Construction Process

Many people are surprised by the number of people and amount of equipment required to build a pipeline. Pipeline construction looks much like a moving assembly line. As one crew completes its work, the next crew will move into position to complete its piece of the construction process. It includes the following components:

Pre-construction survey

Before construction begins, surveys will be done on the environmental features along proposed pipeline segments. Utility lines and agricultural drainages are located and marked to prevent accidental damage during pipeline construction. Next, the pipeline's centerline and the exterior right of way boundaries are staked.

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Clearing and Grading

The Clearing and Grading crew leads the construction spread. 

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 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. Brush is shredded and properly disposed. As may be necessary in agricultural areas, topsoil may also be stripped to a predetermined depth and stockpiled along the sides of the right-of-way.

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Pipe Inspection and Stringing

At steel rolling mills where the pipe is fabricated, pipeline representatives will carefully inspect new pipe to assure that it meets industry and federal government safety standards. For corrosion control, the outside surface will be treated with a protective coating.

The pipe will be transported from the pipe mill to a pipe storage yard in the vicinity of the pipeline location. The pipe lengths typically are 40 to 80 feet long. A stringing crew using specialized trailers will move the pipe from the storage yard to the pipeline right-of-way. The crew will be careful to distribute the various pipe joints according to the design plan since the type of coating and wall thickness can vary based on soil conditions and location. 

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Trenching

The trenching crew will use a wheel trencher or backhoe to dig the pipe trench. The U.S. Department of Transportation (DOT) requires the top of the pipe to be buried a minimum of 30 inches below the ground surface in rural areas, so the depth of the trench will be at least five to six feet deep for pipe 30 to 36 inches in diameter. For less rural areas, the pipe must be buried a minimum of 36 inches. The pipe will be buried even deeper at stream and road crossings.

If the crew finds large quantities of solid rock during the trenching operation, it will use special equipment or explosives to remove the rock. The contractor will use explosives carefully, in accordance with state and federal guidelines, to ensure a safe and controlled blast. 

In cultivated areas the topsoil over the trench will be removed first and kept separate from the excavated subsoil, a process called topsoiling. As backfilling operations begin, the soil will be 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.

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Pipe Bending

The pipe bending crew will use 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 a very smooth, controlled bend in the pipe. All bending is performed in strict accordance with federally prescribed standards to ensure integrity of the bend.

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Welding

The pipe gang and a welding crew will be responsible for welding, the process that joins the various sections of pipe together into one continuous length. The pipe gang uses special pipeline equipment called side booms to pick up each joint of pipe, align it with the previous joint and make the first part (pass) of the weld. The pipe gang then moves down the line to the next section repeating the process. The welding crew follows the pipe gang to complete each weld. In recent years, contractors have used semi-automatic welding units to move down a pipeline and 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 hand-weld at connection points and crossings. 

As part of the quality-assurance process, each welder must pass qualification tests to work on a particular pipeline job, and each weld procedure must be approved for use on that job in accordance with federally adopted welding standards. Welder qualification takes place before the project begins. Each welder must complete several welds using the same type of pipe as that to be used in the project. The welds are then evaluated by placing the welded material in a machine and measuring the force required to pull the weld apart. It is interesting to note that the weld has a greater tensile strength than the pipe itself.

A second quality-assurance test ensures the quality of the ongoing welding operation. To do this, qualified technicians take X-rays of the pipe welds to ensure the completed welds meet federally prescribed quality standards. The X-ray technician processes the film in a small, portable darkroom at the site. If the technician detects any flaws, the weld is repaired or cut out, and a new weld is made. Another form of weld quality inspection employs ultrasonic technology.

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Line pipe is externally coated to inhibit corrosion by preventing moisture from coming into direct contact with the steel. 

Normally, this is done at the mill where the pipe is manufactured or at another coating plant location before it is delivered to the construction site. 

All coated pipe, however, has uncoated areas three to six inches from each end to prevent the coating from interfering with the welding process. Once the welds are made, a coating crew coats the field joint, the area around the weld, before the pipeline is lowered into the ditch. 

Pipeline companies use several different types of coatings for field joints. Prior to application, the coating crew thoroughly cleans the bare pipe with a power wire brush or sandblast to remove any dirt, mill scale or debris. The crew then applies the coating and allows it to dry prior to lowering the pipe in the ditch. Before the pipe is lowered into the trench, the coating of the entire pipeline is inspected to ensure it is free of any defects.

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Lowering In

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 the pipe and carefully lower the welded sections into the trench. Non-metallic slings protect the pipe and coating as it is lifted and moved into position.

In rocky areas the contractor may place sandbags or foam blocks at the bottom of the trench prior to lowering-in to protect the pipe and coating from damage.

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Backfilling

Once the pipe has been placed in the trench, the trench can be backfilled. This is accomplished with either a backhoe or padding machine depending on the soil makeup. As with previous construction crews, the backfilling crew takes care to protect the pipe and coating as the soil is returned to the trench. As the operations begin, the 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, or bring in clean fill to cover the pipe. Once the pipe is sufficiently covered, the coarser soil and rock can be used to complete the backfill. 

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Hydrostatic Testing 

Before the pipeline is put into natural gas service, the entire length of the pipeline is pressure tested using water. The hydrostatic test is the final construction quality assurance test. 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 operating pressure. The test pressure is held for a specific period of time to determine if it meets the design strength requirements and if any leaks are present. Once a test section successfully passes the hydrostatic test, water is emptied from the pipeline in accordance with state and federal requirements. The pipeline is then dried to assure it has no water in it before gas is put into the pipeline.

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Restoration

The final step in the construction process is restoring the land as closely as possible to its original condition. Depending on the projects requirements, this process typically involves decompacting the construction work areas, replacing topsoil, removing large rocks that may have been brought to the surface, completing any final repairs to irrigation systems or drain tiles, applying lime or fertilizer, restoring fences, etc.

The restoration crew carefully grades the right-of-way and in hilly areas, installs erosion-prevention measures such as interceptor dikes, which are small earthen mounds constructed across the right-of-way to divert water.

As a final measure the crew may plant seed and mulches the construction right-of-way to restore it to its original condition.