Foundation, Concrete and Earthquake Engineering

High Strength Steel Pipe: Higher Operating Pressure Gas Line

To achieve higher throughput, the pipe line in service have to withstand higher operating pressure. For this purpose high strength steel pipelines are used. For example, actual pipeline operating pressure varies depending on many factors, including the diameter, wall thickness, and grade of pipe, but for the most part it is typically around 1,000 to 1,500 PSI. Using higher strength steel, such as going from X60 to X80, and keeping everything else the same results in a proportional increase in maximum allowable pressure, or in this case, 33 percent higher pressure. In fact, some of the newer high-strength steel pipelines operate around 2,000 PSI.

The high-strength steel also may allow for a reduction in wall thickness, which reduces costs associated with hauling and handling of material, as well as time spent welding the pipe. In case of welding, the arc energy can interact with the ambient air. A partially ionizing the arc into hydrogen is occurred which can be absorbed by the weld metal. This phenomenon is called hydrogen-assisted cracking (HAC). This situation is exacerbated when the welding of these high-strength steels occurs in locations with relatively high humidity, such as Mississippi. This challenge is overcome by low-hydrogen electrodes.

Now-a-days high-strength steel pipe grades like X80 are widely used for long-distance transmission pipelines, but present a challenge for welding. In the past field girth welds on X70 and lower-strength grades were completed almost exclusively with cellulosic electrodes because of their low cost. However, with high-strength X80, cellulosic electrodes typically are used only for root and hot passes and are not used for fill passes, due to the increased risk of HAC.

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