Large Diameter Pipelines Onshore

Pipelines are not just a mode of fuel transportation but they are an identity which traverses the length and width of geographical spans, impacting the live of flora and fauna of the area it traverses. So it becomes utmost responsibility of pipeline fraternity to analyze each and every aspect of pipeline design and to lay the pipeline in such a manner which results in congenial development, without affecting the delicate balance established by Mother Nature.

Increasing energy demand requires transportation of huge quantity of POL to meet the mammoth requirements of refineries, petrochemical complexes, city gas networks etc. Also, with the increasing difficulties in acquisition of land for pipeline installation, large diameter pipelines are becoming only viable option. As per the industrial practice and for this article, pipelines with size 36” (910 mm) and above are considered as large diameter pipelines.

Large diameter
Large diameter not only eases the operators from meeting the current as well as future requirement but also saves them from the repeat exercise of installation of multiple pipelines. Owing to the large diameter, frictional loses are comparatively less which aids in placing pumping/ compressor stations at a larger interval. Also, CP currents travels longer distances on large diameter pipelines which further reduces the requirement of land for CP stations.

Coming to the management of large diameter pipelines, which is utmost challenging as any spillage or leak in large diameter pipeline can result in catastrophic damages to the nearby area due to large quantity of POL into it. Therefore, large diameter pipelines necessitate highly sensitive pipeline control systems and pipeline safety system. Close monitoring of the pipeline transporting such huge quantity of explosives material no longer remain as the responsibility of the operator only but also integrate in the components of national security.

Installing large diameter pipelines comes with many challenges which shall be analyzed prudently against its benefits. This article presents insight into the aspects that shall be given consideration during conceptualization, design & engineering, installation and operation of large diameter pipeline.

Selection of large diameter pipeline route shall be straighter and shall avoid steep bends &undulations. Large diameter pipelines require higher turning radius for bending the pipe at the turning points, thus at the location of bends wider ROU is required toinstall the pipeline. Acquisition of ROU is one of the main challenges which oil marketing companies (OMCs) are facing these days. The challenges of requirement of wider ROU and land required at crossing location shall be pragmatically analyzed right at the stage of conceptual study and feasibility study of pipeline.

Degree of bend angle which can be fabricated by cold field bending machine reduces with increase in diameter. Therefore, for same turning angle of the pipeline route, large diameter pipeline may require more number of cold field bend as compared to smaller diameter pipe. Angle of bend which can be achieved from a typical double random length of pipe reduces with increase in the diameter of pipeline e.g.

  • 610 mm (24”) size pipe of length 12 meter can be bent to 12o - 14o, whereas
  • 1210 mm (48”) pipe of length 12 meter can be bent to 6o - 7o only.

This also results in requirement for fabrication of higher numbers of hot induction bends which impacts the installation cost and time.

Large diameter pipeline necessitates the requirement for fabrication of thick-walled pipes:-

  • to limit D/t ratio within 100, as well as
  • for sustaining the internal pressure (Barlow’s Formula)
Susceptibility of pipe towards flattening, ovality, buckling and denting increases with rise in D/t ratio (> 100), decreased wall thickness, decreased yield strength, and combinations thereof. Therefore, pipe, having a D/t ratio less than 100, shall be utilized for large diameter pipelines.

With the increasing diameter, the requirements for tighter dimensional tolerances of the line pipes such as pipe ovality, pipe ends tolerance, straightness, and wall thickness tolerances etc. increases exponentially. It is required to ensure that pipe will not only install with ease but also maintain its durability against all types of loads predicted to be sustained during its operation. Therefore, tremendous efforts are required from line pipe manufacturers to produce the high quality line pipes within specified time.

Pipeline design
During conceptualization of large diameter pipeline a study for optimal utilization of pipeline shall be based on a “Break-even Analysis” of capital gain (due to increased overall throughput) compared with increased capital investment (due to increased tonnage).
Line pipes are required to be transported to site or storage yard from pipe mill after production. The high capacity equipment are required for handlingand hauling of large diameter pipes. Transportation of pipes becomes challenging owing to the high weight and volume of large diameter pipes. The number of tiers of pipe during transportation is governed by following criteria:

  • Stacking load and stress generated in the line pipes during transportation
  • Stacking height of the pipes within the storage height of barge/ rail wagon/ trailer
  • Weight carrying capacity of the barge/ rail wagon/ trailer
Pipe transportation on Trailer
Diameter of Pipe Number of Tiers Number of Pipe
1422 mm (56”) 1 1
1219 mm (48”) 2 3
910 mm (36”) 2 4
610 mm (24”) 3 12

Line pipe on trailer
Due to this, more number of rail wagons/ trailers are required for transportation of pipe, thus increasing the pipe transportation time and cost.
Mainline Welding:
Welding is the backbone of pipeline installation activity and high quality welding is extremely important for ensuring high integrity and safe functioning of pipeline. As already explained above, large diameter pipelines are also thick walled pipes. Consequently the weld “Inch Meter” is high and more number of welding passes (e.g. root pass, hot pass, filler pass and capping) are required for girth welding of pipes on field. This results in high welding time and thus increased chances of welding defects. Therefore, to mitigate these challenges, semi-automatic or automatic welding is preferred for large diameter pipelines.

Mainline Automatic Welding
The results of a typical welding time study show the following results:

Pipe Size (NPS) Number of Welds per Day
Automatic Welding Semi-automatic Welding*
910 mm (36”) 45 25
1219 mm (48”) 35 20
1422 mm (56”) 20 8
* Root pass is performed by manual welding process.

Requirement of High Capacity Equipment:
With the advancement of technology in oil & gas exploration and refining facilities are set up in most remote locations. Consequently the pipeline is also required to be laid in challenging geographical terrains such as hilly areas, swampy stretches, forests etc. Installation of large diameter pipes requires excavation of deeper and wider trenches along with backfilling of the same. Therefore heavy earth moving equipment such as excavator, backhoe etc. are required. Deeper and wider trenches calls for additional safety measures such as sheet piling, shoring & strutting by wooden planks for preventing the collapse of trench sides on the personnel working in the trench.

Pipeline installation requires high capacity pipe laying equipment such as:

  • For 36” pipeline - Sideboom of 40-60 metric ton is required
  • For 48” pipeline - Sideboom of 60-90 metric ton is required
  • For 56” pipeline - Sideboom of 100-120 metric ton is required

Pipeline Crossings:
Cross country pipelines traverses various uncharted locations and crossesvarious utilities like water body, railway line, roads etc.Pipeline crossing methodologies such as open-cut, jacking-boring, micro-tunneling, horizontal directional drilling etc are selected depending upon the length of the crossing, sub-soil (geo-tech) properties and ensuring minimal impact on environment.

In case of large diameter pipelines, crossings becomes more complex, especially in the case of trenchless methods. In case of pipeline crossing by trenchless technique, the chances of hole collapse increases as a larger hole needs to be drilled for pipe crossing. Crossing section becomes vulnerable to hole collapse due to longer drilling time and larger hole diameter required for pipe crossing.

In the case of pipeline crossing by Horizontal Directional Drilling methodology, following challenges are faced:

  • With increase in pipe diameter, pulling force required to pull the pipe also increases due to increase in upwards buoyancy force on the pipe. Therefore, generally pipe of diameter larger than 30” is pulled in water filled condition (partially filled/ fully filled/ pipe-in-pipe). These methods require a higher degree of expertise for execution.
  • High capacity Maxi-rigs are required for large diameter crossings.
  • Large diameter hole requires high capacity of mud pumps for maintaining the hole pressure and removal of cuttings from the hole.
Large diameter
  • The overall cost of HDD crossing is also exponentially proportional to the pipe diameter. A typical HDD of 1000 meter for 24” pipe costs $1.2 million (approx.), whereas same HDD with 48” pipe will cost $3 million (approx.).
(Note: all data given above are provided to get the comparative idea and can vary from case to case basis.)
After mechanical completion of pipeline installation activities, pipeline is hydrostatically tested to check leak tightness and strength of the pipeline before commissioning. Hydrotest requires pipeline to be completely filled and pressurize with water. Humongous quantity of water is required for performing hydrostatic testing of large diameter pipeline. Therefore, sources of water shall be classified during routing of the pipeline and pipeline sections shall be prepared such that water source is near the filling point. Filling of the pipeline by supply of water other than natural source near pipeline may have high cost implication. During hydrotesting, water is dosed with corrosion inhibitor to prevent internal corrosion of the pipeline. Corrosion inhibitors pose threat to environment and shall be disposed-off properly. Arranging for disposal of such large quantity of corrosion inhibited water is a challenging task.
In this article we have elaborated upon some challenges which are faced by pipeline industry during the implementation of large diameter pipelines. While deciding the pipe line diameter, apart from the flow assurance study and the throughput requirement, due considerations shall be given to engineering, installation and operational aspects. Direct and indirect challenges associated with implementation of large diameter pipelines may vary from project to project. This article is intended to provide a basic insight only.

Strong coordinated efforts shall be made by operators, designers, pipeline component manufacturers and government authorities/ regulatory bodies for implementation of large diameter pipeline projects and meet the astronomically growing requirements of the expanding world.