ARCHIVE. Steel alloy pipeline is the mode of transport for natural gas and its byproducts. To prevent the escape of hazardous materials and costly maintenance shutdowns, these massive tubes must be airtight and durable. They must also be supple enough for welding and molding. These two demands are hard to meet without breakdowns in the system.
A team of researchers based in Qatar and Canada are looking closely into erosion and how it causes a chemical breakdown, i.e. corrosion, in different grades of alloy steel. Their work promises to make economic and safety impacts on the fuel industry.
By definition, steel alloys involve a combination of substances that, when combined in different ways, display different degrees of heat resistance and durability, said Dr. El-Sadig Mahdi, Associate Professor in the Mechanical and Industrial Engineering Department at Qatar University and principal investigator on a group of studies into corrosion in steel alloy pipelines. A key substance in any steel alloy is chromium.
“In order to increase the corrosion resistance, you would use more chromium,” Dr. El-Sadig explained. “This results in different grades of alloy steel, like x52, which is widely used in pipelines now, and then higher grades like x65 and x100. But increasing the chromium content means that the pipeline is harder, which means that it is also harder to weld. The same goes for titanium and nickel—they increase durability but make it difficult to weld and work with an alloy.”
Dr. El-Sadig, along with researchers from the Corrosion Group, Department of Materials Engineering at the University of British Columbia, Vancouver, Canada, is looking at the erosion factors—such as heat, water and particle movement against the sides of the pipelines—and how these impact the rate of corrosion within their steel alloy walls.
“This is all about chemistry,” Dr. El-Sadig said. “The effects of temperature, the pH, or acidity, of a liquid running through the pipes. And if you mix water, oxygen and higher temperatures, you have catalysts to increase the chemical reactions. Here in Qatar, with the heat, you know the environment is harsh. So in the case of corrosion of steel, there is no way to avoid it here, in fact it’s accelerated.”
Microscopic view of corrosion's effect on a steel surface, courtesy team publication in Corrosion Science 58 (2012) 181-191
The research is critical in Qatar, where every minute of natural gas production and transport counts. When a pipe needs replacing or a leak needs fixing, the shutdown cost can be astronomical. This is not to mention the safety issues when dealing with hazardous chemical leaks if pipes should fail.
“In that way, since you will have corrosion, you want to delay it,” said Dr. El-Sadig. “How can you delay it? Employ high-performance steels, or higher grades of steels that are more expensive. Let’s say if the pipe’s expected lifetime is 10 years, it will bump it up to 12.”
It sounds straightforward: take a look at how long the pipes last, find a pipe that lasts longer and do a cost-benefit analysis. But the types of erosion can vary with every twist and turn of the pipeline, and Dr. El-Sadig and his colleagues are analyzing many factors to see how they might translate into upgrades to alloys along the pipeline.
“That is what we are really running after, the proof in this argument … to look closely at the types of alloys in place and the different factors that complicate the corrosion rate in different places,” Dr. El-Sadig said. “We need to offer the complete information package to the industries.”
It’s important then to know the rate of corrosion along different segments of pipeline, Dr. El-Sadig explained. A big part of the corrosion story involves ‘water hammering,’ when condensation inside the pipe due forms due to a sudden spike in temperature. Where hammering is present, a stream of water rushes at a high speed, resulting in extra friction within some stretches of pipeline. If that fluid contains some particles, he explained, then the friction is higher and so is the amount of erosion and rate of corrosion.
“So what we have to do is find out where exactly the pipeline needs to be thicker than other stretches,” he said, “because I know that if I suggest an upgrade in the alloy, it’s going to be very difficult for the company to implement it. They need special technicians to weld it, and they need also to understand the effect of welding on these pipes.
Natural Gas Pipeline
Natural gas pipeline, photo courtesy Glen Dillon
“Welding involves heating a part of the pipeline, which becomes weaker than other zones. So there are so many factors to consider. We’re tackling all of them and just started a project that looks into welded areas where we can use x65 and x100 alloys, together. I think this is going to offer some nice findings.”
Dr. El-Sadig is working on three National Priorities Research Program (NPRP)-funded projects and is overseeing almost 20 students who are working on Undergraduate Research Experience Program (UREP)-funded projects. His students are working on studies ranging from robotic approaches to scanning the pipes for damage, to using fluid dynamics principles, electron microscope readings and software to analyze what happens at the t joints of the pipelines.
“I can tell you that whenever we deal with QNRF it’s really smooth,” Dr. El-Sadig said.
Setting their sites on offshore pipelines, Dr. El-Sadig and a team of researchers from the United States are applying for funding that would support undersea research on pipelines exposed to different pressures and sea water among other conditions.
Erosion-corrosion behavior of a variety of API X-series transportation pipeline steels for Qatar oil and gas industry.