Fri. Sep 24th, 2021

City gas distribution

Transporting future fuel

#52 Procedure for laying any Gas pipeline

18 min read

Different stages of steel pipeline project are as follows :

a) Conceptualization:

  1. Market surveys for deriving basic assumption for demand.
  2. Initiation of feasibility study for Commercial / Technical viability.

b) Feasibility Report

  1. Route selection & surveys
  2. Hydraulic/simulations
  3. Estimation of the project cost
  4. Market demand assesment
  5. Project Approval

c) Engineering & Planning

  1. Design & sizing
  2. Specification & tender document preparation
  3. Material procurement/Release of work order & contracts
  4. Acquire RoU/Permissions
  5. Project Execution & Commissioning

d) Other details

  1. Pipeline length
  2. Type of Terrain
  3. No. of crossings/type of crossings
  4. No. of valve
  5. Obstruction/limitation
  6. Identification for Authorities for permission

Note 1: Type of surveys

  1. Topographical
  2. Population density index
  3. Soil resistivity

Note 2: Design/Hydraulic & simulation process

Simulation for network hydraulics is done in the SYNERGEE software. This is basically required to derive the diameter of the pipeline in such a way that required pressure at all locations can be achieved and future load projections can be ascertained.

Note 3: Constructional Stages:

  1. Pre-constructional activities
  2. Construction activities
  3. Testing & Commissioning
  4. Handover to Operation & Maintenance Department

Pre-constructional activities :It include the following,

PlanningResource & Equipment
SchedulingScheduling of activity & material ordering & delivery schedule
MobilizationMobilization of manpower & equipment
Procedure/QualificationApproval for a procedure for various activities & following welding testing & Qualification
TrainingSafety Training & Site briefing

Construction activities: It includes the following;

Natural gas pipeline construction steps:

  1. Route design and approval
  2. Surveying and staking the construction right-of-way
  3. Clearing the ground
  4. Grading the area
  5. Trenching
  6. Stringing the pipeline
  7. Bending pipes to fit ground contours
  8. Welding the pipeline segments together
  9. Coating the joints
  10. Lowering the pipeline into place
  11. Installing valves
  12. Backfilling the trench
  13. Hydrostatic testing to ensure the pipeline can handle pressure
  14. Commissioning of the pipeline
  15. Restoration to return the land to its original condition

1. Route Design and Approval

New natural gas pipeline construction requires an extensive design and approval process to ensure the natural gas gets where it needs to go while maintaining an efficient, environmentally-friendly route. The company must acquire land rights or easements for private and state land along the pipeline route. Pipeline construction also requires multiple permits and clearances to protect natural resources and local structures.

The process often involves satellite imagery and aerial photos to determine the route. Pipeline routes typically avoid any areas with potentially sensitive environmental issues, as well as highly populated areas. After scoping out the route on a wide scale, engineers hit the ground, walking the route to inspect the actual topography, vegetation and other unique characteristics of the route. The planning process takes into account historical and cultural sites, wetlands and other potential areas of concern in each community.

The length of the average project means the pipeline will go through a wide range of conditions and environmental differences. As part of the planning phase, engineers determine the best size of pipes for the different areas, as well as the need for specialized coatings on certain sections of the pipeline.

This planning phase is an essential part of the pipeline construction process. It ensures the pipeline gets where it needs to go without impacting the communities or the environment along the route. The intensive planning process also ensures the integrity of the pipeline for increased safety.

2. Surveying the Construction Area

Before breaking ground on the pipeline project, survey personnel shows up to mark the construction right-of-way. All of the preliminary route planning starts to become a reality in this phase. The survey team surveys and stakes out the construction right-of-way and temporary workspaces to allow for construction activities, based on the approval of the project.

This gives the construction personnel a clear framework for the construction activities to minimize the impact on the surrounding areas. The survey personnel also marks the centerline of the trench as a guide for digging.

3. Clearing the Area

The next phase involves clearing the path for the natural gas pipeline and the construction activities. Construction personnel needs a clear work area to get the construction equipment to the site. To minimize the impact on the surrounding area, the personnel removes only the trees and vegetation necessary to complete the construction activities.

If a landowner has fences long the construction right-of-way, the personnel cuts, and braces the existing fences and adds temporary gates and fences when and where necessary to contain livestock.

Topsoil is removed carefully before the trench digging begins. The goal is to preserve the original topsoil from the site to return it to its natural condition without losing the rich top layer of soil. This topsoil goes into a stockpile, where it remains protected until the end of the construction, when workers put it back in place.

Erosion measures, including silt fences along waterways and wetlands, go into effect at this stage.

4. Grading the Work Area

Once cleared, the ground is ready for any necessary grading. The pipeline generally moves with the topography of the land, but some leveling is necessary in certain areas to complete the project. Grading helps avoid excessive pipe bending in areas with steep slopes. If the pipeline project requires the personnel to cut into steep slopes to avoid sharp bends in the pipeline, the original steep contours are returned at the end of the construction phase.

Leveling the ground may also be necessary for the large construction equipment to operate safely. The process involves many very large pieces of equipment, and keeping the personnel safe is paramount to the success of the pipeline project.

5. Trenching the Route

With the above-ground preparation done, the trenching personnel moves in to dig the trench that holds the pipeline. Just like the topsoil that was removed during the clearing phase, the soil removed during trenching goes into storage throughout the construction process. That soil goes back into the ground to fill in around the pipe near the conclusion of the project. These practices help maintain the original state of the area while conserving resources.

The personnel must create a trench deep enough to accommodate the pipeline, plus the required distance from the pipeline to the ground surface — a minimum of 30 inches. A typical pipeline trench measures about two to five feet deep and four to six feet wide in areas with stable soil. Certain areas, such as waterways and road crossings, require greater minimum depth to meet the regulations for natural gas pipelines.  

Trenching personnel deal with a variety of terrain and buried debris. Rocks often fall along the pipeline route, and specialized equipment and explosives help the personnel clear out the rocks. When blasting is required, personnel use explosives under strict guidelines to control the blast.

Rock removed from the trench goes back into the ground as backfill at the end of construction. That rocky backfill is used up to the top of the bedrock profile in that area.

6. Stringing the Pipeline

Stringing the pipeline is essentially a dry run of the layout to piece together all the sections of the pipeline. The sections come in 40- to 80-foot lengths and stay in a stockpile area near the construction right-of-way until needed. The stringing personnel hauls in the pipeline segments using specialized trailers to handle the large sections.

This phase may sound simple, but the process requires precision to ensure the pipeline gets installed properly. Not all pipeline sections are the same. Wall thickness and coatings vary to match the specific soil conditions and other factors along the route. The stringing personnel refers to the design plans to ensure the correct segments go to the correct location.

7. Bending the Pipe

Since a natural gas pipeline route doesn’t only go along flat land, the pipes sometimes need to bend to fit into the topography of the land. This is where the pipe bending personnel enters the process. Using a special bending machine in accordance with standards, the personnel bends certain sections as needed to fit into place. Clamps and hydraulic pressure enable the machine to control the bends for smooth finished results.

8. Welding Pipeline Sections

The welding personnel comes into the construction process at this point to join the pipes into a continuous length. Quality welds are essential in building a safe natural gas pipeline. Welders must pass a qualification test before being allowed to work on the pipeline project to help ensure the quality and safety of the project.

A sideboom picks up and aligns the segments to allow the personnel to make the first welding pass. Additional passes then take place to stabilize the joints. The number of passes required varies depending on the wall thickness, with three or more passes required for some sections.

Every seam goes through a nondestructive inspection process to ensure the weld is perfect. This inspection process typically involves X-rays or ultrasounds to check for issues without affecting the integrity of the weld. When a flaw is found, the the welder either fixes it, or the flaw cut out and done again.

9. Joint Coating

The steel pipelines require special coatings to keep moisture away for corrosion protection. A standard fusion bond epoxy works for most sections of the pipeline. Sections spanning through rocky ground require additional coatings to stand up to the impact and stress that can occur due to the rocks. These coatings may include concrete, abrasive-resistant fusion bond epoxy or polyethylene.

Each length of pipeline arrives at the construction site with the coating applied to within three to six inches from each end. This uncoated area is necessary to allow for proper welding, but it also leaves those sections exposed. To remedy the potential for corrosion, those sections receive a coating on the job site.

After all welding is completed and tested, the coating process can begin. personnel must first clean the joints well to remove dirt and debris that could interfere with the coating. Once applied, the coating must dry before the pipeline goes into the ground.

The personnel also tests the existing coating on the pipeline to ensure the coating is flawless. In addition to a visual inspection for scratches or other faults, the personnel uses a high-voltage tool to detect any defects. Repair coatings go on those affected areas before the pipeline goes into the ground.

10. Lowering the Pipeline

Secure welds and defect-free coatings indicate the assembled pipeline is ready to go down into the trench. Before lowering, an inspector checks for any wildlife, livestock, rocks or debris in the trench that could present a problem. The same sidebooms that helped lift and position the pipeline segments for welding lift and lower the pipeline into place. This process takes extreme coordination, with all sideboom operators working slowly and in unison.

Great care goes into protecting the coating during the lowering process. Special slings that protect the coating go around the pipeline to move the section easily without scraping the coating. Rocky terrain requires the use of sandbags, foam blocks or a padding material, such as sand or soil, along the trench to prevent coating damage.

11. Valve Installation

Special valves to prevent problems and control the flow of natural gas go onto the pipeline at certain points. The valves allow operators to shut off the natural gas flow or isolate a section of pipeline. These valves and other special fittings go into place before the trench gets filled.

12. Backfilling

With the pipeline in position, the personnel begins backfilling the trench to cover the pipeline. The original soil goes back into the trench in the reverse order of how it was removed. This means the subsoil goes in first to keep the layering and composition the same as it was originally. A backhoe or padding machine moves the soil back into the trench, using great caution to ensure no damage occurs to the pipeline or the coating. Once the subsoil fills in the trench, the topsoil goes back on top.

Just as in other phases, rocky terrain requires some extra steps and care to prevent damage to the pipeline coatings. The personnel can screen the soil to eliminate the rocks before backfilling the trench. In lieu of screening, the person may use protective material to cover the pipe before filling the trench with the rocky soil. A final option is to bring in new soil instead of using the old rocky soil.

13. Pipeline Pressure Testing

Natural gas cannot enter the newly constructed pipeline until the personnel completes comprehensive pressure testing based on regulations. The operational pressure of transmission pipelines is typically between 500 and 1,400 pounds per square inch gauge. The purpose of the testing is to ensure the pipeline can withstand the high pressure of natural gas traveling through the line.

Hydrostatic testing uses water to test the pipe at 125 percent of the maximum pressure of the natural gas that will run through the pipeline. The pipe must withstand that pressure for a prescribed amount of time without leaking —  typically at least eight hours. The purpose of the testing is to check for leaks and to ensure the pipe is able to withstand the pressure over an extended period of time.

Each section of the pipeline undergoes hydrostatic testing. personnel block off one segment at a time with test manifolds. If the testing reveals a leak, that section receives repairs. The hydrostatic testing takes place again to ensure the repairs worked to stop the leak. This process continues until the section meets the specifications of testing.

After the section passes, the personnel empties the water and dries the pipeline to remove all water before natural gas enters the pipe. The drying process uses mechanical tools to send pressurized dry air through the pipeline and prevents internal corrosion. The personnel removes the test manifolds and any final tie-ins, and inspections take place.

The water used in the hydrostatic testing may come from a local river, lack or a municipal source. If those options aren’t available, the water gets trucked in to the location. Water often gets passed on to each new section during testing. Before being discharged, the water undergoes analysis to comply with National Pollution Discharge Elimination System discharge permit requirements. In some cases, the water must be treated before being discharged. The water and testing process must meet state and location regulations.

14. Commissioning of the Pipeline

Commissioning is the process of verifying proper operation in the pipeline. The process checks the installation and ensures control and communication systems are in place and functioning properly. Once the pipeline is deemed ready for service, the line is purged of air and loaded with natural gas.

15. Restoration

Once the actual pipeline construction ends and the pipeline is operational, the clean-up process for the construction right-of-way begins. The goal of this final step is to restore the land to its original condition as if no construction happened. personnel tries to have the land cleaned up and restored within 20 days of backfilling the pipeline trench. Weather and site conditions sometimes delay the efforts, but the personnel works hard to get the ground back to normal as quickly as possible.

The personnel doesn’t simply pick up the tools and move on, though. The restoration process involves several steps to ensure the integrity of the area.

Clean-up personnel complete the following tasks:

  • Remove any temporary structures, large rocks brought to the surface during construction and any other debris left from the process
  • Stabilize the construction right-of-way
  • Perform final grading
  • Recreate the original contours of the land as closely as possible to maintain water drainage
  • Replace the removed topsoil
  • Re-seed the area if the timing and weather allow
  • Replace vegetation to help with soil stabilization and to restore the natural look of the area
  • Mulch the newly seeded or planted areas to keep the seeds in place and support growth
  • Install erosion prevention features, particularly on hills, such as interceptor dikes that help divert water
  • Install riprap made of stones or timber near streams or wetlands for soil stabilization
  • Add pipeline markers along fences, waterways and road crossings to mark the location (markers include name of pipeline owner and emergency information)

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