Depositing the Pipeline
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.
Other steps in the constructions process:
- Clearing and grading and trenching
- Stringing and welding pipe segments together
- Depositing the pipeline, backfilling and testing