How are Pipelines Built? From the Perspective of INGAA Foundation

Pipelines cannot be constructed overnight, and the entire construction process can take up to 18 months. perhaps a lot more than that.

Before construction can begin, the company must obtain legal rights to the land along the proposed route, called a right-of-way, from landowners. Natural gas pipelines are constructed in response to the evolving supply and demand dynamics of the natural gas market.  In order to construct an interstate pipeline, a company must receive authorization from the Federal Energy Regulatory Commission (FERC or Commission), which includes a determination that there is a need for the facility and a thorough review of the proposed pipeline route and the environmental impacts associated with the proposed facilities.

 

Construction in Progress

A pipeline construction project looks much like a moving assembly line.  A large project typically is broken into manageable lengths called “spreads,” and utilizes highly specialized and qualified workgroups.  Each spread is composed of various crews, each with its own responsibilities. The tasks include steps in the constructions process as following:

1. Clearing, grading and trenching
   Before any construction can begin, a survey crew carefully surveys and stakes the construction right-of-way to ensure that only the pre-approved construction workspace is cleared. A large project typically is broken into manageable lengths called “spreads,” and utilizes highly specialized and qualified workgroups. This crew is responsible for removing trees, boulders and debris from the construction right-of-way and preparing a level-working surface for the heavy construction equipment that follows.
   
   
   The crew installs silt fence along the edges of streams and wetlands to prevent erosion of disturbed soil. Trees inside the right-of-way are cut down, and the contractor removes or stacks the timber along the side of the right-of-way depending on the landowner’s wishes.
   
   
   The trenching crew typically uses a wheel trencher or backhoe to dig the pipe trench.  DOT requires the top of a pipeline to be buried a minimum of 30 inches below the ground surface.  The pipeline must be buried even deeper at river and road crossings.
   
   
   If the crew finds large quantities of solid rock during the trenching operation, it uses special equipment or explosives to remove the rock.  The crew uses explosives carefully, in accordance with state and federal guidelines, to ensure a safe and controlled blast. 
   
   
   In cultivated areas, the topsoil over the trench is removed first and kept separate from the excavated subsoil, a process called topsoiling. As backfilling operations begin, the soil is returned to the trench in reverse order with the subsoil put back first, followed by the topsoil.  This process ensures the topsoil is returned to its original position.
   
   
2. Stringing and welding pipe segments together
   
   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.
   
   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.
3. Depositing the pipeline, backfilling and testing
   Lowering the welded pipe into the trench demands close coordination and skilled operators.
   
   
   Using a series of side-booms, which are tracked construction equipment with a boom on the side, operators simultaneously lift and carefully lower the welded pipe sections into the trench.  Non-metallic slings protect the pipe and coating as it is lifted and moved into position.
   
   
   
   
   In rocky areas, a contractor may place sandbags or foam blocks at the bottom of the trench prior to placing the pipeline in the trench in order to protect the pipe and coating from damage.
   
   With the pipeline successfully laid in the trench, crews begin backfilling the trench.  This can be accomplished with either a backhoe or padding machine depending on the soil composition.  As with previous construction crews, the backfilling crew takes care to protect the pipeline and coating as the soil is returned to the trench. 
   
   
   Soil is returned to the trench in reverse order, with the subsoil put back first, followed by the topsoil.  This ensures the topsoil is returned to its original position.  In areas where the ground is rocky and coarse, crews screen the backfill material to remove rocks, bring in clean soil to cover the pipeline, or cover the pipe with a protective material to protect it from sharp rocks.
   Before natural gas is transported through a new pipeline, the entire length of the pipeline is pressure tested using water.  This hydrostatic testing is the final construction quality assurance test before the pipeline is put into operation. 
   Requirements for this test are also prescribed in DOT’s federal regulations.  Depending on the varying elevation of the terrain along the pipeline and the location of available water sources, the pipeline may be divided into sections to facilitate the test.  Each section is filled with water and pressured up to a level higher than the maximum pressure at which the pipeline will operate when carrying natural gas. 
   The test pressure is held for a specific period of time to determine if the pipeline meets the design strength requirements and if any leaks are present.  Once a section successfully passes the hydrostatic test, water is emptied from the pipeline and the pipeline is dried to ensure that no water is present when natural gas begins to flow.
4. Restoration
   
   
   
   The final step in the construction process is to restore the right-of-way and easement land as closely as possible to its original condition
   Depending on the requirements of the project, this process typically involves such things as replacing topsoil, removing large rocks that may have been brought to the surface, completing any final repairs to irrigation systems or drain tiles, spreading lime or fertilizer, restoring fences, etc. 
   The restoration crew carefully grades the right-of-way.  In hilly areas, the crew installs erosion prevention measures such as interceptor dikes, which are small earthen mounds constructed across the right-of-way to divert water. 
   The restoration crew also installs riprap, consisting of stones or timbers, along streams and wetlands to stabilize soils.  As a final measure, the crew may plant seed and mulch the construction right-of-way, to ensure the foliage and grassland is restored as close as possible to its original condition.

As one crew completes its work, the next crew moves into position to complete its piece of the construction process.  Each spread may be 30 to 100 miles in length, with the front of the spread clearing the right-of-way and the back of the spread restoring the right-of-way.

Sources:

• http://www.ingaa.org/cms/65.aspx
• http://www.ingaa.org/Topics/Pipelines101/65/67.aspx
• http://www.ingaa.org/Topics/Pipelines101/65/70.aspx
• http://www.ingaa.org/Topics/Pipelines101/65/80.aspx
• http://www.ingaa.org/Topics/Pipelines101/65/84.aspx