Stringing, Welding and Coating Pipeline Segments

Natural gas pipelines are separated into segments typically 40 to 80 feet long.  A stringing crew uses specialized trailers to move the pipe from a storage yard to the pipeline right-of-way. 

The crew meticulously monitors the pipeline design plan to ensure various pipeline segments are distributed properly along the pipeline right-of-way because 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, and heavy wall pipe is required at road crossings and in special construction areas.

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 ensure the integrity of pipe is preserved.

Welding joins the various sections of pipe together into one continuous length.  Special pipeline equipment called a side boom is 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.

In recent years contractors have used semi-automatic welding units to complete the welding process.  Semi-automatic welding, done to strict specifications, still requires qualified welders and personnel are required to set up the equipment and conduct hand welding at connection points and crossings.

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 coating crew coats the remaining portion of the pipeline before it is lowered into the ditch.

Prior to lowering the pipe into the trench, the coating of the entire pipeline is inspected to ensure it is free of defects.

Other steps in the constructions process:

  1. Clearing and grading and trenching
  2. Stringing and welding pipe segments together  
  3. Depositing the pipeline, backfilling and testing
  4. Restoration