Scott Funderburk, Director – Global Marketing Pipeline Segment, The Lincoln Electric Company, explains how proper consumable selection can have a significant effect on welding success on subsea sour service pipelines.
Materials specification plays a crucial role in the success and integrity of welds on subsea pipelines, especially sour service pipelines, particularly when it comes to selecting welding consumables.
A sour service pipeline environment is defined by NACE International specification MR017 as “fluids containing water and hydrogen sulfide (H2S) that is a total pressure of 0.4MPa (65psia) or greater, and if the partial pressure of hydrogen sulfide in the gas is greater than 0.0003MPa (0.05psia).”
More simply put, concentrations of H2S, known as sour gas, are becoming increasingly more prevalent in subsea oil and gas transport conditions. Not only is this gas almost as deadly as carbon monoxide, but it also can be highly damaging to pipelines if the correct materials are not specified. On pipeline exteriors, welds are exposed to lower hydrogen concentrations than those in contact with the sour environment found on surfaces inside the pipeline. Hydrogen sulfide’s corrosiveness can result in sulfide stress corrosion cracking in the welded material.
Consumables for sour service
Typically, high-strength carbon steel, grade X70, is used for such subsea jobs. There are no major differences in welding X70 material compared to lower-grade steel, such as X65. However, welding consumables do need to be selected to obtain the specified mechanical requirements for this particular product.
Low alloy GMAW consumables are designed for semi-automatic or automatic welding of root, hot, fill and cap passes in all positions on up to X80-grade pipe and root passes on up to X100-grade pipe, making this consumable category ideal for high-strength carbon steel used in subsea sour service environments.
Some products are capable of producing Charpy V-Notch impact properties of 69-95 J at -50°C – meeting the tough requirements in demanding pipeline jobs, such as the one Abu Dhabi-based Valentine Maritime (Gulf) LLC (VML) encountered in late 2010 – a 100-km subsea pipeline and cable installation project in the Manifa field for Saudi Aramco.
The company had launched its operations 20 years prior with the acquisition of a derrick lay barge (DLB), the Regina-250. In 2010, it added another barge and was taking another major step forward in its growth by commissioning the construction of a third, even larger, DLB. According to Valentine Maritime’s Project Manager (PM) Subash Nair, a good showing with Saudi Aramco could go a long way towards making sure the fleet of pipelay and derrick barges stayed busy for years to come.
“It was an extremely important project for us as a company because it was a breakthrough with Saudi Aramco,” Nair notes.
Although VML’s crews had successfully installed hundreds of kilometers of subsea pipeline, they had not previously welded the hefty, 42” diameter, 1.25” wall thickness, X70-grade pipe that this project demanded. The project also had some areas that used X65-grade pipe, another suitable option for sour environments, including subsea valve skid tie-in.
On the Manifa pipeline project, VML needed to specify a welding process and materials that would work both with such a heavy-duty pipe and the sour environment, while keeping the project running at a highly productive pace, on time and on budget.
Welding engineers settled upon automated gas metal arc welding (GMAW) in short-circuit mode with a narrow J-groove design to achieve a high rate of welds. However, joints welded by an automated MIG process are more susceptible to lack of fusion defects, so VML needed to build strict controls for the welding details, including selecting the right welding wire.
This wire consistently beat out the competitors by delivering the needed results in all aspects. Competitive products revealed micro porosity above the root up through the cap pass, but Pipeliner 80Ni1 showed strong resistance to this phenomenon. It also met all the mechanical property requirements, including Charpy V-Notch, tensile strength and crack tip opening displacement (CTOD).
“There was a negligible amount of gas pores in the weld metal compared to other similar wires and brands,” the welding engineer noted. “As a result of the consumable’s good wetting nature, LOF was totally eliminated.”
Because of the large diameter and 1.25” wall thickness, AUT with an approved engineering critical assessment (ECA) was the chosen method for NDT from a speed and sensitivity standpoint. This, however, was backed up by an excellent pipeline welding team, which is an in-house capability of VML.
On the job
Once on the job, VML’s team of professional welders worked back-to-back shifts. They averaged more than 79 joints per day on the 42” OD x 1.25” WT pipe, with a repair rate of less than 3 percent. The peak production was 92 joints per day.
Many variables to consider
The specification of consumables for any offshore project, particularly subsea sour service pipelines, takes into careful consideration many selection factors; from welding process, to grade and thickness of the steel to stringent ECA requirements under consideration, choosing the right electrode takes time and testing.
In the end, this time and attention to choosing the specification/process delivers a definitive return on investment. With the right consumable, welds in sour environments will maintain integrity and avoid LOF defects and hydrogen sulfide cracking, among other failures.
Finally, properly specified welding wire can contribute to overall gains not only in quality but also project productivity. Joints welded right the first time can keep a job moving on or ahead of schedule, as the example of VML suggests.
“We were not expecting that level of performance,” the PM said, looking back at the job after the lengthy consumable specification process. “Saudi Aramco also was pleasantly surprised by the quality of production and the efficiency of the work by a contractor for the first time, working under stringent quality and safety requirements of Saudi Aramco, VML now can bid future subsea projects for this esteemed client.”
Article reprinted from World Pipelines, May 2012 View