Underground gas and oil pipelines eventually reach the end of their service life. This creates a challenge – what to do with a former asset once it is decommissioned and/or abandoned to avoid unintended consequences.
This has driven both the oil and gas industry and water/wastewater systems to look for solutions for safely and effectively abandoning pipelines. One solution has been to excavate and remove the abandoned line – this is expensive and time consuming and carries its own environmental costs, in terms of disturbed ground, backfill material and access requirements. Another solution has been to fill the abandoned line with concrete or other flowable fills – this is also costly, as a lot of the pipelines are located in remote areas with no access to ready mix facilities. Another difficulty is getting concrete to flow well enough to fill the large sections of the abandoned pipeline.
Golder is currently developing an alternative based on our long experience in the mining sector. We have well over 20 years of experience helping mining companies deal with a similar problem – the need to avoid collapse of their abandoned underground workings. Mining companies also need to deal with the problem of “mine water” – groundwater that flows through the mine, picking up contaminants along the way, which must then be treated before release.
Golder helps mining companies take the various waste products they produce on site e.g. waste rock, overburden, sand, etc. and engineer a recipe using the waste along with binder e.g. cement and other reagents, plus some water. The resulting “paste,” much like toothpaste in consistency, is pumped underground to fill the abandoned voids and tunnels. There, it cures with little to no change in volume and little to no water bleed, producing backfill strong enough to hold up the rock mass overhead. Backfilling the underground workings with paste also helps to solve the mine water problem as the water cannot easily penetrate through the paste mass.
Paste technology is mature, robust and widely understood. The paste recipes are specific to each application and involve using locally available sources of material that meet the filling requirements e.g. strength and volume requirements.
Using our mining experience, Golder has conducted tests and completed modelling to determine the practicality of the paste solution for pipeline abandonment. In every mining application to date globally, paste has been transported via pipeline so the methodology for producing and moving paste in a pipeline is well understood. The concept in the pipeline abandonment context is to leave the material in the pipeline as opposed to flowing it through to exit. Our results show that it is possible and practical to fill a pipeline with paste and have it fill the void and gain enough strength to provide appropriate support.
One of the chief advantages of paste is its adaptability. Paste recipes can be altered to suit each particular job, depending on factors such as slope of the pipeline, diameter, and the length of pipe to be filled along with strength requirements and access points.
In addition, paste has an advantage over concrete and other flowable fills in terms of costs. Typically the strengths required by the fill are not substantial as it is not structural fill, only void filling that is usually required. So paste can be designed to produce strengths of 200 kPa – 2 MPa which is a typical strength range. This typically equates to 2-5% cement content, a significant savings over traditional concrete.
By using local sand, overburden or borrow material close to the pipeline access points, paste can also be made and injected into the line while avoiding the steady stream of ready mix trucks that may have to come from long distances. Paste is useful in remote or hard to access places as it can be designed to flow long distances and still meet the fill and strength requirements. As an example, in mining applications paste is pumped up to several kilometers.
As with any pipeline abandonment effort, consideration must be given to local regulations, length and diameter of the pipe, third party infrastructure stability requirements, and overall land development and planning, before selecting any one (or more) solutions.
While it is still in development, the idea of paste technology to solve the problem of abandoned pipelines shows good hope as a viable option for the future.
ABOUT THE AUTHOR
Sue Longo (B.S.C.E., MBA), is a Principal and Senior Project Manager at Golder, based in Calgary Alberta. She has over 15 years of experience in domestic and international mining projects, with a focus on backfill and tailings disposal, and pipeline project management in the mining, power, and oil sand industries.