Often, it may be necessary to remove old pipes that have reached the end of their service life. Removing abandoned pipelines can provide many benefits to the owner. While it may seem hard to believe, some abandoned pipes and their surrounding areas still require routine maintenance, since the owner may still be legally responsible to local authorities and private landowners.
With the non-operational pipeline safely removed, the pipeline owner is relieved of all their legal obligations.
However, current removal methods present several challenges. Conventional pipeline removal techniques usually involve extensive excavation to expose the abandoned pipe. The pipe is then cut into several pieces for removal and transportation. Finally, the site is reinstated to its pre-existing condition by backfilling, compacting, and revegetating the surrounding area.
In addition to being costly, open-cut methods may be extremely difficult or even impossible in some cases. Road crossings, river crossings, protected areas and sites with unstable soil conditions are examples of situations where conventional removal methods are impractical.
Trenchless pipeline removal (TPR), as its name implies, involves removing decommissioned pipelines with little to no trenching (or excavation). This technique is especially ideal for applications where it is not possible to remove pipes via open-cut methods.
How Does Trenchless Pipeline Removal Work?
Trenchless pipeline removal, like other trenchless techniques, such as pipe jacking and horizontal directional drilling (HDD), requires relatively small excavations in the form of entry and exit pits. These pits provide a work area to accommodate equipment placement and facilitate removal operations.
The removal of buried pipelines is a fairly complex operation since the magnitude of the soil-to-pipe friction (i.e., skin friction) can be significant. The pulling force necessary to remove underground pipelines without trenching can be calculated by considering this friction value (among other factors).
However, the resistance provided by friction can often result in extremely high (or even impractical) pulling forces.
To facilitate TPR, Italian company, Enereco, developed a removal system which reduces the diameter of the decommissioned pipe, resulting in a drastic decrease in the opposing frictional forces between the surrounding soil and the pipeline.
The Helical Cut
To minimize skin friction during pipe removal, a helical cut is carried out along the length of the pipe in its original position. This is done by inserting a cutting machine into the pipe from the entry pit. This machine consists of a rotating head with a water jet nozzle which sprays high-pressure, high-velocity streams of water capable of cutting through steel.
The water-jet technology allows cutting to be performed in-place without the formation of sparks. As such, existing steel pipes can be cut in the presence of residual gas or hydrocarbons without the risk of fires or explosions.
Additionally, using water as a cutting tool ensures that a clean, consistent cut is maintained throughout the length of the pipeline since the water-jet does not lose its cutting edge.
Mechanical devices, on the other hand, are susceptible to wear, causing them to lose their cutting finish. Additionally, mechanical tools would also be difficult or impossible to remove from inside the pipeline during cutting operations.
As the cutting tool traverses the pipeline, the rotating water-jet nozzle cuts the pipeline, resulting in a helically-shaped cut. This helical cut causes the pipe to act as a spring when pulling and rotating forces are applied to it. And, like a spring, when tensile forces are applied to it, the diameter reduces, significantly reducing skin friction.
As an additional bonus, the water from the jets also serves to lubricate the surrounding soil, further reducing the friction between the soil and the pipe.
Figure 1: Schematic of the cutting process
The Extraction Process
Once the helical cut is complete, the pipe is ready for extraction. Typically, an HDD rig capable of generating the required pulling force and torque is selected. A pulling head is welded to the pipe and connected with the rods of the rig. The rig applies a pulling force on the end of the pipe, which activates the spring effect of the helical cut.
The pipe then elongates and reduces in diameter along its entire length. Where the friction force is lower than the pulling force, the extraction commences. In addition to a pulling force, the rig also applies a twisting force, also known as torque.
The twisting helps to loosen the surrounding soil and reduce friction even further. The extraction process continues until the entire pipe is pulled out from the ground in which it was buried.
Figure 2: Schematic of the extraction process
The final step of TPR is the filling operation. This involves filling the remaining hole with suitable material. Depending on the existing soil conditions, filling can be performed during the extraction phase, or it can be done after extraction is complete.
The former process is typically required where soil investigations indicate that there will be significant instability upon pipe removal. In this case, a suitable mixture of water and clay is used during the extraction phase to help strengthen the walls of the hole and increase stability before filling.
What We've Learned
The innovative TPR system developed by Enereco represents one of the latest advancement in trenchless technologies. Currently, this technique can be used on pipelines that are 10-inches in diameter or larger.
Like all trenchless technologies, this method has the potential to evolve over time. With the increased demand for new oil and gas infrastructure, the market for trenchless pipe removal seems promising as owners of abandoned gas pipelines may seek ways to minimize their legal obligations to landowners.