technique perpendicular to the pipe, after which the electrode

has to be tilted to get the appropriate dragging angle. This requires dedicated skills and does not entirely avoid starting porosity. The Pipeweld DH series use a patented tip design that allows the welder to start with the electrode already in the proper welding direction, without the risk to generate starting porosity.

This special tip design is furthermore supported from enhanced coating strength. The projects supplied so far have reported a complete reduction of electrode scrap rates, which has been on average as high as 30% for the classical tapered tip designs, due to transport damage.

These modifications are now enabling pipeline contractors to use

Figure 2. Deposition comparison (kg/hr at 100% arc time).

Table 1. Pipeweld WPQR tests

Test

No.

Process Bead location Consumable Diameter Passes Welding

time

1 Cellulose Root Pipeweld 6010 Plus 4 mm

6 13.5 minsCellulose Hot pass Pipeweld 9010 Plus 4 mm

FCAW Fill & cap Pipeweld 71T-1 1.2 mm

2 Cellulose Root Pipeweld 6010 Plus 4 mm

6 14.5 minsCellulose Hot pass Pipeweld 9010 Plus 4 mm

DH Basic First fill pass Pipeweld 90DH 4 mm

FCAW Fill & cap Pipeweld 71T-1 2 mm

3 Cellulose Root Pipeweld 6010 Plus 4 mm17 28.5 mins

Cellulose Hot pass, fill & cap Pipeweld 9010 Plus 4 mm

4 Cellulose Root Pipeweld 6010 Plus 4 mm

10 19 minsCellulose Hot pass Pipeweld 9010 Plus 4 mm

DH Basic First fill pass Pipeweld 90DH 4 mm

DH Basic Fill & cap Pipeweld 90DH 4.5 mm

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vertical down basic electrode welding to their full benefit and seriously reduce rework and welding time whilst meeting high requirements on the weld metal. The Pipeweld DH range is available for pipe steels up to API-5L: X80.

‘2 run’ procedureThe ‘2 run’ procedure uses filling with a rutile cored wire directly after the cellulose hot pass. This method is regularly causing issues with gas pops or blowholes coming from the cellulose layer underneath the first FCW fill pass. Deep grinding to clean the weld metal does overcome most of this problem, but not entirely. This grinding is both time consuming and removes quite a bit of weld metal, adding to the further completion time.

‘3 run’ procedureThe ‘3 run’ procedure uses a basic vertical down first fill pass. This avoids heavy grinding and leaves a clean basis for the FCAW process without the need for any other treatment than slag removal. Filling commences with a rutile flux-cored wire that is specially formulated for mechanised pipe welding. This style was originally developed for tie-in welds where, due to physical restrictions (fittings etc) or connecting into the main line at road crossings, the use of the internal line-up clamp and or internal welder was not possible. Contractors soon realised that with the correct manning and number of welding stations this technique resulted in a similar number of joints per day at a lower cost.

Table 1 explains the details of the techniques described with their economical characteristics.

Field welding results have given the following productivity rates:

) 1 pair of welders for root, hot pass and fill using cellulose, Basic DH and FCW resulted in 3 welds/d on a 48 in. 25 mm wall pipe.

) 1 pair of welders for root cellulose, one pair for 1st HP with cellulose, 1 pair for 2nd HP with basic VD and 6 pairs with welding bug and FCAW resulted in 60/65 welds/d on a 48 in. 21.3 mm wall pipe.

) On a 24 in. mainline 14 mm wall thickness with 9 pairs of welders (as above) 120 - 130 joints/d were achieved and

65 joints/d on a 48 in. 17 mm wall thickness with the same crew formation.

Practical conditionsThe main requirements for successfully mechanised pipe welding have to do with the practical conditions under which the wire has to perform. Arc voltage has to be set low in order to get a smooth weld bead in all the pipe positions. However, this low arc voltage shall not create excessive spatter or contact tip damage. At the same time the welding parameters should not be so low that productivity is affected.

The Pipeweld FCW’s are designed to work in mixed gas and overcome all the hurdles above. They are class leading in their productivity, which can reach 4 kg/hr, they have a good welder appeal and have proven themselves to be very robust in field conditions. Their strength levels go up to X80 with overmatched weld metal.

Flux-cored wires are nowadays well suited for pipeline applications. Low-hydrogen operation is possible as well as good impact toughness down to -60 °C. It is easy to obtain flat welds with a good penetration and smooth wetting onto the pipe edges. The brittle slag is easily removed leaving behind a smooth rutile weld appearance. Typical positional welding defects, such as lack of fusion and slag inclusions, are avoided due to the spray arc operation. The wires have a good tolerance to poor joint fit up, which understandably makes then especially suited for tie-in welds.

The combination of cellulose, basic vertical down electrodes and the cored wire from the Pipeweld range has proven to be as cost-effective as automatic solid wire welding, even when using an internal root pass welding clamp. This is mainly due to the relatively low cost equipment and avoiding the high rental costs of internal welding line-up clamps and the additional equipment and technicians to support joint completion. However, this statement is restricted to light to medium wall thicknesses. On heavy wall thicknesses narrow gap pulsed MIG is the most productive solution.

Figure 2 demonstrates the deposition rates that can be expected from the various processes, when using these under ideal pipe welding conditions. What is often not recognised is the productivity of the vertical down electrodes.

This technique is receiving a warm interest due to the global trend to use cellulosic electrodes for root and hot pass applications only. This will not change overnight, however once the pipeline owners requests for limitations on weld metal hydrogen content are heard, appropriate alternative solutions must be found. The technique described here allows the existing skilled welders to make the change with no issues as both processes are high speed vertical down MMA welding processes.

From these field results it can be learned that the use of modified basic vertical down electrodes can significantly reduce welding time and enhance flexibility. The welding time reduction of more than 30% against cellulose is very attractive, especially since savings can be realised with existing skills. It is obvious that filling with FCAW delivers in terms of productivity. Here the modified basic vertical down electrode enables a process switch whilst enhancing productivity and weld quality.

Figure 3. Macro examination of Pipeweld FCAW 15.9 mm wall thickness. Root and hot pass with cellulose, fill and cap with FCAW.

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