Steel pipe used in transporting natural gas is constructed in one of two ways.
For larger pipe typically used for transmission systems, a mill manufactures a steel plate that is formed into a cylindrical shape. A seam is then welded across the top and tested using ultrasonic and/or radiological equipment.
For a smaller pipe typically used in a gathering or distribution system, a mill produces a cylindrical bar of steel that is pierced to create a hole through which the gas will flow. This technique is used to create pipe with diameters ranging from ½ inch to 24 inches.
Regardless of the technique used to create the pipe, all steel line pipe is coated to protect it from corrosion and other damage. Pipe is typically pressure tested in the factory and again in the field prior to installation to ensure that the pipe will not leak.