Steps of Construction

Step 2: Stringing, Welding and Coating Pipeline Segments

 

Natural gas pipelines are assembled in segments typically 40 to 80 feet long.

The crew monitors the pipeline design plan to make sure that the segments of pipe are lined up along the pipeline right-of-way. This process is called stringing. The type of coating and wall thickness can vary based on soil conditions and location. For example, concrete coating may be used in streams and wetlands, while road crossings, populated areas and special construction areas may require heavy wall pipe.

Pipe bending

The pipe bending crew uses a bending machine to make slight bends in the pipe to account for changes in the pipeline route and to conform to the topography.

The bending machine uses a series of clamps and hydraulic pressure to make very smooth, controlled bends in the pipe. All bending is performed in strict accordance with federally prescribed standards to preserve the integrity of the pipe.

Welding

Welding joins the various sections of pipe together into one continuous length. Special pipeline equipment called side booms are used to pick up each pipeline segment and align it with the previous segment. The crew then makes the first part (pass) of the weld. 

The welding crew follows the pipeline along the route until each segment is welded together. Depending on the wall thickness of the pipe, three or more passes may be required to complete each segment weld. Inspectors monitor the weld process to assure quality and safety.

Coating

Natural gas pipelines are externally coated to prevent moisture from coming into direct contact with the steel and causing corrosion. 

This process typically is completed before the pipeline is delivered to the construction site.  All coated pipelines are delivered with uncoated areas three to six inches from each end to prevent the coating from interfering with the welding process.

Once the welds are completed, a crew coats the remaining portion of the pipeline before it is lowered into the ditch.

Prior to lowering the pipe into the trench, inspectors review the coating of the entire pipeline to make sure that it is free of defects.